CA1321530C

CA1321530C - Diamond laser crystal and method of manufacturing the same

Info

Publication number: CA1321530C
Application number: CA000602061A
Authority: CA
Inventors: Takeru Nakashima; Shuichi Satoh; Kazuwo Tsuji
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1988-06-13
Filing date: 1989-06-07
Publication date: 1993-08-24
Anticipated expiration: 2010-08-24
Also published as: JPH0288498A; US4950625A; EP0346794A1; DE68905529D1; DE68905529T2; EP0346794B1

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed herein is a method of manufacturing a diamond laser crystal having excellent laser efficiency.
First, a synthetic type Ib diamond containing at least 60 volume percent of a (111) plane growth sector (43) is prepared. This synthetic diamond is thermally treated under high temperature/high pressure, so that type Ib nitrogen contained in the synthetic diamond is converted to type IaA
nitrogen. Thereafter an electron beam is applied to the synthetic diamond in order to generate vacancies in the synthetic diamond. Finally, annealing is performed on the synthetic diamond to form H3 centers by coupling type IaA
nitrogen contained in the synthetic diamond with the vacancies. According to this method, the H3 centers can be formed in the synthetic type Ib diamond in high concentration, while formation of NV centers which become an obstacle to laser action can be suppressed.

Description

1321~
The present invention relates to a diamond laser crystal which can efficiently achieve laser action and a method of preparing the same, and more particularly, it relates to a method of preparing a diamond laser crystal which is formed with H3 centers in a type Ib diamond.
It is reported by S. C. Rand that H3 centers in a diamond incur laser action (optic Letters, 1985, p. 10). In general, such H3 centers have been synthesized by natural type Ia diamonds. Optical and thermal characteristics thereof have been examined in detail by A. T. Collins: Diamond Research, p. 7 (1979), Journal of Physics D. Applied Physics, 15, p.
1431 (1982) and the like. It is known that most natural diamonds contain type IaA nitrogen and type IaB nitrogen as nitrogen impurities.
The H3 centers are formed from type IaA nitrogen and H4 centers are formed from type IaB nitrogen. The percentages of type IaA nitrogen and the type IaB nitrogen are varied with the diamonds, and those of the H3 centers and the H4 centers are also varied responsively. Thus, it has been difficult to selectively form only H3 centers from natural diamonds.
on the other hand, S. C. Rand has proposed the possibility of forming H3 centers in a synthetic type Ib diamond in Tunable Solid State Laser (Springer Verlag), p.
276. However, there has been no method of independently forming only H3 centers in a synthetic type Ib diamond.
An object of the present invention is to provide a method of preparing a diamond laser crystal, which can form a large quantity of H3 centers with high reproducibility by employing a synthetic type Ib diamond.
Another object of the present invention is to provide a diamond laser crystal which has excellent laser efficiency.
Accordingly, the present invention provides a method of manufacturing a diamond laser crystal comprising the steps of: preparing a synthetic type Ib diamond containing at least 60 volume percent of a (111) plane growth sector; thermally 2 1321~30 treating said synthetic diamond under high temperature/high pressure thereby to convert type Ib nitrogen contained in said synthetic diamond to type IaA nitrogen; irradiating said synthetic diamond with an electron or neutron beam thereby to generate vacancies in said synthetic diamond; and annealing said synthetic diamond to form H3 centers by coupling said type IaA nitrogen and said vacancies in said synthetic diamond.
The synthetic type Ib diamond to be prepared may be doped with boron or nickel. In this case, the absorption coefficient of an infrared absorption peak of the synthetic type Ib diamond at a wavenumber of 1132 cm-' is preferably within a range of 0.8 to 15 cm-~. The synthetic type Ib diamond to be prepared preferably contains the type Ib nitrogen in concentrations of 30 to 600 p.p.m.
Preferably the step of converting the type Ib nitrogen to the type IaA nitrogen includes a process of holding the synthetic diamond under an atmosphere of 3 to 7 GPa in pressure and 1500 to 2500C in temperature for at least five hours.
The electron beam to be applied to the synthetic diamond preferably has an energy of 0.5 to 4 MeV and a dose of 1017 to 1019 e/cm2. The neutron beam to be applied to the synthetic diamond preferably has energy of 0.5 to 4 MeV and a dose of 1015 to 1017 n/cm2.
The annealing treatment is preferably performed under an atmosphere of not more than 10-l Torr. in pressure and 1300 to 1600C in temperature for at least five hours.
In another aspect of the present invention, a method of manufacturing a diamond laser crystal comprises the steps of: preparing a synthetic type Ib diamond containing at least 60 volume percent of a (111) plane growth sector; ho~ng said synthetic diamond under an atmosphere of 3 to 7 GPa in pressure and 1800 to 2500C in temperature for at least five hours; irradiating said synthetic diamond with an electron beam having an energy of 0.5 to 4 MeV and a dose of 10l7 to 10l9 , .

3 132~3~
e/cm2; and annealing said synthetic diamond under an atmosphere of not more than lo-l Torr. in pressure and 1300 to 1600C in temperature for at least five hours.
The aforementioned method of preparing a diamond laser crystal may further comprise the step of holding the synthetic diamond under an atmosphere of not more than lo-l Torr. in pressure and 600 to 1200C in temperature for at least five hours previous to the aforementioned annealing step.
In still another aspect of the present invention, a method of manufacturing a diamond laser crystal comprising the steps of: preparing a synthetic type Ib diamond containing at least 60 volume percent of a (111) plane growth sector and being doped with boron or nickel; irradiating said synthetic diamond with an electron or neutron beam; holding said synthetic diamond under an atmosphere of 3 to 7 GPa in pressure and 1500 to 2500C in temperature for at least five hours after said step of irradiating said synthetic diamond with said electron or neutron beam; irradiating said synthetic diamond with an electron or neutron beam after said step of heat treatment under high temperature/high pressure; and holding said synthetic diamond under a vacuum atmosphere of 1300 to 1600C in temperature for at least five hours.
In the aforementioned method of preparing a diamond laser crystal, the absorption coefficient of the synthetic type Ib diamond to be prepared in an infrared absorption peak is preferably within a range of 0.8 to 15 cm~l at a wavenumber of 1332 cm-l. The synthetic type Ib diamond preferablycontains the type Ib nitrogen in concentration of 30 to 600 p.p.m.
The diamond laser crystal according to the present invention comprises a diamond laser crystal prepared from a synthetic type Ib diamond and containing at least 60 volume percent of a (111) plane growth sector. The synthetic type Ib diamond is preferably doped with boron or nickel. In this case, the absorption coefficient of an infrared absorption peak of the synthetic type Ib diamond at a wavenumber of 1332 cm-l is preferably within a range of 0.8 to 15 cm-~.

4 1321~;30 The invention will be more readily understood from the following description of a preferred embodiment thereof given, by way of example, with reference to the accompanying drawings, in which:
Figure 1 schematically illustrates states of sectors of a synthetic diamond grown from the (100) plane of a seed crystal, as viewed from the (110) plane direction;
Figure 2 shows visible-ultraviolet absorption spectra observed before and after aggregation;
Figure 3 shows visible-ultraviolet absorption spectra of a (111) plane growth sector and a (100) plane growth sector observed after annealing;

Sianificance of Employment of (111) Plane Growth Sector in Synthetic Type Ib Diamond A diamond comprises a (111) plane growth sector which is a region grown in parallel with the (111) crystal plane, a (100) plane growth sector which is a region grown in parallel with the (100) crystal plane and a higher indices plane growth sector. Figure 1 shows respective sectors of typical industrially mass-produced diamonds. Referring to F$gure 1, numeral 11 denotes a seed crystal, numeral 12 denotes a (100) plane growth sector, numeral 12a denotes the (100) crystal plane, numeral 13 denotes a (111) plane growth sector, numeral 13a denotes the (111) crystal plane and numeral 14 denotes a higher indices plane growth sector.
Even if there is little difference between type Ib nitrogen concentration in the (111) plane growth sector and that in the (100) plane growth sector, extreme differences are observed between types and quantities of color centers formed in the sectors through respective treatment steps. In this regard, the inventors have found that the (111) plane growth ~ector i8 advantageous for forming H3 centers. Description is now made as to differences in absorption of impurity nitrogens and formation of color centers between the sectors, which differenc is increased through the respective treatment steps.

' s - 1321~3~
Treatment steps necessary for forming H3 centers in type Ib diamond are those of aggregation, irradiation by an electron or neutron beam and vacuum annealing. The aggreqation step is required for converting dispersed nitrogen atoms (type Ib nitrogen) into pairs of nitrogen atoms (type IaA nitrogen). Vacancies are produced in the diamond by irradiating the synthetic diamond with the electron or neutron beam. The type IaA nitrogens are coupled with the vacancies by the final annealing step, to form H3 centers.
Description is now made of differences in the optical properties between the sectors, caused by the aggregation step. Figure 2 shows visible-ultraviolet absorption spectra observed before and after aggregation.
Referring to Figure 2, numeral 21 denotes the spectrum of the (111) plane growth sector observed before aggregation.
Numeral 22 denotes the spectrum of the (100) plane growth sector observed before aggregation. Numeral 23 denotes the spectrum of the (111) plane growth sector observed after aggregation. Numeral 24 denotes the spectrum of the (100) plane growth sector observed after aggregation. The diamond herein employed has type Ib nitrogen concentrations of 140 p.p.m. in both the (111) plane growth sector and the (lO0) plane growth sector, and there is no difference between the visible-ultraviolet absorption spectra observed before 25 ~aggregation. However, when aggregation is performed by holding this diamond under an atmosphere of 5 GPa in pressure and 2300C in temperature for 20 hours, significant difference i~ caused between the absorption spectra of the (111) plane growth sector and the (100) plane growth sector, as clearly understood from the spectra 23 and 24 shown in Figure 2. Such absorption is caused by nitrogen impurities. Further, this absorption first appears at a wavelength of about 590 nm in the (111) plane growth sector, and is abruptly increased as ~ ~ the wavelength is reduced. In the (100) plane growth sector, on the other hand, absorption first appears at a wavelength of about 610 nm and is loosely increased as the wavelength is ,~, . . ~

, 6 1321~3~
reduced~ The absorption region of H3 centers is 450 to 505 nm, and hence it is understood that the (111) plane growth sector is more preferable since the absorption by nitrogen impurities is small in this wavelength range.
Figure 3 shows absorption spectra of a synthetic diamond which is irradiated with an electron beam having an energy of 2 MeV with a dose of 1ol8 e/cm2 and then annealed in a vacuum at 850C for five hours. Referring to Figure 3, numeral 31 denotes the spectrum of the (111) plane growth sector after annealing, and numeral 32 denotes the spectrum of the (100) plane growth sector after annealing. It is understood from Figure 3 that new absorption phenomenon appear as a result of annealing. One such phenomenon is absorption by H3 centers which are formed by coupling of type IaA
nitrogens and vacancies. Another phenomenon is absorption by NV centers (wavelength range: 450 to 640 nm) formed by coupling of type Ib nitrogens, which are left in the diamond by incomplete aggregation, and the vacancies. The difference between the (100) plane growth sector and the (111) plane growth sector clearly appears also in this case.
In the (111) plane growth sector, absorption by the H3 centers is observed simultaneously with that by the NV
centers. On the other hand, it is shown that only extreme absorption by the NV centers is observed and that few H3 centers are formed in the (100) plane growth sector. It is also shown from this annealing treatment that employment of the (111) plane growth sector is an important factor for formation of the H3 centers.
As hereinabove described, it is possible to prepare a diamond having H3 centers by using a diamond containing only a (111) plane growth sector, through the difference appearing between the sectors in the steps of aggregation and annealing.
In a general synthesizing method utilizing the (100) plane of a seed crystal, the (111) plane growth sector region is narrow and optical measurement cannot be easily performed.
In order to obtain a wide region for the (111) plane growth r~

7 132~
sector, therefore, the (111) plane is employed as the crystal plane of a seed crystal 41 as shown in Figure 4, to synthesize a diamond by the temperature gradient method, for example.
Thus, a diamond having a wide region for the (111) plane growth sector can be obtained. Referring to Figure 4, numeral 41 denotes the seed crystal, numeral 42 denotes the (100) plane growth sector, numeral 42a denotes the (100) plane, numeral 43 denotes the (111) plane growth sector and numeral 43a denotes the (11~) plane.
Treatment Condition for Convertinq Type Ib Nitrogen to Type IaA Nitroaen As hereinabove described, known is a method of diffusing nitrogen atoms under high temperature/high pressure to form nitrogen atom pairs, in order to convert type Ib nitrogen to type IaA nitrogen. It is also known that aggregation is accelerated by irradiation of the electron or neutron beam prior to the high temperature/high pressure treatment (R. M. Chrenko et al.: Nature 270, 1981, p. 141 and A. T. Collins: Journal of Physics C Solid State Physics, 13, 1980, p. 2641). It is further known that conversation from type Ib nitrogen to type IaA nitrogen can be expressed as follows:
kt = 1/C - l/Co where t represents the treatment time, C0 represents initial concentrations of type Ib nitrogen, C represents type Ib nitrogen concentrations after the treatment and k represents the reaction rate constant. The reaction rate constant indicates temperature dependency, which is expressed as follows:
k ~ eXp(-E/(k8-T)) where k~ represents the Boltzmann constant, T represents the temperature and E represents the activation energy.
In order to prepare a laser crystal, it is necessary to convert type Ib nitrogen to type IaA nitrogen as completely as possible. If this conversion is incomplete, residual type 8 - 1321~30 Ib nitrogen is coupled with the vacancies after the step of irradiating the electron or neutron beam and the annealing step to form NV centers. The NV centers have an absorption band in the emission band of the H3 centers, and block laser action of the H3 centers. Therefore, conditions for increasing the reaction rate constant k have been studied.
It is shown from the aforementioned two expressions that conversion to type IaA nitrogen is accelerated as the treatment temperature is increased. However, if the temperature is abnormally increased, a reverse reaction occurs which converts type Ib nitrogen to type IaA nitrogen (T. Evans et al.: Proceeding of the Royal Society of London A 381, 1982, p. 159). Thus, there is an optimum temperature for forming nitrogen atom pairs. Figure 5 shows temperature dependence of experimentally obtained reaction rate constants k. The values shown in Figure 5 result from examinations of temperature dependency of a sample irradiated with an electron beam prior to aggregation and that aggregated without electron irradiation. The treatment pressure was 5 GPa, and residual type Ib nitrogen concentration after treatment was estimated from changes in the signal strength of electron spin resonance (ESR). The following facts have been clarified from the results of the experiment shown in Figure 5:
1) Conversion from type Ib nitrogen to type IaA
nitrogen occurs within a temperature range of 1800C to 2500C, and is most efficiently attained at about 2300C. At lea~t 97% of the nitrogen atoms are paired when the synthetic diamond i8 held at this temperature for at least 20 hours.
2) Within the temperature range of 1800C to 2500C, no significant difference appears between the reaction rate oonstants of the sample irradiated with the electron beam befor~ aggregation and that aggregated without electron irradiation. Thus, it is understood that type IaA nitrogen, which is necessary for forming H3 centers, can be efficiently formed by performing aggregation within the temperature range of 1800C to 2500C.

B~

9 - 1321~30 When a synthetic type Ib diamond doped with boron or nitrogen is employed, the reaction rate constant is significantly increased by irradiation with an electron or neutron beam before aggregation. Figure 6 shows temperature dependence of experimentally obtained reaction rate constants k, similarly to Figure 5. Referring to Figure 6, the solid line 51 shows the temperature dependence of a nondoped synthetic diamond which was aggregated without electron irradiation. The broken line 52 shows the temperature dependence of a nondoped synthetic diamond which was aggregated with electron irradiation. The one-dot chain line 53 shows the temperature dependence of a synthetic diamond doped with boron, which was aggregated with electron irradiation. The energy of the electron beam employed in this experiment was 2 MeV, while the dose or concentration thereof was 1 x 10l8 e/cm2. The reaction time was 40 hours.
From the results shown in Figure 6, the following facts have been clarified:
1) The maximal value of the reaction rate constant ~ is at approximately 2300C.
2) Within a temperature range of not more than 2000C, the reaction rate constant is significantly increased by electron irradiation before aggregation. Particularly in the synthetic diamond doped with boron, the reaction rate constant i5 significantly increased by performing electron lrradiation prior to aggregation, and type Ib nitrogen can be sufficiently converted to the type IaA nitrogen at a temperature of approximately 1500C. The same effect was attained when the synthetic diamond was doped with nickel in place of boron. The same effect was also attained when the synthetic diamond was irradiated with a neutron beam in place of the electron beam. The dose or concentration of the electron beam to be applied is preferably within a range of lol7 to 1ol9 e/cm2- In the case of the neutron beam, the dose or concentration may be within a range of lGI5 to 1ol7 n/cm2 since the same has high ability of generating vacancies.
:

,~

.

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lo 132153~
Treatment Conditions for Coupling Type IaA Nitroaen and Vacancies in Synthetic Diamond The electron or neutron beam is applied to the synthetic diamond for the purpose of introducing the vacancies into the diamond. However, since type Ib nitrogen cannot be completely converted to type IaA nitrogen, NV centers are inevitably formed by the irradiation of the electron or neutron beam and the subsequent annealing treatment. This is because aqgregation is incompletely performed, and results in a residual of type Ib nitrogen. Thus, type Ib nitrogens are coupled with the vacancies to form the NV centers. At a general annealing temperature of 850C for a natural diamond, further, the vacancies are easily coupled with type Ib nitrogens, rather than with type IaA nitrogen. Therefore, the ratio of the number of the NV centers to that of the H3 centers is increased as compared with the concentration ratio of type Ib nitrogen to type IaA nitrogen although type Ib nitrogen is in low concentration, and hence the NV centers form in excess. Thus, the NV centers are inevitably formed when aggregation cannot be completely performed.
The inventors have succeeded in suppressing formation of the NV centers by further increasing the annealing temperature. Figure 7 shows changes of absorption coefficients of H3 centers and NV centers in a case of annealing being performed for five hours at temperature of 850C, 1200C, 1400C and 1600C respectively. Referring to Figure 7, the solid line 61 shows the change in the absorption coefficient of the H3 centers, and the broken line 62 shows the change in the absorption coefficient of the NV centers.
As understood from Figure 7, the absorption coefficient of the NV centers peaks at a temperature of about 1200C, and is abruptly reduced in a temperature range exceeding 1200C while absorption substantially disappears at a temperature of about 1400C. On the other hand, the absorption coefficient of the H3 centers is substantially unchanged up to a temperature of about 1400C, and reduction thereof is started at a if 11 132~0 temperature of about 1460C. This means that, when the annealing temperature is at 1200C, the NV centers are so destabilized, that those once formed are decomposed. When the annealing temperature is set within the range of 1300C to 1600C, therefore, formation of the NV centers can be suppressed so that only the H3 centers are formed. A
preferable annealing temperature is about 1400C. Figure 8 shows an exemplary spectrum which is obtained when annealing is performed at 1400C.
A synthetic type Ib diamond containing type Ib nitrogen of about 120 p.p.m. and doped with nickel was annealed under the same conditions as above to examine changes in absorption coefficients of H3 centers and NV centers. The results were identical to those in Pigure 7.
Sianificance of Dopina of SYnthetic Type Ib Diamond with Boron or Nickel The inventors have found that the H3 centers can be more efficiently formed by performing aggregation, irradiation by an electron or neutron beam and annealing on the synthetic type Ib diamond which is doped with boron or nickel. The action is now described.
In the synthetic type Ib diamond doped with boron or nickel, the absorption coefficient peaks at a wavenumber of 1332 cm-l of infrared absorption. Figure 9 shows an infrared absorption spectrum 71 of the type Ib diamond doped with boron as compared with an infrared absorption spectrum 72 of the nondoped type Ib diamond. As clearly shown from Pigure 9, the absorption coefficient of the type Ib diamond doped with boron peaks at the wavenumber of 1332 cm-l. A
spectrum identical to the spectrum 71 is obtained also when the diamond is doped with nickel in place of boron.
Absorption at 1332 cm~~ is proportional to the amount of doping. However, it is extremely difficult to correctly measure the concentration of boron or nickel, and hence the amount of doping is hereinaftaer replaced by the infrared B

. . . . .

12 132~
absorption coefficient. It has been recognized from measurements heretofore made that the absorption coefficient of 1 cm-l corresponds to boron or nickel concentrations of about 1 to 10 p.p.m.
Figure 10 shows visible-ultraviolet absorption spectra obtained by aggregating samples of both a synthetic type Ib diamond doped with boron and a nondoped synthetic type Ib diamond, irradiating the samples with an electron or neutron beam and annealing the same. Numeral 81 denotes the spectrum of the sample doped with boron, and numeral 82 denotes that of the nondoped sample. As clearly understood from Figure 10, H3 centers, NV centers and H2 centers are observed.
Figure 11 illustrates changes to the absorption coefficients of the respective centers caused by doping the samples with boron or nickel. The dotted line 91 shows the absorption coefficient of the H3 centers, the one-dot chain line 92 shows that of the NV centers and the solid line 93 ~hows that of the H2 centers. The samples employed for obtaining the data shown in Figure 11 had substantially identical type Ib nitrogen concentrations of about 160 p.p.m.
From the results shown in Figures 10 and 11, it is shown that the following effects are attained by doping the diamond with boron or nickel:
i) Accelerated of Formation of H3 Centers The following relation holds between the absorption coefficient a(H3) of the H3 centers and the absorption ¢oerficient a(H2) of the H2 centers:
a(H3) + 3.2a(H2) = constant In other words, the percentage of the H3 centers is increa~ed by doping the diamond with boron or nickel. It may be considered that boron or nickel acts as an acceptor, to which charge transfer from nitrogen occurs in diamond.
ii) Suppression of NV Center Formation As clearly shown in Figures 10 and 11, formation of the NV centers is suppressed by doping the diamond with boron :

..
.
' " .

13 1 32 1 ~ 3 ~
or nickel. It may be considered that sucn a phenomenon results since the conversion to type IaA nitrogen is accelerated and the residual type Ib nitrogen concentration is reduced by doping the diamond with boron or nickel.
Further, the secondary absorption edge (220 to 500 nm) of type Ib nitrogen is reduced because of the reduction of the residual type Ib nitrogen.
The above effect is important for laser action of the ~3 centers. As shown in Figure 9, the amount of doping of boron or nickel can be defined by the infrared absorption peak at 1332 cm-'. The inventors have found that laser action of the H3 centers is possible if the absorption coefficient at this peak is at least 0.8 cm~~. on the other hand, the upper limit of the absorption coefficient at 1332 cm~' is mainly limited by the limit amount of doping of boron or nickel. This upper limit is 15 cm~l. Comparing boron with nickel as the material to be doped, boron is superior to nickel in consideration of the amount capable of doping, control of the amount of doping etc. The method of changing the amount of color center formation by doping synthetic type Ib diamonds with boron or nickel is an absolutely novel technique with no anticipation.

Sianificance of Content of at least 60 Volume Percent of (111) Plane Growth Sector When H3 centers are employed as a laser active material, the diamond laser crystal preferably contains the minimum percentage of the (100) plane growth sector. This is because the NV centers contained in the (100) plane growth sector inevitably resorb emissions from the H3 centers as hereinabove described, and significantly decrease the gain of the H3 center laser.
As hereinabove described, formation of the H3 centers is accelerated by doping synthetic type Ib diamonds with boron or nickel. However, the amount of boron or nickel thus doped is extremely varied with the types of the growth 14 ~ 3~ ;3t~
sectors of the crystal. Both of these elements are select-ively doped in the (111) plane growth sector, while the same are only slightly doped in the (100) plane growth sector.
Therefore, the H3 centers are reluctantly formed while the NV
centers are easily formed in the (100) plane growth sector.
The H3 centers are increased and the NV centers are reduced over the entire crystal by reducing the percentage of the (100) plane growth sector and increasing that of the (111) plane growth sector in the crystal. Through experiments, the inventors have found that the percentage of the (111) plane growth sector must be at least 60 volume percent of the crystal, in order to achieve laser action of the H3 centers.
In order to synthesize such a crystal, the diamond may be synthesized in accordance with the temperature gradient method, for example, by employing the (111) plane as the crystal plane of the seed crystal, as hereinabove described with reference to Figure 4.

Range of Nitrogen Concentration in Synthetic Type Ib Diamond In order to efficiently form H3 centers, the synthetic type Ib diamond preferably contains type Ib nitrogen in concentrations of 30 to 600 p.p.m. As hereinabove described, the sum of the absorption coefficients of the H3 centers and the H2 centers is determined by the initial type Ib nitrogen concentration with no regard to the amount of doping of boron or nickel. The inventors have examined the relationship between the sum of the absorption coefficients of the H3 centers and the H2 centers and the initial type Ib nitrogen concentration. Figure 12 shows the result. The following relational expression is obtained from the result shown in Figure 12:
~ H3) + 3.2a(H2) z 13.21OgN - 18.9 where ~(H3) represents the absorption coefficient of the H3 centers, ~(h2) represents that of the H2 centers, and N
represents the initial type Ib nitrogen concentration. It is ~B

~ 1 ~21 ~3 0 understood from the above expression that type Ib nitrogen concentration must be at least 30 p.p.m. in order to form the H3 centers in the diamond. On the other hand, the upper limit of type Ib nitrogen concentration is defined by the amount of nitrogen which can be doped in the diamond. That is, the upper limit of type Ib nitrogen concentration is approximately 600 p.p.m. The type Ib nitrogen concentration in the diamond was calculated by the infrared absorption peak at 1130 cm~'.

Effect of the Invention According to the present invention, as hereinabove described, H3 centers can be formed in a synthetic type Ib diamond in high concentrations, and the formation of NV
centers which are obstacles to laser action, can be sup-pressed. Thus, the diamond crystal obtained according to thepresent invention is employable as a laser crystal, the wavelength of which is variable within a range of 500 to 600 nm.
Example 1 H3 centers were formed by employing (111) plane growth sectors and (100) plane growth sectors in the same crystals of type Ib diamond samples synthesized by the temperature gradient method, to obtain the results shown in Table 1.
Nitrogen concentration values shown in Table 1 were calculated from infrared absorption coefficients at 1130 cm-~.
Absorption coefficients of nitrogen impurities are those by nitrogen impurities (mainly type Ib nitrogen) at a wavelength of 480 nm, at which absorption by the H3 centers is maximized.
Table 1 show~ both those values observed before and after aggregation. Absorption coefficients of H3 centers are those at a wavelength of 480 nm, at which absorption by the H3 center~ in the phonon side band is maximized. These values ~ are preferably low since the H3 centers appear overlappingly with absorption by the nitrogen impurities after aggregation.

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16 - 1321~3~
Absorption coefficients of NV centers are those at a wavelength of 580 nm, at which absorption by the NV centers in the phonon side band is maximized. These values are preferably suppressed at low levels since absorption by the NV centers partially overlaps with that by the H3 centers, while the absorption region of the NV centers is in the emission region (505 to 600 nm) of the H3 centers.
Treatment conditions for the samples employed in this Example are as follows:
10 Aggregation: the samples were held under pressure of 5 GPa and a temperature of 2300C for 20 hours Electron Irradiation: with energy of 2 MeV and a dose of 1018 e/cm2 Annealing in Vacuum: the samples were held at 850C
for five hours Table 1 , S~m~le No. l 2 . .
Sector (111) Plane(100) Plane Growth Sector Growth Sector _ T~e Ib Nitroeen Concentration140 ~m 140 ~pm Ab~orption Coefficient by Nitrogen Impurit~ at ~- 480 nm 4.1 cm~l 4.2 cm~
25 (be~ore A~ereeation~ _ ~bsorption Coefficient by Nitrogen Impurit~ at 1- 480 nm 1.3 cm 1 3.1 cm~
~a~ter ~eereeation) ~b~orption Coefficient of ~3 Center3.8 cm 1 0 cm 1 30 ~ ~ 480 nm) Ab~orption Coefficient of NV'Center 4.7 cm 1 11.3 cm 1 - S80 nm) .
. Exam~leComn~rative Samnle 35`

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17 1 321 ~3 0 Example 2 Type Ib diamond samples synthesized according to the temperature gradient method were aggregated, to calculate reaction rate constants in conversion from type Ib nitrogen to typè IaA nitrogen. Experiments were made on both samples irradiated with electron beams before aggregation and those aggregated without electron irradiation at various temperatures. The conditions for electron irradiation were 2 MeV in energy and 1018 e/cm2 in dosage.
10 Table 2 shows the results. It is recognized from the results shown in Table 2 that no significant difference appears between the samples irradiated with the electron beams before aggregation and those aggregated with no electron irradiation when the treatment temperature is about 2000~C.
Table 2 SamPle No. 11 12 13 14 15 16 17 rype Ib Nltro~en Conc-ntratlon (PPm) 71 53 j 95 87 52 38 49 Electron Irradletlon No Yes No Yes No No No rr-atJnent remPerature ~C) 1700C 1700C 2000C 20Q0C 2200C 2300C 2500C
rreetment Sln~
(h) 45h 50h lSh , lSh lSh l5h l5h Reactlon Rate Conntant<10 82.5xlO 5 7.1xlO-5 6.6xlO 5 6.7xl S 2.4xlO 3 1.6x10-4 (DPm-~ n~l) Comvaratlve Con~Ar~tlve ExemPle CoQarAtlve Ex~le ~xunple ExamPle SamPle SAmPle SemPle B

Example 3 18 ~ 2 ~ ~ 3 0 (lll) plane growth sectors (type Ib nitrogen concentration: 140 p.p.m.) of type Ib diamond samples synthesized by the temperature gradient method were aggregated S under pressure of 5.0 GPa and a temperature of 2300C for 20 hours. Thereafter electron irradiation was performed under conditions of 2 NeV and lo18 e/cm2. The samples thus obtained were sequentially annealed in vacuum at various temperatures, to examine changes in absorption coefficients of the H3 centers and the NV centers.
Table 3 shows the results. It is recognized from the results shown in Table 3 that the absorption coefficients of the NV centers are abruptly reduced as the annealing temperatures are increased.
Table 3 Sampl- No 21 22 23 24 20 Adn-~ling T-mp-rature 800 & 1200C 1400C 1600C
Annealing Time (h ) 5 5 5 S
Absorption Co-fficient of NV Canter 4 7 cm 5 1 cm 11 0 cm 1 <0 2 om 1 ( -: sao ~) .
Absorption Coefficient of H3 Center 3 8 cm 1 3 7 cm 13 5 cm 1 2 1 cm 1 ( ~ 480 nm) Comparative Coqparative ExampLa Example Salple SamP1Q

~ ~ 35 .
~ , ' . ' Exam~le 4 -- 1321~30 Type Ib diamond samples (2.5 mm in thickness) synthesized by the temperature gradient method were aggregated under pressure of s.0 GPa and a temperature of 2300OC for 20 hours. The samples thus obtained were irradiated with electron beams having energy of 2 MeV with a dose of lo18 e/cmZ. Finally the samples were annealed in vacuum at 1400C
for five hours. As shown in Table 4, the samples were different in volume ratios of regions of (111) plane growth sectors and (100) plane growth sectors.
These samples were subjected to a laser oscillation test. Laser light of 490 nm in wavelength and 40 nsec. in pulse width was employed as the excitation light. No external resonator was employed but Fresnel reflection on diamond crystal end surfaces was utilized. The intensity of the excitation light was 120 MM/cm2 at the maximum, and a sample not oscillating at this value was determined to have achieved no laser action. Table 4 shows the results.
(111) plane growth sectors and (100) plane growth sectors were decided by an X-ray topography, thereby to calculate the percentages thereof.

Table 4 , Sample No. 31 32 33 _ . Volumo Ratio of ~111) Plane Growth Sector 50 % 60 % 100 %
_ Nitrogen Conc~ntration 12? ppm 132 ppm 140 ppm Absorption Co~fficiont .
of H3 Conter 0.9 cm 1 2.3 cm 1 3.5 cm 1 Ab~orption Coefficiont of NV Center 1.3 cm 1 0,4 cm 1 <0.2 cm _ Lasor OscillationNo Yes Yes _ l Comparative Sample Example Example , . . .

Example 5 ~ 3 ~ 3 ~
Type Ib diamond samples doped with boron and nickel were prepared according to the temperature gradient method.
Boron was doped by adding the sum to the carbon sources.
Nickel was doped by adding a large amount of nickel to Fe solvents. Nitrogen concentration values were adjusted by adding FeN to the carbon sources. All of such crystals were grown on (111) planes. Synthesizing temperatures were changed in order to change the ratios of the (100) plane growth sectors to the (111) plane growth sectors in the crystals.
These crystals were worked into rectangular parallelopipeds of 2.0 mm in thickness.
The crystals prepared in the aforementioned manner were irradiated with electron beams under conditions of 2 MeV
and 1018 e/cm2, and thereafter the samples were held under an atmosphere of 5 GPa in prèssure and 1700C in temperature for five hours. Then the samples were again subjected to electron irradiation under conditions of 2 MeV and 3 x 10l8 e/cm2.
Thereafter the samples were annealed in vacuum at 1400C for five hours. Through the aforementioned steps, H3 centers were formed in the diamond samples. The diamond crystals were subjected to a laser oscillation test. Figure 13 schematically illustrates a laser oscillation experimentation apparatus employed for this experiment. Referring to Figure 25 13, numeral 101 denotes a flash lamp dye laser, numeral 102 denotes a diamond laser crystal, numerals 103a and 103b denote a pair of resonators, numeral 104 denotes a lens and numeral 105 denotes outgoing laser light. Pulse excitation light outgoing from the flash lamp dye laser 102 is condensed by the 30 ~en~ 104, to be incident upon the diamond laser crystal 102.
Consequently, the diamond laser crystal 102 generates fluorescence, which is amplified by the pair of resonators 103a and 103b. Finally the outgoing laser light 105 is outputted from the first resonator 103b.
In the apparatus employed for the test, the excitation light was emitted from a flash lamp dye laser r~
~9 21 - 1321~0 having a wavelength of 470 nm. Its energy was about 50 mJ.
Reflection factors of the pair of resonators were 100~ and 97%
respectively.
First, the relationship between the amounts of doping of nickel and boron and laser output intensity levels was examined. Table 5 shows the results.

Table 5 _ Sample No. 41 42 43 44-Type Ib Nitrogen Concentration ~ppm) 130 165 168 165 Percentage of (111) Plane Growth Sector (~) 95 98 100 100 ~:
Dopant Ni Ni+B Ni+B
Amount of Peak Absorption at 1332 cm 1 (cm 1) 0 0.82 1.24 4.33 Absorption Coefficient of H3 C2nter (cm 1) (at 470 nm) 1.1 5.8 6.9 7.3 Absorption Coefficient of NV Center (cm 1) (at 5701un) 11. 2 1.9 O.9 O.2 Laser Output Energy (~J) 0 22 38 ¦ 51 Compar;tive EYa=ple Example IExample ,~

22 -- 1321~0 Then, the relationship between the percentages of the (100) plane growth sectors and laser output energy levels was examined. The diamond crystals were doped with nickel and boron. Table 6 shows the results.
Table 6 Samp1e No 51 ¦52 53 Typa I~ Nitrogen Concentratlon ~ppm~ 142 153 165 P-rcentage of (111) 0 Plane Growth Sector (~) 30 60 100 Dopant Ni~3 Ni+a Ni+B
Amount of eeak Absorption at 1332 c3~ 1 (cm 1) 0 91 2 50 4 33 Absorption Coeffici-nt of H3 Center (c31 1) (at 470 mn) 2 8 4 5 7 3 Absorption Coefficient of Nv Cent2r ~cm 1) (at 570 nm) 8 1 3 9 0 2 Lasor Out~ut Energy (I~J) O 13 51 Comparative Exa~ple Example Sa0ple Further, the relationship between type Ib nitrogen concentration and the laser output energy level was examined.
The diamond crystals were doped with nickel and boron. Table 7 shows the results.

Table 7 9ample No 61 ¦62 63 64 Typ- Ib Nltrog~n Conc-ntration (ppm) 21 30 165 320 Pcrc-ntago of (111) Plan- Growth Sector ~%) 100 100 100 100 oop~nt Ni NiNi~B Ni , =~
Axunt o~ Pealc Absorptlon at 1332 cm 1 ~cm 1) 0 5 0 94 4 33 3 7 Ab50rption Coefflclent o~
H3 C-nt-r ( 1) (at 470 nm) i 0 5 7 3 5 5 Absorption Coefficlent o~

tlV Cunter (cm 1) (at 570 nm) ¦ 0 3 0 1 0 2 0 1 Laser 0utp1t Energy (~) O 11 51 43 Co~parative Example Example ~xamDle Sa~ple 23 ~ 132~53~
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

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Claims

1. A method for manufacturing a diamond laser crystal comprising the steps of preparing a synthetic type Ib diamond having a (111) plane growth sector taking up at least 60% of the diamond's volume; thermally treating said synthetic diamond under high temperature and high pressure sufficient for converting type Ib nitrogen contained in said synthetic diamond to type IaA nitrogen; irradiating said synthetic diamond with an electron or neutron beam thereby to generate vacancies in said synthetic diamond;
and annealing said synthetic diamond to form H3 centers by coupling said type IaA nitrogen and said vacancies in said synthetic diamond, said annealing being performed under a pressure not more than 101 Torr at a temperature within the range of 1300° to 1600° for at least five hours.

2. The method of claim 1, further comprising doping said synthetic type Ib diamond with one of boron and nickel.

3. The method of claim 1, wherein said type Ib nitrogen is present in said synthetic type Ib diamond in a concentration within a range of 30 to 600 p.p.m.

4. The method of claim 1, wherein said type Ib synthetic diamond has an infrared absorption peak at a wavenumber of 1332 cm-1, and an absorption coefficient within a range of 0.8 to 15 cm-1.

5. The method of claim 1, wherein said thermal treating step for converting said type Ib nitrogen to said type IaA nitrogen is performed at a pressure within the range of 3 to 7 GPa and at a temperature within the range of 1500° to 2500° C. for at least five hours.

6. The method of claim 5, wherein said thermal treating step is performed after irradiating said synthetic diamond with said electron or neutron beam.

7. The method of claim 1, wherein said electron beam to be applied to said synthetic diamond has an energy of 0.5 to 4 MeV, and wherein an applied dose is within the range of 1017 to 1019 e/cm2.

8. The method of claim l, wherein said neutron beam to be applied to said synthetic diamond has an energy of 0.5 to 4 MeV, and wherein an applied dose is within the range of 1015 to 1017 n/cm2.

9. A method for manufacturing a diamond laser crystal comprising the steps of preparing a synthetic type Ib diamond having a (111) plane growth sector taking up at least 60% of the diamond's volume; thermally treating said synthetic diamond under a pressure within the range of 3 to 7 GPa at a temperature within the range of 1800° to 2500°
C. for at least five hours; irradiating said synthetic diamond with an electron beam having an energy within the range of 0.5 to 4 MeV at a dose within the range of 1017 to 1019 e/cm2; and annealing said synthetic diamond under a pressure of not more than 10-1 Torr at a temperature within the range of 1300° to 1600° C. for at least five hours.

10. The method of claim 9, further comprising holding said synthetic diamond under said pressure of not more than 10-1 Torr at a temperature within the range of 600° to 1200° C. for at least five hours prior to said annealing step.

11. A method for manufacturing a diamond laser crystal comprising the steps of doping with one of boron and nickel a synthetic type Ib diamond having a (111) plane growth sector taking up at least 60% of the diamond's volume; irradiating said synthetic diamond with an electron or neutron beam; thermally treating said synthetic diamond under a pressure within the range of 3 to 7 GPa at a temperature within the range of 1500° to 2500° C. for at least five hours after said step of irradiating said synthetic diamond with said electron or neutron beam; again irradiating said synthetic diamond with an electron or neutron beam after said thermal treating step; and holding said synthetic diamond under a vacuum atmosphere at a temperature within the range of 1300° to 1600° C. for at least five hours.

12. The method of claim 11, wherein said type Ib nitrogen is present in said diamond in a concentration within a range of 30 to 600 p.p.m.

13. The method of claim 11, wherein said synthetic type Ib diamond has an infrared absorption peak at a wavenumber of 1332 cm-1, and an absorption coefficient within a range of 0.8 to 15 cm-1.

14. The method of claim 11, wherein said electron beam to be applied to said synthetic diamond has an energy of 0.5 to 4 MeV, and wherein an applied dose is within the range of 1017 to 1019 e/cm2.

15. The method of claim 11, wherein said neutron beam to be applied to said synthetic diamond has an energy of 0.5 to 4 MeV, and wherein an applied dose is within the range of 1015 to 1017 n/cm2.

16. A diamond laser crystal prepared from a synthetic type Ib diamond doped with one of boron and nickel, and a (111) plane growth sector taking up at least 60% of the diamond's volume.

17. The diamond laser crystal of claim 16, wherein said diamond laser crystal has an infrared absorption peak at a wavenumber of 1332 cm-1, and an absorption coefficient within a range of 0.8 to 15 cm-1.

18. A method for manufacturing a diamond laser crystal comprising the following steps: preparing a synthetic type Ib diamond having a (111) plane growth sector taking up at least 60% of the diamond's volume;
thermally treating said synthetic diamond under high temperature and high pressure sufficient for converting type Ib nitrogen contained in said synthetic diamond to type IaA nitrogen; irradiating said synthetic diamond with an electron or neutron beam thereby to generate vacancies in said synthetic diamond: first annealing said synthetic diamond under a pressure of not more than 10-1 Torr at a temperature within the range of 600° to 1200° C. for at least five hours, and further annealing said synthetic diamond under said pressure of not more than 10-1 Torr at an increased temperature within the range of 1300° to 1600°
C. for at least five hours after said first annealing step.

19. The method of claim 18, further comprising doping said synthetic type Ib diamond with one of boron and nickel.

20. The method of claim 18, wherein said type Ib nitrogen is present in said synthetic type Ib diamond in a concentration within a range of 30 to 600 p.p.m.

21. The method of claim 18, wherein said type Ib synthetic diamond has an infrared absorption peak at a wavenumber of 1332 cm-1, and an absorption coefficient within a range of 0.8 to 15 cm-1.

22. The method of claim 18, wherein said thermal treating step for converting said type Ib nitrogen to said type IaA nitrogen is performed at a pressure within the range of 3 to 7 GPa at a temperature within the range of 1500° to 2500° C. for at least five hours.

23. The method of claim 22, wherein said thermal treating step is performed after irradiating said synthetic diamond with said electron or neutron beam.

24. The method of claim 18, wherein said electron beam to be applied to said synthetic diamond has an energy of 0.5 to 4 MeV, and wherein an applied dose is within the range of 1017 to 1019 e/cm2.

25. The method of claim 18, wherein said neutron beam to be applied to said synthetic diamond has an energy of 0.5 to 4 MeV, and wherein an applied dose is within the range of 1015 to 1017 n/cm2.