CA1193008A - Information-recording process and apparatus - Google Patents

Abstract

Abstract of the Disclosure Information-recording process in which heating e.g. using a pulsed laser, produces recording marks in the form of areas of surface relief on the surface of a recording medium. The medium is selectively producible in different density states by cooling it at a selected cooling rate from the molten state, and can be switched between said states by controlled exposure to the heating, thus permitting repeated recording and erasing on the medium. This permits very small marks of less than 1 micron diameter to be produced using a diffraction-limited spot of light, and as surface relief of high aspect ratio can be obtained at low power levels, it is possible to achieve high packing densities and high signal to noise ratio at low energy costs.

Description

INFORMATION-RECORDING PROCESS & APPARATUS
This invention relates to information storage devices and, in particular, to an erasable and reusable archival memory using heating, which may be applied to a recording medium using focused radiation, especially a highly focused laser beam, for recording, reading and erasing.
The present invention provides an information-recording process comprising the steps of: providing a recording medium exhibiting first and second stable solid physical states associated with different respective specific volumes and being selectively producible in said states by heating said medium and cooling it from the heated state at selected rates of cooling;
applying heat transiently to a localised volume of said medium adjacent its surface, said volume being sufficiently large in re-lation to the area ofthe surface of the medium that is heated thaton transition between said first and second s-tates, at least one area of changed surface curvature, sufficient to directly and immediately distinguish said localised volume from the surrounding medium, is produced at said surface; said medium being originally in one of sai.d first and second states; and permitting said volume to cool under conditions selected so that said volume is converted to the other of said first and second states.
The invention also provides an information-recording process comprising exposing an optically smooth and homogeneous su.rface of a recording medium to a concentrated flux oE energy, said medium being a solid having first and second solid sta-tes associated with different respective specific volumes and being selectively producible in said states by heating said medium and cooling it from the heated state at selected rates of cooling, quickly increasing said flux of energy to thereby transiently heat a localised volume of said medium adjacent its surface, said volume extending sufficiently deeply into the medium that on transition between the first and second states there is obtained ,,' ~

.It l~u.lst one area on said surface exhibiting a change in surface curva-ture sufficient to ~irec-tly and immediately distinguish said loca]iscd volullle from the surrounding medium; and permitting said volurne to cool at a rate such that a transi-tion between said first and second states is ob-tained, whereby said area O r changed surface curvature on said surface is obtained.
In the preferred form, a pulsed laser beam or other form of focused radiation is used to injeet a pulse of heat energy into a smooth-surfaeed recording medium to produee a recording mark left on the surface of the recording medium in the form of a sharply-defined change in surfaee eurva-ture i.e. a sharply~defined bump or a sharply-defined pit or erater. The recorded mark directly and immediately distinguislles the reeorded volume from the surrounding medium and does not require any proeess of developmen-t or af-ter-treatment to permit playbaek.
This is aehieved employing a medium in which ehanges in densityare produeible on exposure to a eontrolled regime of heating and eooling, and whieh has suffieient transpareney or -transmissivity with respee-t to the radiation tha-t the radiation penetra-tes downwardly through the medium and, owing to the heating effeet of the radiation, transiently melts a volume whose depth is typieally large in eomparison with the diameter of the spot that is melted at the surfaee of the medium.
An important advantage of the presen-t proeess is that on playbaek or reproduetion of the reeorded information a high signal-to-noise ratio ean be obtained. When a spot of light of diameter comparable with the width of the area of ehanged surface curvature, i.e. the bump or pit, is scanned over the surface of the reeording medium, good reflectance is obtainecl from the 3n unmarked, flat or smootll areas of the recorcling mediunl, but when the spot strikes a pit or bump, the light is scattered because of the eurvature of the surface, and therefore ~ signal in the form of a strongly marked drop in -the intensity of reflec-te-l or transmitted light, as measured with a suitable optical sensor, can be detected. Moreover, by employing a novel form of an optical filter, described in more detail hereinafter that cuts down the amount of reflected or transmit-ted light collected from the surfLlce of the medium so Lhat alJ or a majority of tlle amount of -~,, , ., 11~3hL receivec~ by ttle sensor is selectecl from a portion of the bac~-reflected cr transmitted beam which is more heavily modulated thall the rest of the beam, a significant increase in the amplitude of the detected signal can be obtained, corresponding -to an increased contrast between the intensity of beams reflec-ted fromor transmitted by the smooth surface of the medium, and the bump or pit respectively.
Although in the most advantageous applications of the present invention, a laser s-tylus is employed to melt said sharply-defined localised volume of the recording medium, there are alternative, although less convenient, devices for rapidly and transiently hea-ting a localised volume of a meltable medium, e.g. an electron beam, i.e. a concentrated flux of electrons delivered, as is conven-tional, by a current carrying electrode, or a non-laser eleetro~
magnetic radiation beam, and the use of fine eonduetors in or adjaeent to the medium, and the use of all such deviees is within the scope of the present invention.
The invention will now be deseribed in grea-ter detail with reference to the aceompanying drawings in which:
Fig. 1 is a representation of a high resolution trans-mission electron microscope photograph of a thin vertical section through a layer of a presently preferred recording medium formed with a localised ehange in surfaee eurvature in the form of a bump;
Fig. 2 shows in diagrammatic form one example of reeording and play-baek apparatus in accordance with the invention;
Fig. 3 is a graph illustrating the rela-tionship between temperature and specific volume for various morpholocJical s-tates of a typieal solid recording medium; and Figs. ~ through 8 are vertical sections through five different arranc3ements of recording meclium layers employable in -the process of the present invention.
Referring to Fig. 1, a layer 11 comprises a recording medium that is a solid havirlg firs-t and seconcd states associated with different rc-~spective specific volumes ~it may be noted that the specific volume of a material is the inverse of its density). rrhe medium 1l is selectively producible in either of said states by - 3a -melting the medium and cooling it from the molten state at a selected ra-te of cooling. Reference numeral 12 indicates -the boundaries of a directed flux of energy, e.g. a laser beam that is focused on the surface 13 of the recording medium 11. The recording medium 11 is free from irregularities and from any particulate or other inclusions of such size or distribution as would interfere with distinguishing recorded volumes 15 from the surrounding unrecorded medium during the process of playback, in aceordance with the techniques of the present invention, and such a medium is conveniently referred to as "optically homogeneous".
Desirably, the surfaee 13 is optically smooth and homogeneous. By "optically smooth and homogeneous" is meant that the surface 13 does not exhibit any irregularities in contour of its surface, or any particulate or other inclusions such as grains of pigment, etc., of sueh size or distribution within the material itself sueh as would interfere with distinguishing optically between the smooth unreeorded surfaee 13 of the medium and the raised or depressed recording pits and bumps that are created on said surface in aeeordanee with the techniques of the present invention.

30(~

The laser beam 12 is focused to a spot of ligh-t of the smallest-possible diameter coincident with the surface 13 of the recording medium 11. The smallest size of spot that can be achieved is limited, for a laser beam of given wavelength, by the laws of diffraction to a certain size that is dependent on said wavelength, and this smallest-possible sized spot is accordingly referred to as the "diffraction-limited spot". It will be noted that in practice the sides 12 of the laser beam near its region of minimum diameter at the surface 13 are not straight, but form a curved, waisted configuration. Likewise, in the portion 14 of the beam below the surface 13, the ~ides of the laser beam are also curved, as shown in Fig. 1.

In operation, the smooth-surfaced layer of the recording medium 11 is pulsed with the focused laser beam 12 to injec-t a quantity of laser radiation of sufficient intensity to heat to the melting temperature the small volume of material within the medium 11 that is irradiated by the beam. The volume 15 of the medium 11 that is heated by the laser beam, as defined by the beam 14 where it passes through the medium 11, can be made very srnall. Typically, the diameter of the spot that is m~lted at the surface 13 will be less than about 0.65 ~m (0.65 micron), and the duration of the pulse of heating will be of the order of 100 nanoseconds. Owing to the minuteness of the irradiated volume, and because of the sharply-defined characteristics of the laser beamr the amount of heat that is injected into the melted volume 15, and the temperature that it attains, can be closely controlled. Once the pulse of radiation has terminated, the melted volume begins to cool at a rate of cooling that is closely dependent on the temperature to which the melted material is raised. If the material is raised to barely above the melting temperature, the surrounding recording medium in the layer 11 remains relatively cool, and on removal of the laser radiation the melted volume 15 will cool rapidly, whereas if the melted volume 15 is raised to a higher temperature, the surrounding medium will also correspondingly be heated, so that a slower 30~

L.I~C 01 "OOLill(; will. rt~sult when the .].ascr radia~ion is removed. In thLs manner, the rate of cooling from the molten state C~lll be control.led, SO thllt it is possible to se1cctivcly produce on solidification of the melted volume either the first or the secon~ of the different specific volume states in which the medium 11 is capable of existing.
Moreover, it will be noted that, wi-th materials tha-t are capable of undergoing the above-described thermally-ialduced transition, once a mark, e.~. a pit or a bump, has been produced on the surface of the recording medium, the mark can, if desired, be erased by exposing it to localised heating of intensity such that the previously melted volume is re-melted and raised to a temperature from which, owing to the temperature to which the surrounding medium is thereby raised, the material will cool at a rate such that it is restored, at least mainly, to its original specific volume, whereupon there is no longer as grea-t a discrep-ancy between-the specific volume of the re-solidified volume and of the surrounding mass of recording medium and therefore no longer as pronounced a bump or pit as formerly at the surface of 2n the record.ing medium.
Where, in the course of making a recording, the medium is converted from a state of lower specific volume -to a state of higher specific volume, since the melted volume ].5 is confined at least at the sides by the unmelted residue of the recording medium 11, the increase in volume results in a quantity of the melted material being extruded beyond the surface 13, e.g.
upwardly, through the opening that is melted at the surface 13 of the recording medium, so as to form a raised curved bum~ 16 that is left upstanding above the general plane of the surface 13 whell the materi.al has cool.ed. In -the preferre~ form in WlliCll the melting characteristics of the medium 11 are such that a sharply-defined volume 15 is melted, -this results in a sharpl~-defined bump 1.6 the edges of which provide a sharp discontinuity from -the plane of the surface 13. The rnanner of formation of -the bump 16 is somewha-t analogous to the manner in whicll an u;?ward pro~rusion of frozcn milk is obta:irlcd above the neck of a mil.k bottle when the contents are frozen. It will be rloted that this "milk-bottlc effect" is cnhancc~d, and thc hci(3ht ~3(~

of -the bump is increased, when the diameter o:E the spot that is melted at the surface of the material 13, and hence the diameter of the neck through which the molten material is extruded upwardly, is made as small as possible in relation to the volume of material that is melted within the layer of recording medium 11.
It is therefore advantageous that the beam should be brought to focus preferably at a diffraction-limited spot, coincident with -the surface 13 of the recording medium 11, so that a spot of minimum diameter is melted at the surface 13.
As is well known, the minimum signal to noise ratio acceptable for playback will vary according to the nature of the information recorded, and with the present recording process the ease with which a localised change in surface curvature can be detected will depend on the smoothness and homogeneity of the recording medium surface and on the sensitivity of the detector. Accordingly, a minimum acceptable aspect ratio for a recorded mark can no-t be rigidly specified in general. However, it is considered that, in order to permit the recorded mark to be readily distinguished from the background flat surface 13 by optical means, it is preferable that the aspect ratio of the bump 16 should be at least 1:10 i.e.
the vertical extent of the bump in comparison with its width should be at least 1:10, more preferably at least about 1:5 and still more preferably about 1:2.
For the ideal condition in which the diffrac-tion-limited spot is coincident with the surface 13 of the layer 11, the aspect ratio of the bump that is obtained is dependent on three Eactors. Firstly, the aspect ratio will increase as -the angle of convergence of -the incoming beam 12 increases. There are, however, practical limits to the greatest value of numerical aperture that can be used with a laser recorderO In practice, the largest value of numerical aperture that can be used is abou-t 0.85, corresponding -to an angle of convergence of -the beam 12 in Figure 1 of about 116.
Secondly, the aspect ratio of the bump increases with increase in the change in specific volume that occurs on transi-tion between the original and the changed states of the recordin~ medium. It is useful to refer to the change in specific volume in terms of r~ the percentage specific volume change dV~ which is ~efined as - 7 - ~ 8 dV = V x 100 wherein Vl is the magnitude of the difference in specific volume between said first and second states of differing specific volume, and V2 is the original specific volume of said medium 11.
Thirdly, the aspect ratio of the bump increases with increase in the volume 15 of melted medium, and this in turn is dependent on the depth of the volume 15 which is rnelted. The depth -to which the medium can be melted is dictated by the transparency or transmis-sivity of the medium with respect to the particular laser radiation or other form of energy flux that is employed. It may be noted that the transparency or transmissivity varies with the nature of the form of energy, e.g. with the wavelength of laser radiation, but for any given form of energy flux, the transmissivity of the medium with respect to the energy is indicated by the formula ( ) o wherein PO is the power or intensity of the energy flux at the surface 13, P(x) is the power at depth x below the surface, and L
is the distance at which the power of the energy flux has fallen to e (36.8~) of its value at the surface. ~he value L, which is termed the e attenuation length of the material, is a constant for any given recording medium and for the particular form of energy flux/ e.g. for laser radiation of a given wavelength, and in practice it is found that it approximately indicates the depth to which a given recording medium may be melted by a given form of energy flux.
Accordingly, :in order to achieve a bump of aspect ratio of at least 1:10, a medium should be employed that is characterised by the product dV.L attaining at least a certain value. From con-siderations of the geometry of the pear-shaped volume 15 of melted material as indicated in Figure 1, it can readily be calculated that the product dV.L must have a value of at least about 1 ~m, where L is measured in microns. ~lthough in theory there need be no 3~

upper limit -to the maximum value of the product dV.L, the maximum percentage specific volume change that may be encountered in practice is unlikely to exceed about 50%, and for reasons discussed in more detail below, it is preferable that the attenuation length L should not exceed about 3 ~m. Accordingly, in practice the maximum value of dV.~ should preferably not exceed about 150 ~m.
Desirably, dV is at least about 1%, more preferably a-t least about 7%, and the product dV.L is at least about 1 ~m. In -the presently preferred form of the invention, said product is abou-t 7 ~m.
The above discussion has dealt mainly with layers of recording media that are capable of undergoing an increase in specific volume, that is, a decrease in density. The present recording process is, however, also applicable to layers of recording media that are capable of undergoing a decrease in specific volume on melting and cooling at a selected rate. In such case, there is injected the required amount of energy into the melted volume 15 such thatl on termination of the energy pulse, the melted volume cools at a rate of cooling selected to achieve a transition to its other physical sta-te. There is thereupon produced a corresponding decrease in volume of the melted zone 15, so that a depressed pit, as indicated in broken lines by reference numeral 17 in Figure 1 is obtained. As with the procedure for producing raised bumps discussed above, on adjustment of the intensity of the energy pulse injected into the recording medium, the depressed area can be re-melted and raised to a temperature such tha-t a rate of cooling is achieved which results in the re melted volume solidifying back to its original state of speciEic volume, so that the pit is erased and the surface oE the medium is restored -to its original smooth condition.
In selecting materials to be used as recording media in the process of the invention, conventional techniques for measurement of 1 attenuation length may be adapted for the measurement of the small values of e attenuation length, L ~m, that are 3~
g desirable. For example, thin films may be made from the materials to be tested, the films tapering in -thickness towards one edge, so that measurements of the variation in transmissivity of the medium with variation in ~hickness of the film may be made directly. Further, in the case of proposed recording media that can be brought into solution in an appropriate solvent, measurements can be made of the absorption of the laser radiation by solutions of varying concentration, whereby a value corresponding to the e attenuation length of -the pure solid may be obtained by extrapolation.

The desired recording media that meet the requirements of relatively short attenuation length with respect to laser light and of having an optically smooth and homogeneous surface, will normally ha~7e a black, glossy appearance, and all such materials are ~rima facie candidates for use as recording media for the process of the invention.

The capacity of a given material to exist in two different states of specific volume, between which the material is switchable by melting and cooling at a selected rate, can mos-t conveniently be investigated by probing the material with a laser beam stylus, e.g. using a laser beam pulsed at pulses of duration of the order of 100 nanoseconds and of controllable intensity, with said laser beam being focused to a diffraction-limi-ted spot at the surface of the proposed recording medium, or by any other metnod that is capable of transiently melting a small volume of the medium to be tested and permittlng it to cool under conditions such that the melted material is rapidly quenched or is more slowly cooled. It is for example possible to investigate thermally-inducible changes in specific volume on small specimens of the proposed recording medium by melting them and subjecting them to different rates of cooling e.g. by permitting the molten material to cool slowly by exposure to the ambient temperature or by quenching the molten sample by plunging into a bath of a suitable inert coolant liquid such as cold w~ter, and then observillg any changes in the specific volume.
Examples of recording media include biturrlens such as native asphalts, e.g. gilsonite, albertite, and Trinidad pi-tch, petroleum asphaltenes, bituminous dis-tilla-tion residues, e.g.
distillation residues from fuel refineries such as petroleum refinery distilla-tion residues, and bituminous fractions of all these substances, for example fractions obtained chromatogra-phically or by solvent fractionation.
The preierred recording medium amongst -those investiga-ted up to the present time is a film of commercially available sealan-t wax that is a glossy black tar-like solid material obtained, according to applicant's understanding, as petroleum refinery distillation residues, and available from Shell International Chemicals Co. Ltd., London, England SEl 7PG, under the trade mark APIEZON W. The physical properties of APIEZON W as made available by its manufacturer are set out in the Table below. Films of this material are preferably prepared by applying tne material to a substrate by knife-coating, i.e. spreading out the material on the substrate using a spreading device, or dip-coating, i.e.
bringing the substrate into contact with a fluent prepara-tion of the material which is allowed to flow onto and solidify on -the substrate. Preferably, this is done using a solu-tion of the APIEZON W material in an organic solvent, e.g. toluene, or trichloroethane, and permitting the solvent to evaporate, as films prepared in this manner exhibit better recording properties -than the bulk APIEZON W material. Desirably, the solution is made up as a highly viscous, concentrated solution oE the ~PIlZON W in the solvent, preEerably oE viscosity oE at least about 700 centi-poise as these concentrated solutions yield films with better recording properties. Films made from these concentrated solutions provide an excellent recording medium of which small volumes can readily b~ melted and switched to a state of increased specific volume on application of a pulsed laser stylus -there-to. Thus, the material can be rapidly melted and quenched and bumps of raised surface relief can be obtained on the surface of the recording medium.

3~

TABLE
, _ Appro~imate Softening Point, C 85 .
Temperature for Application, C 100 __ . . . ~
Vapour Pressure at 20C, torr 10 8 _ Vapour Pressure at 100C, torr 5 x 10 6 Specific Gravity at 20C/15.5C 1~055 Specific Gravity at 30C/15.5C 1.048 Average Molecular Weight 1214 Coefficient of Volume Expansion per C, 4 over 20C-30C 6.2 x 10 Thermal Conductivity, w/m C 0.189 Specific Heat a-t 25C, cal/g 0.43 joule/g 1.8 _ Loss Tangent 0.015 Permittivity 2.8 _ Volume Resistivity, ohm cms 6.31 x 10 On re-melting the raised relief portions of the recording medium, using a laser beam of higher intensity, the bumps can be flattened out, and the recording thereby erased, as the melted material thereupon cools at a slower rate so that it is restored to its original state of lower specific volume.

It is suggested that these changes in specific volume are associated with changes from an ordered (i.e. crystalline or microcrystalline) state corresponding to a lower specific volume to a less ordered or amorphous state corresponding to an increased specific volurne, and that on melting and rapid cooling or quenching the amorphous state is produced whereas in slower cooling the melted material re~crystallizes and the original higher density, lower specific volume ordered state is achieved.

In any event, it is found that when a recording is made on a film of the APIEZON W material deposited from a solution, as illustrated in the representation of Fig. 1 the pear-shaped melted volume 15 indicated in Fig. 1 exhibits in the transmission electron micrograph a lighter appearance than the surrounding darker unmelted regions, and this is indicative of a lower density, higher specific volume state. Although the crystallinity of APIEZON W wax does not appear to have been previously reported, transmission electron micrographs through thin sections of the material present an appearance that is indicative of the presence of a fine microcrystalline grain structure, with the grain sizes being small in comparison to the wavelength of light, and thus not interfering with the optical smoothness of the surface of the medium.
Moreover, the zones of the medium that have been subjected to the recording process and have been switched to a sta-te of increased specific volume are physically less brittle than the unchanged zones, indicating a transition to the more resilient condition that is associated with the amorphous state.

Tests conducted on the preferred APIEZON W material show that with respect to laser light obtalnable from a -typical laser source, e.g. a blue emission at a wavelength of 4880 A as obtained from an argon ion laser, the APIEZON W material exhibits a 1 attenuation length of about 1.0 ~m. On transition from its lower to its higher specific volume state, there is a percentage specific volume change dV as defined above of about 7~, so that the above-discussed product dV.L has a value of about 7 ~m.

Further examples of recording media include organic materials ~ that have been darkened in color ~y-~y-r41~s~-by appropriate heat-treatment. One example of a recording medium that is capable of forming raised recording marks in the form of bumps when exposed to a laser beam stylus is the shiny black residue of caramelized sugar that is obtainable by heating a sugar e.g. sucrose for pro-longed periods. As with the APIEZON W matcrial, on exposure to a pulse of laser light of relatively low intensity, a raised bump-like mark can be made, and on exposure of the bump to a pulse of higher intensity the bump can be flattened out and the recording can thereby be erased. The response of caramelized sugar is however much inferior, as the height of the bumps that can be formed is much lower and therefore this material is not preferred.

Further examples of recording media include doped polymeric resin materials, preferably a cured epoxy resin material, prepared by impregnating a polymeric resin material with a light-absorbing dopant, until a predetermined degree of light absorption is imparted thereto~ An example of a ma-terial that is capable of undergoing a reduction in specific volume on heating with a laser beam stylus so that depressed pits as indicated by the broken line 17 in Fig. 1 are obtained, is a cured epoxy resin material blackened by doping with iodine. This material is obtained by mixing a conventional two-part epoxy resin adhesive pack e.g.
LEPAGE'S FIVE-MINUTE two-component epoxy resin material to obtain a cured epoxy resin film. The film is then left in contact with iodine vapor e.g. by leaving the film in a sealed enclosure into which crystals of iodine have been introduced, until the film absorbs sufficient of the vapor to render it black. On exposure of the surface of this film to pulses (duration 100 to 200 nano-seconds) ofblue laser light (wavelength 4880 A) focused to a dif-fraction limited spot on the surface of the film, at an intensity of about 2 to 3 m~J at the surface of the film, a recording in the form of crater-like depressed pits can be produced on the surface of the ~L~L93(~

film. The recorded information can be reproduced by sweeping the spot of light along the surface of the film over the pits, operating the laser continuously a-t a lower level of intensity (e.g. about 300 ~W). A pulsed reflection from the surface of the film is obtained. The recording can be erased by sweeping the said diffraction-limited spot of light along the recording, operating the laser continuously with the intensity of said spot being about 3 mW. After erasure, a pulsed reflection is no longer obtained when a spot of low intensity laser light is swept along the track of the former recording.
Apart from recording media that can be selectively switched between amorphous and ordered states, there exist other materials that can undergo a change in specific volume on exposure to a selected regime of heating. For example, certain materials can be made to undergo changes in density on quenching and on heating at annealing temperatures, even though the material remains in the amorphous state, and such materials that can be selectively switched between different states of densi-ty or specific volume through injection of controlled amounts of heat energy into a confined volume of the material are employable in the recording process of the invention. For further description of variable density amorphous materials, reference should be made to Turnbull et al. "Structure of Amorphous ~emiconductors" J. Non-Crystalline Solids vol 8-10, pp. 19-35, 1972.
The presently preferred recording media are, however, materials that undergo a transition between ordered and amorphous states accompanied by a large increase in specific volurne. The desirable properties of the preferred recording media for use in the present invention will now be discussed with reference to materials that are originally in a state of low specific volume (high density) and that can be switched to a higher specific volume state, leading to the production of a bump, but it will be appreciated that similar principles apply also to materials used in recording processes where there is a decrease in specific volume, leading to production of a depressed pit in the recording medium surface.

30~3 ~ 15 -In the preferred materials, the glass -transition temperature, Tg, should be well above normal maximum room temperature i.e.
should be at least about 100F or ~0C so that the amorphous low density sta-te will be stable and permanent under -the full range of ambient temperatures. Therefore, preferably Tg should be a-t least about ~0C.
Moreover, the thermal conduc-tivity (or equivalently, the thermal "diffusivity"~ should be low for a number of impor-tant reasons: it prevents the lateral spread of thermal energy during the bump forming process, allowing high resolution recording; it reduces to economical levels the threshold power levels for writing and erasing; and it makes the local cooling time sensitive to the amount of in~ected heat so that either quenching (recording) or annealing (erasing) can be thermally initiated.
Desirably the thermal conductivi-ty is less than about 5 mW/cmK, more preferably less than about 3mW/cmK.

The characteristic annealing time should be short so that a volume of melted material less than a cubic micron in size will approach its maximum density within a few microseconds. This is a requirement Eor fast thermal reversibility. Further, in order to achieve good sensitivityj the melting process should call for as little heat as possible from the recording laser.
So for thermal efficiency a number of requirements must be met.
In the case of an ordered material, it is necessary to heat the substance beyond the glass transition -temperature of -the amorphous phase to the true thermodynamic melting -tempera-ture, Tm, in order to achieve complete amorphization on quenching.
This is because the last traces of crystallini-ty do not disappear until the melting temperature is reached. Therefore, the melting temperature should not be excessively high. However, in order for crystallization to be kinetically favored during the annealing process, Tg and Tm should be separated by several tens of degrees. Therefore, if Tg is around 100C, say, T should be 3C)~

at least 150C. The two major factors governlng how much heat is required -to achieve melting are the speci-fic hea-t capaci-ty and the latent heat of fusion. Thus, i-t is desirable to select materials in which these two values are low. The material should also have a sharply-defined melting temperature in the crys-talline state and a relatively well defined softening temperature in -the glassy or amorphous state.
The optical attenuation leng-th of the material for -the chosen laser wavelength should be long enough to permi-t a depth of penetration sufficient to produce the net volume expansion required to yield an easily detectable bump having an aspect ratio of at least 1:10 at the surface. To produce a playback signal with high contrast, using a playback beam focused to a diffraction-limited (i.e. wavelength-sized) spot, the heigh-t and width of the bump should be comparable with the wavelength of said playback beam. As discussed earlier, if the material undergoes a relatively large percentage specific volume change or expansion on quenching, the depth of penetration required to produce the desired total expansion will be proportionately less. For example, for efficient high relief recording with blue light at a wavelength of ~ = 4880 A, the attenuation length should preferably be about one micrometer where the percentage specific volume change is about 5 to 10 percent. In the preferred form, wherein the laser beam is focused to a diffraction limited spot at the surface of the recording medium, the e attenuation length will normally be in the range of about lloth to about -twice -the width of the area that is melted at the surface of the recording medium.
At lower attenuation lengths it is unlikely -that sufficient ma-terial is melted to provide a bump of adequately pronounced aspect ratio, and at longer attenuation lengths an excessively large volume of the medium is heated and a pulse of light of very high intensity is required to melt the recording medium. Owing to the high costs of laser power at the present sta-te of the laser art this increases the costs of the recording process beyong acceptable levels. Preferably, the - attenuation length is about 12 -to twice the 0~3~
~ 17 -wid-th of the spot melted at the surface, and in the presen-tly preferred form -the e attenuation length is abou-t twice the value of said width.

In addition to the thermal, morphological, and optical properties mentioned above, the preferred materials should facilitate the fabrication of large area recording media of optical smoothness and homogeneity without resort to expensive processes such as vacuum deposition.

The laser recording process and the apparatus used for recording and playback are illustrated in more detail in Figure 2.

Specifically, a target ~1 comprises any convenient stiff or flexible substrate 22 having deposited thereon the layer 11 of recording medium, a transparent elastic compressible layer 23 e.g. of air or a solid compressible layer e.g. of clear silicone rubber, and a hard transparent dust co~er ~4 e.g. a glass cover plate. The laser beam input-output apparatus comprises a source of polarized continuous wave or pulsed laser light 26, of variable intensity, an expander-collimator 27, a polarizing beamsplitter 28, a quarter~wave plate 29, a high numerical aperture (e.g. 0.65) microscope objective lens 31, a movable and variable-aperture iris diaphragm 32 that serves as a spatial filter, and an optical detector 33. These elements ser~e to generate and focus the heam oE laser li~ht 12 at target 21 and to collect various amounts of the back-reflected lig'nt indicated by arrows 3~ and direct it to the detection system 32, 33~ The states of polarization of the laser radiation from the source 26, and of the incident and reflected beams are indicated by the conventional notation in Fig. 2.

The incident focused laser beam penetrates the surface of the recording medium 11 and is absorbed within a depth which is typically two optical wavelengths. This penetrating laser beam is converted to heat which warms the medium 11 and can ~93~

bring about localized melting if its power density is higher than a certain threshold value.

During an initial wrlting operation the intensity of the laser beam 12 is briefly (e.g. for 100 nsec) pulsed ahove the afore-mentioned melting threshold to cause localized melting at selectedbit sites within the recording medium layer 11 o:E the target 21.
The localized melting servestoamorphise tiny pockets of the layer 11 forming in the case of bump-formation less dense amorphous regions such as the amorphous volume 15. Because of the mechanical iO restraint imposed by the surrounding umnel-ted material 11, the reduction in density in the amorphous region 15 is expressed as a sharply-defined bump 16 at the surface of the layer 11, or as a sharply-defined pit in the case of materials that undergo a decrease in specific volume.

During the reading operation, the intensity of the laser beam is reduced below the melting threshold of the recording material, and either the beam or the target 21 is moved so that the surface of layer 11 is scanned by the spot of focused laser light. As the playback spot moves over the recording surface, about 7% of the incident light is reflected back up to the lens and is directed by the beamsplitter 28 to the optical detector 33 which measures the intensity of the back-reflected light. If the beam falls on a flat area, all of the reflected light is collected by the lens 31 and directed to the detector 33 which measures a relatively high intensity. But, if the beam falls on a bump 16, or on a pit with a downwardly-recessed curved surface, the high localized curvature of -the dome-like surface oE the bump or oE the crater-like surface of the plt acts to spread the bac]c-reflec-ted light 34 over a much wider angle than that subtended by the aperture oE the lens 31.
Thus, most of the back-reflected light falls outside the lens aper-ture, and the detector 33 measures a much reduced intensity. As the playback spot is scanned over the recording surface, a sudden dip in the back-reflected intensity signals the presence of a bump, or pit, - 1'3~ ~ ~ 3 e~uiv.llent to onc bit of inform~tion.
One can substalltially increase the amplitudes of these dips by usin~ a vari.l~le .perture iris 32 ~laced between the lens 31 and the detector 33~ It is a characteristic of the response yielded by the reflec-tion of light from the aforesaid bumps or pits that there is a small por-tion of the back-reflected beam pattern 34 that undergoes a larger change of intensity than the rest of the pattern when a bump or pit moves into the scanning spot. By mounting a variable size aperture iris 32 in a mechanical arrangemen-t which allows its position to be adjusted in a plane perpendicular to the back-reflected beam, and by adjusting the position and size of the aperture, i-t is possible to select that portion of the beam which is most highly modulated as the scanning spot moves across the bump or pit.
This method achieves an improvement of typical]y a factor or two or three in the output signal contrast, and if recording has been performed with the recording power level set well above the record-ing threshold, this technique can easily yield an output signal whose depth of modulation approaches 100%. That is,when the aperture 32 is properly adjusted, the presence of a bump will reduce the in-tensity seen by the detector 33 close to the level it ~ould see if the laser beam were turned off. The use of an adjust-able spatial filter such as the iris 32 is believed novel, and leads to a level of play~ack signal contrast unsurpassed in the prior art.
In the case of materials that form a raised bump during recording, erasing is accomplished by raising the intensity of the laser bean 12 about a fac-tor of two above -the threshold for recording, and scanning the spot over the selected Eeatures.
This measure serves to recrystalliæe the arnorphous melted and solidified volumes :L5 causing the associated bumps to Elatten out. ~or the case where a recording surface of APIE~ON W is moved at a rate which is slow enough so that -the surface does not move much during an exposing pulse, the recording threshold is typically about 1.5mW for a 0.5~1m diameter spot of blue light (~ = 4880A) from an argon ion laser. The threshold for erasing under similar conditions is about 3 mW.

~1~30~

Referring again to Figure 1, the "boundary" of the converying laser beam is represented by the curved lines CD and EF whlch outlines the surface of an hyperboloid of revolution within which the intensity of the light is above the melting threshold of the recording material 11. The peaked symmetrical curve 36 represents the Airy Pattern, and indicates the relative radial intensity distribution of the beam in the vicinity of the recording surface 13. The radial position where the power density of the beam has fallen to one-half of its peak value is marked by the half-power point 37, which defines the half-power diame-ter of the laser beam at the recording surface. ~ecause of -the sharp melting threshold of the recording material, there is a radius beyond which melting does not take place due to the falling off of the intensity distribution 36 at a level corresponding to the melting temperature of the recording material. As the central peak intensity of the laser beam is lowered or raised, the intensity distribution 36 will shift up or down relative to a horizontal line at that level, the points of intersection of -the line with the distribution curve 36 marking the radial boundary of the resulting bump. Thus, by lowering the intensity of the beam to the appropriate level compared to the melting threshold, it is possible to make a bump which is narrower than the half-power spot size, and similarly, by raising the beam intensity a larger bump can be made. Figure 1 illustrates the special case where the melting threshold corresponds to the half power point, resulting in a bump whose diameter is exactly the half-power width of the beam.

It is useful to define the "effec-tive spot size" of the beam as the diameter (at the surface 13) of the melted zone 15. Of course, for a given material with a given melting temperature, the effective spot size will increase as the central peak intensity of the beam is raised. This fact has a strong bearing on the recording process in the following way. For geometrical reasons, ~9~

a smaller melted volume will cool more rapidly than a larger one. Since the amorphization process which leads to the formation of the bump 16 depends on a high ra-te ~f cooling to bring aboui quenchiny in zone 15, it follows that -the effective spot size should be kept to a minimum when recording. Of course, this is in harmony with the goal of maximizing the bit ~,~ packing density. Thus, the s~Y~t~ intensity of the lasex beam should be kept below a certain critical level for best results when recording. This critical level is the threshold for erasing.

As discussed earlier, with recording media of very low thermal conductivity, it is possible to control the rate of cooling of a typical recording "cell" 15 by varying the amount of heat injec-ted by the laser beam during the melting process. The ~aterial tends to condense at a high rate towards a stable lower specific volume state unless it is quenched at an even higher rate, in which case it freezes in a permanent metastable state of higher specific volume. The latter condition, corresponding to the process of recording, will be obtained only if the smallest possible volume is exposed (corresponding to a small effective spot size) and given just enough heat to melt the material in the volume 15. In this case the melting energy will diffuse away quickly, and quenching to the high specific volume state will occur, with the concomitan-t formation of a bump.
However, the former condition, correspondin~ to the erasure process wiil occur if the period of cooling of a melted cell is extended by injecting an excessively large shot of heat during the melting pxocess. The e~cess heat will altex the thermal boundary conditions of the cell causing the cooling period to be extended to the point that the material is ab3e to condense to the lower specific volu~e, and this will cause the surface of the cell to return to its flat state. Delivering a larger injection of heat will be done by raising the intensity of the beam above the level for recording, and this will have the further effect of increasing the effective spot size somewhat.

~930~

If the volume of the melted region is increased as a result of the larger spot size, the rate of cooling will be reduced by this factor as well.

It will be appreciated that with materials that are depositable as optically-smooth layers in the amorphous state~ préc:isely the opposite recording and erasing mechanism takes place with injection of a higher quantity of heat resulting in the transition to a lower specific volume crystalline state, produciny a pit-form recording mark, and the injection of a lower quantity serving to re-convert the material to the higher specific volume amorphous state, and thereby erasing the recording.

The process in which localised volumes of the recording medium are converted from the crystalline to the amorphous sta-te during the recording process is preferred to that in which the conversion is from the amorphous state to the cxystalline state, as it is superior from the point of view or data security in an archival memory system because it is an erase-rewrite mechanism in which the erase operation requires at least as much power density as the recording operation. This provides the maximum margin of safety against accidental erasure by the lower powered read beam. In contrast, with recording processes that invo]ve conversion from the amorphous to the crystalline state, erasing is accomplished by exposing recorded areas to a continuous laser beam intensity above the level used for playback but substantially below that required for recording. Thus, in any memory system based on this latter process the possibility exists that due to long term drift or system faults, the intensity of the playback beam might wander upward to the erase threshold and cause a catastrophic loss of data files.
The preferred forms of the present invention in which the recording ~; medium is APIEZON W or a doped epoxy material exhibit~ the remark-~ .~
able feature that erasing must be done at a power level that is higher than that required for bare threshold recording and equal to that for peak amplitude recording. ~urthermore, the erase process is purely thermal, so that the erase rate is limited only by the ~ 23 -power available from the laser.

~eferring to Fig. 3, this illustra-tes the variation of specific volume wi-th temperature for a preferred recording medium that undergoes a crystalline to amorphous transformation. Point A
represents the pre-recording operating point of the recording material in its low specific volume ~high density) (e.g.
crystalline) state at the ambient -temperature T . In the process of recording, a pulse of light injects just enough heat to push the operating point along the melting curve 38 to the point B, or to a point a little way up the segment 39, which indicates the liquidus. This action brings about rapid melting in the chosen recording cell 15.

At the end of the light pulse, rapid cooling or quenching causes the operating point to move along curve 41 from B to C at the glass transition temperature T , and along curve 42 to the point D. As the temperature of the cell falls below T the material freezes into a metastable amorphous state at point ~. This metastable state wi~l be permanent at the ambient temperature Ta (room temperature) provided the glass transi-tion temperature Tg is far enough above Ta that the material is hard and stable at Ta. It is clear from Fig. 3 that when the material of the cell 15 is switched from the crystalline state at A to the amorphous state at D, there is a permanent net volume expansion. This expansion, accumulated throughout the exposed volume 15, gives rise to the bump 16 as s'~own in Fig. 1. The process of erasure is brought about by exposing the expanded cell to a larger pulse of ,-eat which will push the operating point from D up to some point E beyond point B on the liquidus curve 39. At the end oE
the pulse the operating point slides slowly back to B and then down along the crystallization curve 38 back to its original state A. The resulting net volume loss throughout the cell 15 causes the bump to recede, leaving the top of the cell approximately flush with the surround'ing materia'l. The percent volume change, from the high densi-ty state at A to the lo~

3~

density state at D, varies considerably from material to material, but in the case of recording media that are micro-crystalline polymers the variation is typically between 5 and 10 percent.

S It should be emphasized that the essence of the preferred form of recording process is in the thexmally induced swi-tching from a stable state of high density to a stable state of low density with a concomitant net volume increase which is expressed as a bump. The inverse transition, which is also thermally induced, is the basis of the erase process. The example in Fig. 1 represents one embodiment in which such a bistable density change is induced in a highly crystalline material. That is, the volume increase required for the recording process is associated with a transition from a highly ordered crystalline state, to a disordered or amorphous state.
But, as was pointecl out earlier, there are also materials without structural order which also exhibit such a bistable change of density. However, the highly crystalline materials are preferred as they may be expected to yield more sharply-defined bumps because of the more sharply defined meltingtransition of a crystalline material.

As long as the thickness of the recording layer 11 is greater than the required depth of material to be melted for producing a detectable bump of aspect ratio of at least 1:10, the thickness of ~he layer is not important, unless it is desired to enhance the cooling rate of the melted volume 15 by using a high thermal conductivity substrate as a heat sink. This measure would be required if it is desired to record using a large spot (for example, a 1 ~m spot from a diode laser) on a material such as APIEZON W, which gives its best response for a spot si~e less than 0.7 ~m. In this case, the active layer 11 should be ~o thickcr than the depth of a recording cell 15, so that the bottoms of the cel]s 15 are in good thermal contact with the substrate. However, since the depth of a cell could vary by about 20 or 30 percent from the preferred depth oE about 1 j,m without significantly affecting the response, the thickness of this thinner layer does not have to be stringen-tly controlled.
Fig. 4 shows a config-uration for a recording me~ium with an enhanced quench ra-te. It should be noted that in this and the other figures wherein like reference numerals indicate like parts, the thicknesses of the various layers are not to scale;
only relative thickness is indicated. ~n this example an active layer 11 of the recording medium, on the surface of which a recording mark in the form of a bump 16 is formed, is coated on a material 46 of high thermal conductivity (such as a metal) which forms the combination substrate and heat sink. If -the active layer were APIEZON W or an equivalent material, being switched with a diffraction-limited spot of blue light, the thickness of this layer should be for example about one micrometer, as this allows the bottom of a recording cell 15 to make thermal contact with the heat sink 46. The thickness of the heat sink 46 may be any convenient value. Immediately above the active layer 11, is a compressible, elastic layer 47 such as air, or transparent silicone rub~er. As will be appreciated, the surface of the recording medium on which the recording mark is formed may be an interface between medium 11 and a deformable medium such as the layer 47.
The layer 47 should be sufficiently readily deformable -that its compression does not hamper the upward expansion of a bump 16. I-f this expansion causes a sufficient buildup of stress in layer 47, the resulting downward force on -the surface of the bump can cause it to gradually relax back to the f:Lat sta-te. Since :Ln dppropriate recording media 11 the metastable state is virtually permanent until the material is re-melted, in some materials with suitably stable metastable states, but which have long erase -tirnes the downward return force exerted by layer 47 on the bump 16 might enhance the erasure process, without signi-fican-tly affecting the life-time of the recording.
On top o~ layers 11 and 47, there is a hard transparerlt protective layer 48 whicll must be thick enough -that dust particles falling on its surface are well ou-t of focus for the input-ou-tput laser beam 49. In this case the dust will not significantly affect the signal to noise level of the system. Layer 48 needs to be regarded as part of the optics of the input-output system because it must be used with a microscope objective lens 31 which has been specifically compensated to allow diffraction-limited focusing on the active surface 13 with the dust cover 48 in place. For high numerical aperture objective lenses 31, -the thickness of layer 48 needs to be regulated -to within about one percent over the entire active surface of the recording medium.
The need for a dust cover of well regulated thickness is a common requiremen-t of known laser recording systems which are designed to operate under normal ambient conditions, and the manufacture of such materials is well within the capabilities of those skilled in the art. A further important function of layer 48 and of the compressible layer 47 is to seal off the active surface 13 from oxida-tive or corrosive chemica]s such as oxygen which might degrade the recording medium 11. As with known systems, the dust cover 48 may be removable, and a radiation opaque layer 48 may be used that can be retracted before recording or playback. The system used in Fig. 4 is a top-side reflective playback system.
That is, information is played back by reflecting light 49 from the top surface oE the active medium 11. The reflectance being detected is, in mathematical terms, the disturbed phase reflectance associated with each bump 16. In order for there to be a detectable phase disturbance, it will be apprecia-ted there must be an appreciable refractive index difference be-tween -the layer 11 and the adjacen-t layer 47. In the example i]lustra-ted, dotted center line 51 represents the plane of symmetry of the recording medium. That is, it is double sided.
Fig. 5 illustrates a simplified form of recording medium in which the recording ma-terial 11 acts as its own substrate, and a clear compressible material such as silicone rubber is used as the optically compensated dust protector 47. A material for layer 47 may be chosen so tha-t its top surface 52 can be - 26a -tougilened by inducing molecular crosslinking, rendering it more resistant to damage. Again, this example illustrates a double-sided top surface reflective playback system, having a plane of symmetry 51.

- 2l ~ ,. illusL2-.itcs a double-sided bottom-surface reflective play-back system. In thi.s case, recording is done by focusing the laser beam 49 through thc acti.ve layer 11 to the active surface 1.3 which is at the bottom of the active layer 11. The bump 16 expands downward into the compressible layer 47. The layer 11 in this case must be about one cell-depth -thick, with reference to the cell 15. That is, it should be thick enough to absorb an appreciable portion of the incident laser light so that melting can occur throughout volume of the cell 15, but not thick enough that an excessively large volume o~ material must be melted, as would reduce the sensitivity of the medium.
Also, if layex 11 is too thick, ligh-t reflecting back from the interface 13 would be absorbed before i-t could return to the detector. The particular configuration of Fig. 6 has the disadvantage that the swi-tching effect causes a sligh-t reduction of the refractive index in the volume 15 compared to the surrounding layer 11. As the laser beam scans along through the active medium ll, this slight index change at the top of the cell 15 will be de-tected as a low arnplitude pulse tha-t is longer in duration superimposed on shorter-duration pulse associa-ted with the bump 16. 'I'hat is, a low frequency low amplitude noise signal is produced by reflection at the interface 53. In Fig. 6, both the clear protective layer 52 and the compressible layer 47 act as heat sinks to aid the recording process, especially where their -thermal conductivities are appreciably higher than that of recording medium 11. In a double-sided medium provision can be made so that the light -transmit-ted through recording medium 11 does not affect the active layer of recordinq medium on the opposite side of the plane of symmetry 51. Thi.s rnay be done by making compressible layer 47 opaque by dyeing or doping.
Fig. 7 shows an example of a double-sided ~ransmissive play-back recording medium. In this case -the active layers 11 and lla are made -the depth of one cell 15 thick (one attenuation length) and the recording beam 49 b~in~s about switching in the volume 15 so that a bump is pushed up into the optically compensated

- 2~ -compressible layer 47. Thus, the active recording surface is the interface 13. But instead of de-tecting the presence of a bump 16 by reflecting light off the top of i-t, use is made of the fact that the curvature associated with the surface of the bump 16 causes an equally detectable perturbation in the distribution of light transmitted through the recorded volume 15.
That is, the bump 16 scatters transmitted light as well as reflec-ted light. During playback, the transmitted light 54 w~ll pass through the other recording medium layer lla and out to the detector. If both recording layers 11 and lla are one attenuation length thick then the intensity of the transmi-tted beam 56 will be reduced to approximately 14% of the intensity of the incident beam 49. As is indicated by the consideration that the top-surface reflective system shown in Fig. 4 may be adequate with only the 7~ Fresnel reflection returning from the active surface 13, the above 1~% transmission is ample for detection purposes.
So as not to interfere with -the playback of recordings in recording medium layer lL, the clear spacing layer 58 is made thick enouc3h so that the other active recording medium layer lla is well out of focus when the playback beam 49 is focused on the active surface 13. The recorded features such as bump 70 do not then significantly perturb the outgoing beam 56. With the configuration of Fig. 7, the recording beam 49 causes memory switching in the particular active layer 11 on which it is focused. This has the disadvantage that accidental misEocusing of the recording beam may damage recordings in the other active recording medium layer, but on the other hand, it makes possible two-surface recording without having -to flip -the recording medium, simply by refocusing -the record beam.
A further example of -the lat-ter kind of mul-tiple-layer record-ing medium, in which either reflective or transmissive playback is used is shown in Fig. 8. In this example, the various active layers, 61, 62, and 63, each absorb radiation of a different colour or wavelength. rl'llis arrangement can be used to advantacJe with lasers such as the dye laser which can be tuned to radiate - 29 ~ 3~8 at any wavelength over a wide range. This permi-ts the use of a recording medium such as in Fig. 8 in which a different colour of light is used to record on each of the active layersO This may be done by setting the radiating wavelength of the laser so that i-t falls within the narrow absorption band of thechosen recording layer, and focusing the laser beam on its active surface. For example, to record on layer 61, the laser beam 64 is retuned and refocused at interface 13. However, it will be appreciated that the beam could also be focused inside the recording layer as well.
In either case, since the recording beam penetrates through the entire recording layer, and since each recording layer is sandwiched between two compressible layers, the resulting expansion in the volume 15 will produce a bump at both surfaces of the recording layer, as shown. In order to eliminate interference from features recorded in the layers above the one being probed, it is necessary to make the clear compressible separator layers 67 and 68 thick enough so that only one layer is in focus at a time. The thickness of the separators 67 and 68 should be at least several times the longest wavelength used for recording. But since visible wavelengths are less than one micrometer, the separators 67 and 68 may be thin enough tha-t the thickness of the whole multilayered medium is relatively small so that the medium can be made flexible, if flexibility was desired.
If narrow absorption-band dyes are used in the active layers 61, 62, and 63, the number of such layers would be limited mostly by the wavelength tuning range of the laser. In general, the maximum number of layers in this system is limited by spectral overlap between adjacent absorption bands of the various active layers as well as the total spectral emission range of the laser.
It should be noted that an impor-tant advantage of the present memory concept, in common with most other beam addressable laser recording systems, is that the active portion of the ~ecording medium is structureless, in the sense that bit sites are not predefined by any artifacts in the struc-ture of the film. This feature makes the memory enormously more economical on a cost per bit basis than solid state electronic systems such as the magnetic bubble memories.

~3~
- 29a -Memory systems based on the present recorcling process must, like all known beam addressable memories t include a means to bring about rela-tive motion between the focused spot of laser light acting as -the input-output means and t'ne surface of the reccrding ...

~L~L93~

medium. Thus, the laser beam may be lef-t relatively fixed and the medium may be moved as in disc, drum, or tape systems, or the medium may be left relatively immobile and the bit sites accessed by scanning the laser spot over its surface.

The provision of appropriate devices for scanning the laser spot relative to the recording medium surface is well within the capabilities of those skilled in the art, and forms no part of the present invention.

Although the above disclosure provides ample information to one skilled in the art to permit the operation of a recording process in accordance with the invention, for -the avoidance of doubt a detailed example of the operation of a recording process and of the apparatus used therein will now be given.

In this example, the laser 26 is a model 165/265 Argon Ion Laser from Spectra-Physics, providing a strong blue emission at A = 4880A of the Argon ion laser, and having the following characteristics:
Output polarization: vertical Beam diameter at l/e points: 1.5 mm at 5145A
2Q Beam divergence: 0.5 milliradians Noise rating (nominal): with power stabilizer on:
lOHz to 2M~Iz - O.2% rms typical.
Maximum power available at ~ = ~880; ~ 1.5 watts The laser is run at a typical output power level of 10 mW so that, taking into account the losses between the laser and the recording surface, the power there will be in the vicinity oE

3 mW. From the laser 26, -the beam passes to a modulator which in this example is a model 20 electro-optical modulator obtained from Coherent Associates. This device is a Pockels-Effect modulator with input and output polarizers which are crossed so that the light cominc3 out has its electric field oriented in the horizontal plane. This permits the modulated laser light to be coupled efficiently to the recording surface and also to efficiently redirect the back-reflected light to the detector using a polarizing beamsplitter, as described below. The modulator is driven through a Coherent Associates model 30 Driver, which has a nominal bandwidth extending from ~C to lQ MHz. The modulator driver is controlled by a fast pulse generator, namely ~! ~ a model 2101 Pulse Generator from Tektronix (~or*~l pulse rise time from 10-90~, 5 nsec). This device can be adjusted to provide pulses from ~0 nsec to 40 msec in width.

A master-slave JK flip-flop; namely the CD 4027AE CMOS flip-flop from RCA, is used as a "one and only one" trigger source for the pulse generator and a CD 4047AE CMOS monostable multi-vibrator, from RCA, is triggered by the JIC flip-flop to produce the pulse which either gates or triggers the pulse generator.
The output from the laser 26 can thus produce pulses of a duration controlled by the pulse duration of the pulse generator.

From the modulator the beam passes to an expander collimator 27.
This expands the beam to a diameter somewhat greater than the diameter of the lens 31, i.e. to about 0.7 cm. The expander collimator consists of two microscope objective lenses, one of which focuses the beam from the modulator down to an aperture of a spatial filter. The spatial filter is a ten micron copper pinhGle aperture obtained from Optimation Ltd. It is somewhat smaller than the beam, ~hich is focused onto the aperture. The other microscope objective lens collects the light emerging from the copper pinhole and reconverges it to a collimated beam of diameter about 0.7 cms. ~he effect of the expander collimator 27 is that, with about a ~0% loss in power, the higher ~ourier components of the beam profile are eliminated, yielding a much smoother dlstribution.

From the expander collimator 27, the beam passes to a polarizing beamsplitter cub~ 28 from Perkin-Elmer:

The beamsplitter has the following cnaracteristics:

~L93g~

Maximum Transmittance T to "P" s-tate polari~a-tion: 95 5 Maximum Reflec-tivity R to "S" state polarization: 99 - 1 (entrance and exit faces are coated for minimllm R) Wavelength range: 4880 A - 5145 A
Accep-tance Angle: Normal incidence - 1 Aperture: Central 12 mm The beamsplitting plane of this device has a special multilaver coating so that light which is polarized with its electric field parallel to the vertical axis of this plane (in the "S" state~
is 99% reflected (+ 1~), whereas light wi-th its plane of polarizatioll perpendicular to this axis (in -the "P" state) is 95% transmitted (~ 5%).

The beam passes to a quarter-wave plate 29 which is a rhomboidal piece of glass with parallel input and output faces ~a Fresnel's Rhomb). In this device, by two internal reflections, plane polarized light at its input is converted into circularly polari~ed light at its output.

The lens 31 is a #462097, L.D. Epiplan 40/0.60 lens from Carl Zeiss Canada Ltd., having the following characteristics:

Numerical aperture a : 0.60 Working distance: 4.4 mm Magnification: 40 x Resolution: 0.5 ~m at~ = 4880A

Since the laser 26 is effectively a source at infinity, it is necessary to use in association with lens 31 a negative compensating lens to create a virtual source at ~ 160 mm. The negative lens is a $95,425 lens (diameter: 12mm; focal length:
- 148 mm) from Edmund Scientific Products.

The lens 31 focuses the beam to a diffraction-limited spot on the surface of a layer 11 of APIEZON W wax coated onto a glass slide from a concentra-ted, viscous solution thereof in toluene and permitted to dry to a thickness which is greater than 1 micron.

The coarse focus con rol in the form of a micrometer stage, is 5 used to move the lens 31 along its axis to approximately position its focal plane on the recording surface, and a fine focus control is also used. This is a model ~D-25 Piezoelectric Translator from Jodon Engineering Associates, Inc., Ann Arbor, - Michiyan ~1.3 ~m extension per 100 V, 32 ~m total extension), used for ultra-fine control of the position of the lens focal plane of the lens 31, so that the beam 12 is focused precisely at the surface 13 of the layer 11. A high voltage power supply ~Xepco model Asc425M~ 500 V programmable power supply) is used to drive the piezoelectric fine-focus control. This power supply was set up in the "programmable-by-resistance" mode and a Bourns ~k ~
multiturn potentiometer was used to vary the programming -' resistance, and thereby control the fine focus.

The target 21 including the film of recording medium 11 is mounted on a turntable capable of moving the target relative to the stationary laser beam 12 at a surface speed of about 2 meters per second, whereby the laser beam 12 sweeps along a track on the film 11. This is approximately the minimum surface speed required for recording at 2 megabits per second with a series of 200 nsec pulses occurring once every half a micro-second. That is, ~n the 300 nsec between pulses, the target has moved a little more than the 0.5 ~Im diffraction-limi-ted width of the recording spot, which has been repositioned over a fresh, unrecorded zone.

Under these conditions, fine xecording marks in the form of bumps about 0.25 ~m in height and about 0.5 ~m in width will be close-packed along the recording track with little or no merging. The power level of the light re~uired at said diffraction-limited spot at the recording surface in order to achieve a high-- 3~ -amplitude recording at this sur~ace speed is about 3mW
(corresponding to an intensity of about 15 mW per square micron).

If the laser is operated continuously and the difrac-tion-limited spot is scanned along the recording track wit~ the power level being maintained at about 3 mW (corresponding to -~ about 15 mW per square micron), the bumps are erased and are ~~ replaced by a low ridge about 0.1 11m in height and about 0.5 ~rn ~ide. Information can be re-recorded on the erased track by pulsing the laser, as before, whereby raised bumps, having an elevation of about 0.2 ~m relative to the surface of the ridge, can once more be produced. The cycle of erasure and re-recording can be repeated a large number of times.

The detector system used to provide an output signal comprises the polarizing beamsplitter 28, from which the beam 34 reflected from the surface passes to an outpu-t spatial filter, which is a variable iris aperture 32 from ~dmund Scientific (Aperture: lmm to lcm). The device 32 is mounted with its aperture in the vertical plane on two orthogonally-mounted translation stages so that both the size of the aperture and its position in the back-reflected beam 34 can be finely adjusted.

Optical detector 33 is an 8644/2B 10 stage Special-Line photomultiplier tube from RCA, having the following character-istics:

Spectral Response: S-20 Wavelength of Max. response 4200A
Sensitivity (@ 1500 V cathode-anode):
Cathode: 65 mA/W
Anode: 5.1 x 10 A/W
Current gain (@ 1500 V~: 8 x 10 Anode dark current (@ 1380 V): 1.5 nA
Quantum Efficiency (@ 1500 V, ~ = 4880A): 14%
Rise time (@ 1500 V): 10 ns~c Bandwidth: ~ 35 MHz This high-gain optical sensor converts the intensity changes of the weak beam of light (which, with the detector system described need be no more than about 1 ~W during playback) coming through the output spatial filter 32 into a series of current pulses which are then converted to voltage pulses by a fast video amplifier connected as a current-to-voltage converter. When using less sensitive detector systems the laser can be operated continuously at power levels such that the intensity of the diffraction-limited spot at the surface of the film is at lea.st 300 ~W, so that a high output signal to noise ratio can be obtained.

The video amplifier used is a model 1430 FET operational amplifier from Teledyne Philbrick, having the following characteristics:
Gain Bandwidth Product 100 MHz Settlin~ Time to 1~ for a lOV step 70 nsec Slew Rate 500 V/~sec Output Current Range + 50 mA
Input Bias Current (@ 25C) 500 pA
Input Impedance (@ dc) lOllQ 11 3pF
Noise (referred to Input):
Midband current (1.6 to 160 Hz) 2pA
Wideband voltage (1.6 Hz to 16 MHz) 9~V

~9~

The output signal is obtained as a drop in output from the video amplifier when the laser beam is incident on a bump 16 as compared with -the output ob-tained when the beam is reflected from the smooth surface 13 of the layer ll. A modulated depth of output siynal in -the range of 25% to 100% is achieved.
As no-ted previously, recording marks in the form of crater-like pits can be prcduced in the surface of an iodine-doped cured epoxy resin material using the apparatus and procedures as described in detail above with reference to Figure 2 of the accompanying drawings, and substitu-ting a cured epoxy resin layer, blackened by iodine-vapor doping for the film of APIEZON W.
These crater-like pits can be formed using apulsed beam of light focused to a diffraction-limited spo-t of light with a power level at the surface of the epoxy resin of about 3mW, with a pulse duration of abou-t 100 to 200 nanoseconds and can be erased by sweeping the spo-t along the recording track with the laser operated continuously at the same power level.
The preferred recording media mentioned above, such as bitumens, fractions thereof, doped polymeric resins and organic materials darkened by appropriate heat treatment, such as a caramelized sucrose, are characterized by high color intensity and are there-fore also suitable for use as novel xecording media in known forms of laser thermal recording processes for example those wherein pits are formed irreversibly in the surface of the recording medium by ablation or under the influence of surface tension, or wherein zones of changed ref~ac-tive index are formed.

Claims (175)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An information-recording process comprising the steps of:
providing a recording medium exhibiting first and second stable solid physical states associated with different respective specific volumes and being selectively producible in said states by heating said medium and cooling it from the heated state at selected rates of cooling;

applying heat transiently to a localized volume of said medium adjacent its surface, said volume being sufficiently large in relation to the area of the surface of the medium that is heated that on transition between said first and second states, at least one area of changed surface curvature, sufficient to directly and immediately distinguish said localised volume from the surrounding medium, is produced at said surface; said medium being originally in one of said first and second states; and permitting said volume to cool under conditions selected so that said volume is converted to the other of said first and second states.

2. An information-recording process comprising exposing an optically smooth and homogeneous surface of a recording medium to a concentrated flux of energy, said medium being a solid having first and second solid states associated with different respective specific volumes and being selectively producible in said states by heating said medium and cooling it from the heated state at selected rates of cooling, quickly increasing said flux of energy to thereby transiently heat a localised volume of said medium adjacent its surface, said volume extending sufficiently deeply into the medium that on transition between the first and second states there is obtained at least one area on said surface exhibiting a change in surface curvature sufficient to directly and immediately distinguish said localised volume from the surrounding medium; and permitting said volume to cool at a rate such that a transition between said first and second states is obtained, whereby said area of changed surface curvature on said surface is obtained.

3. A process according to claim 1 in which said area of changed surface curvature has a vertical to transverse aspect ratio of at least 1:10.

4. A process according to claim 3 in which said aspect ratio is at least 1:5.

5. A process according to claim 3 in which said aspect ratio is at least 1:2.

6. A process according to claim 1 in which the conversion from one state to another is obtained by permitting the heated volume to cool at a selected rate of cooling

7. A process according to claim 1 wherein said surface of the medium is optically smooth and homogeneous.

8. A process according to claim 1 wherein said area of changed surface curvature is a sharply-defined area.

9. A process according to claim 1 in which the heating is carried out using a flux of energy concentrated onto the recording medium.

10. A process according to claim 9 in which the energy is laser radiation.

11. A process according to claim 9 in which said flux of energy is concentrated to a spot of minimum diameter substantially co-incident with the surface of the medium, and diverges within the medium below the spot, whereby a volume having a downwardly-increasing cross-section is heated in the medium below the spot.

12. A process according to claim 11 in which said spot is a diffraction limited spot.

13. A process according to claim 9 in which the recording medium has a characteristic value of dV.L

of at least about 1 µm (wherein L is the attenuation length of said energy flux in the recording medium in microns) and (wherein V1 is the magnitude of the difference in specific volume between first and second states of the recording medium, and V2 is the original specific volume of the medium).

14. A process according to claim 13 in which the product does not exceed about 150 µm.

15. A process according to claim 13 in which the product is about 7 µm.

16. A process according to claim 13 in which the attenuation length L is about 1/10th to about twice the width of the area of the surface of the medium that is heated.

17. A process according to claim 16 in which said attenuation length is about 1/2 to about twice said width.

18. A process according to claim 16 in which said attenuation length is substantially the same as said width.

19. A process according to claim 13 in which the percentage specific volume change dV is at least about 1%.

20. A process according to claim 19 in which he percentage specific volume change dV is at least about 7%.

21. A process according to claim 1 in which the thermal conductivity of the medium is less than about 5mW/cm°K.

22. A process according to claim 21 in which the thermal conductivity of the medium is less than about 3 mW/cm°K.

23. A process as claimed in claim 1 in which the heating is carried out using fine conductors.

24. A process according to claim 1 in which two states are ordered and less ordered or amorphous states.

25. A process according to claim 24 in which the bulk of the recording medium is ordered and exhibits an increase in specific volume on transition to the less ordered or amorphous state wherby a bump is obtained on the surface of the recording medium.

26. A process according to claim 1 in which the medium has a softening or melting temperature of at least about 40°C.

27. A process according to claim 1 wherein the medium comprises a bitumen or a fraction thereof.

28. A process according to claim 1 wherein the medium comprises APIEZON W wax.

29. A process according to claim 1 in which the medium comprises a coating of APIEZON W wax deposited on a substrate from a concentrated solution thereof.

30. A process according to claim 1 in which said medium comprises a caramelized sugar.

31. Process according to claim 30 wherein the sugar is sucrose.

32. A process according to claim 1 in which said medium comprises a doped polymeric material prepared by impregnating a polymeric material with a light absorbing dopant to impart a predetermined degree of light absorption thereto.

33. A process according to claim 32 in which the dopant comprises iodine.

34. A process according to claim 32 or 33 in which the polymeric material comprises an epoxy resin.

35. A process according to claim 1 wherein the medium comprises an organic material darkened using heat.

36. A process according to claim 1 wherein the medium exhibits an increase in specific volume, whereby a bump is obtained on said surface.

37. A process according to claim 1 in which said medium exhibits a decrease in specific volume, whereby a pit is obtained on said surface.

38. A process according to claim 1 in which the medium passes through the molten state on conversion between the two solid states.

39. A process according to claim 1 including the further step of erasing the recorded information by exposing the surface of the recording medium to a source of localised heating of a level of intensity effective to re-establish the original stable solid physical state of the recording medium.

40. A process according to claim 1 in which the said surface of the recording medium comprises an interface between the recording medium and a deformable medium.

41. A process according to claim 1 in which the heating is carried out using a concentrated flux of electrons delivered by a current-carrying electrode.

42. Information-recording structure for use in a process according to claim 1, comprising a recording medium having first and second solid states associated with different respective specific volumes and being selectively producible in said states by heating said medium and cooling it from the heated state at selected rates of cooling, whereby on heating a localised volume of said medium adjacent its surface said volume can be changed between said first and second states and at least one area of changed surface relief can be created or erased.

43. A structure according to claim 42 in which said area has a vertical to transverse aspect ratio of at least 1:10.

44. A structure according to claim 43 in which said aspect ratio is at least 1:5.

45. A structure according to claim 43 in which said aspect ratio is at least 1:2.

46. Structure according to claim 42 in which the medium provides a sharply-defined area of changed surface curvature on said heating.

47. Structure according to claim 42 in which the medium undergoes a percentage specific volume change dV of at least about 1%.

48. Structure according to claim 47 in which dV is at least 7%.

49. Structure according to claim 42 in which said states are ordered and less ordered or amorphous states.

50. Structure according to claim 42 in which the medium has a softening or melting temperature of at least about 40°C.

51. Structure according to claim 42 in which the medium exhibits an increase in in specific volume on transition between said states to provide a bump at said surface.

52. Structure according to claim 42 in which the medium exhibits a decrease in specific volume on transition between said states to provide a pit at said surface.

53. Structure according to claim 42 having a recording medium comprising a bitumen or a bituminous fraction thereof.

54. Structure according to claim 42 having a recording medium comprising APIEZON W wax.

55. Structure according to claim 54 in which the medium comprises a coating of APIEZON W wax deposited on a substrate from a concentrated solution thereof.

56. Structure according to claim 42 having a recording medium comprising an organic material darkened using heat.

57. Structure according to claim 42 having a recording medium comprising a caramelized sugar.

58. Structure according to claim 57 in which the sugar is sucrose.

59. Structure according to claim 42 in which the medium comprises a doped polymeric material prepared by impregnating a polymeric material with a light absorbing dopant to impart a predetermined degree of light absorption thereto.

60. Structure according to claim 59 in which the dopant comprises iodine.

61. Structure according to claim 59 or 60 in which the polymeric material comprises an epoxy resin.

62. Structure according to claim 42 having at least one layer of the recording medium disposed between two compressible elastic material layers.

63. Structure according to claim 42 wherein said surface of the recording medium comprises an interface between the recording medium and a compressible elastic material.

64. Structure as claimed in claim 63 in which the elastic material is capable of having its top surface toughened by inducing molecular cross-linking.

65. Structure as claimed in claim 63 in which a film of the recording medium or a film thereof coated on a supporting substrate has thereon a layer of transparent compressible material of sufficient thickness to act as an optical dust cover.

66. Structure as claimed in claim 63, 64 or 65 in which the elastic material comprises a silicone elastomers

67. Structure as claimed in claim 63 comprising a protective barrier layer extending over the deformable medium.

68. Structure as claimed in claim 67 in which the thickness of the protective barrier layer together with the compressible layer is uniform to within about one percent over the entire recordable surface of the recording medium.

69. Structure according to claim 42, 53 or 54 having a protective barrier layer extending over the surface of the medium, the barrier layer being removable, retractable or being radiation-transparent.

70. Structure according to claim 55, 56 or 57 having a protective barrier layer extending over the surface of the medium, the barrier layer being removable, retractable or being radiation-transparent.

71. Structure according to claim 58, 59 or 60 having a protective barrier layer extending over the surface of the medium, the barrier layer being removable, retractable or being radiation-transparent.

72. Structure according to claim 42 wherein the recording medium is free of any particulate or other inclusions or irregularities of such a size or distribution as would interfere with distinguishing the recorded volumes from the surrounding medium during playback.

73. Structure according to claim 42 wherein said surface is optically smooth and homogeneous.

74. Structure according to claim 53, 54 or 55 wherein the recording medium has an optically smooth and homogeneous surface.

75. Structure according to claim 56, 57 or 58 wherein the recording medium has an optically smooth and homogeneous surface.

76. Structure according to claim 59, 60 or 61 wherein the recording medium has an optically smooth and homogeneous surface.

77. Structure according to claim 42 wherein said surface is optically homogeneous.

78. Structure according to claim 53, 54 or 55 wherein said surface is optically homogeneous.

79. Structure according to claim 56, 57 or 58 wherein said surface is optically homogeneous.

80. Structure according to claim 59, 60 or 61 wherein said surface is optically homogeneous.

81. Information recording apparatus comprising a recording structure according to claim 42 and a heating device to inject sufficient energy to transiently heat a localised volume of the recording medium adjacent its surface.

82. Apparatus according to claim 81 wherin the heating device comprises fine conductors in or adjacent to the recording medium.

83. Apparatus according to claim 81 wherein the heating device provides a flux of energy concentrated onto the recording medium.

84. Apparatus according to claim 83 in which said energy flux is concentrated to a spot of minimum diameter substantially co-incident with the surface of said medium, said flux diverging within said medium below said spot of minimum diameter.

85. Apparatus according to claim 83 in which the recording medium has a characteristic value of dV.L

of at least 1 µm (wherein L is the attenuation length of the energy flux in the recording medium in microns and wherein V1 is the magnitude of the difference in specific volume between the two stable states and V2 is the original specific volume of the medium).

86. Apparatus according to claim 85 in which said product does not exceed about 150 µm.

87. Apparatus according to claim 85 in which said product is about 7 µm.

88. Apparatus according to claim 85 in which said attenuation length L is about 1/10th to about twice the width of said heated area.

89. Apparatus according to claim 88 in which said attentuation length is about 1/2 to about twice said width.

90. Apparatus according to claim 89 in which said attenuation length is substantially the same as said width.

91. Apparatus according to claim 85 in which said percentage specific volume change dV is at least about 1%.

92. Apparatus according to claim 91 in which said percentage specific volume change dV is at least about 7%.

93. Apparatus according to claim 81 in which the recording medium has a thermal conductivity of less than about 5mW/cm?K.

94. Apparatus according to claim 93 in which the thermal conductivity is less than about 3mW/cm?K.

95. Apparatus according to claim 81 in which the recording medium is convertible between ordered and less ordered or amorphous states.

96. Apparatus according to claim 81 in which the medium has a softening or melting temperature of at least about 40°C.

97. Apparatus according to claim 81 in which the medium exhibits an increase in specific volume on transition between said states, whereby a bump on the surface of said recording medium is obtained.

98. Apparatus according to claim 81 in which said medium exhibits a decrease in specific volume on transition between said states, whereby a pit is obtained on the surface of said medium.

99. Apparatus according to claim 81 in which the medium passes through a molten state during conversion between the two stable states.

100. Information recording apparatus comprising a recording structure according to claim 53 and a heating device for transiently heating a localises volume of the recording medium adjacent its surface.

101. Apparatus according to claim 100 wherein the heating device provides a flux of energy concentrated onto the recording medium.

102. Apparatus according to claim 101 in which said energy flux is concentrated to a spot of minimum diameter substantially co-incident with the surface of said medium, said flux diverging within said medium below said spot of minimum diameter.

103. Information recording apparatus comprising a recording structure according to claim 54 and a heating device for transiently heating a localised volume of the recording medium adjacent its surface.

104. Apparatus according to claim 103 wherein the heating device provides a flux of energy concentrated onto the recording medium.

105. Apparatus according to claim 104 in which said energy flux is concentrated to a spot of minimum diameter substantially co-incident with the surface of said medium, said flux diverging within said medium below said spot of minimum diameter.

106. Information recording apparatus comprising a recording structure according to claim 56 and a heating device for transiently heating a localised volume of the recording medium adjacent its surface.

107. Apparatus according to claim 106 wherein the heating device provides a flux of energy concentrated onto the recording medium.

108. Apparatus according to claim 107 in which said energy flux is concentrated to a spot of minimum diameter substantially co-incident with the surface of said medium, said flux diverging within said medium below said spot of minimum diameter.

109. Information recording apparatus comprising a recording structure according to claim 59 and a heating device for transiently heating a localised volume of the recording medium adjacent its surface.

110. Apparatus according to claim 109 wherein the heating device provides a flux of energy concentrated onto the recording medium.

111. Apparatus according to claim 110 in which said energy flux is concentrated to a spot of minimum diameter substantially co-incident with the surface of said medium, said flux diverging within said medium below said spot of minimum diameter.

112. Information recording apparatus comprising a recording structure according to claim 60 and a heating device for transiently heating a localised volume of the recording medium adjacent its surface.

113. Apparatus according to claim 112 wherein the heating device provides a flux of energy concentrated onto the recording medium.

114. Apparatus according to claim 113 in which said energy flux is concentrated to a spot of minimum diameter substantially co-incident with the surface of said medium, said flux diverging within said medium below said spot of minimum diameter.

115. Apparatus according to claim 81, 83 or 84 wherein the heating device comprises a laser.

116. Apparatus according to claim 100, 101 or 102 wherein the heating device comprises a laser.

117. Apparatus according to claim 103, 104 or 105 wherein the heating device comprises a laser.

118. Apparatus according to claim 106, 107 or 108 wherein the heating device comprises a laser.

119. Apparatus according to claim 109, 110 or 111 wherein the heating device comprise a laser.

120. Apparatus according to claim 112, 113 or 114 wherein the heating device comprises a laser.

121. Apparatus according to claim 81, 83 or 84 wherein the heating device comprises a current-carrying electrode delivering a concentrated flux of electrons.

122. Apparatus according to claim 100, 101 or 102 wherein the heating device comprises a current-carrying electrode delivering a concentrated flux of electrons.

123. Apparatus according to claim 103, 104 or 105 wherein the heating device comprises a current-carrying electrode delivering a concentrated flux of electrons.

124. Apparatus according to claim 106, 107 or 108 wherein the heating device comprises a current-carrying electrode delivering a concentrated flux of electrons.

125. Apparatus according to claim 109, 110 or 111 wherein the heating device comprises a current-carrying electrode delivering a concentrated flux of electrons.

126. Apparatus according to claim 112, 113 or 114 wherein the heating device comprises a current-carrying electrode delivering a concentrated flux of electrons.

127. Apparatus according to claim 81, 83 or 84 in which said surface of the recording medium comprises an interface between the recording medium and a deformable medium.

128. Information reproducing apparatus comprising a recording structure according to claim 42 and in combination therewith apparatus for reproducing information from the recording medium surface comprising: means for scanning a spot of light over the surface of said medium, an optical sensor for detecting changes in the intensity of light reflected from or transmitted through said surface as said spot scans over an area of surface relief, and a filter interposed between the sensor and said surface, said filter passing to the sensor a selected portion of the cross-sectional area of the beam of light reflected from or transmitted through the surface that exhibits a greater modulation than the aggregate of the reflected or transmitted light collected from the surface.

129. Apparatus according to claim 128 in which the said surface of the recording medium comprises an interface between the recording medium and a deformable optically transparent medium.

130. A thermal recording process wherein a flux of energy is used to transiently heat a localised volume adjacent to the surface of a recording medium, characterized in that the recording medium is a bitumen or a bituminous fraction thereof, an iodine-doped polymeric material or an organic material darkened using heat.

131. A process according to claim 130 wherein the medium is a bitumen.

132. A process according to claim 130 wherein the medium is a bituminous fraction obtained from a bitumen.

133. A process according to claim 130 wherein the medium is an iodine-doped polymeric material.

134. A process according to claim 130 wherein the medium is an iodine-doped epoxy resin material.

135. A process according to claim 130 wherein the medium is an organic material darkened using heat.

136. A process according to claim 135 wherein the medium is a caramelized sugar.

137. A process according to claim 136 wherein the sugar is sucrose.

138. A thermal recording structure comprising a recording medium for use in a thermal recording process wherein a flux of energy is used to transiently heat a localised volume adjacent to the surface of the recording medium, characterized in that the recording medium is a bitumen or a bituminous fraction thereof, an iodine-doped polymeric material or an organic material darkened using heat.

139. A structure according to claim 138 wherein the medium is a bitumen.

140. A structure according to claim 138 wherein the medium is a bituminous fraction obtained from a bitumen.

141. A structure according to claim 138 wherein the medium is an iodine-doped polymeric material.

142. A structure according to claim 141 wherein the medium is an iodine-doped epoxy resin material.

143. A structure according to claim 138 wherein the medium is an organic material darkened using heat.

144. A structure according to claim 143 wherein the medium is a caramelized sugar.

145. A structure according to claim 144 wherein the sugar is sucrose.

146. Process for the manufacture of an information recording structure according to claim 42 comprising providing the recording medium in a form free from particles or other inclusions of such size as would interfere with distinguishing said volume from the surrounding medium, and forming a self-supporting film of or a coating of a film of said medium on a supporting substrate, wherein the recording medium comprises a bitumen or a bituminous fraction thereof, a doped polymeric material or an organic material darkened using heat.

147. Process according to claim 146 comprising knife-coating or dip-coating a supporting substrate with a bitumen or a bituminous fraction thereof to form a recording medium film thereof on the substrate.

148. Process according to claim 146 wherein the film is prepared by knife-coating by spreading out the recording medium on the substrate using a spreading device.

149. Process according to claim 146 wherein the film is prepared by dip-coating by bringing the substrate into contact with a fluent preparation of the recording medium which is allowed to flow onto and solidify on the substrate.

150. Process according to claim 147 wherein the said bitumen or fraction is coated on the substrate as a solution in an organic solvent, and comprising permitting the solvent to evaporate.

151. Process according to claim 150 wherein the solution is a viscous concentrated solution.

152. Process according to claim 151 wherein the viscosity of the solution is at least about 700 centipoise.

153. Process according to claim 150, 151 or 152 wherein the solvent is toluene or trichloroethane.

154. Process according to claim 150, 151 or 152 wherein the solvent is toluene.

155. Process according to claim 150, 151 or 152 wherein the solvent is trichloroethane.

156. Process according to claim 147, 150 or 151 wherein said bitumen is a native asphalt.

157. Process according to claim 147, 150 or 151 wherein said bitumen is a distillation redidue

158. Process according to claim 147, 150 or 151 wherein said bitumen or fraction is a bituminous fraction obtained from a bitumen.

159. Process according to claim 147, 150 or 151 wherein said bitumen or fraction is APIEZON W wax.

160. Process according to claim 146 wherein the recording medium is an organic material darkened using heat.

161. Process according to claim 160 wherein the organic material is a sugar.

162. Process according to claim 161 wherein the sugar is sucrose.

163. Process according to claim 146 wherein the recording medium is a doped polymeric material prepared by impregnating a polymer with a light-absorbing dopant to impart a predetermined degree of light absorption thereto.

164. Process according to claim 163 wherein the recording medium comprises an iodine-doped polymeric material.

165. Process according to claim 163 wherein the polymeric material is doped with iodine by bringing the polymer into contact with iodine vapour until it absorbs sufficient of the vapour to impart a predetermined degree of light absorption thereto.

166. Process according to claim 163 wherein the polymer comprises an epoxy resin.

167. Process according to claim 164 wherein the polymer comprises an epoxy resin.

168. Process according to claim 165 wherein the polymer comprises an epoxy resin.

169. Process according to claim 146, 150 or 160 wherein the film is formed such that at least one surface is optically smooth and homogeneous.

170. Process according to claim 163, 164 or 165 wherein the film is formed such that at least one surface is optically smooth and homogeneous.

171. Process according to claim 166, 167 or 168 wherein the film is formed such that at least one surface is optically smooth and homogeneous.

172. Process according to claim 146 including incorporating fine conductors in the structure, in or adjacent to the recording medium.

173. Process according to claim 146 in which the film of the recording medium is coated with a transparent compressible material of sufficient thickness to act as an optical dust cover.

174. Process according to claim 173 wherein the compressible material is a silicone elastomer.

175. Process according to claim 173 or 174 wherein the compressible material is capable of having its top surface toughened by inducing molecular cross-linking.