Second harmonic wave generating device, light-emitting device, and method for designing second harmonic wave generating device

Abstract

To provide an SHG (second harmonic generation) capable of emitting a laser beam having a wave length shorter than that of 215nm.SOLUTION: The SHG device 1 has an optical crystal layer made of a nonlinear optical crystal, a lower DBR layer disposed on the lower surface of the optical crystal layer, and an upper DBR layer disposed on the upper surface of the optical crystal layer. The lower DBR layer has a double structure of a lower first 1DBR layer disposed on the optical crystalline layer side and a lower second 1DBR layer disposed under the lower first 2DBR layer. The second harmonic reflectivity of the lower 1DBR layer is higher than the second harmonic reflectivity of the lower 2DBR layer. The lower 2DBR layer has a reflectivity for the fundamental wave higher than that of the lower 1DBR layer. The second harmonic generated in the optical crystal layer is emitted through the upper DBR layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a device that emits second harmonic laser light. [Background technology]

[0002] Deep ultraviolet (DUV) light is effective in inactivating viruses and disinfecting bacteria, and has been attracting attention in recent years. In addition, deep ultraviolet laser light can be focused to an extremely small spot diameter, making it useful for ultra-fine lithography, wafer inspection, etc. In particular, an all-solid-state ultra-compact coherent deep ultraviolet light source is desired.

[0003] Non-Patent Document 1 discloses a second harmonic generation (SHG) device in which distributed Bragg reflector (DBR) layers are arranged on the top and bottom surfaces of a nonlinear optical crystal. In this device, the top DBR layer is designed to have a high reflectance for the fundamental wave and a low reflectance for the second harmonic, while the bottom DBR layer has a high reflectance for both the fundamental wave and the SH wave. Both the top and bottom DBR layers have a structure in which a predetermined number of pairs of HfO2 and SiO2 layers are stacked.

[0004] When a fundamental wave is incident on the top surface of the device described in Non-Patent Document 1, the fundamental wave is repeatedly reflected by the top DBR layer and the bottom DBR layer and repeatedly passes through the nonlinear optical crystal. Each time the fundamental wave passes through the nonlinear optical crystal, an SH wave is generated. The SH wave is reflected by the bottom DBR layer, passes through the top DBR layer, and is emitted upward. The spacing between the top and bottom DBR layers and the thickness of the nonlinear optical crystal are designed so that the phases of the SH waves generated from the nonlinear optical crystal are matched. Therefore, coherent SH wave laser light is emitted from the top DBR layer. With this configuration, the device described in Non-Patent Document 1 can emit coherent SH waves with a wavelength of 234 nm. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] "DUV coherent light emission from ultracompact microcavity wavelength conversion device", TOMOAKI NAMBU etc., Vol. 30, No.11, 23 May 2022, Optics Express, pp. 18628-18637 Summary of the Invention [Problem to be solved by the invention]

[0006] Non-Patent Document 1 discloses an SHG device that emits deep ultraviolet laser light with a wavelength of 234 nm, but an SHG device that emits deep ultraviolet laser light with an even shorter wavelength is desired. For example, if vacuum ultraviolet laser light with a wavelength of 200 nm or less could be emitted, the spot diameter of the laser light could be focused to an even smaller size than that of laser light with a wavelength of 234 nm, thereby improving the processing accuracy of ultrafine lithography. However, the DBR used in Non-Patent Document 1, which is made by stacking a predetermined number of pairs of HfO2 layers and SiO2 layers, has difficulty efficiently emitting laser light with wavelengths shorter than 215 nm because the HfO2 layers absorb light with wavelengths shorter than 215 nm.

[0007] It is known that the efficiency of converting a fundamental wave into an SH wave increases with the peak intensity of the fundamental wave, and therefore, it is desirable to use an ultrashort pulse laser with a high peak intensity as the fundamental wave. Furthermore, when considering the use of SH waves for laser processing and other applications, an SHG device that emits SH waves with a high short-pulse output is desirable. However, the SHG device described in Non-Patent Document 1 has a structure optimized for continuous wave excitation. According to experiments conducted by the inventors, when a short-pulse laser is used as the fundamental wave, it is not possible to enhance the fundamental wave to a high intensity within the resonator, and therefore, it was not possible to actually measure and evaluate the normalized wavelength conversion efficiency.

[0008] A first object of the present invention is to provide an SHG device capable of emitting laser light with a wavelength shorter than 215 nm.

[0009] A second object of the present invention is to provide an SHG device that can input a short-pulse laser beam as a fundamental wave and efficiently emit a short-pulse SH wave 102 laser beam. [Means for solving the problem]

[0010] To achieve the first object, a first aspect of the present invention provides a second-harmonic generating device having an optical crystal layer made of a nonlinear optical crystal, a lower DBR layer disposed on the lower surface of the optical crystal layer, and an upper DBR layer disposed on the upper surface of the optical crystal layer. The lower DBR layer and the upper DBR layer each have a structure in which multiple pairs of two dielectric layers with different refractive indices are laminated. The optical crystal layer generates a second harmonic when a fundamental wave passes through it. The upper DBR layer has a reflectance for the fundamental wave equal to or greater than a predetermined reflectance and a reflectance for the second harmonic lower than the predetermined reflectance. The lower DBR layer includes a lower first DBR layer disposed on the optical crystal layer side and a lower second DBR layer disposed below the lower first DBR layer. The reflectance for the second harmonic of the lower first DBR layer is higher than the reflectance for the second harmonic of the lower second DBR layer. The reflectance of the lower second DBR layer for the fundamental wave is greater than the reflectance of the lower first DBR layer for the fundamental wave. The second harmonic wave generated in the optical crystal layer passes through the upper DBR layer and is emitted.

[0011] To achieve the second object, according to a second aspect of the present invention, a second harmonic generating device includes an optical crystal layer made of a nonlinear optical crystal, a lower DBR layer disposed on the lower surface of the optical crystal layer, and an upper DBR layer disposed on the upper surface of the optical crystal layer. The lower DBR layer and the upper DBR layer each have a structure in which multiple pairs of dielectric layers with different refractive indices are stacked. A fundamental wave is irradiated from the outside onto the upper surface of the upper DBR layer. The irradiated fundamental wave is repeatedly reflected between the lower DBR layer and the upper DBR layer and repeatedly passes through the optical crystal layer. The second harmonic wave generated when the fundamental wave passes through the optical crystal layer is transmitted through the lower DBR layer or the upper DBR layer and emitted to the outside. The fundamental wave is a pulse with a predetermined time width. The reflectivity of the upper DBR layer is preset so that the fundamental wave irradiated from the outside and transmitted through the upper DBR layer is highly amplified inside the resonator within the time pulse width of the fundamental wave. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide an SHG device capable of emitting laser light with a wavelength shorter than 215 nm.

[0013] Furthermore, according to the present invention, in an SHG device in which DBR layers are arranged on both the top and bottom surfaces of a nonlinear optical crystal, the number of stacked DBR layers can be optimized, thereby improving the output efficiency of the SH wave 102. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a block diagram showing the configuration of a light emitting device 100 according to a first embodiment of the present invention. [Figure 2] FIG. 2 is an explanatory diagram showing the layer structure of the SHG device 1 of the light emitting device 100 of the first embodiment. [Figure 3] FIG. 2 is an explanatory diagram showing an example of the film thickness of the SHG device 1 of the light emitting device 100 of the first embodiment. [Figure 4] 6(a) and 6(b) are graphs showing the reflection characteristics (theoretical values) of the upper DBR layer 11 of the SHG device 1 of the first embodiment. [Figure 5] 1A is a graph showing the reflection characteristics (theoretical values) of the lower second DBR layer 13 of the SHG device 1 of the first embodiment, and FIG. 1B is a graph showing the reflection characteristics (theoretical values) of the lower first DBR layer 12 of the SHG device 1 of the first embodiment. [Figure 6] 3 is a flowchart showing a design procedure for the SHG device 1 of the first embodiment. [Figure 7] 3 is a flowchart showing the manufacturing process of the SHG device 1 of the first embodiment. [Figure 8] 3(a) to 3(g) are explanatory views showing the manufacturing process of the SHG device 1 of the first embodiment. [Figure 9] 3 is a graph showing the reflection characteristics (actually measured values) of the upper DBR layer 11, the lower first DBR layer 12, and the lower second DBR layer 13 of the SHG device 1 of the first embodiment. [Figure 10] (a) A graph showing the spectrum of the fundamental wave incident on the light-emitting device 100 of embodiment 1 and the emitted SH wave 102, and (b) a graph showing the relationship between the power of the fundamental wave incident on the light-emitting device 100 of embodiment 1 and the power of the emitted SH wave 102. [Figure 11] FIG. 10(a) is a perspective view of an SHG device 210 of a light emitting device according to a second embodiment, and FIG. 10(b) is an explanatory diagram showing a cross-sectional structure. [Figure 12] 10 is a flowchart showing a design procedure for an SHG device 210 according to the second embodiment. [Figure 13] FIG. 10 is an explanatory diagram showing regions I, II, and III of an SHG device 210 according to a second embodiment. [Figure 14] FIG. 10 is an explanatory diagram showing a fundamental wave incident on an SHG device 210 according to the second embodiment and the repeated reflection of the incident fundamental wave. [Figure 15] 10 is a diagram for explaining the spectrum of the fundamental wave incident on the SHG device 210 of the second embodiment and the longitudinal mode (narrow band spectrum). FIG. [Figure 16] 10 is a graph showing the relationship between the number of pairs in the incident-side DBR 111 of the SHG device 210 of the second embodiment, the intensity of the fundamental wave 101 in the SHG layer 110, and the reflectance of the fundamental wave 101 in the incident-side DBR 111. [Figure 17]10 is a graph showing the time evolution of the intensity of the fundamental wave 101 in the SHG layer 110 of the SHG device 210 of the second embodiment. [Figure 18] 10 is a graph showing the spectrum of converted light composed of an SH wave 102 and an SF wave, which is generated from each longitudinal mode of the fundamental wave 101 of the SHG device 210 of the second embodiment. [Figure 19] 10(a) to 10(h) are explanatory diagrams showing the manufacturing process of the SHG device 210 of the second embodiment. [Figure 20] 10 shows transmittance spectra of an incident-side DBR 111 and an exit-side DBR 112 of an SHG device 210 according to the second embodiment. [Figure 21] 10 is a graph showing the relationship between the wavelength and intensity of the SH waves generated in the SHG device 210 of the second embodiment. [Figure 22] 10 is a graph showing the relationship between the power of the fundamental wave 101 and the intensity of the SH wave 102 in the SHG device 210 of the second embodiment. [Figure 23] 10 is a graph showing the relationship between the center wavelength of the fundamental wave 101 in the SHG device 210 of the second embodiment and the SH wave intensity corresponding to that center wavelength. [Figure 24] 10 is a block diagram showing the configuration of a light emitting device 100 according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0016] <<<Embodiment 1>>> A light emitting device 100 according to the first embodiment will be described with reference to FIG.

[0017] The light emitting device 100 is configured to include a light source 2 that emits a fundamental wave 101, and a second harmonic generation (hereinafter referred to as SHG) device 1 that receives the fundamental wave 101 and emits a second harmonic wave 102 (hereinafter referred to as SH wave 102).

[0018] In the light emitting device 100 of the first embodiment, a light source 2 causes a fundamental wave 101 to enter the upper surface of an SHG device 1, and the SHG device 1 emits the generated SH wave 102 from the upper surface. A condenser lens 3 and a dichroic mirror 4 are disposed between the light source 2 and the SHG device 1. The condenser lens 3 condenses the fundamental wave 101 emitted from the light source 2 onto or near the upper surface of the SHG device 1. The dichroic mirror 4 is configured to transmit the fundamental wave 101 and reflect the SH wave 102. The dichroic mirror 4 reflects and deflects the SH wave 102 emitted from the SHG device 1, separating it from the fundamental wave 101. In this embodiment, for convenience, the side of the SHG device 1 closer to the light source 2 is called the top, and the side farther from the light source 2 is called the bottom. However, the orientation of the light emitting device 100 when in use is not limited to the orientation with the light source 2 at the top, and the light emitting device 100 can be placed in any orientation.

[0019] The SHG device 1 of this embodiment can emit SH waves 102 in the wavelength range of 130 to 280 nm, known as deep ultraviolet rays, and can particularly emit deep ultraviolet rays of 214 nm or less and SH waves 102 with wavelengths of 200 nm or less, known as vacuum ultraviolet rays. As an example, the fundamental wave 101 has a wavelength of less than 430 nm. Here, an SHG device 1 that emits SH waves 102 with a wavelength of 199.3 nm, which is in the wavelength range of vacuum ultraviolet rays, from a fundamental wave 101 with a wavelength of 398.5 nm, will be described.

[0020] The SHG device 1 of this embodiment is not limited to one that emits deep ultraviolet light as the SH wave 102. By selecting the wavelength of the fundamental wave 101, it is of course possible to emit ultraviolet light or visible light having a wavelength longer than 280 nm as the SH wave 102.

[0021] Specifically, the SHG device 1 has an optical crystal layer (hereinafter referred to as the SHG layer) 10 made of a nonlinear optical crystal, lower DBR layers 12 and 13 arranged on the lower surface of the SHG layer 10, and an upper DBR layer 11 arranged on the upper surface of the SHG layer 10.

[0022] The SHG layer 10 is made of a nonlinear optical crystal that generates SH waves 102 when a fundamental wave 101 passes through it. The nonlinear optical crystal that makes up the SHG layer 10 is positioned taking into consideration the direction of the electric field of the fundamental wave 101 and the crystal axis so that the maximum nonlinear optical constant can be utilized.

[0023] Here, SrB4O7 (hereinafter referred to as SBO) with an absorption edge of approximately 130 nm and a nonlinear optical constant d = 3.5 pm / V is used as the nonlinear optical crystal constituting the optical crystal 10 to generate SH waves 102 with a wavelength of 199.3 nm. Note that the nonlinear optical crystal is not limited to SBO, and any crystal with an absorption edge shorter than the wavelength desired to be emitted as the SH waves 102 can be selected and used. Note that a crystal with a large nonlinear optical constant d is preferable to improve conversion efficiency. For example, to generate SH waves 102 with a wavelength of 199.3 nm, optical crystals such as SiO2 (absorption edge 150 nm, d = 0.30 pm / V), LaBGeO5 (absorption edge 195 nm, d = 0.57 pm / V), and KBe2BO3F2 (absorption edge 155 nm, d = 0.76 pm / V) can be used. Depending on the wavelength of the SH waves 102 to be generated, CsLiBO 10 It is also possible to use AlN (absorption edge 237 nm, d=1.01 pm / V), AlN (absorption edge 210 nm, d=6.3 pm / V), or β-BaB2O4 (absorption edge 205 nm, d=1.85 pm / V).

[0024] The lower DBR layers 12 and 13 and the upper DBR layer 11 each have a structure in which multiple pairs of two dielectric layers with different refractive indices are stacked, and they reflect light incident in the film thickness direction by distributed Bragg reflection (DBR).

[0025] The upper DBR layer 11 is designed so that the reflectance of the fundamental wave 101 is equal to or greater than a predetermined reflectance, and the reflectance of the SH wave 102 is smaller than the predetermined reflectance.

[0026] In the first embodiment, the lower DBR layers 12, 13 include a lower first DBR layer 12 arranged closer to the SHG layer 10 and a lower second DBR layer 13 arranged below the lower first DBR layer 12. The lower first DBR layer 12 is set so that the reflectance of the SH wave 102 is greater than the reflectance of the SH wave 102 of the lower second DBR layer 13, and the lower second DBR layer 13 is set so that the reflectance of the fundamental wave 101 is greater than the reflectance of the fundamental wave of the lower first DBR layer 12.

[0027] As a result, the fundamental wave 101 incident on the top surface of the upper DBR layer 11 passes from top to bottom through the SHG layer 10, then passes from top to bottom through the lower first DBR layer 12, reaches the lower second DBR layer 13 and is reflected there. The fundamental wave 101 reflected by the lower second DBR layer 13 passes from bottom to top through the lower first DBR layer 12, then passes from bottom to top through the SHG layer 10, reaches the upper DBR layer 11, is reflected there, and then travels downward again. As a result, the upper DBR layer 11 and the lower second DBR layer 13 form a microresonator that repeatedly reflects the fundamental wave 101. The fundamental wave 101 traveling back and forth within the resonator repeatedly passes through the SHG layer 10. Each time the fundamental wave 101 passes through the SHG layer 10, the SHG layer 10 generates an SH wave 102.

[0028] Of the SH waves 102 generated when the fundamental wave 101 passes through the SHG layer 10, the SH waves 102 that travel upward pass through the upper DBR layer 11 and are emitted upward. Of the SH waves 102 generated when the fundamental wave 101 passes through the SHG layer 10, the SH waves 102 that travel downward are reflected by the lower first DBR layer 12 and travel upward, pass through the upper DBR layer of the SHG layer 10, and are emitted upward. As a result, of the second harmonic waves 102 generated in the SHG layer 10, both the light traveling upward and the light traveling downward pass through the upper DBR layer 11 and are emitted from the top surface of the SHG device 1.

[0029] In order to efficiently emit the SH wave 102, the fundamental wave 101 and the SH wave 102 are quasi-phase matched. The conditions for satisfying the phase matching will be described later.

[0030] As described above, the SHG device 1 of the first embodiment has the lower DBR layers 12 and 13 divided into the lower first DBR layer 12 and the lower second DBR layer 13, with the lower first DBR layer 12 reflecting the SH wave 102 and the lower second DBR layer 13 reflecting the fundamental wave 101. This eliminates the need to design a common DBR layer to reflect both the SH wave 102 and the fundamental wave 101. Even the SH wave 102, which is a deep ultraviolet light with a short wavelength (wavelength of 234 nm or less) that is easily absorbed by DBR layers, can be reflected by the lower first DBR layer 12, which is designed to have high reflectivity at the wavelength of the deep ultraviolet SH wave 102. On the other hand, the fundamental wave 101 is reflected by the lower second DBR layer 13, which is designed to have high reflectivity at the wavelength of the fundamental wave 101. Therefore, since the lower second DBR layer 13 does not need to take into consideration the reflectivity of the SH wave 102 of deep ultraviolet light, a material having the property of absorbing the SH wave 102 of deep ultraviolet light with a wavelength of 234 nm or less can be used as the material for the lower second DBR layer 13, and the lower second DBR layer 13 can be designed to reflect the fundamental wave 101 over a wide band and with high reflectivity.

[0031] In this embodiment, the upper DBR layer 11 and the lower first DBR layer 12 have a structure in which a predetermined number of pairs (hereinafter simply referred to as pairs) of MgO layers and SiO layers are stacked. As an example, as shown in Fig. 3, the upper DBR layer 11 has a structure in which 9 to 14 pairs of 57.8 nm-thick MgO layers and 68.0 nm-thick SiO layers are stacked. The lower first DBR layer 12 has a structure in which 9 to 15 pairs of 25.7 nm-thick MgO layers and 33.0 nm-thick SiO layers are stacked.

[0032] Furthermore, the upper DBR layer 11, the lower first DBR layer 12, and the lower second DBR layer 13 can each be formed by stacking multiple pairs of two layers selected from, for example, an MgO layer, an SiO2 layer, a CaF2 layer, an HfO2 layer, a CaF2 layer, an Al2O3 layer, an AlF3 layer, a LaF3 layer, and an MgF2 layer.

[0033] 4(a) and 4(b), the upper DBR layer 11 can reflect the fundamental wave 101 having a wavelength of 398.5 nm with a reflectance of 60% or more (theoretical value) and transmit the SH wave 102 having a wavelength of 199.3 nm with a transmittance of 50% or more (theoretical value). Also, the lower first DBR 12 can reflect the SH wave 102 having a wavelength of 199.3 nm with a reflectance of 95% or more (theoretical value) and transmit the fundamental wave 101 having a wavelength of 398.5 nm with a transmittance of 90% or more (theoretical value), as shown in FIG.

[0034] On the other hand, the lower second DBR layer 13 has a structure in which a predetermined number of pairs of HfO2 layers and SiO2 layers are stacked. As an example, as shown in Figure 3, the lower second DBR layer 13 has a structure in which 15 pairs of HfO2 layers with a thickness of 49.1 nm and SiO2 layers with a thickness of 68.0 nm are stacked.

[0035] As a result, the lower second DBR layer 13 can reflect the fundamental wave 101 with a wavelength of 398.5 nm with a reflectance of 98% or more, as shown in FIG. 5(a).

[0036] Furthermore, an SH-wave antireflection layer 14 made of a dielectric layer of a predetermined thickness is disposed between the upper DBR layer 11 and the SHG layer 10 in order to reduce the reflectance of the SH-wave 102 from the upper DBR layer 11. Here, the SH-wave antireflection layer 14 is made of the same SiO2 layer as one of the layers constituting the upper DBR layer 11, and is designed to have a thickness that minimizes the reflectance of the upper DBR layer 11 for the SH-wave 102. The upper DBR layer 11 is designed to maximize the reflectance of the fundamental wave.

[0037] By inserting a phase adjustment layer 15 between the SHG layer 10 and the lower first DBR layer 12 and adjusting the thickness of the phase adjustment layer 15, it is possible to control the phase shift at the time of reflection by the lower first DBR layer 12 so that the SH wave 102 generated downward in the SHG layer 10 and reflected by the lower first DBR layer 12 and the SH wave 102 generated upward in the SHG layer 10 are in phase. As a result, the SH wave 102 reflected by the lower first DBR layer 12 and the SH wave 102 generated upward in the SHG layer 10 constructively interact with each other and are output from the upper DBR layer 11, and the conversion efficiency η from the fundamental wave 101 to the SH wave 102 is increased. SH can be increased.

[0038] Furthermore, a phase adjustment layer 15 made of a dielectric is disposed between the SHG layer 10 and the lower first DBR layer 12. The phase adjustment layer 15 is made of a material that is transparent to the SH wave 102. For example, the phase adjustment layer 15 can be made of two layers made of the same material as the pair of two dielectric layers that make up the lower first DBR layer 12. That is, if the lower first DBR layer 12 is made of a pair of an MgO layer and an SiO2 layer, the phase adjustment layer 15 can also be made of a pair of an MgO layer and an SiO2 layer. Of course, the phase adjustment layer 15 can also be made of a single layer made of a material that is transparent to the SH wave 102.

[0039] The support substrate 16 may be made of any material as long as it can support the laminate of the SHG layer 10, upper DBR layer 11, lower first DBR layer 12, lower second DBR layer 13, SH-wave antireflection layer 14, and phase adjustment layer 15. Here, a sapphire substrate is used as the support substrate 16.

[0040] Furthermore, adhesive layer 17 may be made of any material that can bond support substrate 16 and lower second DBR layer 13. Here, epoxy resin is used as an example. Alternatively, without using adhesive layer 17, support substrate 16 and lower second DBR layer 13 may be directly bonded by surface activation, hydrophilic bonding, or the like.

[0041] <<Design method>> Next, a method for designing the SHG device 1 will be described with reference to the flow chart of FIG.

[0042] <Step S201> First, the thickness of the SHG layer 10 is designed.

[0043] As described above, the SHG layer 10 is made of the nonlinear crystal SBO (SrBO). The thickness of the SHG layer 10 is set to the coherence length Lc calculated from equation (1). This allows the SHG layer 10 to achieve maximum wavelength conversion efficiency.

[0044] The thickness of the SHG layer 10 may be set to a value obtained by multiplying the coherence length Lc by an odd natural number. By setting the thickness of the SHG layer 10 to an odd multiple of the coherence length Lc, the intensity of the generated SH waves can be increased.

[0045]

number

[0046] Specifically, the thickness of the SHG layer 10 is designed to be 921.1 nm.

[0047] <Step S202> Next, the thickness of each layer of the lower first DBR layer 12 is designed.

[0048] As described above, the lower first DBR layer 12 has a structure in which a predetermined number of pairs of MgO and SiO layers (hereinafter referred to as MgO / SiO) that do not absorb the SH wave 102 with a wavelength of 199.3 nm are stacked. However, since the refractive index difference between the MgO / SiO pairs and the fundamental wave 101 with a wavelength of 398.5 nm is small, it is necessary to stack a large number of MgO / SiO pairs in order to reflect the fundamental wave 101 with high reflectance. Furthermore, MgO / SiO, which is a layer of many MgO / SiO layers, has a narrow reflection band.

[0049] For this reason, in this embodiment, as described above, a structure is adopted in which the lower second DBR layer 13 is formed by stacking a predetermined number of pairs of HfO2 layers and SiO2 layers (hereinafter referred to as HfO2 / SiO2) under the lower first DBR layer 12. The lower second DBR layer 13, which is formed by stacking a predetermined number of HfO2 / SiO2 pairs, has a wide band and high reflectivity for the fundamental wave in the blue band. Therefore, by using a structure (called a hybrid structure) in which the lower second DBR layer is placed under the lower first DBR layer 12, both the fundamental wave 101 and the SH wave 102 can be reflected with high efficiency.

[0050] The thickness of each HfO2 / SiO2 layer in the lower second DBR layer 13 is set to λ p , the refractive index of each material for the fundamental wave is n p In this case, λ p / 4n p However, if the above reflectance can be obtained, the film thickness of each material may be changed independently.

[0051] Similarly, the thickness of each layer of MgO / SiO2 in the lower first DBR layer 12 is set to λ SH , the refractive index of each material for the fundamental wave and the second harmonic wave is n SH In this case, λ SH / 4n SH However, as long as the above reflectance can be obtained, the film thickness of each material may be changed independently.

[0052] Since the SHG device 1 is a reflective type in which the fundamental wave 101 enters from the top surface and the SH wave 102 exits from the top surface, the lower DBR layer including the lower first DBR layer 12, the lower second DBR layer 13, and the phase adjustment layer 15 is designed so that the reflectivity for the fundamental wave 101 and the SH wave 102 is very close to 1.

[0053] The design of the upper DBR layer will be explained in steps S203-1 and S203-2.

[0054] <Step S203-1> The period (total thickness of the pairs constituting the DBR layer) and duty ratio (thickness of the high refractive index layer / period) of the upper DBR layer 11 are designed. In this embodiment, the period of the upper DBR layer 11 refers to the thickness of one pair of MgO / SiO2.

[0055] <Step S203-2> Next, the number of pairs in the upper DBR layer 11 is designed.

[0056] As described above, the upper DBR layer 11 has a structure in which a predetermined number of MgO / SiO2 pairs are stacked. The structure of the upper DBR layer 11 is determined to maximize the SH wave power emitted from the device, depending on the structure of the lower DBR layer including the lower first DBR layer 12, the lower second DBR layer 13, and the phase adjustment layer 15.

[0057] In steps S203-1 and S203-2, for example, when the reflectance of the lower DBR structure is optimized to be 1, the ideal thicknesses of the MgO / SiO2 layers in the upper DBR layer 11 are 57.8 nm / 68.0 nm, and the number of stacked pairs is 14.

[0058] However, it is known that increasing the number of MgO / SiO2 layers in the upper DBR layer 11 causes the transmittance of the SH wave 102 to become lower than the theoretical value due to the influence of scattering, and therefore, in the embodiment, nine pairs were adopted.

[0059] <Step S204> Next, the thicknesses of the SH wave antireflection layer 14 and the phase adjustment layer 15 are designed.

[0060] The SH-wave antireflection layer 14 is made of an SiO2 layer. As described above, the upper DBR layer 11 is designed to maximize the reflectance of the fundamental wave. In this embodiment, the SH-wave antireflection layer 14 is disposed between the upper DBR layer 11 and the SHG layer 10, and the thickness of the SH-wave antireflection layer 14 is adjusted to minimize the reflectance of the SH wave 102 incident on the upper DBR layer 11 from the SHG layer 10.

[0061] As described above, the phase adjustment layer 15 is made of a MgO / SiO2 pair. The thickness of each MgO / SiO2 layer is designed so that the SH waves 102 reflected by the lower first DBR layer 12 and the SH waves 102 generated upward in the SHG layer 10 are in phase and reinforce each other, maximizing the intensity of the SH waves 102 emitted from the top surface of the SHG device 1.

[0062] <Step S205> Next, the conversion efficiency η of the entire SHG device 1 from the fundamental wave 101 to the SH wave 102 is calculated. SH The duty ratio, period, and number of stacked pairs of the upper DBR layer 11 are changed, and steps S203-2, S204, and S205 are repeated to obtain the conversion efficiency η SH Maximize.

[0063] By repeating steps S203 to S205, the conversion efficiency η SH The thickness of the upper DBR layer 11 and the number of stacked pairs that maximize the value are found.

[0064] <Step S206> The conversion efficiency η calculated in step S205 SH The thickness of the upper DBR layer 11 and the number of stacked pairs that maximize the thickness, the thickness of the SHG layer 10 determined in steps S201, S202, and S203, the thickness of each layer of the lower DBR layers 12 and 13 and the number of stacked pairs, the thickness of the SH-wave antireflection layer 14, and the thickness of the phase adjustment layer 15 are determined for the structure of the SHG device 1.

[0065] <<Manufacturing method>> A method for manufacturing the SHG device 1 designed by the above design method will be described with reference to the process flow of FIG. 7 and the structural diagrams of each process of FIGS. 8(a) to 8(g).

[0066] <Step S1> First, an SBO substrate that will become the SHG layer 10 is prepared as shown in FIG. 8(a).

[0067] The SBO substrate is made of an SBO crystal grown by the TSSG (top seeded solution growth) method, with a b-plane area of ​​10 × 15 mm. 2 Use pieces cut out so that they fit together.

[0068] <Step S2> Next, one of the ±b planes of the SBO crystal is mirror-polished with diamond slurry (Fig. 8(b)).

[0069] <Step S3> On the b-plane selected in step S2 of the polished SBO crystal, a SiO2 layer and an MgO layer are deposited in this order by sputtering as the phase adjustment layer 15. Subsequently, a designed number of MgO / SiO2 pairs that will become the lower first DBR layer 12 are deposited on the surface of the phase adjustment layer 15 (FIG. 8(c)).

[0070] Furthermore, on the surface of the lower first DBR layer 12, HfO2 / SiO2 films that will become the lower second DBR layer 13 are laminated in the designed number of pairs (FIG. 8(d)).

[0071] <Step S4> A sapphire substrate, which is the support substrate 16, is bonded to the surface of the lower second DBR layer 13 formed in step S3 using an epoxy resin as an adhesive layer 17 (FIG. 8(e)).

[0072] <Step S5> The b-face of the SBO crystal that was not selected in step S2 is mirror-polished with diamond slurry (FIG. 8(f)). This polishing ensures that the film thickness of the SBO crystal (SHG layer 10) matches the designed coherence length.

[0073] <Step S6> In step S5, a SiO2 film is formed by sputtering on the b-face of the polished SBO crystal, the face not selected in step S2, to form the SH-wave antireflection layer 14, and then a MgO / SiO2 film is further formed on the surface of the b-face by laminating the designed number of pairs to form the upper DBR layer. The film formation method may be, for example, electron beam (EB) evaporation or atomic layer deposition (ALD).

[0074] In this manner, the SHG device 1 of the first embodiment can be manufactured.

[0075] The transmittance of each of the upper DBR layer 11, the lower first DBR layer 12, and the lower second DBR layer 13 of the SHG device 1 manufactured by the above manufacturing method was measured using a sample formed on a quartz substrate with the same layer structure and under the same film-forming conditions as the above manufacturing method. The results are shown in the graph of FIG.

[0076] Furthermore, as shown in the spectrum in Figure 10(a), when a fundamental wave 101 with a peak wavelength of 398.5 nm was irradiated from a light source 2 onto the top surface of an SHG device 1 actually manufactured using the above manufacturing method, it was confirmed that an SH wave 102 with a peak wavelength of 199.3 nm, which is in the wavelength range of vacuum ultraviolet light, was emitted from the top surface of the SHG device 1.

[0077] Furthermore, when the ratio of the power of the SH wave 102 to the power of the fundamental wave 101 was calculated, the power of the SH wave 102 was proportional to the square of the power of the fundamental wave 101, as shown in Figure 10(b). This indicates that the SH wave 102 is generated by a second-order nonlinear optical process. Furthermore, the SHG conversion efficiency was 2.6% / W when the power of the fundamental wave 101 was 0.42 mW.

[0078] This makes it possible to provide an SHG device 1 that can emit laser light with a wavelength shorter than 215 nm with high efficiency.

[0079] <<<Embodiment 2>>> A second harmonic generation (SHG) device according to a second embodiment of the present invention and a method for designing the same will now be described.

[0080] The inventors designed and manufactured a second harmonic generation device (SHG device) as disclosed in Non-Patent Document 1, and conducted experiments to confirm its operation. This device has a structure optimized for continuous wave (CW) excitation, so it could not be excited efficiently with an ultrashort pulse laser. The inventors believe that the reason for this is that when an ultrashort pulse laser is incident on an SHG device, the pulse is turned off and the supply of the fundamental wave to the SHG device (irradiation time) ends before the SHG device reaches a high-intensity steady state.

[0081] Therefore, in this second embodiment, the inventors focus on the reflectivity of the upper DBR layer, which controls the amount of fundamental wave supplied to the SHG device. In this embodiment, the reflectivity of the upper DBR layer 11 is designed so that the intensity of the fundamental wave 101 is greatly increased inside the SHG device during the pulse-on time (pulse width time) of the fundamental wave 101. Specifically, by setting the reflectivity of the upper DBR layer 11 lower than when a continuous wave fundamental wave is used, the intensity of the fundamental wave 101 is greatly increased inside the SHG device during the pulse-on time.

[0082] On the other hand, in the case of the reflective SHG device described in the first embodiment, it is optimal for the upper DBR layer 11 to have a reflectance of 0 for the second harmonic wave 102. It is also optimal for the lower DBR layers 12 and 13 to have a reflectance of 1 for the fundamental wave 101 and the second harmonic wave 102. In the case of a transmissive SHG device that emits the second harmonic wave 102 from below, it is optimal for the upper DBR layer 11 to have a reflectance of 1 for the second harmonic wave 102, and it is optimal for the lower DBR layers 12 and 13 to have a reflectance of 1 for the fundamental wave 101 and a reflectance of 0 for the second harmonic wave 102.

[0083] The optimum reflectivity of the upper DBR layer 11 varies depending on the thickness of other layers such as the lower DBR layer.

[0084] Furthermore, since ultrashort pulse lasers have a wider spectral width than continuous waves, it is necessary to consider the generation of sum frequencies in addition to second harmonics.

[0085] Taking these into consideration, in the second embodiment, the SHG device is designed taking into account the reflectivity of the fundamental wave 101 of the incident DBR layer. This aims to maximize the normalized wavelength conversion efficiency of the SHG device when an ultrashort pulse laser is incident as the fundamental wave.

[0086] 11 , the SHG device of the second embodiment includes an optical crystal layer 110 made of a nonlinear optical crystal, an output-side DBR layer 112 disposed on the lower surface of the optical crystal layer 110, and an input-side DBR layer 111 disposed on the upper surface of the optical crystal layer 110. The output-side DBR layer 112 and the input-side DBR layer 111 each have a structure in which multiple pairs of layers, each pair having multiple dielectric layers with different refractive indices, are stacked. A phase adjustment layer 115 is disposed between the output-side DBR layer 112 and the input-side DBR layer 111.

[0087] In the second embodiment, the fundamental wave 101 is a pulse wave with a predetermined duration.

[0088] Fundamental wave 101 is incident on the upper surface of incident-side DBR layer 111. The incident fundamental wave 101 is repeatedly reflected between output-side DBR layer 112 and incident-side DBR layer 111. As a result, fundamental wave 101 passes through optical crystal layer 110 repeatedly.

[0089] The reflectivity of the incident-side DBR layer 111 is set in advance so that the fundamental wave 101 that is irradiated from outside and transmitted through the incident-side DBR layer 111 is amplified to a high intensity within the device within the time pulse width of the fundamental wave 101.

[0090] The fundamental wave 101 is supplied to the inside of the SHG device 210 for the time width of the pulse, and is gradually strengthened while being repeatedly reflected between the incident-side DBR layer 111 and the output-side DBR layer 112 .

[0091] In the second embodiment, the SHG device 210 is designed as follows so that the fundamental wave 101 is enhanced to a high intensity during the time width of one pulse (pulse-on time).

[0092] <Step S301> First, the time evolution of the electric field strength of fundamental wave 101 in optical crystal layer 110 is calculated for the time period during which the pulse of fundamental wave 101 is irradiated (pulse-on time).

[0093] Specifically, first, the time T required for the fundamental wave 101 to make one round trip between the output-side DBR layer 112 and the input-side DBR layer 111 is round The calculated time T round The number of times that fundamental wave 101 travels back and forth between output-side DBR layer 112 and input-side DBR layer 111 during pulse-on time τ is calculated by dividing pulse-on time τ of fundamental wave 101 by τ. The time evolution of the electric field strength of fundamental wave 101 in optical crystal layer 110 is calculated using the number of times calculated.

[0094] The time evolution of the electric field strength of the fundamental wave 101 is also calculated during the pulse-off period of the fundamental wave 101, and the time evolution of the electric field of the fundamental wave 101 is calculated for all the pulse-on and pulse-off periods.

[0095] <Step S302> Next, when incident-side DBR layer 111 has a predetermined reflectance, the electric field intensity of converted light emitted from optical crystal layer 110 by the electric field of a predetermined fundamental wave 101 is calculated based on the time evolution of the electric field intensity of fundamental wave 101 calculated in step S301. The electric field intensity of converted light is calculated while the reflectance of incident-side DBR layer 111 is changed to a plurality of reflectances within a predetermined range.

[0096] This device is excited by an ultrashort pulse laser with a wide wavelength range, so the electric field intensity is calculated using converted light that includes not only second harmonic 102 emitted from optical crystal layer 110 but also the sum frequency.

[0097] Specifically, the fundamental wave 101 is divided into a plurality of longitudinal modes (narrowband spectrum), and the second harmonic generated from each longitudinal mode is calculated. Also, the sum frequency generated from any two longitudinal modes is calculated.

[0098] <Step S303> From the calculated electric field intensity of the converted light, the reflectance of the incident-side DBR layer 111 at which the electric field intensity of the converted light is maximized is calculated.

[0099] This makes it possible to design an SHG device 210 that can obtain converted light (second harmonic 102 and sum frequency wave) with maximum electric field intensity when a pulsed laser beam is incident as fundamental wave 101.

[0100] The method for designing the SHG device 210 of the second embodiment will be described in more detail below.

[0101] FIG. 11(a) is a perspective view of the SHG device 210 as seen from the support substrate side, and FIG. 11(b) is a cross-sectional view of the SHG device 210 shown in FIG. 11(a).

[0102] A pulse with a center wavelength of 850 nm is used as the fundamental wave 101 to generate a second harmonic wave 102 (hereinafter referred to as SH wave 102) with a center wavelength of 425 nm. The optical crystal layer 110 (hereinafter referred to as SHG layer 110) is an a-plane GaN SHG layer with a length (thickness) as the coherence length. The incident-side DBR layer 111 (hereinafter referred to as incident-side DBR 111) is a distributed Bragg reflector (DBR) formed by stacking multiple pairs of ZrO2 and SiO2 layers. The phase adjustment layer 115 has a structure in which ZrO2 and SiO2 layers are stacked. The exit-side DBR layer 112 (hereinafter referred to as exit-side DBR 112) is composed of a DBR formed by stacking multiple pairs of TiO2 and SiO2 layers.

[0103] The fundamental wave 101 incident perpendicularly from above on the SHG device 210 is amplified in the SHG layer 110 by the incident-side and output-side DBRs 111 and 112 having reflectivities that will be optimized below.

[0104] When the fundamental wave 101 passes through the SHG layer 110, a forward SH wave 102f traveling in the direction of the output side DBR 112 and a backward SH wave 102b traveling in the direction of the input side DBR 111 are generated.

[0105] The backward SH wave 102b is reflected by the incident-side DBR 111. The phase adjustment layer 115 between the SHG layer 110 and the incident-side DBR 111 adjusts the phase of the backward SH wave 102b to match the phase of the forward SH wave 102f traveling toward the exit-side DBR 112. This allows the SH wave 102 to be generated efficiently without using a periodic polarity inversion structure.

[0106] By setting the reflectances of the incident-side and exit-side DBRs 111 and 112 for the SH wave 102 close to 1 and 0, respectively, the SH wave 102 is selectively emitted from the lower side of the SHG device 210 .

[0107] Regardless of whether the fundamental wave 101 is CW or pulsed, the optimum reflectance of the fundamental wave 101 of the output DBR 112 is 1. Therefore, the output DBR 112 is made of a structure in which multiple pairs of TiO2 / SiO2, which are transparent to the fundamental wave 101 and SH wave 102 and have a large refractive index difference, are laminated. As a result, the output DBR 112 can achieve a high reflectance and wideband DBR due to the large refractive index difference between TiO2 / SiO2.

[0108] On the other hand, when considering excitation of the SHG device 210 by a pulsed laser, the reflectivity of the incident-side DBR 111 for the fundamental wave 101 has an optimum value that depends on the pulse width and spectrum width.

[0109] In the SHG device 210 of this embodiment, the reflectance of the fundamental wave 101 of the incident-side DBR 111 is set to 0.66. In order to perform fine control of the reflectance in the band of the fundamental wave 101 with a central wavelength of 850 nm, the incident-side DBR 111 has a structure in which multiple ZrO2 / SiO2 pairs, which have a smaller refractive index difference than the TiO2 / SiO2 pairs of the output-side DBR 112, are stacked.

[0110] <Design of SHG device 210 under pulse excitation> This section describes a method for designing a microcavity SHG device 210 when a pulsed laser is used as the fundamental wave 101. Here, a transmissive device having the structure shown in Figures 11(a) and (b) is described, but a reflective device can also be designed in a similar manner.

[0111] For ease of explanation, the upper side of the SHG device 210 is referred to as region I, the SHG layer 110 as region II, and the lower side of the SHG device 210 as region III, as shown in FIG.

[0112] The direction of light wave propagation is defined as positive, going from region I to region II. The reflection coefficient and transmission coefficient are complex numbers, and must be handled with care, taking into account the direction of incidence of the light wave on the structure.

[0113] The transmission coefficient of the fundamental wave 101 from region I to region II is t 12 ω The reflection coefficient of the fundamental wave 101 present in region II at the interface between region I and region II is r 21 ω The reflection coefficient of the SH wave 102 existing in the region II at the interface between the region I and the region II is r 21 2ω The reflection coefficient of the fundamental wave 101 present in the region II at the interface between the region II and the region III is r 23 ω The reflection coefficient of the SH wave 102 existing in the region II at the interface between the region II and the region III is r 23 2ω The transmittance of the SH wave 102 at the interface between the regions II and III is t 23 2ω These reflection coefficients, transmission coefficients, and transmittances can be calculated for the actual structure of the SHG device 210 using the transfer matrix method. The time evolution of the intensity of the fundamental wave 101 in the SHG layer 110 when the SHG device 210 is pulse-excited is calculated. Here, calculations are performed for the central wavelength (850 nm) of the fundamental wave 101. The time evolution of the intensity of the fundamental wave 101 in the SHG layer 110 is calculated for the pulse-on time region where the fundamental wave 101 is supplied to the SHG device 210 and the pulse-off time region where the fundamental wave 101 is not supplied. The length of the pulse-on time is approximated as the pulse full width at half maximum (FWHM) τ of the excitation fundamental wave 101 laser.

[0114] First, the number of times N that the fundamental wave 101 can make a round trip within the SHG device 210 within the pulse-on time is calculated. This value is calculated by multiplying the time T required for the fundamental wave 101 to make one round trip within the SHG device 210. round It is calculated by dividing τ by (see equation (2)).

[0115] N=τ / T round ···(2)

[0116] T round is obtained by adding up the time taken for the fundamental wave 101 to travel back and forth between the incident and output side DBRs 111, 112, including the SHG layer 110 and the phase adjustment layer 115. The time taken for the fundamental wave 101 to travel back and forth between the SHG layer 110 and the output side DBRs 111, 112, including the phase adjustment layer 115, is calculated from the refractive index and thickness of the wavelength conversion crystal. The time taken for the fundamental wave 101 to travel back and forth between the incident and output side DBRs 111, 112, including the phase adjustment layer 115, is calculated when the intensity of the fundamental wave 101 incident on the DBR structure is 1 / e 2 The position where this occurs is defined as the bottom of the DBR reflection, and is calculated from the refractive index and thickness of each layer.

[0117] The number of times N that the fundamental wave 101 can travel back and forth within the SHG device 210 within the pulse-on time τ calculated from equation (2) depends on the resonator life of the SHG device 210, but the longer the laser pulse width, the greater the enhancement due to resonance.

[0118] Hereinafter, the explanation will be given assuming that the number of times that the fundamental wave 101 can travel back and forth within the SHG device 210 within the pulse-on time is N. Here, N is a positive integer calculated by equation (2).

[0119] The time evolution of the intensity of the fundamental wave 101 in the SHG layer 110 in the pulse-on time domain is obtained.

[0120] A conceptual diagram of the calculation method is shown in Fig. 14. The complex electric field amplitude of the incident fundamental wave 101 is defined as E0, and the complex electric field amplitude of the traveling fundamental wave 101 at the upper end of the SHG device 210 after traveling n times inside the SHG device 210 is defined as E n Let us assume that: E n are E0 and E n-1 Using this, it is expressed by equation (3).

[0121]

number

[0122] Next, the time evolution of the intensity of the fundamental wave 101 in the SHG layer 110 in the pulse-off time domain is calculated.

[0123] After the fundamental wave 101 has traveled back and forth within the resonator N times, it changes from pulse-on to pulse-off. In the pulse-off time region, the fundamental wave 101 is not supplied, so the complex electric field amplitude E of the traveling fundamental wave 101 at the upper end of the SHG device 210 after traveling N+m times within the resonator is N+m is E N+(m-1) Using this, it can be expressed by equation (4).

[0124]

number

[0125] The time evolution of the electric field (complex electric field amplitude) of the fundamental wave 101 over the entire time (pulse-on time and pulse-off time) can be calculated using the above equations (3) and (4) (see step S301 in FIG. 12).

[0126] Next, the electric field of the SH wave 102 emitted from the electric field of the fundamental wave 101 can be calculated as follows (see step S302 in FIG. 12).

[0127] The intensity of the SH wave 102 generated from the forward and backward fundamental waves 101 present in the SHG layer 110 at each time is calculated.

[0128] The complex amplitudes of the forward fundamental wave 101 propagating from top to bottom in the SHG region (region II) and the backward fundamental wave 101 propagating from bottom to top are respectively denoted as A f ω and A b ω The complex amplitude A of the forward SH wave 102f is f 2ω and the complex amplitude A of the backward SH wave 102b b 2ω is A f ω and A b ω Here, A ω and A 2ω represents the complex amplitude of the fundamental wave and SH wave, and the square of their absolute values ​​represents their respective powers. Also, E(x,y) is the electric field normalized so that the power is 1.

[0129] The phase of the forward fundamental wave at the upper end of the SHG region and the phase of the backward fundamental wave at the lower end of the SHG region are respectively φ l ω , φ L ω Let the length of the SHG region be L c Then, the complex amplitude A of the forward SH wave at the bottom of the SHG region is f 2ωcan be expressed by the following equation (5): where κ is a nonlinear coupling coefficient.

[0130]

number

[0131] Similarly, the complex amplitude A of the backward SH wave at the upper edge of the SHG region b 2ω can be expressed by the following equation (6).

[0132]

number

[0133] That is, the complex amplitude A of the SH wave 102 generated inside the SHG region (region II) at the bottom end (boundary between regions II and III) of the SHG layer 110 (region II) is in 2ω is the complex reflection coefficient r of the incident DBR 111 for the SH wave 102. l 2ω (=r 21 2ω ) is expressed as in equation (7).

[0134]

number

[0135] The SH wave 102 generated inside the SHG region (region II) is multiple-reflected and then emitted to the outside (region III) of the SHG device 210. That is, the complex amplitude A out 2ω can be expressed by the following equation (8).

[0136]

number

[0137] Therefore, the wavelength conversion efficiency η of the SHG device 210 is SH is the power of the fundamental wave 101 incident on the SHG device 210, P in Then, it can be expressed by equation (9).

[0138]

number

[0139] The intensity and wavelength conversion efficiency η of the SH wave 102 extracted from the SHG device 210 SH The reflectivity t of the incident DBR 111 for the fundamental wave 101 can be maximized. 12 ω can be explored.

[0140] The SHG device 210 was designed using the design methods of S301 and S302. The fundamental wave 101 was extraordinary light, with a central wavelength λ0 of 850 nm. A femtosecond titanium sapphire laser (SpectraPhysics: MaitaiHP) was used as the pump light source. The repetition rate and Δλ of the laser were 80 MHz and 13 nm, respectively. The pulse width τ of the pump light, measured using an autocorrelator, was 217 fs.

[0141] We also measured the wavelength dispersion of the refractive index of TiO2, ZrO2, and SiO2. Samples were prepared by depositing TiO2, ZrO2, and SiO2 thin films on sapphire substrates using a sputtering device, and the wavelength dispersion of the refractive index of these thin film samples was measured using a spectroscopic ellipsometry device (JA Woollam: M-2000U).

[0142] The wavelength dispersion equations for the refractive index of TiO2, ZrO2, and SiO2, fitted using the Cauchy equation, are shown in equations (17), (18), and (19). Here, λ is the wavelength. Furthermore, using the Tauc-Lorentz equation, we confirmed that the extinction coefficients of these materials are below the detection limit in the wavelength range of 400 to 900 nm.

[0143]

number

number

number

[0144] First, we designed the SHG layer 110 and the output-side DBR 112. The thickness of the SHG layer 110, made of a-plane GaN, is 1243.5 nm, which is the coherence length. The output-side DBR 112 is configured by stacking multiple TiO2 / SiO2 pairs. The output-side DBR 112 has an optimal reflectance of 1 for the fundamental wave 101. On the other hand, the optimal reflectance for the SH wave 102 is 0. We searched for a structure for the output-side DBR 112 that could achieve pairs with such reflectances. As a result, we found that the optimal number of stacked pairs for the output-side DBR 112 was 8, with a TiO2 thickness of 75.0 nm and an SiO2 thickness of 172.0 nm. The reflectances for the fundamental wave 101 and the SH wave 102 were 0.9999 and 0.069, respectively.

[0145] Next, we designed the reflectivity of the incident-side DBR 111. First, we set the thickness of the ZrO2 layer and the SiO22 layer of the incident-side DBR 111 to λ0 / 4n ZrO2 and λ0 / 4n SiO2 Next, the film thickness of the phase adjustment layer 115 was determined so as to satisfy the resonance conditions of the fundamental wave 101. As a result, even if the number of pairs of the incident-side DBR 111 is changed, the phase relationship does not change and the resonance conditions of the fundamental wave 101 do not collapse.

[0146] The number of pairs in the incident-side DBR 111 was changed within a predetermined range (0 to 20 pairs), and the intensity of the fundamental wave 101 in the SHG layer 110 and the reflectance of the fundamental wave 101 from the incident-side DBR 111 were calculated. The results are shown in the graph in Fig. 16. From Fig. 16, it can be seen that the intensity of the fundamental wave 101 inside the SHG layer 110 is maximized when the reflectance of the fundamental wave 101 from the incident-side DBR 111 is approximately 0.63.

[0147] Next, we designed the structure of the incident-side DBR 111. We searched for a structure that has a reflectance close to the fundamental wave 101 reflectance (approximately 0.63) calculated above, has high reflectance for the SH wave 102, and satisfies the constructive interference conditions for the SH wave 102. For example, a structure with 5 pairs of incident-side DBR 111, ZrO2 thickness of 47.0 nm, SiO2 thickness of 229.0 nm, ZrO2 phase adjustment layer 115 thickness of 35.6 nm, and SiO2 phase adjustment layer 115 thickness of 300.5 nm satisfies the resonance conditions for the fundamental wave 101 and the constructive interference conditions for the SH wave 102. Furthermore, the reflectance of the fundamental wave 101 and the reflectance of the SH wave 102 are 0.6600 and 0.8452, respectively.

[0148] Fig. 17 shows the time evolution of the intensity of the fundamental wave 101 in the SHG layer 110 in this structure of the incident-side DBR 111. As is clear from Fig. 17, the intensity of the fundamental wave 101 in the SHG layer is enhanced during the pulse-on time of the fundamental wave 101 (pulse width τ: 217 fs).

[0149] Furthermore, when the reflectance of the fundamental wave 101 of the incident-side DBR 111 was set to be smaller than 0.6600 and the intensity of the fundamental wave 101 in the SHG layer 110 was calculated, the rise in intensity of the fundamental wave 101 in the SHG layer 110 was faster but the maximum intensity reached was smaller compared to Figure 17. On the other hand, when the reflectance of the fundamental wave 101 of the incident-side DBR 111 was set to be larger than 0.6600, the rise in intensity of the fundamental wave 101 in the SHG layer 110 was slower, but if the supply of the fundamental wave continued until a steady state was reached, the intensity of the fundamental wave 101 became larger.

[0150] From these facts, it is clear that it is important to design the reflectivity of the fundamental wave 101 of the upper DBR in consideration of the pulse width of the pump fundamental wave 101 laser.

[0151] Next, we calculated the dependence of the intensity of the emitted SH wave 102 on the wavelength of the fundamental wave 101 without considering the spectral width of the fundamental wave 101. We found that the wavelength tolerance was approximately 11 nm, and that efficient excitation was possible using a pulsed laser with a wavelength width of 13 nm. The design results are summarized in Table 1.

[0152] [Table 1]

[0153] Finally, the η SH was calculated. However, in step S302 of FIG. 12, when calculating the intensity of the SH wave 102, it is necessary to take into consideration the spectrum width of the fundamental wave 101.

[0154] Hereinafter, the η of the microcavity SHG device 210 taking into consideration the spectrum width of the fundamental wave 101 will be SH In an ultrashort pulse laser, there are multiple longitudinal modes in the pulse, spaced apart by the reciprocal of the period. Here, the envelope of the fundamental wave 101 spectrum is expressed by the following equation (10): 2 where λ and I0 are the wavelength and proportionality constant.

[0155]

number

[0156] λ0 and Δλ are the center wavelength and spectral FWHM of the fundamental wave 101, respectively. For example, when λ0 and Δλ are 850 nm and 13 nm, and a femtosecond pulse laser with a repetition rate of 80 MHz is used as the fundamental wave 101, the theoretical spectrum is calculated as shown in Figure 15. When the fundamental wave 101 is expressed in the time domain as peak Approximating this to a square wave pulse of P peak is expressed by equation (11).

[0157]

number

[0158] In equation (11), P average is the average power of the fundamental wave 101, and T is the pulse interval. From equations (10) and (11), the peak power P of the longitudinal mode of mode number k is peak k is the longitudinal mode intensity I k Ratio between longitudinal modes I k / Σ k I k Using this, it is expressed by equation (12).

[0159]

number

[0160]

number

[0161] By applying equations (12) and (13) to equations (3) and (4), the longitudinal mode intensity I of mode number k is obtained. k The time evolution of the SHG layer 110 can be calculated.

[0162] When the fundamental wave 101 has multiple longitudinal modes, η SH In the calculation, it is necessary to consider not only second harmonic generation (SHG) but also sum frequency generation (SFG). In the entire time domain, the energy of the SH waves 102 generated from each longitudinal mode and the energy of the SF waves generated from any two longitudinal modes are calculated.

[0163] The total energy of the emitted SH wave 102 and SF wave generated from one pulse of the fundamental wave 101 is the energy τP of one pulse of the fundamental wave 101. peak Dividing by η SH Here, since the pulse widths of the fundamental wave 101, SH wave 102, and SF wave are different, η SH Energy is used to calculate.

[0164] The complex amplitudes Ap, q of the SF waves generated from the complex amplitudes Ap and Aq of the fundamental waves 101 of mode numbers p and q are calculated using the following equations (14) to (16).

[0165]

number

number

number

[0166] Here, Ep,q(x,y), Ep(x,y), and Eq(x,y) are the electric fields of the SF wave normalized to have a power of 1 and the longitudinal mode of the fundamental wave 101 of mode numbers p and q. κp,q is a nonlinear coupling coefficient, and βp and βq are the propagation constants of the longitudinal mode of the fundamental wave 101 of mode numbers p and q.

[0167] When the spot diameter of the fundamental wave 101 is 3.3 μm, η SH was calculated to be 0.20% / W. For the Fresnel reflection of the SH wave 102, only the interface between the synthetic quartz substrate and the air, which has a large refractive index difference, was considered. Figure 18 shows the spectrum of the converted light composed of the SH wave 102 and SF wave generated from each longitudinal mode of the fundamental wave 101. In Figure 18, the SH wave 102 and SF wave are collectively referred to as the SH wave 102 for convenience. The spectral FWHM was estimated to be 3.1 nm.

[0168] The SHG device 210 of the second embodiment was actually fabricated and its performance was evaluated. Schematic diagrams of the fabrication process are shown in Figures 19(a) to 19(h).

[0169] First, as shown in FIG. 19(a), a-plane GaN was grown on an r-plane sapphire substrate using a metalorganic vapor phase epitaxy (MOVPE) apparatus.

[0170] As shown in Figure 19(b), the GaN was mirror-polished using colloidal silica slurry until the thickness reached a value close to the coherence length.

[0171] As shown in FIG. 19(c), a TiO2 / SiO2 phase adjustment layer 115 and an emission-side DBR 112 were formed on the polished surface of the GaN using a sputtering device.

[0172] 19(d), the emission-side DBR 112 and the support substrate 116 were bonded using epoxy resin. The support substrate 116 was a synthetic quartz substrate.

[0173] As shown in Figures 19(e)-(f), the r-plane sapphire substrate was peeled off by a laser lift-off (LLO) process.

[0174] As shown in Figure 19(g), the damaged layer generated during LLO was removed by polishing with colloidal silica slurry, and the film thickness was adjusted to match the coherence length.

[0175] As shown in FIG. 19(h), ZrO2 / SiO2 was laminated using a sputtering device to form the incident-side DBR 111.

[0176] Through the above process, the SHG device 210 was completed.

[0177] Next, some details of the above process will be described.

[0178] <GaN polishing in Figure 19(b)> The a-plane GaN used in the examples was grown to a thickness of approximately 3.5 μm on a 2-inch r-plane sapphire substrate using an MOVPE system (AIXTRON: CCS3x2). The 2-inch sample was cut into 10 × 9 mm pieces using a dicing system. 2 The GaN c-axis was aligned parallel to the short side of the sample. The cut samples were ultrasonically cleaned using trichloroethane and acetone for 15 minutes each, and then rinsed with ethanol and ultrapure water.

[0179] Observation of the cleaned GaN surface using differential interference and white light interference microscopes confirmed the macroscopic unevenness of the GaN surface resulting from epitaxial growth. The root-mean-square (rms) roughness of a 105 μm square measured using the white light interference microscope was a large value of 19.0 nm.

[0180] This uneven structure has a maximum height difference of about 100 nm, which can cause scattering of light waves.

[0181] Next, we attempted to remove the uneven structure on the GaN surface by chemical mechanical polishing (CMP) using a polishing machine (Musashino Electronics: MA-200D) and colloidal silica slurry (Musashino Electronics: Compol EXIII). The polishing conditions were a rotation speed of 150 rpm and a weight of 0.7 kg. During this process, the GaN film thickness was thinned to approximately 1.2 μm, which is the coherence length.

[0182] White light interference microscope images of the polished GaN surface confirmed that the uneven surface structure had been removed by polishing. The rms roughness over a 105 μm square was 0.8 nm, a significant improvement in flatness compared to the GaN surface before polishing.

[0183] Next, the sample was immersed in room temperature hydrofluoric acid (Hydrogen Fluoride: HF) for 1 minute to remove the colloidal silica slurry remaining on the GaN surface. The rms roughness of a 105 μm square was 0.5 nm.

[0184] By removing the remaining colloidal silica slurry, the flatness of the GaN surface was further improved compared to that before the HF treatment. CMP using colloidal silica slurry successfully formed an a-plane GaN surface with extremely high surface flatness.

[0185] <Film formation on the emission side DBR 112 in Figure 19(c)> A sputtering device (Shibaura Mechatronics: CFS-4EP-LL) was used to form a TiO2 / SiO2 film on the surface of the output-side DBR 112. The SiO2 sputtering conditions are shown in Table 2.

[0186] Simultaneously with the fabrication of the SHG device 210, the transmittance spectrum of the output-side DBR 112 sample formed on a synthetic quartz substrate was measured. The measured spectrum is shown in Figure 20. The transmittance spectrum was measured using a spectrophotometer (JASCO: V-650). The transmittance of the synthetic quartz substrate itself has been removed from the transmittance spectrum in Figure 20 by calibration.

[0187] From the transmittance spectrum of FIG. 20, it can be seen that the transmittance of the fundamental wave 101 of the output-side DBR 112 is 0.003 or less, and the reflection band is formed as designed.

[0188] On the other hand, the transmittance of SH waves 102 was 0.73, which was slightly smaller than the theoretical value of 0.93. The main reason for this is thought to be non-uniformity in the film thickness of each layer constituting the DBR, which is caused by fluctuations in the film formation rate of the sputtering equipment.

[0189] [Table 2]

[0190] <Adhesion of the support substrate 116 in FIG. 19(d)> The sample and support substrate 116 were bonded to the output side DBR 112 using epoxy resin (Epoxy Technology: EPO-TEK301).

[0191] The support substrate 116 was made of synthetic quartz, which is transparent to the fundamental wave 101 and the SH wave 102 .

[0192] According to the technical data sheet, the epoxy resin has a transmittance of 0.99 or more in the visible light region and 0.95 or more in the infrared region. The sample and the sapphire substrate were placed face-to-face with the epoxy resin between them, and vacuum degassing was performed in a desiccator for 45 minutes. The sample was then heated and left to stand overnight in an oven at 65°C, completing the bonding of the emission-side DBR 112 and the support substrate 116.

[0193] <Figures 19(e) to (f) Laser lift of r-plane sapphire substrate> Next, LLO was attempted. The LLO laser light incident on the backside of the sapphire substrate passes through the sapphire substrate and reaches the GaN interface. The GaN absorbs this laser light and separates into Ga and N2.

[0194] In other words, by using LLO, it is possible to peel off the sapphire substrate without damaging the GaN other than at the interface.

[0195] Next, we attempted to peel off the r-plane sapphire substrate. In the sample after LLO, Ga is present at the interface between the sapphire substrate and GaN. The melting point of Ga is 29.76°C, so the sapphire substrate can be peeled off by heating the sample. The sample was immersed in ultrapure water heated to 65°C, and the Ga was melted, allowing the sapphire substrate to be peeled off. We were able to successfully peel off the sapphire substrate without any major cracks.

[0196] <Figure 19(g) Polishing using colloidal silica slurry> When a white light interference microscope image was taken of the sample after peeling, the rms roughness of a 105 μm square was 1.9 nm, and laser marks during LLO were observed. Although this level of surface roughness is not expected to have a significant effect on the operation of the SHG device 210, we attempted to remove it by CMP using colloidal silica slurry.

[0197] The polishing depth of the GaN layer was approximately 50 nm, and the overall thickness of the GaN layer was maintained close to the coherence length. The rms roughness of the 105 μm square layer was 0.8 nm, successfully removing the uneven structure originating from the LLO.

[0198] <Figure 19(h) Film formation on the incident side DBR111> A ZrO2 / SiO2 phase adjustment layer 115 and an incident-side DBR 111 were fabricated using a sputtering device.

[0199] 20 shows the transmittance spectrum of the incident-side DBR 111 sample, including the phase adjustment layer 115 formed on the synthetic quartz substrate at the same time as the fabrication of the SHG device 210. The transmittance spectrum was measured using a spectrophotometer (JASCO: V-650). At this time, the transmittance spectrum of the synthetic quartz substrate itself was removed by calibration.

[0200] 20, the transmittances of the fundamental wave 101 and the SH wave 102 were 0.30 and 0.05. Here again, there was a discrepancy between the theoretical value and the actual measured value, but the error was within a few percent and it was determined that this would not have a significant effect on the operation of the SHG device 210, so the SHG device 210 was completed.

[0201] When the completed SHG device 210 was observed, some cracks originating from the LLO were found, but the SHG device 210 had been successfully fabricated with an area yield of over 90%.

[0202] Furthermore, it was confirmed from the cross-sectional SEM image that an SHG device having an SHG layer 110 with a coherence length close to the designed value was successfully fabricated.

[0203] <Evaluation> When a pulse of the fundamental wave 101 with a central wavelength of 856 nm was incident, blue SH light emitted from the SHG device 210 was clearly observed with the naked eye.

[0204] First, we investigated the polarization direction of the SH wave 102 generated when the SHG device 210 was excited with the extraordinary fundamental wave 101. As shown in Figure 21, we confirmed that the SH wave 102 generated by the extraordinary fundamental wave 101 is extraordinary light. This indicates that this phenomenon is an SHG process mediated by the maximum nonlinear optical coefficient d33 of GaN. From Figure 21, the FWHM of the SH wave 102 spectrum was approximately 2.4 nm.

[0205] This is narrower than the theoretical FWHM of 3.1 nm shown in Figure 18. The reason for this is thought to be that the film thickness of the SHG layer 110 at the measurement point was thinner than the coherence length, which was the design value, making the resonance of the fundamental wave 101 sharper. This corresponds to an increase in the number of times the fundamental wave 101 is amplified. Another thought to be the cause is that the transmittance of the SH wave 102 through the output-side DBR 112 was smaller than the theoretical value, as shown in Figure 20, which caused interference between the SH waves 102 within the SHG layer 110.

[0206] Next, we measured the dependence of the SH wave 102 intensity on the fundamental wave 101 power. The measurement results are shown in Figure 22. As is clear from Figure 22, it was confirmed that the SH wave 102 intensity is proportional to the square of the fundamental wave 101 power. This indicates that this phenomenon is a second-order nonlinear optical process.

[0207] Next, use an optical power meter to measure η SH Actual measurements were carried out.

[0208] When the average power of the fundamental wave 101 was 12.4 mW, the average power of the generated SH wave 102 was 225 nW. The average power of the fundamental wave 101 and the SH wave 102 was calibrated taking into account the transmittance of each optical component. From this, the measured η SH was calculated to be 0.15% / W. This value is SH This agreed with a high degree of accuracy, 0.20% / W.

[0209] Here, the transmittance of the incident-side DBR 111 at a wavelength of 856 nm is 0.33, which is approximately equal to the designed transmittance of 0.34 at a wavelength of 850 nm.

[0210] Actual measurement η SH But, theoretical η SH The reason for this is that the FWHM of the SH-wave 102 spectrum is narrower than the theoretical value. SH and the measured η SH To make this comparison, it is necessary to measure the d33 value of the a-plane GaN used in this study using the maker fringe method or the wedge method.

[0211] Next, we investigated the dependence of the SH wave 102 intensity on the central wavelength of the fundamental wave 101. The average power of the fundamental wave 101 was fixed at 6.2 mW, and the central wavelength of the fundamental wave 101 was changed, and the spectrum of the generated SH wave 102 was observed using a CCD spectrometer. The fundamental wave 101 was treated as extraordinary light. When the central wavelength of the fundamental wave 101 was changed, the intensity of the SH wave 102 changed significantly, as shown in Figure 23. The intensity of the SH wave 102 has a peak at 856 nm, which corresponds to the resonance wavelength of the fundamental wave 101, and therefore, it is possible to determine the η due to resonance enhancement. SH The FWHM was 6.5 nm, which was narrower than the theoretical value of 11 nm.

[0212] As explained using Figure 21, the cause of this is thought to be that the film thickness of the SHG layer 110 at the measurement point was thinner than the coherence length, which was the design value, and that the SH waves 102 interfered with each other within the SHG layer 110.

[0213] Finally, the beam shapes of the fundamental wave 101 and SH wave 102 emitted from the SHG device 210 were observed using a CCD camera. The fundamental wave 101 and SH wave 102 were collimated. It was also confirmed that the SH wave 102 had a beam shape close to a Gaussian beam. This is because the structure of the SHG device 210 of this embodiment is not affected by walk-off. This characteristic is one of the major advantages of the SHG device 210 of this embodiment compared to the beam shape of the SH wave 102 generated from birefringent phase-matched (BPM) crystals such as BaB2O4 (BBO) and LiB3O5 (LBO), which are strongly affected by walk-off.

[0214] In this example, we demonstrated that blue SHG with a wavelength of 428 nm can be obtained by pulse pumping an SHG device 210 using a-plane GaN, which was designed and fabricated for pulse pumping. The findings obtained from this example are summarized below.

[0215] ·Design method of microcavity type SHG device 210 under pulse excitation and η SH We have established a method for calculating η. We have designed an a-plane GaN vertical microcavity SHG device 210 for pulsed excitation and SH As a result of the calculation, η SH was calculated to be 0.2% / W.

[0216] We successfully fabricated an a-plane GaN vertical microcavity SHG device 210 consisting of a DBR with an extremely flat GaN surface and a reflection band as designed. Furthermore, there was a point within the SHG device 210 where the thickness of the a-plane GaN SHG layer 110 matched the coherence length.

[0217] By exciting the SHG device 210 with a pulsed laser with a central wavelength of 856 nm, we succeeded in generating blue SHG with a central wavelength of 428 nm. We confirmed that the SH wave 102 intensity is proportional to the square of the fundamental wave 101 power. From the polarization characteristics of the SH wave 102, we confirmed that SHG via d33 was achieved. Since the SH wave 102 intensity has a peak around the resonance wavelength of the fundamental wave 101, η SHWe confirmed the resonance enhancement of η SH (0.15% / W) is the theoretical η SH It was confirmed that the SH wave 102 has a beam shape close to a Gaussian beam.

[0218] <<<Embodiment 3>>> The SHG device of the third embodiment will be described.

[0219] The SHG device of the third embodiment has a configuration in which the design method of the second embodiment is applied to a reflective SHG device. In the above-mentioned second embodiment, a transmissive SHG device in which a fundamental wave is incident from the top surface and converted light (SH wave 102 and sum frequency wave) is emitted from the bottom surface is illustrated and described in Figures 11(a) and (b). However, the design method of the second embodiment can also be applied to a reflective SHG device in which a fundamental wave is incident from the top surface and converted light (SH wave 102 and sum frequency wave) is emitted from the top surface.

[0220] By applying the second embodiment to a reflective SHG device, the SHG device of the third embodiment can be configured to use a fundamental wave 101 with a wavelength of less than 560 nm and emit SH waves 102 in the wavelength range of 130 to 280 nm, known as deep ultraviolet rays. When the SHG device of the third embodiment is configured to emit SH waves 102 in the wavelength range of 130 to 280 nm, it is preferable to divide the lower DBR layer into a first lower DBR layer 12 and a second lower DBR layer 12, as in the first embodiment, and configure them to reflect deep ultraviolet rays and the fundamental wave, respectively.

[0221] <<<Embodiment 4>>> The light emitting device of the fourth embodiment will be described with reference to FIG.

[0222] 24, the light emitting device of the fourth embodiment is a transmissive device, and is configured to include the SHG device 1 of the first embodiment, a light source 2, a condenser lens 3, and a dichroic mirror 4. The condenser lens 3 is disposed between the lower surface of the SHG device 1 and the light source 2.

[0223] The light source 2 emits a fundamental wave 101. The fundamental wave 101 is condensed by the condenser lens 3 and incident on the lower surface of the SHG device 1 of the first embodiment. The SHG device 1 generates an SH wave 102, which is emitted from the upper surface of the SHG device 1.

[0224] The structure of the SHG device 1 is the same as that of the SHG device 1 of the first embodiment, and therefore a description thereof will be omitted.

[0225] The condenser lens 3 condenses the fundamental wave 101 emitted from the light source 2 onto or near the bottom surface of the SHG device 1. The fundamental wave 101 passes through the lower second DBR layer 13 and the lower first DBR layer 12 to enter the SHG layer 10, where it is repeatedly reflected and amplified by the lower second DBR layer 13 and the upper DBR layer 11. Similar to the SHG device 1 of the first embodiment, the SH wave 102 generated from the SHG layer 10 is reflected by the lower first DBR layer 12, passes through the upper DBR layer 11, and is emitted from the top surface of the SHG device 1.

[0226] The dichroic mirror 4 is configured to transmit the fundamental wave 101 and reflect the SH wave 102. The dichroic mirror 4 transmits the fundamental wave 101 component emitted together with the SH wave 102 from the upper surface of the SHG device 1 and separates it from the SH wave 102.

[0227] The dichroic mirror 4 may be configured to transmit the SH wave 102 and reflect the fundamental wave 101.

[0228] Moreover, instead of the dichroic mirror 4, a filter for removing the fundamental wave 101 may be disposed.

[0229] In the third embodiment, in order to improve the efficiency of incidence of the fundamental wave 101 into the SHG device 1, the reflectance of the lower second DBR layer 13 may be made lower than that of the SHG device 1 of the first embodiment.

[0230] <<<Embodiment 5>>> A light emitting device according to a fifth embodiment will be described.

[0231] The light emitting device of the fifth embodiment has a structure in which a light emitting layer (light source) that emits a fundamental wave 101 is disposed between the lower first DBR layer 12 and the upper DBR layer 11 of the SHG device 1 of the first and third embodiments.

[0232] In the light emitting device of embodiment 5, a light emitting layer that emits fundamental wave 101 is disposed inside SHG device 1, and therefore there is no need to allow fundamental wave 101 to be incident from the outside by passing it through upper DBR layer 11 or through lower second DBR layer 13 and lower first DBR layer 12. Therefore, the reflectance of fundamental wave 101 of upper DBR layer 11 and lower second DBR layer 12 can be set to a value extremely close to 1. [Explanation of symbols]

[0233] 1. SHG device 2 light source 3. Condenser lens 4 Dichroic mirror 10 SHG layer (optical crystal layer) 11 Upper DBR layer 12 Lower 1st DBR layer 13 Lower 2nd DBR layer 14 SH wave anti-reflection layer 15 Phase Adjustment Layer 16 Support substrate 17 Adhesive layer 100 Light-emitting device 101 Fundamental wave 102 SH waves 210 SHG device 110 SHG layer (optical crystal layer) 111 Incident side DBR layer 112 Output side DBR layer 114 SH wave anti-reflection layer 115 Phase Adjustment Layer 116 Support substrate 117 Adhesive layer

Claims

1. an optical crystal layer made of a nonlinear optical crystal, a lower DBR layer disposed on a lower surface of the optical crystal layer, and an upper DBR layer disposed on an upper surface of the optical crystal layer; each of the lower DBR layer and the upper DBR layer has a structure in which a plurality of pairs of laminated dielectric layers each having a refractive index different from each other are laminated; the optical crystal layer generates a second harmonic wave when a fundamental wave passes through the optical crystal layer; the upper DBR layer has a reflectance for the fundamental wave equal to or greater than a predetermined reflectance and a reflectance for the second harmonic wave smaller than the predetermined reflectance; the lower DBR layer includes a lower first DBR layer disposed on the optical crystal layer side and a lower second DBR layer disposed below the lower first DBR layer, the lower first DBR layer has a reflectance for the second harmonic wave greater than a reflectance for the second harmonic wave of the lower second DBR layer; the lower second DBR layer has a reflectance for the fundamental wave greater than a reflectance for the fundamental wave of the lower first DBR layer; The second harmonic generation device, wherein the second harmonic generated in the optical crystal layer is emitted after passing through the upper DBR layer.

2. 2. The second harmonic generating device according to claim 1, the fundamental wave is incident on the upper surface of the upper DBR layer, is repeatedly reflected by the upper DBR layer and the lower second DBR layer, and repeatedly passes through the optical crystal layer; A second harmonic generating device characterized in that, of the second harmonics generated when the fundamental wave passes through the optical crystal layer, the light traveling upward passes through the upper DBR layer and is emitted upward, and, of the second harmonics generated when the fundamental wave passes through the optical crystal layer, the light traveling downward is reflected by the lower first DBR layer and travels upward, passes through the upper DBR layer and is emitted upward.

3. 2. The second harmonic generating device according to claim 1, wherein the optical crystal layer between the lower DBR layer and the upper DBR layer has a thickness in the vicinity of the coherence length and in the vicinity of an odd multiple of the coherence length.

4. 2. The second harmonic generating device according to claim 1, wherein a second harmonic anti-reflection layer made of a dielectric layer of a predetermined thickness is disposed between the upper DBR layer and the optical crystal layer in order to reduce the reflectivity of the upper DBR layer for the second harmonic.

5. 2. The second harmonic generating device according to claim 1, wherein the fundamental wave has a wavelength of less than 430 nm; the upper DBR layer and the lower first DBR layer have reflectivities for the fundamental wave and the second harmonic wave that are equal to or greater than predetermined values; The second harmonic generating device is characterized in that the reflectivity of the second lower DBR layer for the fundamental wave is equal to or greater than a predetermined value.

6. 2. The second harmonic generating device according to claim 1, wherein the upper DBR layer comprises an MgO layer and a SiO ２ A plurality of pairs of laminated layers are stacked together, The lower first DBR layer is made of an MgO layer and a SiO ２ A plurality of pairs of laminated layers are stacked together, The lower second DBR layer is made of HfO ２ layer and SiO ２ 1. A second harmonic generating device comprising: a layer and a laminated layer, and a plurality of pairs of laminated layers.

7. 2. The second harmonic generating device according to claim 1, wherein the upper DBR layer, the lower first DBR layer, and the lower second DBR layer are made of an MgO layer, an SiO.sub.2 layer, and a SiO.sub.2 layer, respectively. ２ layer, CaF ２ layer, HfO ２ layer, CaF ２ layer, Al ２ O ３ layer, AlF ３ layer, LaF ３ layer, and MgF ２ A second harmonic generating device characterized in that a plurality of pairs of layers, each pair being made up of two selected layers, are laminated.

8. 3. The second harmonic generating device according to claim 2, wherein a phase adjustment layer made of a dielectric is disposed between the optical crystal layer and the lower first DBR layer, and the phase adjustment layer is made of a single or multiple dielectric layers.

9. an optical crystal layer made of a nonlinear optical crystal, an output-side DBR layer disposed on a lower surface of the optical crystal layer, and an input-side DBR layer disposed on an upper surface of the optical crystal layer; the output-side DBR layer and the input-side DBR layer each have a structure in which a plurality of pairs of dielectric layers each having a refractive index different from each other are laminated; a fundamental wave is irradiated onto the upper surface of the incident-side DBR layer from the outside, and the irradiated fundamental wave is repeatedly reflected between the output-side DBR layer and the incident-side DBR layer, thereby repeatedly passing through the optical crystal layer; a second harmonic wave generated when the fundamental wave passes through the optical crystal layer is transmitted through the output-side DBR layer or the input-side DBR layer and emitted to the outside, the fundamental wave is a pulse wave having a predetermined time width, A second harmonic generating device characterized in that the reflectivity of the incident-side DBR layer is a preset reflectivity such that the fundamental wave, which is irradiated from outside and transmitted through the incident-side DBR layer, is amplified to a high intensity within the time of the temporal pulse width of the fundamental wave.

10. A method for designing a second harmonic generating device having an optical crystal layer made of a nonlinear optical crystal, an output-side DBR layer disposed on a lower surface of the optical crystal layer, and an input-side DBR layer disposed on an upper surface of the optical crystal layer, wherein a pulsed fundamental wave is irradiated onto the upper surface of the input-side DBR layer and repeatedly reflected between the output-side DBR layer and the input-side DBR layer, thereby generating a second harmonic wave from the optical crystal layer, comprising: Calculating the time evolution of the electric field strength of the fundamental wave in the optical crystal layer for at least a time period during which the pulse of the fundamental wave is irradiated and a time period after the irradiation; calculating, for each of a plurality of predetermined reflectivities of the incident-side DBR layer, an electric field intensity of converted light emitted from the optical crystal layer due to the electric field of the fundamental wave, based on time evolution of the electric field intensity of the fundamental wave; determining, from the calculated electric field intensity of the converted light, the reflectance of the incident-side DBR layer at which the electric field intensity of the converted light is maximized; A method for designing a second harmonic generating device, comprising:

11. 11. The method for designing a second harmonic generating device according to claim 10, wherein the step of calculating the time evolution of the electric field intensity of the fundamental wave in the optical crystal layer comprises: dividing a pulse-on time τ of the fundamental wave by a time T required for the fundamental wave to make one round trip between the output-side DBR layer and the input-side DBR layer; ｒｏｕｎｄ a second harmonic generating device design method, characterized in that the number of times the fundamental wave travels back and forth between the output-side DBR layer and the input-side DBR layer during the pulse-on time τ is determined by dividing by τ, and the determined number of times is used to calculate the time evolution of the electric field strength of the fundamental wave within the optical crystal layer.

12. 12. The method for designing a second harmonic generating device according to claim 11, wherein the converted light includes a second harmonic wave and a sum frequency wave.

13. 13. The method for designing a second harmonic generating device according to claim 12, wherein the step of calculating the electric field intensity of the converted light comprises dividing the fundamental wave into a plurality of longitudinal modes, and calculating, as the converted light, a second harmonic generated from each longitudinal mode and a sum frequency generated from any two longitudinal modes.

14. A light emitting device having a light source that emits a fundamental wave and a second harmonic generating device that receives the fundamental wave and emits a second harmonic, A light emitting device, wherein the second harmonic generating device is the second harmonic generating device according to claim 1 .

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