阅读说明：本技术 一种基于双层铌酸锂薄膜的光子线脊波导倍频芯片及其制备方法 (Photon line ridge waveguide frequency doubling chip based on double-layer lithium niobate thin film and preparation method thereof ) 是由 王磊 张秀全 陈�峰 胡卉 于 2021-07-08 设计创作，主要内容包括：本发明涉及一种基于双层铌酸锂薄膜的光子线脊波导倍频芯片及其制备方法。所述基于双层铌酸锂薄膜的光子线脊波导倍频芯片包括由顶层铌酸锂薄膜,中层二氧化硅、下层硅基衬底组成的复合结构,其特征在于,所述铌酸锂薄膜为双层铌酸锂薄膜结构,双层铌酸锂薄膜中的上层铌酸锂薄膜与下层铌酸锂薄膜的自发极化方向相反,铌酸锂薄膜呈脊波导结构。本发明提供的基于双层铌酸锂薄膜的光子线脊波导倍频芯片具有双层铌酸锂薄膜结构,上下两层铌酸锂薄膜的自发极化方向相反,消除了倍频光的高阶模上下两个旁瓣在模式重叠积分中相互抵消的效应,从而大大提高了模式相位匹配过程的转换效率。(The invention relates to a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film and a preparation method thereof. The photon line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film comprises a composite structure consisting of a top-layer lithium niobate film, a middle-layer silicon dioxide and a lower-layer silicon-based substrate, and is characterized in that the lithium niobate film is of a double-layer lithium niobate film structure, the spontaneous polarization directions of an upper-layer lithium niobate film and a lower-layer lithium niobate film in the double-layer lithium niobate film are opposite, and the lithium niobate film is of a ridge waveguide structure. The photon line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film has a double-layer lithium niobate film structure, the spontaneous polarization directions of the upper and lower layers of lithium niobate films are opposite, and the effect that the upper and lower side lobes of a high-order mode of frequency doubling light are mutually offset in mode overlapping integration is eliminated, so that the conversion efficiency of a mode phase matching process is greatly improved.)

1. A photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film is characterized by comprising a composite structure consisting of a top-layer lithium niobate film, middle-layer silicon dioxide and a lower-layer silicon-based substrate, wherein the top-layer lithium niobate film is of a double-layer lithium niobate film structure, the spontaneous polarization directions of an upper-layer lithium niobate film and a lower-layer lithium niobate film in the double-layer lithium niobate film are opposite, and the top-layer lithium niobate film is of a ridge waveguide structure.

2. The double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip of claim 1, wherein the total thickness of the double-layer lithium niobate film is 560 to 600nm, the thickness of the upper lithium niobate film is 260 to 280nm, and the thickness of the lower lithium niobate film is 300 to 320 nm.

3. The double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip of claim 1, wherein the tangential directions of the lithium niobate film are x-cut and z-cut.

4. The double-layer lithium niobate thin film-based photonic line ridge waveguide frequency doubling chip of claim 1, wherein the width of the ridge waveguide structure is 0.9-1.4 μm.

5. The method for preparing the double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip according to claim 1, comprising the following steps:

depositing a silicon dioxide buffer layer on a silicon-based substrate, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form a double-layer lithium niobate thin film structure; cleaning the double-layer lithium niobate thin film, performing electron beam photoresist spin coating and electron beam exposure on the surface of the double-layer lithium niobate thin film to form an etching mask required by dry etching, and forming a ridge waveguide structure on the upper layer lithium niobate thin film through the dry etching; and then, carrying out optical grinding and polishing on two end faces of the double-layer lithium niobate film, and then carrying out optical fiber end face coupling and ultraviolet glue curing to obtain the photonic line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

6. The method of claim 5, further comprising the steps of:

cleaning the double-layer lithium niobate thin film, and plating a titanium film and a chromium film on the double-layer lithium niobate thin film in sequence; and then carrying out thermal annealing at 240-360 ℃ for 2-4 h, and then continuing to carry out electron beam photoresist spin coating and electron beam exposure on the surface of the chromium film to form an etching mask required by dry etching.

7. The method of claim 5, wherein the cleaning is performed by: firstly, washing a double-layer lithium niobate film by using deionized water to remove large inorganic particles; then ultrasonic cleaning is carried out by using soapy water to remove organic contamination and inorganic particles; finally, the film is rinsed with deionized water and blown dry with nitrogen.

8. The preparation method according to claim 5, wherein a titanium film and a chromium film are plated by an electron beam evaporation plating method, wherein the thickness of the titanium film is 8-20 nm, and the thickness of the chromium film is 150-300 nm.

9. The method according to claim 5, wherein the electron beam resist is a negative resist or a positive resist, and the thickness of the resist spin coating is 300 to 700 nm; the dry etching is argon ion beam etching or inductively coupled plasma etching; the optical grinding and polishing process comprises the following steps: firstly, respectively carrying out coarse grinding and fine grinding on the brown corundum grinding powder of W14 and the brown corundum grinding powder of W7, then carrying out coarse polishing by using diamond grinding fluid, and finally carrying out fine polishing by using silicon dioxide suspension with the granularity of 100 +/-10 nm to obtain a smooth and flat end face.

10. The preparation method according to claim 5, comprising the following steps:

(1) depositing a silicon dioxide buffer layer on a silicon-based substrate, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form a double-layer lithium niobate thin film structure; cleaning a double-layer lithium niobate film sample to remove inorganic large particles and organic contamination on the surface;

(2) plating a titanium film on the surface of the double-layer lithium niobate film, wherein the titanium film is used as an intermediate layer;

(3) plating a chromium film on the titanium film, wherein the chromium film is used as a main mask layer;

(4) carrying out thermal annealing on the sample obtained in the step (3) at 240-360 ℃ for 2-4 h;

(5) carrying out electron beam photoresist spin coating and electron beam exposure on the surface of the chromium film to form an etching mask required by dry etching;

(6) performing dry etching on the sample obtained in the step (5) to form a ridge waveguide structure;

(7) carrying out optical grinding and polishing on two ends of the sample vertical to the ridge waveguide structure;

(8) carrying out a light transmission experiment on the waveguide to test the performance of the optical waveguide;

(9) and after the performance test is qualified (test items and qualified standards can be set according to actual needs), performing optical fiber end face coupling and ultraviolet glue curing on the two polished end faces, and taking the two ends of the optical fiber jumper wire as an input end and an output end respectively to obtain the photonic wire ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

Technical Field

The invention relates to a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film and a preparation method thereof, belonging to the technical field of preparation methods of optoelectronic devices.

Background

The lithium niobate crystal has a larger second-order nonlinear coefficient, is easy to grow a single crystal with large volume and good optical uniformity, and can process optical signals in a second order or even a higher order; meanwhile, the lithium niobate crystal can be prepared into a periodic polarization structure by means of external electric field polarization and the like, so that quasi-phase matching is realized. According to the dispersion characteristic of a lithium niobate material, a polarization domain structure with a polarization period of 8-10 microns is required for realizing frequency doubling at a wavelength of 1.55 microns in a titanium-diffused lithium niobate waveguide. However, in the photonic structure based on the lithium niobate thin film, a polarization domain structure with a polarization period less than 4 microns is required to realize frequency doubling of 1.55 microns, which puts more severe requirements on the polarization process, and meanwhile, in the quasi-phase matching process, the effective nonlinear coefficient of the second-order nonlinear conversion is reduced to 2/pi of the nonlinear coefficient of the material, which undoubtedly reduces the efficiency of the nonlinear conversion. Three photons in the quasi-phase matching process of the lithium niobate thin film generally interact in a form of a fundamental mode, so that mode overlapping integration is ideal.

The lithium niobate thin film integrated photonics structure generally adopts specific morphological structures such as ridge waveguide, micro-ring and micro-disk. The ridge waveguide is an important basic structure, and the ridge waveguide can be used as a direct functional device, such as an electro-optical modulator, a nonlinear frequency converter, a connector between functional devices, and the like, and can also be used for constructing other devices through spatial rotation, such as a micro-ring resonant cavity, and the like. The lithium niobate thin-film ridge waveguide is often multimode at short wavelengths, which provides great convenience for us to utilize the multimode modal dispersion characteristics.

The mode phase matching in the lithium niobate thin film ridge waveguide utilizes the mode dispersion characteristic of the multi-mode ridge waveguide to compensate the material dispersion effect of the lithium niobate, thereby realizing the phase of three photons in the second-order nonlinear processAnd (6) matching. Because the momentum of only three photons exists in the phase matching momentum space, the maximum nonlinear coefficient d of the lithium niobate crystal can be utilized for mode phase matching as long as the polarization of the three photons meets the requirement₃₃(27 pm/V). In addition, the distribution of the high-order mode is greatly different from that of the fundamental mode.

Chinese patent application CN106094263A discloses a periodically poled LNOI ridge waveguide and a method for making the same. The invention relates to a technology for preparing a periodically polarized ferroelectric domain structure on a lithium niobate single crystal film and preparing a ridge waveguide by utilizing a dry etching technology. The method is characterized in that a ferroelectric domain structure is prepared in a single-layer lithium niobate thin film based on an external electric field, the used mechanism is quasi-phase matching, and mode phase matching and nonlinear frequency conversion cannot be realized in a lithium niobate thin film ridge waveguide.

Chinese patent document CN110764188A discloses a method for preparing a lithium niobate ridge optical waveguide. In the patent, a proton exchange method is adopted to change the domain structure of the surface of the lithium niobate, thereby improving the surface stability of the lithium niobate in HF/HNO₃And etching the surface to form a ridge structure after a period of time of etching by the etching selection ratio in the etching liquid. The planar waveguide forming method related to the patent is a proton exchange method, the etching selection ratio of the wet etching is low, the line width precision is low, the method is suitable for preparing the ridge waveguide on a lithium niobate material, and the effect of preparing the ridge waveguide on a lithium niobate double-layer film is not ideal.

Chinese patent application CN109149047A discloses a method for preparing an on-chip low-loss ultrafine ridge waveguide, which combines ultrafast laser pulse and chemical mechanical polishing to prepare a ridge waveguide on a lithium niobate thin film. The lithium niobate thin film related to the patent is a single layer, and from the viewpoint of a ridge waveguide forming mechanism, a method used by the patent is chrome mask combined chemical mechanical polishing, and a pattern generating method used by the patent is ultrafast laser direct writing, but the method is not ideal in the precision of submicron pattern generation.

Chinese patent application CN110989076A discloses a thin film lithium niobate single polarization waveguide and a preparation method thereof. The thin-film lithium niobate single-polarization waveguide structure comprises an upper cladding, a lithium niobate thin-film waveguide core layer, a lower cladding and a substrate layer from top to bottom, wherein the refractive indexes of the upper cladding and the lower cladding are respectively smaller than that of the lithium niobate thin-film waveguide core layer, and the lithium niobate thin-film waveguide core layer comprises a ridge waveguide and groove-shaped regions positioned on two sides of the ridge waveguide; the preparation method comprises the following steps: s1, preparing a graphical etching hard mask on the thin film lithium niobate through photoetching; s2, removing partial lithium niobate materials on two sides of the ridge waveguide by dry etching by means of the etching hard mask; s3, removing the etching hard mask; and S4, covering a low-refractive-index cladding material above the ridge waveguide. The thin-film lithium niobate single-polarization waveguide still has a single-layer structure, and the nonlinear frequency conversion efficiency is low.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film and a preparation method thereof, which can prepare a nonlinear lithium niobate film waveguide chip with micron-order and performance comparable to quasi-phase matching.

The invention adopts the following technical scheme:

a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film comprises a composite structure consisting of a top-layer lithium niobate film, middle-layer silicon dioxide and a lower-layer silicon-based substrate, wherein the top-layer lithium niobate film is of a double-layer lithium niobate film structure, the spontaneous polarization directions of an upper-layer lithium niobate film and a lower-layer lithium niobate film in the double-layer lithium niobate film are opposite, and the top-layer lithium niobate film is of a ridge waveguide structure.

According to the invention, the total thickness of the double-layer lithium niobate thin film is 560-600 nm, the thickness of the upper layer lithium niobate thin film is 260-280 nm, and the thickness of the lower layer lithium niobate thin film is 300-320 nm.

According to the invention, the tangential direction of the lithium niobate thin film is preferably x-cut or z-cut.

According to the invention, the width of the ridge waveguide structure is preferably 0.9-1.4 μm.

A preparation method of a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film comprises the following steps:

depositing a silicon dioxide buffer layer on a silicon-based substrate, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form a double-layer lithium niobate thin film structure; cleaning the double-layer lithium niobate thin film, performing electron beam photoresist spin coating and electron beam exposure on the surface of the double-layer lithium niobate thin film to form an etching mask required by dry etching, and forming a ridge waveguide structure on the upper layer lithium niobate thin film through the dry etching; and then, carrying out optical grinding and polishing on two end faces of the double-layer lithium niobate film, and then carrying out optical fiber end face coupling and ultraviolet glue curing to obtain the photonic line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

According to the preferable preparation method of the double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip, the preparation method further comprises the following steps:

cleaning the double-layer lithium niobate thin film, and plating a titanium film and a chromium film on the double-layer lithium niobate thin film in sequence; and then carrying out thermal annealing at 240-360 ℃ for 2-4 h, and then continuing to carry out electron beam photoresist spin coating and electron beam exposure on the surface of the chromium film to form an etching mask required by dry etching.

According to the invention, the cleaning process is preferably as follows: firstly, washing a double-layer lithium niobate film by using deionized water to remove large inorganic particles; then ultrasonic cleaning is carried out by using soapy water to remove organic contamination and inorganic particles; finally, the film is rinsed with deionized water and blown dry with nitrogen.

According to the invention, the titanium film and the chromium film are preferably plated by an electron beam evaporation plating method. The adopted electron beam evaporation coating equipment can be commercial general equipment;

according to the invention, the thickness of the titanium film is preferably 8-20 nm, and the thickness of the chromium film is preferably 150-300 nm.

According to the invention, the electron beam photoresist is preferably a negative photoresist or a positive photoresist, and the thickness of the photoresist in spin coating is 300-700 nm.

According to the invention, the dry etching is argon ion beam etching or inductively coupled plasma etching.

According to the invention, the optical grinding and polishing process is preferably as follows:

firstly, respectively carrying out coarse grinding and fine grinding on the brown corundum grinding powder of W14 and the brown corundum grinding powder of W7, then carrying out coarse polishing by using diamond grinding fluid, and finally carrying out fine polishing by using silicon dioxide suspension with the granularity of 100 +/-10 nm to obtain a smooth and flat end face.

A preparation method of a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film specifically comprises the following steps:

(1) depositing a silicon dioxide buffer layer on a silicon-based substrate, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form a double-layer lithium niobate thin film structure; cleaning a double-layer lithium niobate film sample to remove inorganic large particles and organic contamination on the surface;

(2) the titanium film is plated on the surface of the double-layer lithium niobate film and serves as an intermediate layer, so that the adhesion between the chromium film and the double-layer lithium niobate film can be enhanced, and the process stability is improved;

(3) plating a chromium film on the titanium film, wherein the chromium film is used as a main mask layer, and the chromium film has stronger corrosion resistance and can enhance the etching selection ratio in dry etching;

(4) carrying out thermal annealing on the sample obtained in the step (3) at 240-360 ℃ for 2-4 h, so as to enhance the density of the chromium film;

(5) carrying out electron beam photoresist spin coating and electron beam exposure on the surface of the chromium film to form an etching mask required by dry etching;

(6) performing dry etching on the sample obtained in the step (5) to form a ridge waveguide structure;

(7) carrying out optical grinding and polishing on two ends of the sample vertical to the ridge waveguide structure;

(8) carrying out a light transmission experiment on the waveguide to test the performance of the optical waveguide;

(9) and after the performance test is qualified (test items and qualified standards can be set according to actual needs), performing optical fiber end face coupling and ultraviolet glue curing on the two polished end faces, and taking the two ends of the optical fiber jumper wire as an input end and an output end respectively to obtain the photonic wire ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

The present invention is not described in detail, and the prior art can be adopted.

The invention has the technical characteristics that:

the inventor of the application discovers, through research on mode phase matching based on the lithium niobate thin-film ridge waveguide, that if the second-order nonlinear coefficient of lithium niobate can be inverted in the depth direction (namely spontaneous polarization is deflected), high-efficiency lithium niobate mode phase matching can be realized. Furthermore, the inventor finds that the second-order nonlinear coefficient of the lithium niobate can be inverted in the depth direction through a double-layer film structure through calculation, so that the high-efficiency phase matching of the lithium niobate mode is realized, and the nonlinear conversion efficiency is improved. The double-layer lithium niobate film in the invention is an important technological innovation in the field of lithium niobate films, and breaks through the traditional understanding of the single distribution of the nonlinear characteristic and the electro-optic characteristic of the lithium niobate film in the thickness direction by researchers in the past, thereby bringing the realization and verification of a new mechanism in nonlinear optical conversion and providing a brand-new platform for the lithium niobate photonics.

The invention has the beneficial effects that:

1. the photon line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film has a double-layer lithium niobate film structure, the spontaneous polarization directions of the upper and lower layers of lithium niobate films are opposite, and the nonlinear coefficients d of the upper and lower layers of lithium niobate films are different according to the properties of lithium niobate crystals₃₃The negative sign of the double-frequency light source is opposite, the special design eliminates the mutual offset effect of the upper side lobe and the lower side lobe of the high-order mode of the double-frequency light in the mode overlapping integral, thereby greatly improving the conversion efficiency of the mode phase matching process; in addition, because only three momentum of interacting photons exist in the momentum space of the mode phase matching, and no additional momentum exists, the mode phase matching is direct phase matching, and the effective nonlinear coefficient of the mode phase matching is the nonlinear coefficient d of the lithium niobate material₃₃So that the prepared photon line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film has high performance, low transmission loss, micron order and higher conversion efficiency,the normalized conversion efficiency reaches 3600-5600%/W/cm²Compared with the existing single-layer lithium niobate film structure, the structure is improved by 18-113%.

2. The invention adopts the double-layer lithium niobate film structure, and the double-layer lithium niobate film structure can directly utilize the preparation process of a relatively mature single-layer lithium niobate film, thereby greatly reducing the difficulty of new platform preparation, reducing the process complexity, optimizing the compatibility and flexibility of the whole preparation process, and directly utilizing various test means of the single-layer lithium niobate film to represent various performance parameters of the double-layer lithium niobate film.

3. The effective phase matching mode in the lithium niobate ridge waveguide is mainly a quasi-phase matching structure based on a periodic polarization structure, which needs a relatively precise ferroelectric domain polarization process, if the polarization process cannot obtain an ideal duty ratio (1:1), the quasi-phase matching efficiency is greatly reduced, and the domain structure of the lithium niobate generates steps due to the difference of etching efficiency in the dry etching process, so that the transmission loss is increased. The ridge waveguide frequency doubling chip based on the double-layer lithium niobate film avoids a complex and harsh electric field polarization reversal process, and avoids extra loss caused by steps generated by etching.

Description of the drawings:

FIG. 1 is a schematic cross-sectional structure diagram of a double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip.

Fig. 2 is a schematic view of a three-dimensional structure of a double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip.

Fig. 3 is a process flow chart of preparing a photonic line ridge waveguide frequency doubling chip based on a double-layer lithium niobate thin film in example 1.

FIG. 4 is a schematic cross-sectional view of a photonic line ridge waveguide frequency doubling chip based on a double-layer lithium niobate thin film prepared in example 1;

wherein: the arrow represents the spontaneous polarization direction of the upper lithium niobate thin film and the lower lithium niobate thin film; (a) the spontaneous polarization direction of the upper lithium niobate film is rightward, and the spontaneous polarization direction of the lower lithium niobate film is leftward; (b) the spontaneous polarization direction of the upper lithium niobate film is leftward, and the spontaneous polarization direction of the lower lithium niobate film is rightward.

FIG. 5 is a schematic view of the strip structure in examples 1 to 3;

wherein: (a) is a uniform stripe pattern, (b) is a grating coupler, and (c) is an end-face adiabatic coupler.

Fig. 6 is a schematic cross-sectional structure view of a photonic line ridge waveguide frequency doubling chip based on a double-layer lithium niobate thin film in embodiment 5;

wherein: the arrow represents the spontaneous polarization direction of the upper lithium niobate thin film and the lower lithium niobate thin film; (a) the spontaneous polarization direction of the upper lithium niobate film is upward, and the spontaneous polarization direction of the lower lithium niobate film is downward; (b) the spontaneous polarization direction of the upper lithium niobate film is downward, and the spontaneous polarization direction of the lower lithium niobate film is upward.

Fig. 7 is a schematic diagram of a method for testing the performance of a waveguide device in an experimental example.

FIG. 8 is a diagram showing a high-order mode distribution of fundamental light and frequency doubled light of the waveguide device of example 1 in an experimental example;

in the figure: 1. an upper lithium niobate film layer, a lower lithium niobate film layer, 3, silicon dioxide, 4 and a silicon-based substrate.

Detailed Description

The present invention is further illustrated by, but not limited to, the following examples.

Example 1

As shown in fig. 1-2, a photonic line ridge waveguide frequency-doubling photonic chip based on a double-layer lithium niobate film comprises a composite structure composed of a top-layer lithium niobate film, a middle-layer silica 2 and a lower-layer silicon-based substrate 4, wherein the top-layer lithium niobate film is of a double-layer lithium niobate film structure, the spontaneous polarization directions of an upper-layer lithium niobate film 1 and a lower-layer lithium niobate film 2 in the double-layer lithium niobate film are opposite, and the top-layer lithium niobate film is prepared into a ridge waveguide structure by dry etching.

The total thickness of the double-layer lithium niobate thin film is 580nm, the thickness of the upper layer lithium niobate thin film is 280nm, and the thickness of the lower layer lithium niobate thin film is 300 nm.

The width of the ridge waveguide structure is 1 μm.

As shown in fig. 3, the preparation method of the photon line ridge waveguide frequency doubling chip based on the double-layer lithium niobate thin film comprises the following steps:

(1) depositing a silicon dioxide buffer layer on a silicon-based substrate with the length of 2cm and the width of 1cm, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form an x-cut double-layer lithium niobate thin film structure; washing the double-layer lithium niobate film by using deionized water to remove large inorganic particles; then ultrasonic cleaning is carried out by using soapy water to remove organic contamination and inorganic particles; finally, washing with deionized water and drying with nitrogen;

(2) plating a titanium film with the thickness of 15nm on the surface of the double-layer lithium niobate film by using an electron beam evaporation coating machine;

(3) plating a chromium film with the thickness of 100nm on the titanium film by using an electron beam evaporation coating machine;

(4) carrying out thermal annealing on the sample obtained in the step (3) at 200 ℃ for 3 h;

(5) spin-coating an electron beam photoresist with the thickness of 300nm on the surface of the chromium film, and preparing a strip structure on the surface of the chromium film by using a commercial electron beam exposure machine (e-beam lithography machine) to form an electron beam photoresist mask;

(6) placing the exposed sample into a reactive ion beam etching machine for dry etching, and etching the chromium film and the titanium film by using an electron beam photoresist as a mask; etching the lithium niobate thin film by using the chromium film as a mask, wherein the etching depth is 400nm, and forming a ridge waveguide structure;

(7) respectively carrying out coarse grinding and fine grinding on two ends of a sample vertical to the ridge waveguide structure by using brown corundum grinding powder of W14 and brown corundum grinding powder of W7, then carrying out coarse polishing by using diamond grinding fluid, and finally carrying out fine polishing by using silicon dioxide suspension liquid with the granularity of 100nm to obtain a smooth and flat end face;

(8) carrying out a light transmission experiment on the waveguide to test the performance of the optical waveguide;

(9) and after the performance test is qualified (test items and qualified standards can be set according to actual needs), performing optical fiber end face coupling and ultraviolet glue curing on the two polished end faces, and taking the two ends of the optical fiber jumper wire as an input end and an output end respectively to obtain the high-performance photonic line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

The double-layer lithium niobate film-based photonic line ridge waveguide frequency doubling chip prepared in this example is shown in fig. 4, and the strip structure in step (5) is shown in fig. 5(a) and is a strip pattern with a uniform width of 100 nm. The strip structure may also be a grating coupler as shown in fig. 5(b) or an end-face adiabatic coupler as shown in fig. 5 (c). In the embodiment, the ridge waveguide formed by the x-cut lithium niobate double-layer film can avoid a complex and harsh external electric field polarization technology, and meanwhile, the pump light and the frequency doubling light are both TE polarized, so that the maximum nonlinear coefficient of a lithium niobate material can be utilized, and the nonlinear conversion efficiency, compatibility and flexibility of the device are greatly improved.

Example 2

A preparation method of a photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film comprises the following steps:

(1) depositing a silicon dioxide buffer layer on a silicon-based substrate with the length of 2cm and the width of 1cm, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form an x-cut double-layer lithium niobate thin film structure; washing the double-layer lithium niobate film by using deionized water to remove large inorganic particles; then ultrasonic cleaning is carried out by using soapy water to remove organic contamination and inorganic particles; finally, washing with deionized water and drying with nitrogen;

(2) spin-coating an electron beam photoresist with the thickness of 200nm on the surface of the double-layer lithium niobate film, and preparing a strip structure on the surface of the lithium niobate by using a commercial electron beam exposure machine to form an electron beam photoresist mask;

(3) placing the exposed sample into a reactive ion beam etching machine for dry etching, and etching the lithium niobate thin film by using an electron beam photoresist as a mask, wherein the etching depth is 350nm to form a ridge waveguide structure;

(4) respectively carrying out coarse grinding and fine grinding on two ends of a sample vertical to the ridge waveguide structure by using brown corundum grinding powder of W14 and brown corundum grinding powder of W7, then carrying out coarse polishing by using diamond grinding fluid, and finally carrying out fine polishing by using silicon dioxide suspension liquid with the granularity of 90nm to obtain a smooth and flat end face;

(5) carrying out a light transmission experiment on the waveguide to test the performance of the optical waveguide;

(6) and after the performance test is qualified (test items and qualified standards can be set according to actual needs), performing optical fiber end face coupling and ultraviolet glue curing on the two polished end faces, and taking the two ends of the optical fiber jumper wire as an input end and an output end respectively to obtain the high-performance photonic line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

In this embodiment, the electron beam resist is directly used as the resist mask on the x-cut lithium niobate double-layer film, which can reduce the process complexity, improve the pattern transfer precision, and improve the yield. The strip structure of step (3) is shown in fig. 5(b), and is a grating coupler.

Example 3

A preparation method of a high-performance photon line ridge waveguide frequency doubling chip based on a double-layer lithium niobate film comprises the following steps:

(1) depositing a silicon dioxide buffer layer on a silicon-based substrate with the length of 2cm and the width of 1cm, and preparing an upper lithium niobate thin film and a lower lithium niobate thin film which have opposite spontaneous polarization directions on the silicon dioxide buffer layer by a bonding method to form an x-cut double-layer lithium niobate thin film structure; washing the double-layer lithium niobate film by using deionized water to remove large inorganic particles; then ultrasonic cleaning is carried out by using soapy water to remove organic contamination and inorganic particles; finally, washing with deionized water and drying with nitrogen;

(2) spin-coating an electron beam photoresist with the thickness of 400nm on the surface of the double-layer lithium niobate film, and preparing a strip structure on the surface of the lithium niobate by using a commercial electron beam exposure machine to form an electron beam photoresist mask;

(3) placing the exposed sample into a reactive ion beam etching machine for dry etching, and etching the lithium niobate thin film by using the electron beam photoresist as a mask, wherein the etching depth is 500nm to form a ridge waveguide structure;

(4) respectively carrying out coarse grinding and fine grinding on two ends of a sample vertical to the ridge waveguide structure by using brown corundum grinding powder of W14 and brown corundum grinding powder of W7, then carrying out coarse polishing by using diamond grinding fluid, and finally carrying out fine polishing by using silicon dioxide suspension liquid with the granularity of 110nm to obtain a smooth and flat end face;

(5) carrying out a light transmission experiment on the waveguide to test the performance of the optical waveguide;

(6) and after the performance test is qualified (test items and qualified standards can be set according to actual needs), performing optical fiber end face coupling and ultraviolet glue curing on the two polished end faces, and taking the two ends of the optical fiber jumper wire as an input end and an output end respectively to obtain the high-performance photonic line ridge waveguide frequency doubling chip based on the double-layer lithium niobate film.

In the embodiment, the electron beam photoresist is directly used as the photoresist mask on the x-cut lithium niobate double-layer film, so that the process complexity can be reduced, the pattern transfer precision can be improved, and the yield can be improved; the pump light and the frequency doubling light of the waveguide chip are both TM polarized. The strip structure of step (3) is shown in fig. 5(c), and is an end-face adiabatic coupler.

Example 4

The process was as described in example 2, except that: in the step (1), a z-cut double-layer lithium niobate thin film structure is formed.

The photonic line ridge waveguide frequency doubling chip based on the double-layer lithium niobate thin film prepared in this example is shown in fig. 6.

Comparative example 1

Physical promulgation of International journal of academic applications, Y.Niu, C.Lin, X.Liu, Y.Chen, X.Hu, Y.Zhang, X.Cai, Y. -X.Gong, Z.Xie, and S.Zhu, "Optimizing the efficacy of a radiocalcally poled LNOI waveform usage in a site monitoring of the ferroelectric domains," applied. Phys.Lett.116,101104(2020).

In this document, researchers have implemented frequency doubling in the optical communication band by using quasi-phase matching in an x-cut single-layer lithium niobate thin film, and normalized conversion efficiency thereof is 3061%/W/cm²。

Comparative example 2

International journal of academic, c.wang, c.langrock, a.marandi, m.jankowski, m.zhang, b.desiatov, m.m.fejer, and M."Ultrahigh-efficiency wavelength conversion in nanophotonic periodically poled lithium niobate waveguides,"Optica 5,1438(2018).

In this document, researchers have utilized quasi-phase matching in x-cut single-layer lithium niobate thin films to realize frequency doubling generation and detection of optical communication bands, and normalized conversion efficiency thereof is 2600%/W/cm²。

Examples of the experiments

The waveguide devices obtained in examples 1 to 4 and comparative examples 1 to 2 were tested for their relevant performance under the same conditions, and the following performance data were obtained, as shown in table 1.

The specific test method is as follows: as shown in fig. 7, a wavelength tunable laser and a tapered fiber are used to couple near-infrared laser into the waveguide devices obtained in examples 1 to 4 and comparative examples 1 to 2, the coupling efficiency is kept stable, the wavelength is scanned, a frequency doubling signal and a fundamental frequency optical signal are recorded at the same time, and after the test is completed, data is processed to obtain a frequency conversion half-width and a normalized conversion efficiency, wherein a high-order mode distribution diagram of the fundamental frequency light and the frequency doubling light of the waveguide device in example 1 is shown in fig. 8.

Table 1: performance data sheet

Item	Frequency conversion wavelength (nm)	Frequency conversion half wave width (nm)	Normalized conversion efficiency
				Example 1	1550.5nm	2.3nm	5540％/W/cm2
Example 2	1551.2nm	3.2nm	4730％/W/cm2
				Example 3	1554.3nm	2.6nm	3620％/W/cm2
Example 4	1552.6nm	3.1nm	4850％/W/cm2
				Comparative example 1	1470nm	3.4nm	3061％/W/cm2
Comparative example 2	1510nm	4.6nm	2600％/W/cm2

In comparative example 1, a researcher prepared a periodic domain inversion structure with a period of 6 microns on a lithium niobate thin film by using external electric field polarization, the thickness of a single-layer lithium niobate thin film was 600nm, the width of the top of the prepared ridge waveguide was 1.4 microns, single-mode transmission in an optical communication band was supported, domain polarization inversion treatment was required to be performed on the single-layer lithium niobate thin film before the preparation of the ridge waveguide, and the performance of the ridge waveguide device was closely related to the polarization quality.

In comparative example 2, a researcher prepared a periodic domain inversion structure with a period of 4 microns on a lithium niobate thin film by using external electric field polarization, the thickness of a single-layer lithium niobate thin film was 600nm, the width of the top of the prepared ridge waveguide was 1.44 microns, single-mode transmission under an optical communication band was supported, domain polarization inversion treatment was required to be performed on the single-layer lithium niobate thin film before preparing the ridge waveguide, and the duty ratio of the prepared domain inversion structure was 0.39, so the normalized conversion efficiency actually measured by the device was 2600%/W/cm²And is 57% of theoretical predicted value, which shows that the polarization quality directly determines the performance of the ridge waveguide device taking quasi-phase matching as a conversion mechanism.

In examples 1 to 4, on the premise of ensuring process applicability, the control wavelength of the optical quantum is equivalent to that in comparative examples 1 to 2, meanwhile, periodic polarization treatment of the lithium niobate thin film is avoided, and by forming a double-layer lithium niobate thin film structure with the spontaneous polarization directions of the upper lithium niobate thin film and the lower lithium niobate thin film being opposite, the normalized conversion efficiency of the high-performance photon linear ridge waveguide frequency doubling chip based on the double-layer lithium niobate thin film prepared in examples 1 to 4 reaches 3600 to 5600%/W/cm²Compared with the comparative example 1, the preparation method improves the preparation efficiency by 18-81%, and compared with the comparative example 2, the preparation method improves the preparation efficiency by 39-113%, and simultaneously reduces the process complexity and optimizes the compatibility and flexibility of the whole preparation process.

As can be seen from fig. 8, in embodiment 1, the TE01 higher-order mode of the high-performance photonic line ridge waveguide frequency doubling chip frequency doubling light based on the double-layer lithium niobate thin film has two peaks in the entire mode field region, and the phases of the light field vibrations in the corresponding region are inverted, so that the two opposite second-order nonlinear coefficients of the two layers of lithium niobate thin films are used to compensate, thereby obtaining high-efficiency frequency doubling conversion efficiency.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.