On-chip nonlinear guided wave mode conversion device based on ferroelectric domain engineering and preparation method

Abstract

The invention discloses an on-chip nonlinear guided wave mode conversion device based on ferroelectric domain engineering and a preparation method, the device comprises a Z-cut film lithium niobate waveguide, the waveguide comprises Z-cut film lithium niobate, a silicon dioxide layer and a silicon substrate in sequence from top to bottom, the Z-cut film lithium niobate layer has a two-dimensional ferroelectric domain structure in which positive and negative domains are periodically and alternately arranged, an electrode structure for preparing the two-dimensional ferroelectric domain structure is formed by arranging n + 1 groups of comb-shaped electrodes in a staggered manner, the electrodes are mutually insulated, each group of comb-shaped electrodes are periodically arranged, the width of the electrodes is 1 / 4 of a polarization period, and n is the number of nodes of a target transverse electric field. The Z-cut film lithium niobate waveguide with a two-dimensional ferroelectric domain structure is prepared on a film lithium niobate material platform by utilizing a high-voltage electric field polarization technology and a semiconductor preparation process, so that the problem of nonlinear conversion from a waveguide fundamental wave fundamental mode to a second harmonic high-order mode is solved, and the method has expansibility.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor integrated optoelectronic devices, and in particular to an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering and a preparation method thereof. Background Art

[0002] Integrated photonic chips use photons as information carriers and have the characteristics of large bandwidth, high speed and low power consumption. The degrees of freedom of photons such as polarization, wavelength, orbital angular momentum, transverse spatial mode and propagation path can be used for information encoding, thereby improving the capacity of information transmission and processing. They have been widely used in optical communications, optical interconnection and optical sensing. Among these degrees of freedom, the transverse spatial mode of multimode waveguides has become a research hotspot in the field of integrated optics due to their orthogonal, discrete and finite dimensional characteristics. The nonlinear interaction between waveguide modes provides new design ideas for the multifunctional integration of integrated photonic chips, such as nonlinear mode demultiplexing and mode-selective nonlinear frequency conversion. In addition, the generation of mode entangled sources and high-dimensional super-entangled states through spontaneous parametric down-conversion on the chip will have important application prospects in quantum information technology.

[0003] Achieving efficient nonlinear frequency conversion between lateral spatial modes in waveguides remains a key challenge. Unlike nonlinear frequency conversion between fundamental modes, when the fundamental mode interacts nonlinearly with higher-order modes, in addition to the phase matching condition, the nonlinear overlap integral between waveguide modes is also crucial. For example, in second-order nonlinear waveguides, it is usually difficult to produce efficient nonlinear mode conversion between the fundamental mode and the first-order mode, because the antisymmetric distribution of the electric field of the first-order mode causes the nonlinear overlap integral to be close to 0, which is much lower than the nonlinear mode conversion efficiency between fundamental modes. To solve this problem, in recent years, a variety of innovative solutions have emerged based on the emerging thin-film lithium niobate integrated photonic platform. For example, heterogeneous integration of TiO2, stacking of 180° spontaneous antiparallel polarized lithium niobate films, and layered polarization on commercial X-cut thin-film lithium niobate (Paper 1: Semi-Nonlinear Nanophotonic Waveguides for Highly Efficient Second-Harmonic Generation [J]; Paper 2: Efficient Second Harmonic Generation in a Reverse-Polarization Dual-Layer Crystalline Thin Film Nanophotonic Waveguide [J]; Paper 3: Efficient photon-pair generation in layer-poled lithium niobatenanophotonic waveguides [J]). These schemes aim to break the second harmonic TE 01 The antisymmetry of the electric field distribution improves the nonlinear overlap integral and realizes the fundamental TE in the second harmonic generation process through the mode phase matching method. 00 To the second harmonic TE 01 However, the above scheme requires precise control of waveguide mode dispersion to achieve phase matching. In addition, due to the complexity of the process, it is difficult to expand to the conversion between the fundamental mode and higher-order modes, and cannot meet the application requirements of nonlinear waveguide mode conversion in integrated photonic chips. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide an on-chip nonlinear waveguide mode conversion device and preparation method based on ferroelectric domain engineering, and to prepare a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure on a thin-film lithium niobate material platform by utilizing high-voltage electric field polarization technology and semiconductor preparation technology, thereby solving the problem of nonlinear conversion of the waveguide fundamental mode to the second harmonic high-order mode, and having scalability.

[0005] Technical solution: To achieve the above-mentioned purpose, the present invention discloses an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering, comprising a Z-cut thin film lithium niobate waveguide. The Z-cut thin film lithium niobate waveguide structure comprises, from top to bottom, a Z-cut thin film lithium niobate, a silicon dioxide layer, and a substrate, wherein the Z-cut thin film lithium niobate layer has a two-dimensional ferroelectric domain structure in which positive and negative domains are periodically alternating.

[0006] Among them, the geometric dimensions of the cross-section of the Z-cut thin film lithium niobate waveguide are: the thickness of the Z-cut thin film lithium niobate layer h1 is 300-700nm, the waveguide top width w is 2-3um, the waveguide etching depth h2 is 300-500nm, the waveguide side wall inclination angle is 65-85°, the silicon dioxide layer thickness is 1.9-3.0um, and the substrate thickness is 500-550um.

[0007] The present invention provides a method for preparing the on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering, comprising:

[0008] S1: Prepare overlay marks on the surface of Z-cut thin film lithium niobate samples;

[0009] S2: Prepare polarized electrode structure on the surface of Z-cut lithium niobate thin film sample according to the overlay mark;

[0010] S3: applying an electric field to the Z-cut lithium niobate thin film sample having a polarization electrode structure, so that the lithium niobate thin film sample in the area covered by the polarization electrode undergoes ferroelectric domain inversion; removing the polarization electrode structure to obtain a ferroelectric domain inversion structure;

[0011] S4: According to the overlay mark, a waveguide shape profile is prepared on the Z-cut lithium niobate thin film sample having a ferroelectric domain inversion structure region, and a waveguide structure is etched out to further obtain a Z-cut lithium niobate thin film waveguide having a two-dimensional ferroelectric domain structure;

[0012] S5: Polishing the end surface of the prepared Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

[0013] Among them, the method for preparing the overlay mark described in S1 is: preparing the overlay mark photoresist pattern on the surface of the Z-cut thin film lithium niobate sample by ultraviolet lithography, successively coating a layer of chromium and gold metal on the photoresist pattern by electron beam evaporation, and removing the photoresist using N-methylpyrrolidone solution to obtain the overlay mark structure.

[0014] Among them, the method for preparing a polarized electrode structure on the surface of a Z-cut thin film lithium niobate sample described in S2 is: according to the overlay mark, the polarized electrode shape contour is prepared on the surface of the Z-cut thin film lithium niobate sample by electron beam lithography, and a nickel metal electrode is evaporated on the photolithographic polarized electrode contour by electron beam evaporation; the photoresist is removed by N-methylpyrrolidone solution to obtain a polarized electrode structure.

[0015] Among them, the method for obtaining the ferroelectric domain inversion structure described in S3 is: placing a Z-cut thin film lithium niobate sample with a polarization electrode structure on a copper sheet on a high-temperature hot stage, connecting the polarization electrode to the positive electrode of the polarization circuit, and connecting the copper sheet to the negative electrode of the polarization circuit, applying a 100-400V electric field to cause ferroelectric domain inversion in the Z-cut thin film lithium niobate in the area covered by the polarization electrode; using dilute hydrochloric acid to remove the polarization electrode to obtain a ferroelectric domain inversion structure.

[0016] Among them, the method for obtaining a Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure described in S4 is: according to the overlay mark, electron beam lithography is used to prepare a waveguide shape contour on the Z-cut thin film lithium niobate sample with a domain inversion structure area; the waveguide structure is etched on the surface of the Z-cut thin film lithium niobate sample by ion beam etching, and the photoresist is removed by N-methylpyrrolidone solution and water bath ultrasound is performed to obtain a Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

[0017] The polarized electrode structure is composed of n+1 groups of comb-shaped electrodes arranged in an alternating manner, and the electrodes are insulated from each other. Each group of comb-shaped electrodes is arranged periodically, and the electrode width is 1 / 4 of the polarization period. n is the number of nodes of the target transverse electric field.

[0018] Beneficial effects: The present invention has the following advantages: The nonlinear waveguide mode conversion device of the present invention utilizes the spatially modulated property of the second-order nonlinear coefficient of the Z-cut thin film lithium niobate, and designs a corresponding two-dimensional ferroelectric domain structure according to the transverse electric field distribution of the second harmonic target mode, which not only meets the phase matching condition, but also significantly improves the nonlinear overlap integral, thereby being able to efficiently realize the nonlinear conversion of the fundamental mode to the second harmonic high-order mode; compared with the existing scheme, the present invention does not require precise control of the waveguide dispersion, and has good scalability, and is expected to promote the practical application of nonlinear mode conversion in integrated photonic chips. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the cross-sectional structure of the Z-cut thin-film lithium niobate waveguide in the present invention;

[0020] Figure 2 It is a schematic diagram of the two-dimensional ferroelectric domain structure and the transverse electric field distribution diagram of the fundamental wave and the second harmonic in the present invention;

[0021] Figure 3 Schematic diagram of waveguide mode evolution in a simulation experiment in an example of the present invention;

[0022] Figure 4 Schematic diagram of the structure of the polarized electrode in the example of the present invention;

[0023] Figure 5It is a schematic diagram of the process flow of preparing a Z-cut thin-film lithium niobate waveguide having a two-dimensional ferroelectric domain structure in an example of the present invention;

[0024] Figure 6 The normalized efficiency result of the frequency doubling in the optical test of the Z-cut lithium niobate waveguide with a two-dimensional ferroelectric domain structure prepared in the example of the present invention is shown in FIG.

[0025] Figure 7 The fundamental wave TM in the optical test of the Z-cut lithium niobate waveguide with a two-dimensional ferroelectric domain structure prepared in the example of the present invention 00 and Second HarmonicTM 10 Pattern

[0026] Figure 8 This is a confocal image of a two-dimensional ferroelectric domain structure prepared on a Z-cut lithium niobate thin film in an example of the present invention;

[0027] Fig. 9 This is a scanning electron microscope characterization image of the Z-cut thin film lithium niobate waveguide prepared in an example of the present invention. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is described in detail below in conjunction with the embodiments and drawings.

[0029] The on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering of the present invention comprises a Z-cut thin film lithium niobate waveguide, the cross-sectional structure of the Z-cut thin film lithium niobate waveguide is as follows: Figure 1 As shown, from top to bottom, it includes Z-cut thin-film lithium niobate, silicon dioxide layer, and silicon substrate. Among them, the Z-cut thin-film lithium niobate layer has a two-dimensional domain structure, which means that in the yx plane, the z direction is the domain growth direction, the inverted ferroelectric domain structure runs through the thin-film lithium niobate in the z direction, and the positive and negative domains are periodically arranged alternately.

[0030] In the present invention, the Z-cut thin film lithium niobate waveguide has the following geometric dimensions: h1 is 300-700nm, the waveguide top width w is 2-3um, the waveguide etching depth h2 is 300-500nm, the waveguide side wall inclination angle is 65-85°, the silicon dioxide layer thickness is 1.9-3.0um, and the silicon substrate thickness is 500-550um.

[0031] In order to realize efficient nonlinear conversion between waveguide modes of different orders and promote the practical application of nonlinear mode conversion in integrated photonic chips, the present invention takes a Z-cut thin film lithium niobate waveguide cross-section structure with a thickness h1 of 500nm, a waveguide top width of 2.3um, a waveguide etching depth h2 of 300nm, a waveguide side wall inclination angle of 75°, a silicon dioxide layer thickness of 2um, and a silicon thickness of 525um as an example, and designs three special ferroelectric domain structures according to the effective nonlinear coefficients and lateral electric field distribution between different modes in the waveguide, which can respectively realize the conversion from fundamental wave TM00 Mode to Second HarmonicTM i0 (i=1,2,3) mode conversion.

[0032] Three special ferroelectric domain structures are distributed as follows Figure 2 As shown in the figure, the lateral region of the waveguide is evenly divided into i+1 regions. The lateral distribution structure of the first ferroelectric domain includes one positive domain and one negative domain; the lateral distribution structure of the second ferroelectric domain includes two positive domain regions and one negative domain region, and the positive and negative domains are arranged alternately; the lateral distribution structure of the third ferroelectric domain includes two positive domain regions and two negative domain regions, and the positive and negative domains are arranged alternately. These three ferroelectric domain structures are all arranged periodically in the longitudinal distribution. Among them, the longitudinally distributed ferroelectric domain structure can compensate for the phase mismatch in the nonlinear mode conversion process, and the transversely distributed ferroelectric domain structure can break the spatial symmetry of the high-order mode electric field, thereby improving the nonlinear overlap integral, thereby realizing an effective nonlinear conversion of the fundamental mode to the second harmonic high-order mode.

[0033] In this embodiment, the Z-cut thin-film lithium niobate waveguide with the three ferroelectric domain structures was simulated by Lumerical FDTD software. The mode evolution process is shown in FIG. Figure 3 As shown. In the designed domain structure waveguide, the fundamental wave TM 00 The mode is effectively switched to the second harmonicTM i0 (i=1,2,3) mode, the theoretical normalized efficiency is up to 4300%W -1 cm -2 .

[0034] Taking the first ferroelectric domain structure as an example, in order to polarize this two-dimensional ferroelectric domain structure, the present invention designs a comb-shaped electrode structure, such as Figure 4 As shown. The electrode width is 1 / 4 of the polarization period. In order to achieve the best polarization effect, the polarization electrode parameters are: electrode length 27um, upper and lower electrode spacing 0.5um, period 2.4um, electrode width 0.6um, and applied polarization voltage 400V. The process flow of preparing Z-cut thin film lithium niobate waveguide with this two-dimensional ferroelectric domain structure is as follows Figure 5 As shown, it is implemented by the following steps:

[0035] 1) preparing an overlay mark, preparing an overlay mark photoresist pattern on the surface of a Z-cut lithium niobate thin film sample by ultraviolet lithography, coating a 30nm / 70nm thick chromium / gold layer that is difficult to remove on the photolithographic structure by electron beam evaporation; removing the photoresist by N-methylpyrrolidone (NMP) solution to obtain an overlay mark structure;

[0036] 2) According to the overlay mark, the polarization electrode shape contour is prepared on the surface of the Z-cut lithium niobate thin film sample by electron beam lithography, and a 100nm nickel metal electrode is evaporated on the photolithographic polarization electrode contour by electron beam evaporation; wherein the comb-shaped polarization electrode structure is as follows Figure 4 shown.

[0037] 3) using NMP solution to remove the photoresist and obtain a comb-shaped polarized electrode structure;

[0038] 4) Place a lithium niobate film sample with a polarized electrode structure on a copper sheet on a high-temperature hot stage (300°C), connect the polarized electrode to the positive electrode of the polarization circuit, connect the copper sheet to the negative electrode of the polarization circuit, and apply a 400V electric field to cause ferroelectric domain reversal in the lithium niobate in the area covered by the polarized electrode;

[0039] 5) using dilute hydrochloric acid to remove the electrodes and obtain a ferroelectric domain inversion structure;

[0040] 6) According to the overlay mark, a waveguide shape profile is prepared on the Z-cut lithium niobate thin film sample having a domain inversion structure region by electron beam lithography;

[0041] 7) A waveguide structure was etched on the surface of the Z-cut thin film lithium niobate sample by ion beam etching, the photoresist was removed by NMP solution, and water bath ultrasound was performed for 2 hours to obtain a Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

[0042] 8) The prepared Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure is end-polished.

[0043] Optical tests were performed on the prepared Z-cut thin film lithium niobate waveguide, such as Figure 6 As shown, 1565.6nm is the frequency doubling point, the fundamental power is 140uW, the frequency doubling power is 13nW, and the normalized efficiency is 2250%W -1 cm -2 , the fundamental wave TM captured by the CMOS camera 00 and Second HarmonicTM 10 Mode such as Figure 7 The prepared two-dimensional ferroelectric domain structure and the prepared Z-cut thin-film lithium niobate waveguide were subjected to confocal imaging and electron microscope scanning, respectively. The obtained confocal images of the polarization region are shown in Figure 8 As shown in Figure 1, the ferroelectric domain structure is uniform and the duty cycle is about 50%. The characterization of lithium niobate waveguide under electron microscope is shown in Figure 1. Fig. 9 As shown, the waveguide sidewall is smooth and the experimentally measured waveguide loss is about 0.5db / cm.

[0044] The present invention uses the TM C-band communication 00 Mode conversion to near infrared TM i0(i=1,2,3) mode. In other embodiments, other waveguide forms based on the solution provided by the present invention are also applicable to the present application.

[0045] In order to prepare a Z-cut thin-film lithium niobate waveguide with periodic alternating arrangement of positive and negative domains in a two-dimensional plane, the corresponding electrode structure is composed of n+1 groups of comb-shaped electrodes arranged in an alternating manner, and the electrodes are insulated from each other. Each group of comb-shaped electrodes is arranged periodically, and the electrode width is 1 / 4 of the polarization period. n is the number of nodes of the target transverse electric field.

[0046] The second harmonic generation in the Z-cut thin-film lithium niobate waveguide of the present invention can be described by a simplified three-wave coupling equation. Under the small signal approximation, the field amplitude of the second harmonic can be expressed as:

[0047]

[0048] Where A1 and A2 are the fundamental and second harmonic amplitudes, E1 and E2 represent the normalized electric field distribution of the fundamental and second harmonic, which can be described as Δβ represents the wave vector mismatch in the second harmonic generation process, L is the interaction length along the transmission direction x, is the effective second-order nonlinear coefficient, c is the speed of light in vacuum, ω1 is the fundamental frequency, and n2 is the second harmonic effective refractive index.

[0049] In order to achieve efficient second harmonic generation of nonlinear waveguide modes in Z-cut thin-film lithium niobate waveguides, the phase matching condition must be met, that is, the wave vector mismatch should be proportional to the second-order nonlinear coefficient χ that is periodically inverted along the x direction. (2) In addition to the phase matching condition, the nonlinear overlap integral also needs to be considered, which is defined as follows:

[0050]

[0051] Among them, E 1,z ,E 2,z represents the z component of the fundamental wave and the second harmonic electric field respectively. d(y,z) is the x (2) Normalized distribution in the yz plane. From formula (2), it can be seen that the nonlinear overlap integral is related to the electric field components in the z direction of the fundamental wave and the second harmonic, and is also related to χ (2) Related to the lateral distribution in the yz plane.

[0052] For a one-dimensional periodically poled thin-film lithium niobate waveguide (i.e., d(y,z) is a constant), when the fundamental wave is the fundamental mode and the second harmonic is a high-order mode, the nonlinear overlap integral will be significantly reduced due to the characteristics of the electric field distribution of the high-order mode, thereby affecting the nonlinear mode conversion efficiency. However, by modulating the d(y,z) distribution through high-voltage electric field polarization technology, so that its distribution on the yz plane matches the electric field distribution of the high-order mode, thereby breaking the spatial symmetry of the electric field distribution of the high-order mode, the nonlinear overlap integral can be effectively improved, thereby significantly enhancing the nonlinear mode conversion efficiency.

[0053] The present invention utilizes the spatially modulated second-order nonlinear coefficient of Z-cut lithium niobate thin film, and designs a corresponding two-dimensional ferroelectric domain structure based on the transverse electric field distribution of the second harmonic target mode, which not only meets the phase matching conditions, but also significantly improves the nonlinear overlap integral, thereby being able to efficiently realize the nonlinear conversion of the fundamental mode to the second harmonic high-order mode. Compared with the existing solutions, the present invention does not require precise control of waveguide dispersion and has good scalability, which is expected to promote the practical application of nonlinear mode conversion in integrated photonic chips.

Claims

1. An on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering, characterized in that: It comprises a Z-cut thin film lithium niobate waveguide, wherein the Z-cut thin film lithium niobate waveguide structure comprises, from top to bottom, a Z-cut thin film lithium niobate, a silicon dioxide layer, and a substrate, wherein the Z-cut thin film lithium niobate layer has a two-dimensional ferroelectric domain structure in which positive and negative domains are periodically alternately arranged.

2. The on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 1, characterized in that: The geometric dimensions of the cross-section of the Z-cut thin film lithium niobate waveguide are: the thickness of the Z-cut thin film lithium niobate layer h1 is 300-700nm, the waveguide top width w is 2-3um, the waveguide etching depth h2 is 300-500nm, the waveguide side wall inclination angle is 65-85°, the silicon dioxide layer thickness is 1.9-3.0um, and the substrate thickness is 500-550um.

3. A method for preparing an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering as claimed in claim 1, characterized in that: include: S1: Prepare overlay marks on the surface of Z-cut thin film lithium niobate samples; S2: Prepare polarized electrode structure on the surface of Z-cut lithium niobate thin film sample according to the overlay mark; S3: applying an electric field to the Z-cut lithium niobate thin film sample having a polarization electrode structure, so that the lithium niobate thin film sample in the area covered by the polarization electrode undergoes ferroelectric domain inversion; removing the polarization electrode structure to obtain a ferroelectric domain inversion structure; S4: According to the overlay mark, a waveguide shape profile is prepared on the Z-cut lithium niobate thin film sample having a ferroelectric domain inversion structure region, and a waveguide structure is etched out to further obtain a Z-cut lithium niobate thin film waveguide having a two-dimensional ferroelectric domain structure; S5: Polishing the end surface of the prepared Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

4. The method for preparing an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 3, characterized in that: The method for preparing the overlay mark described in S1 is: preparing the photoresist pattern of the overlay mark on the surface of the Z-cut lithium niobate thin film sample by ultraviolet lithography, coating a layer of chromium and gold metal on the photoresist pattern in sequence by electron beam evaporation, and removing the photoresist by N-methylpyrrolidone solution to obtain the overlay mark structure.

5. The method for preparing an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 3, characterized in that: S2 The method for preparing a polarized electrode structure on the surface of a Z-cut lithium niobate thin film sample is as follows: according to the overlay mark, a polarized electrode shape contour is prepared on the surface of the Z-cut lithium niobate thin film sample by electron beam lithography, and a nickel metal electrode is evaporated on the photolithographic polarized electrode contour by electron beam evaporation; and the photoresist is removed by N-methylpyrrolidone solution to obtain a polarized electrode structure.

6. The method for preparing an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 3, characterized in that: The method for obtaining the ferroelectric domain inversion structure described in S3 is: placing a Z-cut lithium niobate thin film sample with a polarization electrode structure on a copper sheet on a high-temperature hot stage, connecting the polarization electrode to the positive pole of the polarization circuit, connecting the copper sheet to the negative pole of the polarization circuit, applying a 100-400V electric field to cause ferroelectric domain inversion in the Z-cut lithium niobate thin film in the area covered by the polarization electrode; using dilute hydrochloric acid to remove the polarization electrode to obtain the ferroelectric domain inversion structure.

7. The method for preparing an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 3, characterized in that: S4 The method for obtaining a Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure is as follows: according to the overlay mark, electron beam lithography is used to prepare a waveguide shape contour on the Z-cut thin film lithium niobate sample with a domain inversion structure region; the waveguide structure is etched on the surface of the Z-cut thin film lithium niobate sample by ion beam etching, and the photoresist is removed by N-methylpyrrolidone solution and water bath ultrasound is performed to obtain a Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

8. The method for preparing an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 3, characterized in that: The polarization electrode structure is composed of n+1 groups of comb-shaped electrodes arranged in an interlaced manner, and the electrodes are insulated from each other. Each group of comb-shaped electrodes is arranged periodically, and the electrode width is 1 / 4 of the polarization period. n is the number of nodes of the target transverse electric field.

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