CA1252926A

CA1252926A - Ferroelectric liquid crystal devices

Info

Publication number: CA1252926A
Application number: CA000507587A
Authority: CA
Inventors: John M. Geary
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1985-04-26
Filing date: 1986-04-25
Publication date: 1989-04-18
Also published as: JPS62502636A; KR880700299A; HK44392A; SG84091G; WO1986006507A1; EP0221112A1; EP0221112B1; KR940006158B1; DE3677459D1; US4707078A

Abstract

FERROELECTRIC LIQUID CRYSTAL DEVICES

Abstract In a display device utilizing ferroelectric (e.g., chiral, smectic C material), a stabilizing electric field perpendicular to the cell surfaces forces the molecules to orient themselves in one of two states both of which are parallel to the cell surfaces. A particular state is selected by a switching electric field also oriented perpendicular to the cell surfaces. The stabilizing field has a half-cycle time shorter than the switching time of the ferroelectric material, whereas the switching field duration is longer than or equal to the switching time of the material. The invention enables the use of relatively thick (e.g., ? 2 µm) ferroelectric cells. (FIG. 5).

Description

S~2{3~i FE~ROEIECTRIC LIQIiID CRYSTAL ~EVICES

Back~round of the Invention ____ ____ ________ ________ This invention relates to liquid crystal devices utilizing ferroelec-tric li~uid crystal ~LC) materials.
Although the follo~rin~ description relates primarilY to liquid crystal displays (LCDs), the primary intended application, the invention also contemPlates use in other applications such as optical shutters.
A display device utilizin~ ferroelectric, chiral, smectic C materials has been sugqested ~Y
N. A. Clark et al, ~lied ~hysics Letters Vol. 36, p. 89~ (1980). In this device, the smectic material is layered, and the layers are aligned perpendicular to the glass surfaces of the cell as shown in FIG. 1.- The liquid crystal molecules li2 flat on the sur~aces and are restricted at the surfaces to only tvo positions (i.e., states S1 and S~) out of the cone of possible orientations (FIG. 2) that the chiral, smectic C state allows. The surfaces of the cell are closely sPaced (about 1 ~m se~aration) so that the bulk of the sample follows the molecular orientation at the surface, thereby creating the two "surface-stabilized states" iAentified by Clark et al, _u~r3. The influence of the surfaces also helps to su~press the helix of the chiral smectic C material so that the t~o states are not disrupted by pitch bands. The ferroelectric fixed diPole moment (M1,M2), which is inherent at the chiral center, points normal to the surfaces, uP in one state (S1) and down in the other (S2).
Hence, the davice can be s~itched between the t~o states vith a pulsed electric field applied Yia electrodes on the surface~ The influence of the closely spaced surfaces causes the switched state to latch so that the state is maintained after tha pulse is over. Thus, the device is -` ~2~;~92f~

- 2 -bistable in that a latched state is maintained for time uhich is longer than the time between adjacent write and erase ~ulses or between adiacent refresh pulses, whlchever - is greater. Appropriately oriented polarizers are used to produce a visible contrast between the two states.
Fabrication of the above display ~ay present difficul-ties o~ing to the required close (about 1 ~m) spacing of tha cell surfaces. A ferroelectric display with a larger cell spacing in the range of 5 ~m to 10 ~m, as used in Presently manufactured twisted nematic LCDs, ~ould ~e desirable~ However, when such larger spacings are used with conventional ferroelectric IC materials, a pitch band texture forms which renders the device less desirable. Even i~ the pitch bands are eliminated, the desired two-state behavior is not observed in thick cells (i.e., cells ~ 2 ~m thick). Instead, at the end of the switching pulse, the material quicklY reverts to a complex twisted state akin to the intermediate state sometimes seen in thin ferroelectric cells. In these twisted states, the molecular orientation varies around the smectic C cone as one goes from one cell surface to the other. ~hat is needed in thick cells is a means for holding the molecular orientation in the bulk of the cell ~o those two orientations allowed close to the cell surfaces.
SUmma____f_tke_Inven l_n In accordance with the present invention, the negative dielectric anisotropy of ferroelectric LCs is exploited to induce bulk two-state behavior. Nore specifically, an electric field applied perpendicular to the cell surfaces acts on a LC molecule with negative dielectric anisotropy so as to orient the core axis o~ the molecule normal to the electric field and thus parallel to the cell surfaces. The core axis is the x-raY axis as defined bY R. Bartolino et al in Annals de ~hysi~ue, ~ol~ 3, p. 383 (197~)o In a smectic C phase LC material, for example, ~he cone of possible molecular orientatior.s - 12S~

allows iust two such states, ~hich are identical to the two homogeneous surface-induced orientations. However, for a ferroelectric LC~ the field will also act on the fixed dipole rnoment of the molecules and thus tend to favor one o~ the two states. In accordance with one aspect of this invention, this ef~ect is circumvented by using a stabiliYing electric field which has a half cycle time substantially shorter than the time for the LC
material to switch from one stable state to another, hereinafter termed the "s~itching" time. The stabilizing field preferably has a substantially zero average over the duration of the switching pulse discussed below (e.g~, the stabilizing field is generated by an AC voltage signal such as a sinusoid). ~hus, the influence of such a field on the fi~ed dipole moment will also average out to zero.
But, since the effect of an electric field on dielectric anisotropy is indePendent of the sign of the field, the stabilizing field ~ill still act to orient the core axis of the molecules perpendicular to the field lines, and thus tend to restrict them to the two states that are parallel to the cell surfaces~ This effect acts throughout the bulk of the cell, thus producing field-stabil-zed states aven in thick samples. In accordance with another aspect of mY invention, these states are ~S s~itched by superimposins a s~itching electric field on the stabilizing field. The duration of the s~itching field~ which is illustratively generated by a pulsed voltage signal, should be substantially longer than or equal to the switching time o~ the LC material. Since all ~the molecular orientations between these states are not perpendicular to the field, they are energetically disadvantageous and create a threshold potential between the two states.
~C devices in accordance with this invention sho~ latching and sharp s~itching thresholds as ~ell as improved contrast and switching speed for field-stabilized states. Of these characteristics, the enhanced s~itching s;~

speed is particularly advantageous in matrix addressing schemes for LCDs, and the improved contrast is especially useful in optical shutters.
In accordance with one aspect of the invention there is provided a liquid crystal device comprisingO a liquid crystal cell containing a ferroelectric liquid crystal material having a characteristic switching time, and means for appl~ing an electric field to said material, CHARACTERIZED IN T~IAT said applying means includes, in combination, means for applying a stabilizing electric field which has a half-cycle time shorter than the switching time of said material, and means for applying a switching electric field which has a duration longer than or equal to the switching time of said material.
Brief Description of the Drawing "
The invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanyiny drawing, in which:
FIG. 1 is a schematic, isometric view of a LC
cell showing the field-stabilized states in accordance with one aspect of my invention;
FIG. 2 shows the cone of possible orientations of the molecules of a ferroelectric, smectic C material;
FIGS. 3 and 4 are top views of a layered ferroelectric, smectic C material showing the molecular orien-tation in the layers in the absence of pitch bands;
FIG. 5 is a schematic of a LCD in accordance with one embodiment of my invention depicting the switching and stabilizing sources connected to an illustrative cell;
FIG. 6 shows the pro~ection of a typical molecule onto a plane p~rallel to the smectic layers. The orientation of the molecule's projected image is given by the an~le Q;

~z~
- 4a -FIG. 7 is a graph calculated potential ~ as a fun~tion of Q; and FIG. 8 is a graph of threshold curves for single bipolar write pulses and groups of 1000 bipolar disturb pulses. Change in transmission (in arbitrary units) is plotted as a function of pulse magnitude YO. Curves are shown for a superimposed AC signal of 40 V and 20 V
: amplitudes.
Detailed Description The operation of the stabilizing field in producing bistable states, and the operation of the switching field in controlling the particular state of the LCD, in accordance with one aspect of this invention, are :

~2S;;~9Z~
. .

~ 5 --best understood with reference to E`IGS~ 1-4. As shown in ~I~. 2, each molecule in each laYer of a chiral, smectic C
material has a cone of possible orientations~ with the axis ~ of the cone lying parallel to the major surfaces of the cell. This cone should not be confused with the cone of orientations associated with focal conic defect Si~2S
where the s~ectic laYers themselves twist. In two of the illustrative states S3 and S4, the molecule does not lie in a plane parallel to the surfaces. There are, of ' 10 course, an in~inite number of other orientations ~hich are similar to S3 and S4. In contrast, two of the orientations designated by states S1 and S2 correspond to the molecule lIing in a plane 50 parallel to the cell surfaces~ ~hese two orientations are preferred and correspond to the stable states S1 and S2~ In general, however, all orientations falling on the surface of the ; cone are possible and the displaY would have a correspondingly large number of possible states.
The "switching time" of the LC material is defined as the time for the LC molecules to switch from state Sl to S2, or conversely, and is primarily a function of electric field strength and cell temoerature. In addition, it is also strongly dependent on cell thickness in thin cells but only weakly dependent in thick cells.
In order to restrict the molecules to only th'O
states Sl and S2, a stabilizing electric field ESt is applied perpendicular to the cell surfaces as one component of khe total field E shown in FIGS. 1 and 5. As a result, the molecules throughout th~ bul~ of the LC
material are confined to only two possible states. These states S1 and S2 are displaced from axis A by the smectic tilt angle ~ thereto. In one state S1 the ferroelectric fixed dipole moment M1 is directed upwardly, ~hereas in the other state S2, the fixed dipole moment ~'l2 is directed downwardlyO In both cases ~1 is perpendicular to the cell surfaces. The effect of the stabilizing field on the negative dielectric anisotropy of the LC material is -" ~LZ5~9;2~
~ ,;
independent of the sign of -the field. Consequently, the stabilizing field acts to orient the molecules perpendicular to the Eield lines and thus restricts them ; to the two states S1 and S2 Ihat are parallel to the cell surfaces. This effect acts throughout the bulk of the cell, thus producing stable states even in thick (i.e., 2 ~m) samples~
A top view of the molecular orientation in these states is shown in FIGS. 3 and 4. FIG. 3 shows molecules in se~arate layers oriented as in state S1, uhereas FIG. 4 depicts molecules in separate layers oriented as in state S2. Of course, the particular pattern of orientations is controlled by the pattern of the electrodes as well'as by the selection of which electrodes to energize.
Turning now to FIG. 5, there is sho~n schematically a LCD comprising a ~C cell 10 to which a pair of voltage sources are connected, a stabilizing source 12 and a switching source 14 in series with one 2C another. Although shown as separate units, these sources can readily be designed as a single unit (e.g., integrated circuit). The sNitchins source 14 generates one component of the electric field E, a s~itching electric field EsW
which has a duration longer than or egual to the switching time of the LC material measured at the switching field stren~th. On the other hand, the stabilizing source 12 qenerates another component of the electric field E, a stabilizing electric field ~st ~hich has a half-cycle time substantially shorter than the switching time of the LC
material measured at the combined switching and stabilizing field strengths. Except for marginal designs, however, it is adeguate -to relate the half-cycl~ time to the s~itching time at the stabilizing field strength alone. In addition, Est preferably has a substantially zero average value over the duration of the switching field.

~5Z9;2~

The timing of the stabili~ing and switching fields is largely dictated bY the need to stabilize (latch) the states after switching them. For latchin~, the stabillzing field maY ~e applied after the switching field providefl the delay bet~een the t~o is short relative to the relaxation time of tne molecules. Howe1Jer~ to enhance ~he threshold between states S1 and S2, -the stabilizing field should be applied continuously during the display operation. A continuous stabilizing field is, of course, also suitable from the stand~oint of latching.
On the other hand, the shape of the voltage waveforms is no-t critical; for example, either field may be generated as a sinusoid or a sguare wave voltage of appropriate duration relative to the switching time of the ~C
material. The amplitude of the wave~orms, on the other hand, is rela~ed to considerations such as s~itching speed (higher voltage stabilizing signals allo~ the use of shorter duration switching pulses) a~d threshold voltage (the peak switching signal voltage should e~ceed the 2Q threshold). The actual amplitude used in a particul~r case depends also on the negative dielectric anisotropy of the ~C material and maY be determined empirically from a suitahle control sample.
The cell itself includes the LC materlal 20 bounded bY confinement means, e.~., a pair of spaced, parallel, transParant plates 22 and 24. Illustratively, the plates com~rise glass but plastic maY also be suitable. The interior, facing major surfaces of the plates have conductive material patterned into electrodes 26 and 28 the shapes of dhich define the individual picture elements (pels) of the disPlay. If light is to Pass through either set of electrodes, then the material of that set should be trans~arent. In addition, the cell 10 has means for providing optical contrast, illustratively polari~ers 30 and 32 on opposite sides thereof. Tn one arrangement suitable for thick cells, the polarizers are parallel to one another, but are ~s~9~

oriente~ at the tilt angle ~ with respect to the layers (i,e., axis A of FIG. 1). The thickness of the cell is chosen so that the orthogonal optical components experience a retardation of nA/2, where A is the wavelength of the llght and n is an odd integer.
~epending upon the ~ype of LC material, however, the polarizers may be omitted (e.g., in certain guest-host displa~s)O
The LC material 20, in accordance with the invention, is ferroelectric and has a negative dielectric anisotropy. IllustrativelY, the LC material has a tilted, smectic phase including for example smectic Phases C, F, G, I or J which are well known in the art. From the standpoint of switchillg speed, however, Phase C smectic materials are much faster than the others and hence are to be preferred. LC materials containing a single chiral component as well as those containing a mixture of chiral components are suitable. One class of such materials constitutes ferroelectric LC mixtures which are formed from component chiral smetic C materials with opposin~
t~rist senses, and thus have relati~elY long pitches.
Examples of such LC mixtures include, without limitation, the following:

~Z5;~Z~i , g F~M 4 C~3 (1) C10H21o- ~ -0-C- 0 -CH CH C H

(2) C10H210- 0 -CH=CH-C-0- 0 -C-0-CH2CH C2~5

(3) C8H170~ 0 -C-0- 0 -C-0-CH CH C H

0 C~13 (4~ C11H~30- 0 -CH=CH-C-0- 0 -C-0-CH2CH C2H5 `!
i (5) C11H230- 0 -CH=CH-C-0- 0 -C-0-(CH2)~CH (CH2)3CH(CH3)2 O
(6) CloH210~ 0 -0-C- 0 -0-(cH2)2cH (CH2)3cH(cH ) The constituents (1) through (6) were combined in the following weight percentages~ 0~5%, ~2) 24.0%, ~3) 17.8%, (4~ 10.0~, (5~ 19.7~, and (6) 8.0%.
` A total of about 1 gm of the mixture wac ~eighed into a vial, heated to melting in the isotropic phase, agitated and then cooled to room temperature. The mixture was then ready for use. The FEN 4 mixture has a negati~e dielectric anistropy and exh.ibits the desired bistable characteristics in accordance ~ith this invention but may not he o~timum because the double bonds in conslituents ~2S~926 (2), (4) and (5) are sensitive to W radiation. However, the FEM 23 mixture below also has a negative dielectric anisotropy (without W sensitivity) and hence has exhibited similar bistable properties.

(7) CloH2lO~ O -C-O- O -0-(CH2)2*CH (CH2)3CH~CH3)2 (8) CloH2lO~ o -C-O- O -O-CH CH C H
, O C~3 (9) CloH21O~ O - O -C-O-CH CH C H

The constituents (7) through (9) were combined as above in the following weight percentages: (7) 59.3%, (8) 27.7~ and (9) 13.0%.
The asterisk in constituents (l) through (9) denotes the active chiral center.
The switching time of FEM 4 is about 3.0 msec at 1.2 V/~m in a 25 ~m thick cell at 25C and that of FEM 23 is about 0.5 msec at 4.5 V/~m in a 5 um thick cell at 23C.
Although not shown in FIG. 5, in one embodiment the cell lO is provided with alignment layers on the interior surfaces covering the electrodes and the transparent plates. Alignment techniques similar to those described by J. Patel et al, Ferroelectrics, Vol. 58-59, p.457 (1984) can be utilized. A
. . . . .
preferred alignment technique is described in U.S. Patent No.

4,664,480 which issued on May 12, 1987 to J.M. Geary et al.
In accordance with one procedure taught therein, one interior surface of ~s~

with one ~rocedure tauc1ht tnerein, one interlor surface of the cell is coa-ted witl1 an aligning layer (e.g., a crystalline polYmer which is rubbed with a cloth, for example). The other interior surface is coated with a non-aligning laYer (e.g., an amorphous PolYmer) that cannot align the ~C material (even if rubbed) and is left unrubbed. For exa~ple, one interior surface Inay be coated with an ali~ning layer of rubbed polYethylene terephthalate (PET) whereas the other interior surface is coated ~ith a non-aligning layer of unrubbed polymethylmethacrylate (PMMA).
The__y_of_Fi ld=StabiliZe__s------The following derivation demonstrates how field-stabili~ed states are achieved in the bulk of a chiral, smectic C material. Similar comments apPlY to other tilted-phase, smectic materials. In FIG~ o the smectic layers are aligned parallel to the Plane of the paper, and the molecules are free to rotate around axis A
; normal to the paper. Although the molecule is in yeneral oriented in three dimension~ on the surface of a cone (FIG. ~), OIIly the projection of the molecule onto the plane of the smectic layers is shown in FIG. 6.
An electric field E o~erates on both the ferroelectric fixed dinole as well as the induced dipole.
ApplyinQ torque to the fixed di~ole can cause switching and is rela-ted to pE, where p is magnitude of the dipole moment. On the other hand, ap~lyin~ torQue to the i~duced dipole can ~roduce field-stabilized states and, as sho~n below, is related to a6~ , t~here ~ is the dielectric anisotropy. In accordance with this invention, the switching field Es~ acts on the fixed dipole and has a duration loncler than or equal to the characteristic s~itching time of the IC material at that field strength.
~o a~oid well-kno~n degradation of the LC material, E
preferably h~s no DC component. In contrast, the stabilizing field ESt acts on the induced dipole and has a half-cycle time substantially shorter than the switching ~2S;~26 .

time of the LC material~ E t has no DC component not onlY
to avoid degradation but also to avoid spurious switching.
That is, a DC component of ESt ~ould act on the fixed dipole and over time the integrated effect might be to cause the molecule ~o switch states.
TQ better understand these phenomena, let us proceed with the calculation, assuming, for purposes of the calculation only, that the sta~ilizing field t~kes the form of a sinusoid ESt = Epsin ~t, where E t is applied parallel to the smectic layers and perpendicular to the cell surfaces.
To calculate the torque on the molecules around their axis A of permitted rotation, one needs to know the effective components of the dielectric constant normal to this axis~ In most LC phases, there are only two dielectric constants, one parallel to the long molecular axis and one perpendicular to it. In the chiral smectic C
state, howe~er, there are in general three different dielectric constants as shown in FIG. 2: the usual ~arallel component 6p and two perpendicular (_ormal) Components 6n1 and 6n2' where ~n1 is defined to be parallel to the fixed dipole moment M, and 6n2 is defined to be perpendicular to the fixed dipole moment.
Evaluating the effective components of these three dielectric constants in a plane normal to the axis A of permitted rotation, we obtain two projected components:

2 co~2 6proj(p~ psin ~ ~ Cn2 ~ (1) proj(n) 6n1 (2) where ~ is the smectic tilt angle. The trigonometric functions are squared because the tilt angle affects both the size of the induced moment and the component o~

~S~26 electric field that acts on that moment. The projected dielectric anisotropy is now proj ~proj(p) 6proj(n) . (3) In rationalized YKS units, the torgue (per unit volume) TSt due to the time varying field Est is st [~roi6oEst(t)cos~l[~Est(t)sin9)]

where ~0 is the permittivitY of a vacuum, or ;

T = ~ ~~~2~i~Est(t)sin2~r (5) ;

where ~ is the orientation of the projected molecule as shoT~n in FIG~ 6. For large ~, TSt is replaced with its ti~e avera~ed Yalue I t:

Tst ~ ~ ~-~4-i--Epsin2~ . ~6) This apPro~imation is justified if the time for a half cycle of the stabi~izing field is substantiallY smaller than the swi-tchin~ time of the ~C material at a peak field of Ep. For a stabilizing fie~d var~ing at this rate, the tor~ue on the fixed ferroelectric dipole averages out to zero.
An effective potential ~st is de~ined as ~2~926 d'~st d~ ~st , (7) and ~aking the derivative called for in e~uation (7) yields st(~ = ~ ~~ 8- --Epcos2~ . (8) This effactive potential is plotted in FIG. 7.
Because of its 2~ dependence, ~st e~hibits a double potential well with t~ro stable states in each full cycle of ~. The nature of these states dePends on the sign of ~proj. If the dielectric anisotropy ~proj is negative (as it is in FIG. 6), then the stable states are at = +30O Thus, the molecules will lie perpendicular to the applied field and parallel to the surfaces of the cell. If Q~pr j is positive, then the s~ates are at ~ = 0 and ~ = 180, so that the Projected director is parallel to the field lines.
The t~o permitted molecular configurations in the case of negative ~Proj are similar to the -two states of Clark et al, s_pra~ But, in accordance with this invention, the states are induced by a stabilizing field acting throughout the bulk of the LC material, rather than by a short range surface interaction. Conse~uentlY, the disolay cell can be made much thicker than the 1 ~m spacing typical of the Clark et al dis~lay. The double potential well form of the potential energy diagram of FIG. 7 indicates that the stabilizing field will encourage latching even in thick cells and, importantly, the double ~ell implies a threshold between these two states.

~S2~26 GkServati_n__f-Field=stabiliz-d-states Field-stabilized states corresPonding to the neyative ~ case have been observed in several pro~
ferroelectric ~C materials such as FEM 23 described

5 earlier. Another LC material ~hich exhibits these states very clearly is a long pitch mixture identified earlier as FEM 4. The follo~ing experiments ~ere performed using FEM
4 cells.
For purposes of experimentation, a wedge cell f 10 having a thickness which varied from 5 ym at the left to 7 ~m at the right was filled with FEM 4. ~ith 40 V
applied across the cell at about 25C, -the switchin~ time ranges from about 0.45 msec to Q.63 msec. At 8~ V, however, the switching time would he halved approximately.
15 The smectic layers were aligned Parallel to the vertical diagonal of a square electrode, and the sample was somewhat rotated with regard to crossed polarizer axes.
Alignment was obtained by a technique in Nhich one surface ~as coated with polyethelene terephthalate tPET) and 20 rubbed, and the other surface ~as coated with polymethylmethacrylate (PMMA) and was left unrubbed. When no field was applied, two complex domain states were faintly Yisible. These states, which are typical of un~wound thick ferroelectric samples, exhibited a variation 25 o~ molecular orientation from one surface of the cell to the other~ They did not show extinction at any orientation. But, when a sinusoidally varying stabilizing field was applied t 40 V peak, 15 kHz, 33 ~sec half-cycle time), these states straightened out into simple 30 homogeneous domains with good extinction. The symmetry of the two states was demonstrated by rotating the sample to either side of the polarizer axisr The field-stabilized states in the above FE~ 4 sample could be s~itched and exhibited latching after the 35 switching pulse was overO S~itchin~ source 14 (FIG~ 5) generated bipolar pulses (i.e., two conti~uous rectangular pulses of equal (1.5 msec~ duration and magnitude, but of ~ZS~6 opposite sign) which were suPerimposed on the sinusoid generated by stabilizing source 12. Pulses of this form, ~rhich are useful in practical disPlay addressing schemes, actually switch the ~C material twice, leaving it in the field-stabilized state corresponding to the trailin~
polarity. In one experiment, latched light and dark conditions of the display ~ere ~roduced by hipolar pulsss of two different senses (positive pulse follo~ed by negative, and negative pulse followed by positive). The switching pulse ma~nitude was 40 V and the entire bipolar Rulse lasted 3 msec.
In order to matrix address or multiplex a display, a single addressing Pulse of a particular amplitude complete~y switches a displaY element, while numerous smaller disturb pulses (the byproduct of addressing other elements of the disPlaY) should not significantly switch the element. Clearly, a sharp switching threshold is desirable for effective multi~lexing.
AccordinglY~ field-stabilized states in a FEM 4 cell with a 5 ~m spacing were tested for threshold behavior. Results ~ere obtained for lar~e (40 V peak) and small (20 V pe~k) sinusoidal stabilizin~ fields, both at a frequency of 5 kHz (100 ~sec half-cycle time). ~s before, 25 the s~itching time at L~O V and 25C is about 0.45 msec, hut is about 0.90 msec at 20 V. The effects of (a) single hipolar write pulses, and (b~ sequences of 1000 bipolar disturb pulses ~lere studied as a function of pulse amplitude~ The write and disturb pulses ~ere o~ the same form, corresponding to a simple 2:1 address scheme. Li~ht transmission ~as measured with the samPle placed betwaen crossed polarizers. The sample was rotated to one side (as in the Previous experiment) to produce a contrast between the two field-stabilize~ states. The samPle was first put in the state of minimum light transmission by a single 40 V hipolar reset pulse, and then signals of type ~a) or (b~ ~ere applied. The resultin~ increase in li~ht ~l2S2~6 transmission is plotted versus pulse magnitude in FIG. ~.
The data for the 40 V sinusoid amplitude revealed an extremely sh.arp swi~ching threshold. The write response ~curve I) went from virtually zero to close to the maximum for only a 1 V increase in switching pulse magnitude from 17 V to 18 V. More importantly, however, the disturb response (curve II) was equally steep and was very close to the write curve. Under these circumstances, a 1000 element dis~lay can be easily multiPlexed. With the smaller 2n V sinusoid amplitude, the write response (curve III) was still reasonably sharp, but the disturb ; response (curve IV) was broadened and disPlaced to lo~er voltages. The less desirable performance at 20 V is attributable to the fact -that decreasing the peak am~litude b~ a factor of tuo reduces the torque on the molecules by a factor of four ~see equation (6)]. Still, one is iust able to bracket the write and disturb curves ~ithin a factor of two (16 V and 32 V), so that 1000:1 multiplexing is still possible, even with the louer amplitude sinusoid.
The threshold behavior exhibited in FIG. B, especially for the larYer sinusoid amPlitude, seerns to confirm the predictions of the calculations performed earlier. However, the observed behavior does not fully ~5 correspond to the simple -threshold concePt illustrated by the potential well drawing of FIG. 7. ~hereas the above FEkl 4 sample exhibited good threshold for pulse switching signals, it was possible to switch the cell's state at almost an~ amPlitude if the switching signal ~as aPplied as sustained DC/ In a Practical LCD, however, DC is generally avoided to Prevent degradation of the LC
material. Hence, ~hat is seen is not a static threshold, but rather a dYnamic threshold, dependent on the brief duration o F the applied switchii1g pulses. ~hen sustained DC ~as used, switching was observed to occur by the motion of domain walls which nucleate on de~ects and proPagate across the sample. The sim~le potential well modQl of ~S~2~

FIG. 7 clearly cannot anticipate such effects since it presllmes a comPletely uniform material where no defects exist and where domain walls, being boundaries between regions of differing molecular orientation, cannot be present. Evidently the s~itching caused by brief, relativel~ high amplitude pulses results in something closer to bulk sYitching~ ~here the model of FIG. 7 is more appropriate. I~ is important to note, however, that a static threshold is not necessarY for the multiplexing of displays, ~here brief s~itching pulses are desira~le to begin with.
An interesting feature of the data of FIG. 8 is the imPlication of increased s~itching speed in the case of the higher amplitude (40 V) sinusoid. The pulse amplitude V required to write was reduced by ~earl~ a factor of two bY increasing the sinusoid amplitude from 20 V to 40 V, which implies an increase in the write speed if the write pulse voltage were kept fixed as the sinusoid am~litude ~as increased. The reasons for this effect are not well understood at Present.
It is to ~e understood that the above-described arran~ements are merely illustrative of the many possible specific embodiments ~hich can be devised tO represent application ~f the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these Principles by those skilled in the art without departing from the sPirit and scope of the inventionO In particular, ~hile special emphasis was placed on the utility of my invention for thicX (2 2 ~m) celLs, other attributes of field-stabilized states ma~ be useful in thin (< 2 ~m) cells as ~ell.

Claims

Claims:

1. A liquid crystal device comprising:
a liquid crystal cell containing a ferroelectric liquid crystal material having a characteristic switching time, and means for applying an electric field to said material, CHARACTERIZED IN THAT said applying means includes, in combination, means for applying a stabilizing electric field which has a half-cycle time shorter than the switching time of said material, and means for applying a switching electric field which has a duration longer than or equal to the switching time of said material.

2. The device of claim 1 wherein said applying means generates said stabilizing field with a substantially zero average value over the duration of said switching field.

3. The display of claim 1 wherein said cell includes a pair of parallel, spaced, major surfaces which bound said material, said material comprises a layered, smectic, tilted phase liquid crystal, said layers being oriented perpendicular to said surfaces, said stabilizing field causes the molecules in said layers to lie in planes parallel to said surfaces, each molecule in each plane having a pair of stable states, and said switching field causes said molecules to switch between said states.

4. The display of claim 1, 2 or 3 wherein said ferroelectric liquid crystal comprises a chiral smectic C
phase liquid crystal.

5. The display of claim 1 wherein said ferroelectric liquid crystal material comprises a mixture of liquid crystal components containing chiral liquid crystal material.

6. The display of claim 5 wherein said components have opposing twist senses resulting in a relatively long pitch.

7. The display of claim 3 further including an aligning layer on one of said cell surfaces and a non-aligning layer on the other of said cell surfaces, said aligning layer comprising a crystalline polymer which produces alignment of the liquid crystal molecules along a predetermined direction, and said non-aligning layer comprising an amorphous polymer which does not produce such alignment.

8. The display of claim 7 wherein said crystalline polymer comprises PET and said amorphous polymer comprises PMMA.

9. The display of claim 1, 2 or 3 wherein the thickness of said cell is greater than about 2 µm.

10. The display of claim 1, 2 or 3 wherein said electric fields are applied essentially perpendicular to said major surfaces of said cell.

11. The display of claim 1, 2 or 3 wherein said applying means applies said stabilizing field and said switching field during time intervals which are separated by a time less than the relaxation time of said liquid crystal material.

12. The display of claim 1, 2 or 3 wherein said applying means applies said stabilizing field essentially continuously during the operation of said display.

13. A liquid crystal display comprising a liquid crystal cell containing a ferroelectric liquid crystal material which has a characteristic switching time, said cell including a pair of parallel, spaced, transparent plates which bound said material, electrodes formed on the interior major surfaces of each of said plates, an aligning layer formed on one of said major surfaces and said electrode thereon, and a non-aligning layer formed on another of said major surfaces and said electrodes thereon, means coupled to said electrodes for applying an electric field to said material in a direction perpendicular to said surfaces, said applying means including, in combination, means for applying a switching electric field pulse which has a duration longer than or equal to the switching time of said material, and means for applying a stabilizing electric field which has a substantially zero average value and which has a half-cycle time shorter than the switching time of said material, said material comprising a chiral, smectic, tilted phase liquid crystal formed in layers which are oriented perpendicular to said surfaces, said stabilizing field causing the molecules in said layers to lie in planes parallel to said major surfaces, each molecule in each plane having a pair of stable states, said switching field causing said molecules to switch between said states, and said aligning layer comprising a crystalline polymer which produces alignment of said molecules along said direction, and said non-aligning layer comprises an amorphous polymer which produces no such alignment.

14. The display of claim 13 wherein said liquid crystal material comprises a chiral, smectic C phase liquid crystal.

15. The display of claim 13 wherein said liquid crystal material comprises a mixture of liquid crystal components each of which is a chiral liquid crystal but of opposing twist senses.

16. The display of claim 13, 14 or 15 wherein the thickness of said cell is greater than about 2 µm.

17. The display of claim 13, 14 or 15 wherein said applying means applies said stabilizing field and said switching field during time intervals which are separated by a time less than the relaxation time of said liquid crystal material.

18. The display of claim 13, 14 or 15 wherein said applying means applies said stabilizing field essentially continuously during the operation of said display.