Nonlinear wavelength conversion device and laser beam generation device using the same

Abstract

To make input light incident at a phase matching angle larger than an internal total reflection critical angle to generate harmonic or sum frequency light without using a prism coupling device, in a nonlinear wavelength conversion device using a nonlinear optical crystal material that is virtually impossible to perform polishing on any surface other than a specific surface.SOLUTION: A grating 102 is formed on a polishable surface or a cleaved surface of a nonlinear optical crystal material 101, and input light 201 diffracted by the grating 102 is propagated at a phase matching angle inside the nonlinear optical crystal material 101.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a wavelength conversion device for generating deep ultraviolet light, vacuum ultraviolet light, etc., and a laser light generating apparatus using the same. [Background technology]

[0002] The demand for deep ultraviolet laser light sources is higher than ever before, and the demand for wavelength conversion technology using nonlinear optics has been very high in recent years. The nonlinear optical materials that are currently in industrial use for generating deep ultraviolet light are BBO (β-BaB2O4), CLBO (CsLiB6O 10 ), LBO (LiB3O5), LB4 (Li2B4O7), KDP (KH2PO4), KD*P, etc. These nonlinear optical materials all have the following problems: 1) the optical absorption inside the material increases toward the deep ultraviolet region, making them opaque, and 2) the wavelengths that can satisfy the phase matching conditions described below are limited to a certain degree on the long wavelength side. Therefore, at present, the only method available for generating light with a wavelength of 200 nm or less is a method using sum frequency mixing.

[0003] For example, if you try to obtain an output with an even shorter wavelength by sum frequency mixing, it will be the sum frequency of deep ultraviolet light and infrared light, whose wavelengths have already been shortened. Since the efficiency of nonlinear wavelength conversion is strongly affected by the light intensity, it is necessary to focus the input light into a small spot in order to obtain a practical conversion efficiency. However, focusing deep ultraviolet light into a small spot causes damage and deterioration of the crystal material, and there are limitations to stable operation.

[0004] In order to obtain short-wavelength light from input light with as long a wavelength as possible, it is effective to directly generate a second harmonic, but it is not possible to obtain phase matching for generating a second harmonic with the nonlinear optical materials mentioned above.

[0005] In sum frequency mixing, if the wavelengths of the input light are λ1 and λ2 and the wavelength of the sum frequency output is λ3, the energy of a photon is proportional to the frequency of the light, ν=1 / λ, and therefore the law of conservation of energy gives the relationship shown in equation (1) below.

[0006] (1 / λ1)+(1 / λ2)=1 / λ3···(1)

[0007] For a specific desired wavelength λ3, there are an infinite number of combinations of (λ1, λ2), but the shortest of λ1 and λ2 is when λ1 = λ2, which corresponds to the direct generation of the second harmonic.

[0008] In both second harmonic generation and sum frequency mixing, it is necessary to satisfy the law of conservation of momentum, known as phase matching. This is derived from the laws of conservation of energy and conservation of momentum. The law of conservation of energy states that the sum of the frequencies of light must be the same at the input and output, while the law of conservation of momentum states that the sum of the k-vectors (wave vectors) must be the same at the input and output.

[0009] The law of conservation of momentum is expressed by equation (2), where k is the wave number of each light. k1 + k2 = k3 (2)

[0010] Originally, formula (2) is a formula related to vector quantities. Although non-collinear interactions are theoretically possible, they are not used in the second harmonic generation that is dealt with in this invention, so we will not go into detail here. Instead, we will assume collinear interactions (where the direction of all k vectors is the same) and treat the wave number k as a scalar quantity. If the refractive index of the medium through which each wavelength λ (λ1, λ2, λ3) propagates is n (n1, n2, n3), then the relationship k = 2πn / λ is satisfied, and by dividing both sides of formula (2) by 2π, we obtain the relationship in formula (3). (n1 / λ1)+(n2 / λ2)=n3 / λ3...(3)

[0011] In the case of second harmonic generation, λ1 = λ2 = 2λ3, and n1 = n2, so the phase matching condition in equation (3) is reduced to n1 = n3, that is, the refractive index for the input light and the harmonic are equal.

[0012] In general, the refractive index of a material varies depending on the wavelength, so it is not self-evident that the above conditions are satisfied. In birefringent crystal materials, the refractive index varies depending on the polarization, and the refractive index of a light ray called extraordinary light changes depending on the propagation direction in the crystal material, so these conditions are satisfied in a specific direction in the crystal material. Details of the above concept are described, for example, in "A. Yariv, "Quantum Electronics" 3rd Edition, Wiley, Chap.16".

[0013] In a crystal material with "normal dispersion," in which the refractive index increases as the wavelength becomes shorter, the conditions under which the second harmonic can be phase-matched are as follows:

[0014] (For negative uniaxial crystalline materials) n e (λ1 / 2) <n o (λ1) <n o (λ1 / 2) (4)

[0015] (For positive uniaxial crystal materials) n o (λ1) <n o (λ1 / 2) <n e (λ1) (5)

[0016] A similar condition exists for biaxial crystalline materials.

[0017] In other words, there exists a shortest wavelength at which phase matching is possible in the second harmonic generation process due to the refractive index dispersion of the crystal material. Table 1 shows the shortest second harmonic wavelengths for each of the nonlinear optical materials mentioned above.

[0018] [Table 1]

[0019] As can be seen from Table 1, the shortest second harmonic wavelength among nonlinear optical materials currently in industrial use is 206 nm for BBO.

[0020] In the above explanation, we have described "normal dispersion" materials, whose refractive index increases as the wavelength becomes shorter. However, "anomalous dispersion" materials, whose refractive index decreases as the wavelength becomes shorter, have absorption problems, especially in the deep ultraviolet wavelength region, and are not suitable as materials for wavelength conversion.

[0021] At the end of the last century, KBe2BO3F2 (KBBF) crystals and RbBe2BO3F2 (RBBF) crystals were discovered and reported, and they attracted great expectations as new crystal materials for deep ultraviolet light generating devices (see Patent Document 1, Non-Patent Document 1, Non-Patent Document 2, etc.).

[0022] These materials all have an absorption edge wavelength of 160 nm or less and have strong anisotropy, so the shortest wavelength for direct second harmonic generation is 161 nm for KBBF and 170 nm for RBBF, which makes them suitable for use as crystal materials for generating deep ultraviolet light. However, these crystal materials also have strong mechanical anisotropy, so they all have strong cleavage properties. Therefore, optical polishing is extremely difficult on surfaces other than the cleavage plane (c-plane).

[0023] The phase matching angles for generating 193.4 nm light (ArF laser wavelength) and 177.4 nm light (sixth harmonic of 1064 nm light), which are of industrial interest, are 55.2 degrees and 64.1 degrees for KBBF and 61.3 degrees and 73.1 degrees for RBBF, both of which are larger than the critical angle of total internal reflection determined by the refractive index of the crystal material (both are approximately 1.50). Therefore, incidence from the c-plane cannot be achieved at the phase matching angle.

[0024] To solve this problem, Chen et al. made it possible to achieve phase-matching angle incidence by making a prism glass material optically contact with the cleaved and polished c-plane of a crystal, and many patents and papers were published on short wavelength generation (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0025] [Patent Document 1] U.S. Patent No. 6,859,305 [Patent Document 2] U.S. Pat. No. 4,711,512 [Patent Document 3] JP 2015-049376 A [Patent Document 4] Patent Publication No. 2021-173878 [Patent Document 5] Special Publication No. 2022-551549 [Non-patent literature]

[0026] [Non-Patent Document 1] C. Chen,et. al., “New development of nonlinear optical crystals for the ultraviolet region with molecular engineering approach,” J. Appl. Phys.vоl.77, pp.2268-2272(1995). [Non-Patent Document 2] C. Chen,et. al., “Deep UV nonlinear optical crystal: RbBe2(BO3)F2,” J.Opt.Soc.Am.B26, pp.1519-1525(2009). Summary of the Invention [Problem to be solved by the invention]

[0027] However, in order to achieve optical contact, the bonding surfaces must be polished with extremely high precision (flatness, surface roughness), which poses a high technical barrier to practical use of the optical crystal as a deep-ultraviolet light generating device. In addition, because it contains beryllium (Be), which is difficult to handle, there are only limited facilities available for polishing, and polishing technology has not been developed to the same extent. As a result, even now, more than a quarter century after its invention, there has been little progress in practical use.

[0028] The present invention solves these technical problems and provides a nonlinear wavelength conversion device for generating deep ultraviolet light with a shorter wavelength in a simpler manner.

[0029] Table 2 shows the phase matching angle θ when generating light with wavelengths of 193.4 nm / 177.4 nm using KBBF / RBBF crystals. phasematch and the refractive index n, and the critical angle of total internal reflection θ at that wavelength. TIR As shown in Table 2, the phase coupling angle θ phasematch is the critical angle of total internal reflection θ TIR Greater than.

[0030] Total internal reflection critical angle θ TIR and the refractive index n is sinθ TIR = 1 / n, and the larger angle θ>θ TIR Therefore, θ phasematch >θ TIR In the case of , the phase matching angle θ phasematch This means that light cannot enter from the outside at an angle equivalent to this.

[0031] As mentioned above, KBBF / RBBF have strong mechanical anisotropy, and therefore both have strong cleavage. Therefore, it is extremely difficult to optically polish them on any surface other than the cleavage plane (c-plane). In other words, it is difficult to make light incident on these nonlinear crystal materials at a phase matching angle from the optically polished c-plane, which is a factor that prevents the practical use of nonlinear optical crystal materials that can only be polished on specific planes, such as KBBF / RBBF crystals.

[0032] In the above description, two wavelengths that are important in industry, such as 193.4 nm and 177.4 nm, are exemplified, but the present invention is not limited to these wavelengths. phasematch >θ TIR The relationship is sinθ phasematch >(1 / n). Table 2 shows the phase matching angle (θphasematch ) and refractive index (n), and the critical angle of total internal reflection at that wavelength (θ TIR )

[0033] [Table 2]

[0034] In order to satisfy these phase matching angles, a device shown in FIG. 1, known as a prism coupled device (PCD), has been reported in which a synthetic quartz or calcium fluoride prism cut at an appropriate angle is brought into contact with the c-face of a KBBF / RBBF crystal (Patent Document 1).

[0035] The figure shows an input side prism 1, a medium material 2 for facilitating contact, a nonlinear optical crystal material 3, an output side prism 4, a light beam 201 of input light of wavelength λ1, and a light beam 202 of input light of wavelength λ1 and a light beam 203 of generated light of wavelength λ1. H A light ray 202 in the crystal material of a harmonic of .lambda.1, a light ray 203 of input light of wavelength .lambda.1 emitted from the device, and a light ray 204 of input light of wavelength .lambda.1 emitted from the device. H A light ray 204 of a harmonic of λ1 is shown. In this device, an input side prism 1 and an output side prism 4 are in contact with both sides of a nonlinear optical crystal material 3, such as KBBF, whose two parallel surfaces are optically polished, either directly or through an intermediate material 2. An input light 201 of wavelength λ1 is incident on the input side prism 1 at a specific angle and reaches the interface with the nonlinear optical crystal material 3. The refractive indexes of this prism and the nonlinear optical crystal material are selected to be relatively close, and the input light is transmitted into the crystal material. The incident angle of the input light 201 is adjusted so that the input light 202 in the crystal material after refraction is in a direction that satisfies the desired phase matching condition with respect to the crystal axis. The input light that reaches the opposite surface of the nonlinear optical crystal material 3 is refracted again by contact with the prism 4, which has a refractive index close to the nonlinear optical crystal material 3, without total internal reflection, and is emitted as a light ray 203 to the outside of the device. When phase matching is satisfied, the generated wavelength λ HThe harmonic reaches the interface with prism 4 and is refracted between the prism, which has a refractive index somewhat different from that of the input light, but again does not undergo total internal reflection and exits the device as ray 204.

[0036] When using a prism-bonded device, extremely high-precision optical polishing is required to achieve optical contact, which has been an obstacle to practical use.

[0037] Generally, for use as a laser device, a surface roughness of about 10 nm (rms) is required for the optical surface, which is relatively easy to obtain. However, for optical contact, a surface roughness of about several angstroms is required, and even with materials such as synthetic quartz, which is frequently used industrially and has well-developed polishing technology, advanced technology is required to achieve this. In addition, the development of polishing technology for KBBF and RBBF has not progressed due to the compositional characteristics of the material, which contains beryllium, and in addition, due to the characteristics of cleavage and mechanical properties (softness), it is currently extremely difficult to obtain a good surface roughness and flatness.

[0038] The problem to be solved by the present invention is to provide a nonlinear wavelength conversion device that can receive input light at a phase matching angle larger than the critical angle of total internal reflection and can extract a harmonic wave without requiring such high-precision optical polishing.

[0039] An object of the present invention is to provide such a device, thereby providing a stable and inexpensive deep ultraviolet and vacuum ultraviolet laser light source. [Means for solving the problem]

[0040] The device disclosed herein is a nonlinear wavelength conversion device for generating deep ultraviolet light using KBBF / RBBF crystals or the like, characterized in that a diffraction structure (e.g., a grating) having a diffraction effect is formed at the interface between the crystal material for nonlinear wavelength conversion and air (or vacuum), and input light is made incident at an angle close to perpendicular and diffracted in a phase-matching direction within the crystal material.

[0041] The technique of forming a grating on the surface of a transparent glass material, diffracting light at an angle equal to or greater than the critical angle for total internal reflection, and propagating the light within the glass material is known in the fields of head-up displays and optical waveguides (see, for example, Patent Documents 2 to 5).

[0042] However, conventional techniques in the fields of head-up displays and optical waveguides aim to make light enter (or exit) a waveguide at an angle equal to or greater than the critical angle (total reflection angle) between the waveguide and air in order to propagate the light inside the waveguide, and there is no precedent for a technical approach that takes into account the diffraction conditions of a grating, etc., aimed at matching the phase matching conditions of the nonlinear optical crystal material into which the light enters, as in the present invention.

[0043] An outline of the first nonlinear wavelength conversion device for generating deep ultraviolet light according to the present disclosure is shown in Figure 2. This shows the device configuration for generating second harmonic waves.

[0044] An input grating 102 is formed on a part or the entirety of the input surface of a nonlinear optical crystal material 101, and an output grating 103 is formed on the opposite surface. Input light 201 is diffracted by the input grating and propagates as a light ray 202 within the nonlinear optical crystal material. The light ray 202 forms an angle θ with a normal 104 to the optical surface of the nonlinear optical crystal material (corresponding to the c-axis when the nonlinear optical crystal material is a KBBF / RBBF crystal), and this angle θ is the angle of incidence θ of the input light 201 with respect to the grating 102. ext It is determined by the wavelength λ1 of the input light and the pitch Λ of the grating. In other words, it is possible to adjust θ by adjusting the angle of incidence to the device (nonlinear optical crystal material), and by making this the desired phase matching angle, for example, the wavelength λ H = λ1 / 2 harmonic is generated and diffracted by the output grating 103, resulting in a light beam 203 with the same wavelength λ1 as the input light and a light beam with the generated wavelength λ H The harmonics exit the device as rays 204, each at a different angle. Effect of the Invention

[0045] The nonlinear wavelength conversion device according to the present invention does not require a device with optical contacts that require extremely high-precision optical polishing, and enables the generation of deep ultraviolet light or vacuum ultraviolet light by harmonic generation at a phase matching angle equal to or greater than the internal critical angle. [Brief description of the drawings]

[0046] [Figure 1] FIG. 1 is a diagram showing a cross-sectional configuration of a prism coupling device (PCD). [Diagram 2] FIG. 2 is a diagram showing a cross-sectional configuration of the first nonlinear wavelength conversion device for generating deep ultraviolet light. [Diagram 3] FIG. 3 is a diagram for explaining the relationship between the diffraction angle, the wavelength, and the grating pitch. [Figure 4] FIG. 4 is a diagram showing a cross-sectional configuration of the second nonlinear wavelength conversion device for generating deep ultraviolet light. [Diagram 5] FIG. 5 is a diagram illustrating the application of a nonlinear wavelength conversion device for generating deep ultraviolet light to sum frequency mixing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0048] 2 is a conceptual diagram of a first nonlinear wavelength conversion device for generating deep ultraviolet light. The figure shows a nonlinear optical crystal material 101, an entrance grating (first diffraction structure) 102, an exit grating (second diffraction structure) 103, a normal 104 to the optical surface of the nonlinear optical crystal material, a light ray 201 of input light of wavelength λ1, and a wavelength λ of input light of wavelength λ1 and a wavelength λ of generated light. H A light ray 202 in the crystal material of a harmonic of .lambda.1, a light ray 203 of input light of wavelength .lambda.1 emitted from the device, and a light ray 204 of input light of wavelength .lambda.1 emitted from the device. H A harmonic ray 204 is shown.

[0049] The nonlinear wavelength conversion device of the first example is a nonlinear wavelength conversion device having a nonlinear optical crystal material 101 having a first optical surface S1 and a second optical surface S2 facing each other, and has a first diffraction structure 102 (incident side grating) on ​​at least one of the first optical surfaces S1 of the nonlinear optical crystal material 101 for diffracting input light 201 incident from outside the nonlinear optical crystal material 3, and the input light 201 of wavelength λ1 incident from outside the nonlinear optical crystal material 101 is diffracted by the first diffraction structure 102 at an angle θ (angle θ ext ) and the direction of this angle θ is the wavelength λ H This is a direction that satisfies the phase matching condition for generating light of the direction of incidence. The first diffractive structure 102 diffracts the input light in a predetermined direction. Although the above-mentioned opposing first optical surface S1 and second optical surface S2 are parallel, they do not need to be strictly parallel, and may be approximately parallel. In this example, approximately parallel means that the angle (acute angle) between the normals of the two surfaces is 1 degree or less.

[0050] The nonlinear wavelength conversion device of the second example has a second diffractive structure 103 on a second optical surface S2 on the opposite side to a first optical surface S1 on which an input light 201 is incident in a nonlinear optical crystal material 3, and the second diffractive structure 103 (output side grating) converts the input light 201, the second harmonic (wavelength λ H ), or both, are diffracted outside the nonlinear optical crystal material 3 .

[0051] In the figure, the same grating is formed on the entrance surface (left side in the figure) and the exit surface (right side in the figure), so the input light 201 is diffracted as output light 203 in the same direction as it entered, and the wavelength λ H The second harmonic (second harmonic 204) is diffracted in a direction different from that of the output light 203. This device can be called a GCD (Grating Coupled Device) in contrast to a conventional PCD.

[0052] Here, the relationship between the grating pitch Λ and the diffraction angle and the incident angle will be described.

[0053] n is the refractive index of the nonlinear optical crystal material, and θ ext is the incident angle of the input light, θ int If is the diffraction angle inside the nonlinear optical crystal material, and Λ is the pitch of the gratings (entrance side grating, exit side grating), then the relationship shown in Fig. 3 is established. Fig. 3 shows the space (e.g., air or vacuum) 301 outside the crystal material, crystal material 302 with a refractive index n, grating 303 formed on the crystal material surface, normal 304 to the crystal material surface, light ray 401 incident on the device, light ray 402 propagating inside the crystal material, equiphase surface 501 (wavelength λ) of the light ray incident on the crystal material, equiphase surface 502 (wavelength λ / n) of the light ray propagating inside the crystal material, and grating pitch 503. The light ray 401 with wavelength λ is incident at an angle θ ext This light is diffracted by the grating 303 with pitch Λ and is refracted at a refraction angle θ int The light propagates as a ray 402. int , θ ext The relationship is expressed by the following equation (6).

[0054] Λ=λ / (n sinθ int -sinθ ext ) ···(6)

[0055] A preferred grating is one that has an incident angle θ ext For θ int is the phase matching direction θ phasematch As a specific example, in order to generate light of 193.4 nm / 177.4 nm, when KBBF / RBBF is used as the nonlinear optical crystal material and the incident angle of the input light to the device is set to ±5°, the specific pitch Λ of the preferable grating derived from formula (6) and the output angle θ of the harmonic when a grating of the same pitch is also formed on the output surface are out are shown in Table 3. These are merely examples of the present invention, and the present invention is not, of course, limited to these numerical examples.

[0056] Because the input light and the harmonics are diffracted by the grating and emitted at different angles, it is possible to obtain only the harmonics without using wavelength separation means such as a prism or dichroic mirror. Also, by making the pitch of the grating on the emission side (second diffraction structure) different from that of the grating on the incidence side (first diffraction structure), it is possible to extract the harmonics in, for example, approximately the same direction as the input light (Figure 4).

[0057] In the example in Table 3, the incidence angle θ ext is set to ±5°, but the angle of incidence of the input light can be set appropriately taking into consideration the arrangement of the entire device, the conditions for creating the grating, etc. The input light may be made to enter perpendicular to the surface of the nonlinear optical crystal material (the surface on which the grating is formed), but making the light enter at a slight incline also has the effect of reducing the influence of return light due to surface reflection.

[0058] To generate 193.4 nm / 177.4 nm light, KBBF / RBBF is used. The pitch Λ of the grating when the input light incident angle is set to ±5°, and the output angle θ of the harmonic when a grating with the same pitch is also formed on the output surface. out is shown in Table 3.

[0059] [Table 3]

[0060] From Table 3, it is clear that for the target wavelengths (193.4 nm / 177.4 nm), it is possible to diffract light at a desired angle by forming a grating with a pitch of approximately 0.24 to 0.34 μm.

[0061] These gratings can be created using visible light, for example, laser light with a wavelength of 532 nm, by using a conventionally known interference exposure method. It is also possible to make the exit angle nearly parallel to the incident angle by changing the pitch of the grating on the exit side. Since the pitch of the grating for a device generating 193 nm light is larger than 0.275 μm, it is sufficient to create the grating by interference exposure using general 532 nm light. It is also possible to make the exit angle nearly parallel to the incident angle by changing the pitch of the grating on the exit side.

[0062] It should be noted here that when generating deep ultraviolet light using KBBF or RBBF, the allowable width of phase matching is about 0.01°cm, so unlike head-up displays, optical waveguides, etc., high precision is required for the propagation direction within the nonlinear optical crystal material. In addition, considering the manufacturing error of the grating pitch, in actual use, the phase matching is adjusted by finely adjusting the angle of incidence with respect to the grating. Therefore, when configuring a laser emission device using the nonlinear wavelength conversion device of the present invention, it is preferable to have a mechanism for adjusting the angle of incidence of input light incident on the nonlinear optical crystal material.

[0063] The grating as described above can be fabricated, for example, by applying a photoresist to the surface of the crystal material, creating a grating pattern by interference exposure, and then etching according to the exposure pattern, and then removing the photoresist. Alternatively, if the photoresist is suitable for the wavelength used, it may be used as the grating as it is. It is also possible to form a film of a dielectric material, such as silicon oxide (SiO2) or calcium fluoride (CaF2), that is close to the refractive index of the crystal material and transparent at the wavelength used, on the surface of the crystal material by a process such as deposition or sputtering, and then pattern this dielectric material by lithography, and form a grating by etching. It is also possible to fabricate a grating with a finer pitch by adhering a prism material with a relatively high refractive index to the photoresist and exposing it to interference.

[0064] In the above-described embodiment, the grating is intended to receive one wavelength λ1 and generate diffracted light, but a similar device can also receive light of two wavelengths (λ1 and λ2) and obtain output light of sum frequency wavelength λ3. In this case, the relationship ((1 / λ3)=(1 / λ1)+(1 / λ2)) shown in formula (1) also holds. Since the two input lights need to be spatially overlapped within the crystal material, the propagation direction within the crystal material needs to be the same. In order to diffract light of different wavelengths in the same direction, the two input lights entering from outside the device can be made to enter from different directions.

[0065] Such an embodiment is shown in FIG. 5. In the figure, a light ray 201 of input light of wavelength λ1, a light ray 205 of input light of wavelength λ2, a light ray 202 in a crystal material of wavelength λ3 (or a light ray of a harmonic of wavelength λ1 or wavelength λ2 in a crystal material) of a sum frequency or difference frequency generated by input light of wavelength λ1 and input light of wavelength λ2, a light ray 203 of input light of wavelength λ1 output from the device, a light ray 204 of a harmonic of wavelength λ3 output from the device, and a light ray 206 of input light of wavelength λ2 output from the device are shown. Compared to the case of direct second harmonic generation shown in FIG. 2, input light 205 of a different wavelength is incident on the same point on the device and propagates as light ray 202 in the same direction in the nonlinear optical crystal material. By setting the propagation direction at this time to a direction that satisfies the phase matching condition of sum frequency mixing, it is possible to generate sum frequency light of a new wavelength. For this purpose, it is sufficient to make the input light of λ1 and λ2 incident from different angles in advance, taking into consideration the difference in diffraction angle.

[0066] In the third example of the nonlinear wavelength conversion device, the nonlinear wavelength conversion device is a nonlinear optical crystal material 101 having a first optical surface S1 and a second optical surface S2 facing each other, and has a first diffraction structure 102 that diffracts light incident on the nonlinear optical crystal material 101 from the outside on at least one of the first optical surfaces S1 of the nonlinear optical crystal material 101, and input light 201 (205) of different wavelengths λ1 and λ2 incident from the outside of the nonlinear optical crystal material 101 at different angles is diffracted by the first diffraction structure 102 in the inside of the nonlinear optical crystal material 101 in approximately the same direction that forms an angle θ with the normal 104 of the first optical surface S1 (light ray 202), and the angle θ is an angle that satisfies the phase matching condition for generating light of wavelength λ3 of sum frequency or difference frequency from the input light of different wavelengths λ1 and λ2. Note that, in this example, approximately the same direction means that the angle (acute angle) formed by the two directions is 1 degree or less.

[0067] In the fourth example of the nonlinear wavelength conversion device, the nonlinear optical crystal material 101 also has a second diffraction structure 103 that diffracts incident light on a second optical surface S2 opposite to the first optical surface S1 where the input light 201 (205) of different wavelengths λ1 and λ2 is incident, and the second diffraction structure 103 diffracts the input light 201 (205) of different wavelengths λ1 and λ2, the light of wavelength λ3 which is the sum or difference frequency generated from the input light 201 (205) of different wavelengths, or all of these light (203, 204, 206) to the outside of the nonlinear optical crystal material 101.

[0068] The nonlinear wavelength conversion device of the fifth example is a nonlinear wavelength conversion device of any one of the first to fourth examples, in which an incident angle of a light beam propagating in a direction satisfying a phase matching condition at an interface between the nonlinear optical crystal material 101 and air (or a protective film, a medium material) from the inside of the nonlinear optical crystal material 101 to the interface is θ int , where n is the refractive index of the nonlinear optical crystal material, sinθ int >1 / n, and the condition for total internal reflection within the nonlinear optical crystal material 101 is satisfied.

[0069] A sixth example of a nonlinear wavelength conversion device is any one of the first to fifth examples of the nonlinear wavelength conversion device, in which the nonlinear optical crystal material is KBe2BO3F2 (KBBF), RbBe2BO3F2 (RBBF), or (NH4)Be2BO3F2 (ABBF).

[0070] A seventh example of a nonlinear wavelength conversion device is a nonlinear wavelength conversion device according to any one of the first to sixth examples, in which the first diffractive structure (and / or the second diffractive structure) is a grating.

[0071] The nonlinear wavelength conversion device of the eighth example is a nonlinear wavelength conversion device of any one of the first to sixth examples, in which the first diffraction structure (and / or the second diffraction structure) is a grating formed by etching using a pattern created by lithography. After a resist is applied onto the optical surface of the nonlinear optical crystal material and the resist is patterned to have a stripe opening, the nonlinear optical crystal material within the opening can be etched. By this etching, a grating having irregularities is formed on the surface of the nonlinear optical crystal material.

[0072] A ninth example of a nonlinear wavelength conversion device is a grating formed by forming a dielectric film on a first optical surface of a nonlinear optical crystal material by vapor deposition or sputtering, and then etching the dielectric film by a pattern created by lithography, in the nonlinear wavelength conversion device of any one of the first to sixth examples. The grating of the dielectric film can have a stripe shape.

[0073] A nonlinear wavelength conversion device of a tenth embodiment is the same as the nonlinear wavelength conversion device of the ninth embodiment, except that the material of the dielectric film is SiO2, Al2O3, CaF2, MgF2, or LaF3.

[0074] The nonlinear wavelength conversion device of the eleventh example is a nonlinear optical crystal material having cleavage in any one of the nonlinear wavelength conversion devices of the first to tenth examples. When the nonlinear optical crystal material is a KBBF crystal or an RBBF crystal, each crystal has an a-axis, a b-axis, and a c-axis, and has a cleavage plane perpendicular to the c-axis. The cleavage plane becomes the first optical surface S1 and the second optical surface S2. When forming a diffraction structure (diffraction grating), an example is considered in which the longitudinal direction of the stripes of the diffraction grating is set to the a-axis, and the pitch direction of the stripes is set to the b-axis. An example is considered in which the longitudinal direction of the stripes of the diffraction grating is set to the b-axis, and the pitch direction of the stripes is set to the a-axis. An example is considered in which the longitudinal direction of the stripes of the diffraction grating is set to a direction that forms a predetermined angle (for example, 0 degrees to 30 degrees) with the a-axis or b-axis, and the pitch direction of the stripes is set to a direction perpendicular to the longitudinal direction of the stripes.

[0075] The laser light generating device can include any one of the nonlinear wavelength conversion devices of the first to eleventh examples described above, and a mechanism for adjusting the angle of incidence of input light to the nonlinear wavelength conversion device. Many mechanisms, such as an acousto-optical element, can be listed as mechanisms for changing the traveling direction of the laser light and the angle of incidence to the device.

[0076] The laser light generating device is capable of emitting laser light having a wavelength shorter than 200 nm.

[0077] As described above, in the nonlinear wavelength conversion device according to the embodiment, a grating is formed on a polishable surface or a cleaved surface of a nonlinear optical crystal, and input light diffracted by the grating is propagated inside the nonlinear optical crystal at a phase matching angle. In a nonlinear wavelength conversion device using a nonlinear optical crystal material that is practically impossible to polish on any surface other than a specific surface, a prism coupling device is not used, and input light is made incident at a phase matching angle larger than the critical angle of total internal reflection to generate harmonic or sum frequency light.

[0078] In the present invention, it is desirable to form the grating on the nonlinear optical crystal material so that the lattice vector faces the same direction as the projection of the desired phase matching direction onto the optical surface. Even if the phase matching direction is a uniaxial crystal, depending on the symmetry of the crystal, the effective nonlinearity may differ depending on the azimuth angle even at the same zenith angle, and the wavelength conversion efficiency may depend on the azimuth angle. Therefore, it is desirable to form the grating so that the diffracted light propagates in the crystal in the phase matching direction taking into account this azimuth angle.

[0079] In the above embodiment, the case where a grating is used has been described, but the material formed on the surface of the nonlinear optical crystal material is not limited to a grating, and it goes without saying that the same effect can be obtained as long as the material has a diffraction function that diffracts incident light at a predetermined angle. For example, by forming a two-dimensional lattice-shaped diffraction structure, diffracted light can be generated in multiple positions and directions. In addition, by forming a hologram, it is possible to guide a light beam of a desired shape to a desired location. [Explanation of symbols]

[0080] 1...incident prism, 2...intermediate material, 3...nonlinear optical crystal material, 4...exit prism, 101...nonlinear optical crystal material, 102...incident grating (first diffraction structure), 103...exit grating (second diffraction structure), 104...normal, 201...light ray, 202...light ray, 203...light ray, 204...light ray, 205...light ray, 206...light ray, 301...external space, 302...crystal material, 303...grating, 304...normal, 401...light ray, 402...light ray, 501...equal phase surface, 502...equal phase surface, 503...pitch.

Claims

1. 1. A nonlinear wavelength conversion device having a nonlinear optical crystal material having opposing first and second optical surfaces, a first diffractive structure for diffracting light incident from outside the nonlinear optical crystal material on at least one of the first optical surfaces of the nonlinear optical crystal material; The wavelength λ of the nonlinear optical crystal material is incident from the outside. １ is diffracted by the first diffractive structure in the nonlinear optical crystal material in a direction that forms an angle θ with the normal to the first optical surface, and the direction of the angle θ is a wavelength λ １ The wavelength λ is the second harmonic of the input light. Ｈ A nonlinear wavelength conversion device characterized in that the direction satisfies the phase matching condition for generating light of the wavelength of 100 nm.

2. a second diffractive structure on the second optical surface opposite to the first optical surface on which the input light is incident in the nonlinear optical crystal material; 2. The nonlinear wavelength conversion device according to claim 1, wherein the second diffractive structure diffracts the input light, the second harmonic wave, or both, out of the nonlinear optical crystal material.

3. 1. A nonlinear wavelength conversion device having a nonlinear optical crystal material having opposing first and second optical surfaces, a first diffractive structure for diffracting light incident on the nonlinear optical crystal material from the outside, on at least one of the first optical surfaces of the nonlinear optical crystal material; The nonlinear optical crystal material is provided with different wavelengths λ １ and wavelength λ ２ The input light is diffracted by the first diffractive structure in the nonlinear optical crystal material in substantially the same direction making an angle θ with the normal to the first optical surface, and the angle θ is set to be equal to or smaller than the wavelengths λ １ and wavelength λ ２ The wavelength λ of the sum or difference frequency from the input light ３ A nonlinear wavelength conversion device characterized in that the angle satisfies the phase matching condition for generating light of

4. The nonlinear optical crystal material has different wavelengths λ １ and wavelength λ ２ a second diffractive structure for diffracting the incident light also on the second optical surface opposite to the first optical surface on which the input light is incident, The second diffractive structure is configured to have a different wavelength λ １ and wavelength λ ２ The wavelength λ of the sum or difference frequency generated from the input light of different wavelengths ３ 4. The nonlinear wavelength conversion device according to claim 3, wherein the nonlinear optical crystal material diffracts at least one of the light components.

5. The angle of incidence of a light ray propagating in a direction satisfying the phase matching condition at the interface between the nonlinear optical crystal material and air from inside the nonlinear optical crystal material to the interface is defined as θ ｉｎｔ , where n is the refractive index of the nonlinear optical crystal material, sinθ ｉｎｔ >1 / n 2. The nonlinear wavelength conversion device according to claim 1, wherein the relationship: ##EQU1## is established, and the condition for total internal reflection within the nonlinear optical crystal material is satisfied.

6. The nonlinear optical crystal material is KBe ２ B.O. ３ F ２ (KBBF), RbBe ２ B.O. ３ F ２ (RBBF), or (NH ４ ) Be ２ B.O. ３ F ２ 2. The nonlinear wavelength conversion device according to claim 1, wherein the nonlinear wavelength conversion device is an ABBF.

7. 2. The nonlinear wavelength conversion device according to claim 1, wherein the first diffractive structure is a grating.

8. 2. The nonlinear wavelength conversion device according to claim 1, wherein the first diffractive structure is a grating formed by etching according to a pattern created by lithography.

9. 2. The nonlinear wavelength conversion device according to claim 1, wherein the first diffraction structure is a grating formed by forming a dielectric film on the first optical surface of the nonlinear optical crystal material by deposition or sputtering, and then etching the dielectric film using a pattern created by lithography.

10. The material of the dielectric film is SiO ２ , Al ２ O ３ , CaF ２ , MgF ２ , or LaF ３ 10. The nonlinear wavelength conversion device according to claim 9,

11. 2. The nonlinear wavelength conversion device according to claim 1, wherein the nonlinear optical crystal material is a crystal having cleavage properties.

12. A nonlinear wavelength conversion device according to any one of claims 1 to 11, a mechanism for adjusting an incident angle of an input light to the nonlinear wavelength conversion device; A laser light generating device comprising:

13. 13. The laser light generating device according to claim 12, which emits a laser light having a wavelength shorter than 200 nm.

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