Application of LiNH4SO4 crystal in deep ultraviolet quasi-phase matching optical devices and its preparation method

Abstract

The present invention relates to a method for preparing optical devices using LiNH4SO4 quasi-phase-matched crystals and their use. The LiNH4SO4 crystals are used in the preparation of deep-ultraviolet quasi-phase-matched optical devices. LiNH4SO4 quasi-phase-matched crystals have advantages such as a short UV absorption cutoff edge, moderate nonlinear optical effects, low refractive index dispersion, stable physical and chemical properties, and excellent mechanical properties, making them suitable for use in deep-ultraviolet quasi-phase-matched optical devices.

Description

Technical Field

[0001] The invention relates to the application of LiNH4SO4 crystal in deep ultraviolet quasi-phase matching optical devices and a preparation method thereof. Technical Background

[0002] The nonlinear optical effect of a crystal refers to an effect in which the frequency of a laser beam with a certain polarization direction changes when it passes through a nonlinear optical crystal in a certain direction. Crystals exhibiting this effect are called nonlinear optical crystals. The nonlinear optical effect of crystals can be used to create nonlinear optical devices such as second harmonic generators, up- and down-converters, and optical parametric oscillators. In the second-order nonlinear process, the different wavelengths of the fundamental and frequency-doubled light lead to different propagation speeds in the crystal, causing a phase difference. This causes energy to flow back and forth between the fundamental and frequency-doubled light, preventing effective enhancement. This is called phase mismatch.

[0003] Currently, nonlinear optical crystals used in the ultraviolet band are primarily based on the principle of birefringence, which allows the refractive index of the fundamental and frequency-doubled light to be equal in a certain direction, resulting in a phase mismatch Δk = 0, known as birefringence phase matching. However, birefringence phase matching has disadvantages such as high birefringence requirements and the ability to only utilize nonlinear optical coefficients in specific directions. By constructing a structure in which the nonlinear coefficients are periodically reversed with the coherence length in the nonlinear optical crystal, the phase of the frequency-doubled light is reversed by π every time the coherence length passes, allowing energy to continuously flow from the fundamental to the frequency-doubled light. This is known as quasi-phase matching. Compared to birefringence phase matching, quasi-phase matching has the advantages of lower birefringence requirements and a richer range of reciprocal lattice vectors.

[0004] Existing quasi-phase-matched crystals mainly include BaMgF4, LiNbO3, LiTaO3, KTiOPO4, and LaBGeO5. Among them, the cutoff edges of LiNbO3, LiTaO3, and KTiOPO4 are all above 300nm, which cannot achieve output in the deep ultraviolet band. In addition, due to the large refractive index dispersion of crystals such as LBGO and BaMgF4, the polarization period required to achieve quasi-phase-matched output in the deep ultraviolet band is very small, and the processing is very difficult. Therefore, it is still urgent and necessary to explore deep ultraviolet quasi-phase-matched crystals with excellent comprehensive performance. Summary of the Invention

[0005] The present invention aims to provide the application of LiNH₄SO₄ crystals in deep-ultraviolet quasi-phase-matched optical devices and a preparation method thereof. LiNH₄SO₄ quasi-phase-matched crystals have advantages such as a short UV absorption cutoff edge, moderate nonlinear optical effects, low refractive index dispersion, stable physical and chemical properties, and excellent mechanical properties, making them suitable for use in deep-ultraviolet quasi-phase-matched optical devices.

[0006] The technical solutions of the present invention are as follows:

[0007] Application of LiNH4SO4 crystals in deep ultraviolet quasi-phase-matched optical devices.

[0008] A quasi-phase matching optical device is made of the LiNH4SO4 crystal.

[0009] A method for preparing a quasi-phase matching optical device comprises the following steps:

[0010] (1) Cutting the LiNH4SO4 crystal into thin slices along the direction perpendicular to the LiNH4SO4 crystal b, polishing the two surfaces of the thin slice perpendicular to the direction of the LiNH4SO4 crystal b, and then evenly coating the two surfaces with silver paste, and then placing them in a sample box for polarization;

[0011] (2) The crystal obtained in step (1) and the polarization sample box are then heated to 140-150° C., and an external voltage of 4-4.5 kV is applied and maintained for 10-15 minutes to complete the single domain formation of the wafer;

[0012] (3) then removing the silver paste on the crystal surface obtained in step (2), and coating the two surfaces with a metal film of the same thickness and material, and then photolithography one of the surfaces according to the coherence length calculated according to the refractive index dispersion equation of the LiNH4SO4 crystal to prepare the corresponding electrode structure to form the grating electrode required for polarization;

[0013] (4) Then, a high-voltage pulse power supply is used to apply a square wave electric field to the wafer multiple times to complete periodic polarization, and finally the metal film on the surface of the wafer is removed to obtain the quasi-phase matching optical device.

[0014] Preferably, the metal film is Cr or Al with a thickness of 80-120 nm.

[0015] Preferably, after polishing in step (1), the thickness of the slice along the b direction of the LiNH4SO4 crystal is 1-1.5 mm.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) LiNH4SO4 quasi-phase-matched crystals have the advantages of short UV absorption cutoff edge, moderate nonlinear optical effect, low refractive index dispersion, stable physical and chemical properties and good mechanical properties;

[0018] (2) The quasi-phase matching device made of the quasi-phase matching crystal of the present invention can be used in several high-tech fields, such as laser lithography, micromachining, photochemistry, high-resolution spectral analysis, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a working principle diagram of a deep ultraviolet quasi-phase matching optical device made of LiNH4SO4 crystal, where 1 is the laser, 2 is the incident laser beam, 3 is the deep ultraviolet quasi-phase matching optical device made of LiNH4SO4, 4 is the generated laser beam, and 5 is the filter. DETAILED DESCRIPTION

[0020] The present invention will be further described below with reference to the following examples and accompanying drawings. Those skilled in the art will appreciate that the following examples are not intended to limit the scope of protection of the present invention, and any improvements and variations based on the present invention are within the scope of protection of the present invention.

[0021] Examples 1-6 are about the preparation of LiNH4SO4 quasi-phase matching devices.

[0022] Example 1

[0023] The LiNH4SO4 quasi-phase matching device was obtained by electric field polarization method. The specific operation is carried out as follows:

[0024] a) Cut the LiNH4SO4 crystal perpendicular to the b direction and polish it into a wafer with a thickness of 1 mm. Apply silver paste evenly on the upper and lower surfaces of the wafer and place it in a sample box for polarization;

[0025] b) The wafer in step 1 was heated to 150° C. and an external voltage of 4 kV was applied for 15 minutes to complete the monodomainization of the wafer.

[0026] c) Remove the silver paste from the wafer surface and mark the positive and negative sides: +b side and -b side. Coat the +b side of the wafer with Cr, Al, or other metal to a thickness of 100 nm. Photolithography is performed on the +b side to create an electrode structure with a period of 6.8 μm, forming the grating electrode required for polarization.

[0027] d) A metal film of the same thickness as the +b surface is deposited on the -b side of the wafer as a negative electrode. A high-voltage pulsed power supply then applies a square wave electric field to the wafer a predetermined number of times to complete periodic poling. The metal electrode on the wafer surface is then removed. This results in a deep ultraviolet quasi-phase-matched device with a conversion wavelength of 532 nm to 266 nm.

[0028] Example 2

[0029] The LiNH4SO4 quasi-phase matching device was obtained by electric field polarization method. The specific operation is carried out as follows:

[0030] a) Cut the LiNH4SO4 crystal perpendicular to the b direction and polish it into a wafer with a thickness of 1 mm. Apply silver paste evenly on the upper and lower surfaces of the wafer and place it in a sample box for polarization;

[0031] b) The wafer in step 1 was heated to 140° C. and an external voltage of 4.5 kV was applied for 10 minutes to complete the monodomainization of the wafer.

[0032] c) Remove the silver paste from the wafer surface and mark the positive and negative sides: +b side and -b side. Coat the +b side of the wafer with Cr, Al, or other metal to a thickness of 100 nm. Photolithography is performed on the +b side to create an electrode structure with a period of 1.4 μm, forming the grating electrode required for polarization.

[0033] d) A metal film of the same thickness as the +b surface is deposited on the -b side of the wafer as a negative electrode. A high-voltage pulsed power supply then applies a square wave electric field to the wafer a predetermined number of times to complete periodic poling. The metal electrode on the wafer surface is then removed. This results in a deep ultraviolet quasi-phase-matched device with a conversion wavelength of 354.6 nm to 177.3 nm.

[0034] Example 3

[0035] The LiNH4SO4 quasi-phase matching device was obtained by electric field polarization method. The specific operation is carried out as follows:

[0036] a) Cut the LiNH4SO4 crystal perpendicular to the b direction and polish it into a wafer with a thickness of 1 mm. Apply silver paste evenly on the upper and lower surfaces of the wafer and place it in a sample box for polarization;

[0037] b) The wafer in step 1 was heated to 145° C. and an external voltage of 4.2 kV was applied for 12 minutes to complete the monodomainization of the wafer.

[0038] c) Remove the silver paste from the wafer surface and mark the positive and negative sides: +b side and -b side. Coat the +b side of the wafer with Cr, Al, or other metal to a thickness of 80 nm. Photolithography is performed on the +b side to create an electrode structure with a period of 6.8 μm, forming the grating electrode required for polarization.

[0039] d) A metal film of the same thickness as the +b surface is deposited on the -b side of the wafer as a negative electrode. A high-voltage pulsed power supply then applies a square wave electric field to the wafer a predetermined number of times to complete periodic poling. The metal electrode on the wafer surface is then removed. This results in a deep ultraviolet quasi-phase-matched device with a conversion wavelength of 532 nm to 266 nm.

[0040] Example 4

[0041] The LiNH4SO4 quasi-phase matching device was obtained by electric field polarization method. The specific operation is carried out as follows:

[0042] a) Cut the LiNH4SO4 crystal perpendicular to the b direction and polish it into a wafer with a thickness of 1 mm. Apply silver paste evenly on the upper and lower surfaces of the wafer and place it in a sample box for polarization;

[0043] b) The wafer in step 1 was heated to 150° C. and an external voltage of 4 kV was applied for 15 minutes to complete the monodomainization of the wafer.

[0044] c) Remove the silver paste from the wafer surface and mark the positive and negative sides: +b side and -b side. Coat the +b side of the wafer with Cr, Al, or other metal to a thickness of 80 nm. Photolithography is performed on the +b side to create an electrode structure with a period of 1.4 μm, forming the grating electrode required for polarization.

[0045] d) A metal film of the same thickness as the +b surface is deposited on the -b side of the wafer as a negative electrode. A high-voltage pulsed power supply then applies a square wave electric field to the wafer a predetermined number of times to complete periodic poling. The metal electrode on the wafer surface is then removed. This results in a deep ultraviolet quasi-phase-matched device with a conversion wavelength of 354.6 nm to 177.3 nm.

[0046] Example 5

[0047] The LiNH4SO4 quasi-phase matching device was obtained by electric field polarization method. The specific operation is carried out as follows:

[0048] a) Cut the LiNH4SO4 crystal perpendicular to the b direction and polish it into a wafer with a thickness of 1 mm. Apply silver paste evenly on the upper and lower surfaces of the wafer and place it in a sample box for polarization;

[0049] b) The wafer in step 1 was heated to 150° C. and an external voltage of 4 kV was applied for 15 minutes to complete the monodomainization of the wafer.

[0050] c) Remove the silver paste from the wafer surface and mark the positive and negative sides: +b side and -b side. Coat the +b side of the wafer with Cr, Al, or other metal to a thickness of 100 nm. Photolithography is performed on the +b side to create an electrode structure with a period of 2.3 μm, forming the grating electrode required for polarization.

[0051] d) A metal film of the same material and thickness as the +b surface is deposited on the -b side of the wafer as a negative electrode. A high-voltage pulsed power supply is used to apply a square wave electric field to the wafer a certain number of times to complete periodic poling, and the metal electrode on the wafer surface is removed. This results in a deep ultraviolet quasi-phase-matched device with a conversion wavelength of 400 nm to 200 nm.

[0052] Example 6

[0053] The LiNH4SO4 quasi-phase matching device was obtained by electric field polarization method. The specific operation is carried out as follows:

[0054] a) Cut the LiNH4SO4 crystal perpendicular to the b direction and polish it into a wafer with a thickness of 1 mm. Apply silver paste evenly on the upper and lower surfaces of the wafer and place it in a sample box for polarization;

[0055] b) The wafer in step 1 was heated to 150° C. and an external voltage of 4 kV was applied for 15 minutes to complete the monodomainization of the wafer.

[0056] c) Remove the silver paste from the wafer surface and mark the positive and negative sides: +b side and -b side. Coat the +b side of the wafer with Cr, Al, or other metal to a thickness of 80 nm. Photolithography is performed on the +b side to create an electrode structure with a period of 2.3 μm, forming the grating electrode required for polarization.

[0057] d) A metal film of the same material and thickness as the +b surface is deposited on the -b side of the wafer as a negative electrode. A high-voltage pulsed power supply is used to apply a square wave electric field to the wafer a certain number of times to complete periodic poling, and the metal electrode on the wafer surface is removed. This results in a deep ultraviolet quasi-phase-matched device with a conversion wavelength of 400 nm to 200 nm.

[0058] Attachment Figure 1 This is a brief description of the nonlinear optical device fabricated using LiNH₄SO₄ crystals. Laser 1 emits a light beam 2, which enters a periodically poled LiNH₄SO₄ crystal 3. The resulting output light beam 4 passes through a filter 5, yielding the desired laser beam. This nonlinear optical laser can be a frequency-doubling generator, an up / down frequency converter, or an optical parametric oscillator.

[0059] The above specific implementation methods are only detailed explanations of the technical solutions of the present invention. The present invention is not limited to the above embodiments. Those skilled in the art should understand that any improvements and substitutions based on the above principles and spirits on the basis of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing a deep ultraviolet quasi-phase matching optical device, characterized in that: The steps include: (1) Cutting the LiNH4SO4 crystal into thin slices along the direction perpendicular to the b direction of the LiNH4SO4 crystal, polishing the two surfaces of the thin slice perpendicular to the b direction of the LiNH4SO4 crystal, and then evenly coating the two surfaces with silver paste, and then placing them in a sample box for polarization; (2) The crystal obtained in step (1) and the polarization sample box are then heated to 140-150°C, and an external voltage of 4-4.5 kV is applied and maintained for 10-15 minutes to complete the single domain formation of the wafer; (3) Then, the silver paste on the crystal surface obtained in step (2) is removed, and a metal film with the same thickness and material is plated on the two surfaces. Then, one of the surfaces is photoetched according to the coherence length calculated according to the refractive index dispersion equation of the LiNH4SO4 crystal to prepare the corresponding electrode structure to form the grating electrode required for polarization; (4) A high-voltage pulse power supply is then used to apply a square wave electric field to the chip multiple times to complete periodic polarization, and finally the metal film on the surface of the chip is removed to obtain the quasi-phase matching optical device.

2. The method for preparing a deep ultraviolet quasi-phase matching optical device according to claim 1, characterized in that: The metal film is Cr or Al with a thickness of 80-120 nm.

3. The method for preparing a deep ultraviolet quasi-phase matching optical device according to claim 2, wherein: After polishing in step (1), the thickness of the slice along the b direction of the LiNH4SO4 crystal is 1-1.5 mm.

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