Infrared optical material, method for manufacturing the same, wavelength conversion element using infrared optical material and laser equipment using wavelength conversion element

Abstract

To provide: a new infrared optical material; a method for manufacturing the same; a wavelength conversion element using the infrared optical material; and a laser equipment using the wavelength conversion element.SOLUTION: An infrared optical material expressed by a general formula GeS2 consists of a single crystal belonging to a monoclinic system. A method for manufacturing the infrared optical material comprises: using a raw material including at least one kind selected from a group consisting of disulfide germanium glass expressed by the general formula GeS2, a crystal phase expressed by the general formula GeS2 and belonging to a monoclinic system and a crystal phase expressed by the general formula GeS2 and belonging to an orthorhombic crystal system; and growing a crystal by a unidirectional solidification crystal growth method.SELECTED DRAWING: Figure 17

Description

[Technical Field]

[0001] The present invention relates to an infrared optical material using germanium disulfide, a method for producing the same, a wavelength conversion element using the same, and a laser device using the same. [Background technology]

[0002] Chalcogenide glass is known as an infrared optical material (see, for example, Non-Patent Document 1 and Non-Patent Document 2). According to Figure 4 of Non-Patent Document 1, it is reported that the transmission threshold wavelength of sulfide glass, which is a chalcogenide glass, is 11 to 12 μm. According to Figure 2 of Non-Patent Document 2, the transmission threshold wavelength of germanium disulfide (GeS2) glass is estimated to be 12 μm.

[0003] Although the infrared transmission properties of germanium disulfide glass are superior to those of silica in the longer wavelength range, further improvements in the transmission properties in the longer wavelength range are expected to enable its application in further applications.

[0004] Germanium disulfide is also known to exist in three crystalline states in addition to glass (see, for example, Non-Patent Document 3). Non-Patent Document 3 reports that germanium disulfide exists in three crystalline states: monomorphic α-GeS2 (low pressure / high temperature type), orthorhombic β-GeS2 (low pressure / low temperature type), and tetragonal GeS2 (high pressure / high temperature type), all of which are represented by the general formula GeS2. However, there has been no report on the possibility of infrared optical applications of these germanium disulfides. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Hirohei Kadono, Chemistry and Education, Vol. 63, No. 1, 2015 [Non-patent document 2] Shuichi Shibata,Ceramic Data Book,2012,Vol.40,No.94,pp151-154 [Non-patent document 3] Saji Manabu et al., Nagoya Institute of Technology Bulletin, Vol. 32, 1980 Summary of the Invention [Problem to be solved by the invention]

[0006] In view of the above, an object of the present invention is to provide a novel infrared optical material, a method for producing the same, a wavelength conversion element using the same, and a laser device using the same. [Means for solving the problem]

[0007] The infrared optical material according to the present invention is represented by the general formula GeS2 and is made of a single crystal belonging to the monoclinic system, thereby solving the above-mentioned problems. The single crystal has space group P21 / c symmetry, The lattice constants a, b, and c are a=0.67061±0.05 nm b=1.60877±0.05 nm c=1.14242±0.05 nm may satisfy values in the range The lattice constant β is β=91.069±0.3° may satisfy values in the range At a thickness of 0.1 mm, the transmittance of light with a wavelength of 22 μm may be 35% or more. At a thickness of 0.1 mm, the average transmittance of light having a wavelength of 15 μm or more and 22 μm or less may be 50% or more. Any one of the faces of the single crystal may be parallel to the (001) plane. The method for producing the above-mentioned infrared optical material according to the present invention uses at least one raw material selected from the group consisting of germanium disulfide glass represented by the general formula GeS2, a crystalline phase represented by the general formula GeS2 and belonging to a monoclinic system, and a crystalline phase represented by the general formula GeS2 and belonging to an orthorhombic system, and grows crystals by a directional solidification crystal growth method, thereby solving the above-mentioned problems. the unidirectional solidification crystal growth method is a vertical Bridgman method, Growing the crystal may include placing the raw material in a crucible and melting it, and lowering the crucible to grow a crystal from the molten raw material. The raw materials may be melted at a temperature of 840°C or higher. The crucible may be pulled down at a rate in the range of 0.5 mm / h to 1.5 mm / h. The raw material may be germanium disulfide glass represented by the general formula GeS2 and / or a crystalline phase represented by the general formula GeS2 and belonging to the monoclinic system. A wavelength conversion element according to the present invention comprises the above infrared optical material and solves the above problems. The laser device according to the present invention comprises a laser light output unit that emits fundamental wave light, and a wavelength conversion element that converts the fundamental wave light into light having a frequency different from the frequency of the fundamental wave light, and the wavelength conversion element includes at least the wavelength conversion element described above, thereby solving the above-mentioned problem. [Effects of the Invention]

[0008] The infrared optical material of the present invention is represented by the general formula GeS2 and is composed of a single crystal belonging to the monoclinic system. By forming it into a single crystal, the transmission limit wavelength becomes 22.5 μm, and it can function as an infrared optical material. Such an infrared optical material can be used for infrared windows, lenses, filters, etc. Furthermore, since the infrared optical material of the present invention does not have centrosymmetrical structure, it becomes a nonlinear optical crystal and functions as a wavelength conversion element. Using such a wavelength conversion element, a laser device can be provided.

[0009] The method for producing an infrared optical material of the present invention is characterized by using at least one raw material selected from the group consisting of germanium disulfide glass represented by the general formula GeS2, a crystalline phase represented by the general formula GeS2 and belonging to a monoclinic system, and a crystalline phase represented by the general formula GeS2 and belonging to an orthorhombic system, and growing a crystal by a directional solidification crystal growth method. The production method of the present invention is advantageous for practical use because it allows the use of conventional crystal growth methods. [Brief explanation of the drawings]

[0010] [Figure 1] Schematic diagram showing the crystal structure of a single crystal constituting the infrared optical material of the present invention. [Figure 2] Another schematic diagram showing the crystal structure of a single crystal constituting the infrared optical material of the present invention. [Figure 3] 1 is a schematic diagram showing the crystal structure of a single crystal constituting the infrared optical material of the present invention. [Figure 4] FIG. 1 is a flowchart showing a method for producing an infrared optical material of the present invention. [Figure 5] Schematic diagram showing the process for producing the infrared optical material of the present invention. [Figure 6] Schematic diagram showing a laser device of the present invention. [Figure 7] Schematic diagram showing another laser device of the present invention. [Figure 8] Schematic diagram showing another laser device of the present invention. [Figure 9] A diagram showing the appearance of the obtained sample [Figure 10] Figure showing the XRD pattern of the obtained sample [Figure 11] SEM image of the obtained sample [Figure 12] Temperature distribution in the Bridgman method in Example 1 [Figure 13] Diagram showing the appearance of the crystals in Example 1 [Figure 14] XRD pattern of the cleavage plane of the crystal of Example 1 [Figure 15] DTA profile of the crystals of Example 1 [Figure 16] Graph showing the visible transmittance spectrum of the crystal of Example 1 [Figure 17] Graph showing the infrared transmission spectrum of the crystal of Example 1 DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are designated by like numbers and their description will be omitted.

[0012] (Embodiment 1) In the first embodiment, an infrared optical material and a method for producing the same of the present invention will be described.

[0013] FIG. 1 is a schematic diagram showing the crystal structure of a single crystal constituting the infrared optical material of the present invention. FIG. 2 is another schematic diagram showing the crystal structure of the single crystal constituting the infrared optical material of the present invention. FIG. 3 is yet another schematic diagram showing the crystal structure of the single crystal constituting the infrared optical material of the present invention.

[0014] 1 to 3 are schematic diagrams showing the crystal structure of a single crystal constituting the infrared optical material of the present invention when viewed along the x-axis, y-axis, and z-axis, respectively. The single crystal contains germanium (Ge) and sulfur (S), is represented by the general formula GeS2, and belongs to a monomorphic crystal system. The present inventors have discovered that by forming germanium disulfide into a single crystal having a monomorphic crystal system, the transmission limit wavelength becomes 22.5 μm, and the material functions as an infrared optical material.

[0015] The single crystal of the infrared optical material of the present invention has the crystal structure parameters shown in Table 1. Preferably, the single crystal has the symmetry of the space group P21 / c, and the lattice constants a, b, and c are a=0.67061±0.05 nm b=1.60877±0.05 nm c=1.14242±0.05 nm This stabilizes the crystal and makes it an excellent infrared optical material.

[0016] The single crystal preferably has a lattice constant β of: β=91.069±0.3° This stabilizes the crystal and makes it an excellent infrared optical material.

[0017] [Table 1]

[0018] The infrared optical material of the present invention preferably has a transmittance of 35% or more at a wavelength of 22 μm when the single crystal has a thickness of 0.1 mm. There is no particular upper limit, and it may be 100%. This is advantageous for infrared applications. More preferably, the transmittance of 38% or more at a wavelength of 22 μm when the single crystal has a thickness of 0.1 mm. More preferably, the infrared optical material of the present invention has an average transmittance of 50% or more at a wavelength of 15 μm or more and 22 μm or less when the single crystal has a thickness of 0.1 mm. There is no particular upper limit, and it may be 100%. Even more preferably, the average transmittance of 55% or more at a wavelength of 15 μm or more and 22 μm or less.

[0019] In the infrared optical material of the present invention, the single crystal is used in a bulk shape such as a prism or cylinder, and one of the faces of the single crystal is preferably a crystal plane parallel to the (001) plane. Because such single crystals are easy to cleave, for example, when used as a wavelength conversion element, a crystal having a (001) plane is rotated and phase-matched for use. Furthermore, when used as a filter or lens, the (001) plane should be the surface.

[0020] The infrared optical material of the present invention is composed of a single crystal having no centrosymmetrical structure, and therefore functions as a nonlinear optical crystal. That is, the infrared optical material of the present invention is a laser material, particularly a laser material for infrared wavelengths in the range of 15 μm to 22 μm.

[0021] Next, a method for producing the infrared optical material of the present invention will be described.

[0022] The infrared optical material of the present invention is produced by growing a crystal by a directional solidification crystal growth method using at least one raw material selected from the group consisting of germanium disulfide glass represented by the general formula GeS2, a crystalline phase represented by the general formula GeS2 and belonging to a monoclinic system, and a crystalline phase represented by the general formula GeS2 and belonging to an orthorhombic system. The inventors of the present application have found that by using the above-mentioned raw materials, it is possible to obtain a single crystal of germanium disulfide, which is a high-temperature stable phase, at room temperature and atmospheric pressure.

[0023] FIG. 4 is a flowchart showing the method for producing the infrared optical material of the present invention. FIG. 5 is a schematic diagram showing the process for producing the infrared optical material of the present invention.

[0024] Directional solidification crystal growth methods include the vertical Bridgman method and the vertical gradient freeze method, but the vertical Bridgman method is preferred, as it allows single crystals to be obtained with a high yield.

[0025] Specifically, the raw material is placed in crucible 510 and melted (step S410, FIG. 5(A)). Next, crucible 510 is lowered and a crystal is grown from the melted raw material (step S420, FIG. 5(B)).

[0026] As mentioned above, there are no particular limitations on the raw materials, as long as they are at least one material selected from the group consisting of germanium disulfide glass represented by the general formula GeS2, a crystalline phase represented by the general formula GeS2 and belonging to the monoclinic system (low-pressure, high-temperature type α-GeS2), and a crystalline phase represented by the general formula GeS2 and belonging to the orthorhombic system (low-pressure, low-temperature type β-GeS2). Any of these may be used alone or in combination. The combination may be a mixture or a composite. There are also no limitations on the mixing ratio when combined. The crystalline phase in the raw materials is polycrystalline.

[0027] The raw material is preferably germanium disulfide glass represented by the general formula GeS2 and / or a crystalline phase represented by the general formula GeS2 and belonging to the monoclinic system (low-pressure, high-temperature α-GeS2). These materials are easy to obtain and prepare, and can be used to grow the above-mentioned single crystals with good yield.

[0028] The raw material may be prepared prior to melting. Such raw material is prepared by mixing germanium (Ge) metal and sulfur (S) powder in a molar ratio of Ge:S = 30 mol%-35 mol%:60 mol%-70 mol%, heating the mixture at a temperature between 1000°C and 1200°C for 1 hour to 5 hours, then holding the mixture at a lower temperature between 900°C and 1100°C for 5 hours to 24 hours, and then cooling. The initial heating causes Ge and S to react to form GeS2, and holding the mixture at a lower temperature for a long period of time results in a homogeneous molten GeS2.

[0029] For example, if the mixture is heated and held, followed by quenching, germanium disulfide glass represented by the general formula GeS2 is obtained, and if the mixture is heated and held, followed by furnace cooling, a composite of germanium disulfide glass and a monoclinic crystalline phase (low-pressure, high-temperature α-GeS2) represented by the general formula GeS2 is obtained.

[0030] The crucible 510 is not particularly limited as long as it is made of a material that is not reactive with the raw materials, but is preferably a quartz crucible (quartz ampoule). For example, when preparing the above-mentioned raw materials using a quartz crucible, crystal growth can be performed as is without removing the raw materials.

[0031] In step S410, the raw material is preferably melted at a temperature of 840°C or higher. Since the melting point of the target monoclinic GeS2 single crystal is 840°C, there is no particular limitation as long as the temperature is 840°C or higher. However, taking into consideration the furnace to be used and efficiency, the upper limit of the temperature may be 900°C or lower. This promotes melting of the raw material.

[0032] 5(A), the center of the furnace 520 that heats the crucible 510 has the highest temperature, and should be set to 840° C. or higher. The temperature gradient is usually about 100° C. / cm in the vertical Bridgman method, but in the present invention, a range of 20° C. / cm to 30° C. / cm can be used.

[0033] In step S420, as shown in FIG. 5(B), the lower part of the furnace 520 is set to a lower temperature than the central part, for example, in the range of 580° C. or higher and lower than 840° C., and the crucible 510 is lowered to the lower part of the furnace 520.

[0034] In step S420, the crucible 510 is preferably pulled down at a rate in the range of 0.5 mm / h to 1.5 mm / h, thereby growing a crystal in a direction parallel to the (001) plane.

[0035] (Embodiment 2) In the second embodiment, a wavelength conversion element and a laser device will be described as applications of the infrared optical material of the present invention.

[0036] The laser device of the present invention includes a laser beam output unit that emits fundamental light, and a wavelength conversion element that converts the fundamental light from the laser beam output unit into light having a frequency different from that of the fundamental light. The wavelength conversion element includes at least a wavelength conversion element made of the infrared optical material described in the first embodiment.

[0037] FIG. 6 is a schematic diagram showing a laser device of the present invention. 6 is a schematic diagram showing a laser device that uses the infrared optical material of the present invention as a wavelength conversion element and performs second harmonic oscillation. The laser device of the present invention includes at least a laser light output unit 610 that emits fundamental light with a frequency ω, and a wavelength conversion element 620 that converts the wavelength of the fundamental light from frequency ω to light with a frequency 2ω, which is twice the frequency. Since the wavelength conversion element 620 is the infrared optical material described above, a description thereof will be omitted.

[0038] The laser light output unit 610 is not particularly limited as long as it emits fundamental light having a wavelength (frequency ω) ranging from visible light to infrared light, and such a laser light output unit can use well-known light sources such as semiconductor lasers, solid-state lasers, and fiber lasers.

[0039] FIG. 6 shows an optical system including a focusing lens 630 that focuses the fundamental wave light from the laser light output unit 610, a dispersion mirror 640 that disperses the fundamental wave light (frequency ω) and wavelength-converted light (frequency 2ω) that are transmitted through the wavelength conversion element 620 without being converted and transmits only the fundamental wave light, and a reflection mirror 650 that reflects and transmits the wavelength-converted light dispersed by the dispersion mirror 640, but the optical system is not limited to these, and any optical system may be provided as needed.

[0040] The infrared optical material of the present invention transmits light with wavelengths of 358 nm or more and 22.5 μm or less, and therefore, when a CO2 laser emitting fundamental light with a wavelength of 10.6 μm (frequency 28.3 THz) is used, the wavelength can be converted to light with a wavelength of 5.3 μm (5300 nm) (frequency 56.6 THz). In this case, the infrared optical material of the present invention transmits light up to the visible range, and therefore has the advantage that weak visible light, such as a red He-Ne laser or a green laser, can be used as alignment light for the CO2 laser.

[0041] FIG. 7 is a schematic diagram showing another laser device of the present invention. 7 is a schematic diagram showing a laser device that uses the infrared optical material of the present invention as a wavelength conversion element and generates a difference frequency. The laser device of the present invention includes at least a laser light output unit 710 that emits fundamental light of frequencies ω1 and ω2, and a wavelength conversion element 720 that converts the wavelength of the fundamental light of frequencies ω1 and ω2 into light of a difference frequency ω1-ω2. The wavelength conversion element 720 is the infrared optical material described above, so a description thereof will be omitted.

[0042] The laser light output unit 710 includes a first light source 730 that emits fundamental light having a wavelength (frequency ω1) ranging from visible light to infrared light, and a second light source 740 that emits another fundamental light having a wavelength (frequency ω2) ranging from visible light to infrared light. Such a laser light output unit can use well-known light sources such as a semiconductor laser, a solid-state laser, or a fiber laser.

[0043] Although FIG. 7 shows a coupler 750 that combines fundamental wave light from a first light source 730 and another fundamental wave light from a second light source 740, any optical system such as a reflective mirror, a dispersive mirror, etc. may be provided.

[0044] The infrared optical material of the present invention has an average transmittance of 50% or more for light in the wavelength range of 15 μm or more and 22 μm or less. Therefore, when using, for example, a laser light source that emits fundamental light (also called pump light) with a wavelength of 1.55 μm (frequency 193.4 THz) and a laser light source that emits another fundamental light (also called signal light) with a wavelength of 1.5 μm or more and 2.3 μm or less (frequency 199.9 THz to 130.3 THz), wavelength conversion to light with a wavelength of 4 μm or more and 17 μm or less (frequency 74.9 THz to 17.6 THz) is possible.

[0045] FIG. 8 is a schematic diagram showing another laser device of the present invention. 8 is a schematic diagram showing a laser device that uses the infrared optical material of the present invention as a wavelength conversion element and performs parametric oscillation. The laser device of the present invention includes at least a laser light output unit 810 that emits fundamental light (pump light) with a frequency ωp, and a wavelength conversion element 820 that converts the frequency ωp of the fundamental light to lower frequencies ωs and ωi (ωi = ωp - ωs). The wavelength conversion element 820 is the infrared optical material described above, so a description thereof will be omitted.

[0046] The laser light output unit 810 is not particularly limited as long as it emits fundamental light having a wavelength (frequency ωp) ranging from visible light to infrared light, and such a laser light output unit can use well-known light sources such as semiconductor lasers, solid-state lasers, fiber lasers, etc.

[0047] 8, an incident-side mirror 830 and an exit-side mirror 840 are shown at both ends of the wavelength conversion element 820 to form a resonator, but a resonator is not essential. Also, as shown in FIG. 8, any optical system such as a dispersion mirror 850 and a reflection mirror 860 may be provided.

[0048] The infrared optical material of the present invention has an average transmittance of 50% or more for light in the wavelength range of 15 μm or more and 22 μm or less. Therefore, for example, when a laser light source emitting fundamental wave light (pump light) with a wavelength of 2.8 μm (frequency 107.1 THz) is used, the wavelength can be converted to light (idler light) with a wavelength of 3.3 μm or more and 19 μm or less (frequency 90.8 THz to 15.8 THz).

[0049] 6 to 8, examples have been described in which the infrared optical material of the present invention is used alone as a wavelength conversion element, but another wavelength conversion element may be combined with the infrared optical material of the present invention. This makes it possible to provide a laser light source that converts to a desired wavelength. Such modifications will be understood by those skilled in the art.

[0050] As described above, the infrared optical material of the present invention has a transmission threshold wavelength of 22.5 μm, and therefore is effective as an optical material in the infrared region other than laser devices, and can be applied to, for example, infrared windows, lenses, filters, etc.

[0051] The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples. [Example]

[0052] [Synthesis of raw materials] As raw materials for the synthesis of infrared optical materials, (1) germanium disulfide glass represented by the general formula GeS2, and raw materials represented by the general formula GeS2 containing a monoclinic crystalline phase (low-pressure, high-temperature type α-GeS2), and (2) germanium disulfide glass represented by the general formula GeS2 were prepared.

[0053] (1) Synthesis of germanium disulfide glass represented by the general formula GeS2 and raw materials containing the crystalline phase (low-pressure, high-temperature type α-GeS2) belonging to the monoclinic system represented by the general formula GeS2.

[0054] Germanium (Ge) metal (manufactured by Mitsubishi Materials Corporation, purity: 99.999%, resistivity: 50 Ω·cm or more) and sulfur (S) powder purified by distillation according to M. Nakamura et al., J. Alloy. Comp., 326-328, 591, 2024 were mixed to satisfy the molar ratio of Ge:S = 33.3 mol%:66.7 mol%.

[0055] 5.0 g of the mixed powder was vacuum-sealed in a quartz ampoule and heated at 1080°C for 2 hours in a horizontal electric furnace while being cooled with water. The temperature of the electric furnace was then lowered to 1000°C and maintained at that temperature for 12 hours. The electric furnace was then turned off and allowed to cool.

[0056] The obtained samples were observed, and the X-ray diffraction patterns were measured using a powder X-ray diffractometer (MiniFlex, manufactured by Rigaku Co., Ltd.). The details and composition of the samples were observed using a scanning electron diffractometer (SEM, JEOL Ltd., JSM-6500F) equipped with an energy dispersive X-ray spectrometer (EDX, JEOL Ltd., JED-2300). The results are shown in Figures 9 to 11.

[0057] FIG. 9 shows the appearance of the obtained sample.

[0058] In FIG. 9, the sample is shown in grayscale, and is transparent and yellow overall (areas shown as lighter shades), with scattered black areas (areas shown as darker shades).

[0059] FIG. 10 shows the XRD pattern of the obtained sample.

[0060] Figure 10 shows the XRD pattern of the yellow transparent region and the XRD pattern of the region containing black. No diffraction peaks were observed in the XRD pattern of the yellow transparent region. This indicates that the yellow transparent region is not a crystalline phase but a glass phase.

[0061] On the other hand, diffraction peaks were observed in the XRD pattern of the black region. Furthermore, the characteristic diffraction peak at 15.48° corresponded to the 002 peak of the monoclinic GeS2 phase. This indicated that the black region was a crystalline phase (single phase), consisting of a single GeS2 phase belonging to the monoclinic system. Only the 002 peak, which is the strongest line, was observed. This is thought to be due to the sample having a strong preferred orientation during milling, which caused the other peaks to become so small that they were not observed compared to the 002 peak.

[0062] FIG. 11 shows an SEM image of the obtained sample.

[0063] In Figure 11, regions 1110 and 1120 correspond to the black regions, and regions 1130 and 1140 correspond to the yellow regions. Composition analysis using EDX mounted on an SEM confirmed that the atomic ratio of Ge:S in regions 1110 and 1120 was 1:2, and that the atomic ratio of Ge:S in regions 1130 and 1140 was also 1:2, confirming that they had the same composition. This indicated that the obtained sample consisted of a GeS2 glass phase and a GeS2 crystalline phase (GeS2 single phase).

[0064] From the above, the obtained sample was a composite represented by the general formula GeS2, in which a monoclinic crystalline phase was dispersed in germanium disulfide glass represented by the general formula GeS2. In the following Example 1, this composite was used as the raw material.

[0065] (2) Synthesis of germanium disulfide glass raw material represented by the general formula GeS2 As in (1) above, Ge metal and S powder were mixed and heated at 1080°C for 2 hours, and then the temperature of the electric furnace was lowered to 1000°C, held for 12 hours, and quenched.

[0066] The sample thus obtained was observed, and its XRD pattern was measured. The obtained sample was a yellow, transparent bulk body. The above-mentioned black region was not visually confirmed. The XRD pattern (not shown) was identical to the XRD pattern of the yellow, transparent region shown in Figure 10, and the obtained sample was found to be germanium disulfide glass represented by the general formula GeS2. In the following Example 2, this glass was used as the raw material.

[0067] [Example 1] In Example 1, an infrared optical material was produced using the raw material prepared in (1) (a composite in which α-GeS 2 was dispersed in germanium disulfide glass) according to the method shown in FIG.

[0068] 5.0 g of the prepared raw material was vacuum-sealed in a quartz ampoule (outer diameter 13 mm, length 70 mm) as a crucible. The quartz ampoule was then placed in a vertical Bridgman apparatus, and crystals were grown by the Bridgman method using the temperature distribution shown in Figure 12.

[0069] FIG. 12 is a diagram showing the temperature distribution in the Bridgman method of Example 1.

[0070] As shown in Figure 12, a temperature gradient was set in the electric furnace so that the temperature was 680°C at the top, 898°C in the middle (near the center), and 550°C at the bottom. The growth conditions were as follows: The temperature gradient near the melting point was set to approximately 30°C / cm, and the raw materials in the quartz ampoule were melted and homogenized. The quartz ampoule was then lowered to the bottom of the furnace at a rate of 1.0 mm / h to grow a crystal. The quartz ampoule was not rotated. After growth was completed, the quartz ampoule was removed from the electric furnace and rapidly cooled to room temperature while the temperature (maximum temperature) of the center of the electric furnace was maintained at 898°C to maintain the high-temperature stable phase.

[0071] The crystal thus obtained (referred to as the crystal of Example 1) was cleaved in the growth direction and observed. The X-ray diffraction pattern of the cleaved surface was measured using a powder X-ray diffractometer (MiniFlex, manufactured by Rigaku Corporation). A small piece (0.49 mm × 0.17 mm × 0.07 mm) was cut from the crystal and subjected to precise crystal structure analysis using a single crystal X-ray diffractometer (VariMax DW with Saturn, manufactured by Rigaku Corporation). The results are shown in Figures 13 and 14 and Table 1.

[0072] A portion (0.345 g) of the crystal of Example 1 was cut out and subjected to differential thermal analysis (DTA). A small piece (4 mm × 3 mm × 0.1 mm) was cut out from the crystal, cleaved, and the transmission spectrum was measured using a spectrophotometer (visible range: PerkinElmer, Lamuda 900; infrared range: JASCO, FT / IR-6800). The results are shown in Figures 15 to 17.

[0073] [Example 2] In Example 2, an infrared optical material was produced using the raw material (germanium disulfide glass) prepared in (2) according to the method shown in Figure 2. The production conditions were the same as in Example 1 except for changing the raw material, so a description thereof will be omitted. As in Example 1, the obtained crystal (referred to as the crystal of Example 2) was observed, subjected to crystal structure analysis, and its transmission spectrum was measured.

[0074] These results will be summarized below. FIG. 13 is a diagram showing the appearance of the crystals of Example 1.

[0075] As shown in grayscale in Figure 13, the crystal of Example 1 has a light brown color, and the black regions seen in the raw material are not visible. Figure 13 shows a sample in which the crystal of Example 1 was cleaved in the growth direction, with the initial stage of crystal growth at the right end and the end of crystal growth at the left end. Figure 13 shows that the cleavage plane extends from the initial stage of crystal growth to the end of crystal growth, suggesting that the crystal of Example 1 is a single crystal. Although not shown, the crystal of Example 2 also had a similar appearance to Example 1.

[0076] FIG. 14 is a diagram showing an XRD pattern of the cleavage plane of the crystal of Example 1.

[0077] In Figure 14, only the 001 peak corresponding to the GeS2 phase belonging to the monoclinic system was observed in the XRD pattern of the crystal of Example 1. Although not shown, only the 001 peak was observed in the XRD pattern of the crystal of Example 2. This indicates that the cleavage plane of the crystals of Examples 1 and 2 was the (001) plane, the obtained crystals were single crystals expressed by the general formula GeS2, and the growth direction was parallel to the (001) plane.

[0078] The crystal structure data obtained by crystal structure analysis is shown in Table 1, and the crystal structure is shown in Figure 1. Table 1 describes the crystal system, space group, lattice constant, type of atom, and atomic position, and this data can be used to determine the shape and size of the unit cell and the arrangement of atoms within it.

[0079] The crystal of Example 1 belongs to the monoclinic system and the space group P21 / c (space group number 14 in the International Tables for Crystallography), and the lattice constants a, b, c, α, β, and γ are respectively a=0.67061±0.05 nm b=1.60877±0.05 nm c=1.14242±0.05 nm α=90° β=91.069°±0.3° γ=90° The atomic positions were as shown in Table 1. The crystal of Example 2 also belonged to the space group P21 / c and satisfied the above-mentioned lattice constants.

[0080] FIG. 15 is a diagram showing the DTA profile of the crystals of Example 1.

[0081] The DTA profile during the temperature increase (10°C / min) is shown in Figure 15. Only a melting peak was observed in the DTA profile, and the melting point was found to be 840±1°C.

[0082] From the above, it was shown that by adopting the manufacturing method shown in Figure 2, it is possible to grow a single crystal represented by the general formula GeS2 and belonging to the monoclinic system without using a seed crystal.

[0083] FIG. 16 is a diagram showing the transmission spectrum of the crystal of Example 1 in the visible region. FIG. 17 is a diagram showing the infrared transmission spectrum of the crystal of Example 1.

[0084] 16 and 17, it was found that the sample of Example 1 transmits light with wavelengths of 358 nm or more and 22.5 μm or less. Surprisingly, it was found that the transmission threshold wavelength was 22.5 μm, which was 10 μm wider than the 12 μm previously reported in Non-Patent Documents 1 and 2.

[0085] It is generally known that if the material is the same, the transmission characteristics do not change regardless of whether it is glass or crystal. For example, according to Figure 8-1-6 on page 238 of Maruzen Publishing's New Glass Handbook, ISBN 4621036025, the transmission limit wavelength in the infrared region of silica glass (fused quartz, silicate glass) is about 4.5 μm, and that of quartz (crystalline quartz) is also about 4.5 μm, so the transmission limit wavelength does not change. From this, it can be said that the fact that the transmission limit wavelength of monoclinic GeS2 single crystal has expanded to 22.5 μm is a phenomenon unique to GeS2.

[0086] According to FIG. 17, the crystal of Example 1 (thickness: 0.1 mm) had a transmittance of 39% for light with a wavelength of 22 μm. Furthermore, the crystal of Example 1 (thickness: 0.1 mm) had an average transmittance of 57% or more for light with a wavelength of 15 μm or more and 22 μm or less. Although not shown, the crystal of Example 2 also had a transmission threshold wavelength exceeding 20 μm, and exhibited excellent infrared transmittance similar to the crystal of Example 1. From this, it was found that the crystals of Examples 1 and 2 function as infrared optical materials made of single crystals of the monoclinic system, represented by the general formula GeS2, and are particularly applicable to infrared windows, lenses, and filters. [Industrial Applicability]

[0087] The infrared optical material of the present invention has a transmission limit wavelength of 22.5 μm, making it particularly effective as an optical material in the infrared region, and can be used for, for example, infrared windows, lenses, filters, etc. Furthermore, since the infrared optical material of the present invention does not have centrosymmetrical structure, it can be used as a nonlinear optical crystal and a wavelength conversion element. By combining the wavelength conversion element with a laser light source that emits fundamental wave light, it is possible to provide a laser device, particularly in the infrared region. [Explanation of symbols]

[0088] 510 Crucible 520 Furnace 610, 710, 810 Laser light output unit 620, 720, 820 wavelength conversion element 630 Condenser Lens 640, 850 dispersion mirror 650, 860 Reflective mirror 730 First Light Source 740 Second Light Source 750 Coupler 830 Incident mirror 840 Exit mirror

Claims

1. General formula GeS ２ and is an infrared optical material consisting of a single crystal belonging to the monoclinic system.

2. The single crystal is in the space group P2 １ / c symmetry, The lattice constants a, b, and c are a=0.67061±0.05 nm b=1.60877±0.05 nm c=1.14242±0.05 nm The infrared optical material according to claim 1 , wherein the value satisfies the range of:

3. The lattice constant β is β＝９１．０６９±０．３° The infrared optical material according to claim 2, wherein the value satisfies the range of:

4. 2. The infrared optical material according to claim 1, wherein the transmittance of light with a wavelength of 22 μm at a thickness of 0.1 mm is 35% or more.

5. 5. The infrared optical material according to claim 4, wherein the average transmittance of light having a wavelength of 15 μm or more and 22 μm or less at a thickness of 0.1 mm is 50% or more.

6. 2. The infrared optical material according to claim 1, wherein any one of the faces of the single crystal is a face parallel to the (001) plane.

7. A method for producing an infrared optical material according to any one of claims 1 to 6, comprising the steps of: General formula GeS ２ Germanium disulfide glass represented by the general formula GeS ２ and a crystalline phase belonging to the monoclinic system, and a crystalline phase represented by the general formula GeS ２ and growing a crystal by a directional solidification crystal growth method using at least one raw material selected from the group consisting of crystalline phases belonging to an orthorhombic system, which are represented by the formula:

8. the unidirectional solidification crystal growth method is a vertical Bridgman method, The crystal growing step comprises: placing the raw material in a crucible and melting it; lowering the crucible and growing a crystal from the molten raw material; The method of claim 7, comprising:

9. The method according to claim 8, wherein the raw material is melted at a temperature of 840°C or higher.

10. The manufacturing method according to claim 8, wherein the crucible is lowered at a rate of 0.5 mm / h to 1.5 mm / h.

11. The raw material has the general formula GeS ２ and / or a germanium disulfide glass represented by the general formula GeS ２ The method according to claim 7, wherein the crystalline phase is represented by the formula:

12. A wavelength conversion element comprising the infrared optical material according to any one of claims 1 to 6.

13. a laser light output unit that emits fundamental wave light; a wavelength conversion element that converts the fundamental wave light into light having a frequency different from the frequency of the fundamental wave light; Equipped with A laser device, wherein the wavelength conversion element includes at least the wavelength conversion element according to claim 12 .

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