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

Abstract

This invention discloses an on-chip nonlinear waveguide mode conversion device and its fabrication method based on ferroelectric domain engineering. The device includes a Z-cut thin-film lithium niobate waveguide, which, from top to bottom, comprises a Z-cut thin-film lithium niobate, a silicon dioxide layer, and a silicon substrate. The Z-cut thin-film lithium niobate layer has a two-dimensional ferroelectric domain structure with periodically alternating positive and negative domains. The electrode structure for fabricating this two-dimensional ferroelectric domain structure consists of n+1 groups of comb-shaped electrodes arranged alternately, with the electrodes insulated from each other. Each group of comb-shaped electrodes is periodically arranged, with the electrode width being 1 / 4 of the polarization period, and n being the number of nodes in the target transverse electric field. This invention utilizes high-voltage electric field polarization technology and semiconductor fabrication processes to fabricate a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure on a thin-film lithium niobate material platform, thereby solving the nonlinear conversion problem from the fundamental mode to the second harmonic higher-order mode of the waveguide, and also exhibits scalability.

Description

Technical Field

[0001] This invention relates to the field of semiconductor integrated optoelectronic device technology, and in particular to an on-chip nonlinear waveguide mode conversion device and its fabrication method based on ferroelectric domain engineering. Background Technology

[0002] Integrated photonic chips utilize photons as information carriers, featuring high bandwidth, high speed, and low power consumption. The degrees of freedom of photons, such as polarization, wavelength, orbital angular momentum, transverse spatial modes, and propagation paths, can be used for information encoding, thereby increasing the capacity for information transmission and processing. They have already been widely applied in optical communication, optical interconnection, and optical sensing. Among these degrees of freedom, the transverse spatial modes of multimode waveguides have become a research hotspot in integrated optics due to their orthogonality, discreteness, and finite-dimensional characteristics. Nonlinear interactions between waveguide modes provide new design ideas for the multifunctional integration of integrated photonic chips, such as nonlinear mode demultiplexing and mode-selective nonlinear frequency conversion. Furthermore, generating mode entanglement sources and high-dimensional hyperentangled states on the chip through spontaneous parametric downconversion holds significant application prospects in quantum information technology.

[0003] Achieving efficient nonlinear frequency conversion between transverse spatial modes in waveguides remains a key challenge. Unlike nonlinear frequency conversion between fundamental modes, when a fundamental mode interacts nonlinearly with a higher-order mode, the nonlinear overlap integral between waveguide modes is equally crucial, in addition to the phase-matching condition. For example, in a second-order nonlinear waveguide, efficient nonlinear mode conversion between the fundamental mode and a first-order mode is often difficult because the antisymmetric distribution of the electric field in the first-order mode causes the nonlinear overlap integral to approach zero, far lower than the nonlinear mode conversion efficiency between fundamental modes. To address this issue, several innovative solutions have emerged in recent years based on the emerging thin-film lithium niobate integrated photonics platform. For example, heterogeneous integration of TiO2, stacking of 180° spontaneously antiparallel polarized lithium niobate films, and layered polarization techniques on commercially available X-cut lithium niobate films (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 enhances the nonlinear overlap integral, and the fundamental TE wave during second harmonic generation is achieved through mode phase matching. 00 To the second harmonic TE 01 However, the above schemes require precise control of waveguide mode dispersion to achieve phase matching. Furthermore, due to fabrication complexity, it is difficult to extend to conversion between the fundamental mode and higher-order modes, failing to meet the application requirements of nonlinear guided wave mode conversion in integrated photonic chips. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide an on-chip nonlinear guided wave mode conversion device and its fabrication method based on ferroelectric domain engineering. By utilizing high-voltage electric field polarization technology and semiconductor fabrication process, a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure is fabricated on a thin-film lithium niobate material platform, thereby solving the nonlinear conversion problem from the fundamental mode of the waveguide to the second harmonic higher-order mode, and it has scalability.

[0005] Technical solution: To achieve the above objectives, the present invention provides an on-chip nonlinear guided wave 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. The Z-cut thin-film lithium niobate layer has a two-dimensional ferroelectric domain structure with positive and negative domains arranged in a periodic alternation.

[0006] The cross-sectional geometry of the Z-cut thin-film lithium niobate waveguide is as follows: the thickness h1 of the Z-cut thin-film lithium niobate layer is 300-700 nm, the top width w of the waveguide is 2-3 μm, the waveguide etching depth h2 is 300-500 nm, the waveguide sidewall tilt angle is 65-85°, the silicon dioxide layer thickness is 1.9-3.0 μm, and the substrate thickness is 500-550 μm.

[0007] The present invention provides a method for fabricating an on-chip nonlinear waveguide mode conversion device applied to the above-mentioned ferroelectric domain engineering, comprising:

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

[0009] S2: Based on the overlay markings, a polarized electrode structure is prepared on the surface of the Z-cut thin film lithium niobate sample;

[0010] S3: Apply an electric field to the Z-cut thin film lithium niobate sample with a polarized electrode structure to cause ferroelectric domain inversion in the thin film lithium niobate sample in the polarized electrode covered area; remove the polarized electrode structure to obtain the ferroelectric domain inversion structure;

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

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

[0013] The method for preparing the overlay mark described in S1 is as follows: a photoresist pattern for the overlay mark is prepared on the surface of a Z-cut thin film lithium niobate sample using ultraviolet lithography; a layer of chromium and gold metal is sequentially deposited on the photoresist pattern by electron beam evaporation; and the photoresist is removed using an N-methylpyrrolidone solution to obtain the overlay mark structure.

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

[0015] The method for obtaining the ferroelectric domain inversion structure described in S3 is as follows: a Z-cut thin-film lithium niobate sample with a polarized electrode structure is placed on a copper sheet on a high-temperature hot stage, the polarized electrode is connected to the positive terminal of the polarization circuit, the copper sheet is connected to the negative terminal of the polarization circuit, and an electric field of 100-400V is applied to cause the Z-cut thin-film lithium niobate in the polarized electrode-covered area to undergo ferroelectric domain inversion; the polarized electrode is removed using dilute hydrochloric acid to obtain the ferroelectric domain inversion structure.

[0016] The method for obtaining a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure, as described in S4, is as follows: based on the overlay markings, the waveguide shape profile is prepared on the domain inversion structure region of the Z-cut thin-film lithium niobate sample using electron beam lithography; the waveguide structure is etched on the surface of the Z-cut thin-film lithium niobate sample by ion beam etching; the photoresist is removed using N-methylpyrrolidone solution and ultrasonically treated in a water bath to obtain a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

[0017] The polarization electrode structure consists of n+1 groups of comb-shaped electrodes arranged alternately, with the electrodes insulated from each other. Each group of comb-shaped electrodes is arranged periodically, with the electrode width being 1 / 4 of the polarization period, and n being the number of nodes in 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 second-order nonlinear coefficient of Z-cut thin-film lithium niobate and designs a corresponding two-dimensional ferroelectric domain structure based on the transverse electric field distribution of the second harmonic target mode. This not only satisfies the phase matching condition but also significantly improves the nonlinear overlap integral, thereby enabling efficient nonlinear conversion from the fundamental mode to the second harmonic higher-order mode. Compared with 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. Attached Figure Description

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

[0020] Figure 2 This is a schematic diagram of the two-dimensional ferroelectric domain structure in this invention, as well as a diagram of the transverse electric field distribution of the fundamental and second harmonic waves;

[0021] Figure 3 This is a schematic diagram of the waveguide mode evolution in the simulation experiment of this invention.

[0022] Figure 4 This is a schematic diagram of the polarization electrode structure in an example of the present invention;

[0023] Figure 5This is a schematic diagram of the process flow for fabricating a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure in an example of the present invention.

[0024] Figure 6 The figure shows the frequency doubling normalization efficiency results of the Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure prepared in the example of this invention during optical testing.

[0025] Figure 7 The fundamental wave TM in optical testing of the Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure prepared in the example of this invention. 00 and second harmonic™ 10 Pattern

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

[0027] Figure 9 This is a scanning electron microscope (SEM) characterization image of the Z-cut thin-film lithium niobate waveguide prepared in the example of this invention. Detailed Implementation

[0028] The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings.

[0029] The on-chip nonlinear guided wave mode conversion device based on ferroelectric domain engineering described in this invention includes a Z-cut thin-film lithium niobate waveguide, the cross-sectional structure of which is as follows: Figure 1 As shown, from top to bottom, it includes a Z-cut thin film lithium niobate, a silicon dioxide layer, and a silicon substrate. The Z-cut thin film lithium niobate layer has a two-dimensional domain structure, meaning that in the yx plane, the z-direction is the domain growth direction, and the inverted ferroelectric domain structure penetrates the thin film lithium niobate in the z-direction, with positive and negative domains arranged in a periodic alternation.

[0030] In this invention, the cross-sectional geometry of the Z-cut thin-film lithium niobate waveguide is as follows: h1 is 300-700nm, waveguide top width w is 2-3um, waveguide etching depth h2 is 300-500nm, waveguide sidewall tilt angle is 65-85°, silicon dioxide layer thickness is 1.9-3.0um, and silicon substrate thickness is 500-550um.

[0031] To achieve efficient nonlinear conversion between different waveguide modes and promote the practical application of nonlinear mode conversion in integrated photonic chips, this invention takes a Z-cut thin-film lithium niobate waveguide cross-section structure with a thickness h1 of 500 nm, a waveguide top width of 2.3 μm, a waveguide etching depth h2 of 300 nm, a waveguide sidewall tilt angle of 75°, a silicon dioxide layer thickness of 2 μm, and a silicon thickness of 525 μm as an example. Based on the effective nonlinear coefficients and transverse electric field distribution between different modes in the waveguide, three special ferroelectric domain structures are designed, which can respectively realize nonlinear conversion from the fundamental wave TM00 Mode to Second Harmonic™ i0 (i = 1, 2, 3) pattern conversion.

[0032] The distribution of three special ferroelectric domain structures is as follows: Figure 2 As shown, the waveguide lateral region is divided into i+1 equal regions. The first type of ferroelectric domain lateral distribution structure includes one region each of positive and negative domains; the second type includes two positive domain regions and one negative domain region, with positive and negative domains alternating; the third type includes two positive domain regions and two negative domain regions, with positive and negative domains alternating. All three ferroelectric domain structures are periodically arranged in the longitudinal direction. The longitudinally distributed ferroelectric domain structure can compensate for phase mismatch during nonlinear mode conversion, while the laterally distributed ferroelectric domain structure can break the spatial symmetry of the electric field of higher-order modes, thereby improving the nonlinear overlap integral and achieving effective nonlinear conversion from the fundamental mode to the second harmonic higher-order mode.

[0033] In this embodiment, simulation experiments were conducted on Z-cut thin-film lithium niobate waveguides with these three ferroelectric domain structures using Lumerical FDTD software. The mode evolution process is as follows: Figure 3 As shown. In the designed domain structure waveguide, the fundamental wave TM 00 The mode was effectively converted to second harmonic™. i0 For the (i=1,2,3) pattern, the theoretical normalization efficiency is as high as 4300%W. -1 cm -2 .

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

[0035] 1) Preparation of overlay markings: Photoresist patterns for overlay markings were prepared on the surface of Z-cut thin film lithium niobate samples using ultraviolet lithography. A 30nm / 70nm thick chromium / gold layer that is difficult to remove was deposited on the photolithographic structure by electron beam evaporation. The photoresist was removed using N-methylpyrrolidone (NMP) solution to obtain the overlay marking structure.

[0036] 2) Based on the overlay marks, the polarization electrode shape contour was prepared on the surface of the Z-cut thin-film lithium niobate sample using electron beam lithography, and a 100nm nickel metal electrode was deposited on the lithographic polarization electrode contour by electron beam evaporation; wherein the comb-shaped polarization electrode structure is as follows: Figure 4 As shown.

[0037] 3) The photoresist was removed using NMP solution to obtain a comb-shaped polarized electrode structure;

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

[0039] 5) The electrodes were removed using dilute hydrochloric acid to obtain a ferroelectric domain inversion structure;

[0040] 6) Based on the overlay marks, the waveguide shape profile was prepared on the domain inversion structure region of the Z-cut thin film lithium niobate sample by electron beam lithography;

[0041] 7) Waveguide structures were etched on the surface of Z-cut thin-film lithium niobate samples by ion beam etching, the photoresist was removed by NMP solution and the samples were ultrasonically treated in a water bath for 2 hours to obtain Z-cut thin-film lithium niobate waveguides with two-dimensional ferroelectric domain structures.

[0042] 8) Polish the end face of the prepared Z-cut thin-film lithium niobate waveguide with two-dimensional ferroelectric domain structure.

[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 overtone point, the fundamental power is 140uW, the overtone power is 13nW, and the normalized efficiency is 2250%W. -1 cm -2 Fundamental wave™ captured by CMOS camera 00 and second harmonic™ 10 Pattern such as Figure 7 As shown in the figure. Confocal imaging and electron microscopy were performed on the prepared two-dimensional ferroelectric domain structure and the prepared Z-cut thin-film lithium niobate waveguide, respectively. The obtained confocal images of the polarization region are shown in the figure. Figure 8 As shown, the ferroelectric domain structure is uniform, with a duty cycle of approximately 50%. The characterization image of the lithium niobate waveguide under an electron microscope is shown below. Figure 9 As shown, the waveguide sidewalls are smooth, and the experimentally measured waveguide loss is approximately 0.5 dB / cm.

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

[0045] To fabricate a Z-cut thin-film lithium niobate waveguide with periodically alternating positive and negative domains in a two-dimensional plane, the corresponding electrode structure consists of n+1 groups of comb-shaped electrodes arranged alternately, with the electrodes insulated from each other. Each group of comb-shaped electrodes is arranged periodically, with the electrode width being 1 / 4 of the polarization period, and n being the number of nodes in the target transverse electric field.

[0046] The generation of second harmonics in the Z-cut thin-film lithium niobate waveguide described in this 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, and E1 and E2 represent the normalized electric field distributions of the fundamental and second harmonics, which can be described as... Δβ represents the wave vector mismatch during the generation of the second harmonic, and L is the interaction length along the propagation direction x. ω1 is the effective second-order nonlinear coefficient, c is the speed of light in vacuum, ω1 is the fundamental frequency, and n2 is the effective refractive index of the second harmonic.

[0049] To achieve effective second harmonic generation of nonlinear waveguide modes in Z-cut thin-film lithium niobate waveguides, a phase-matching condition must be met, i.e., the wave vector mismatch should be through a second-order nonlinear coefficient χ that is periodically reversed along the x-direction. (2) The provided reciprocal vector compensation. In addition to the phase matching condition, the nonlinear overlap integral also needs to be considered, and its definition is as follows:

[0050]

[0051] Among them, E 1,z E 2,z These represent the z-components of the fundamental and second harmonic electric fields, respectively. d(y,z) represents the x-component in the waveguide. (2) The normalized distribution in the yz plane. From equation (2), it can be seen that the nonlinear overlapping integral is related to the electric field components in the z-direction of the fundamental and second harmonic waves, and also to the χ-direction. (2) The lateral distribution in the yz plane is related.

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

[0053] This invention utilizes the spatially modulated second-order nonlinear coefficients of Z-cut thin-film lithium niobate to design a corresponding two-dimensional ferroelectric domain structure based on the transverse electric field distribution of the second harmonic target mode. This structure not only satisfies the phase-matching condition but also significantly improves the nonlinear overlap integral, thereby enabling efficient nonlinear conversion from the fundamental mode to higher-order second harmonic modes. Compared with existing solutions, this invention does not require precise control of waveguide dispersion and has good scalability, potentially promoting the practical application of nonlinear mode conversion in integrated photonic chips.

Claims

1. An on-chip nonlinear guided wave mode conversion device based on ferroelectric domain engineering, characterized in that, The waveguide includes a Z-cut thin-film lithium niobate waveguide, which, from top to bottom, comprises a Z-cut thin-film lithium niobate layer, a silicon dioxide layer, and a silicon substrate layer. The Z-cut thin-film lithium niobate layer contains a two-dimensional ferroelectric domain structure, used to achieve selective nonlinear conversion from the fundamental mode to a target second harmonic guided mode (TM). 20 Pattern or TM 30 model; The two-dimensional ferroelectric domain structure is arranged in a periodic alternation of positive and negative domains in the transmission direction, which is used to compensate for the phase mismatch during the conversion of the fundamental mode to the target second harmonic guided mode; When the target second harmonic waveguide mode is TM 20 In the mode, the two-dimensional ferroelectric domain structure includes three transverse domain regions—positive domain, negative domain, and positive domain—in a transverse direction perpendicular to the propagation direction, with a period of 2.7 μm and a duty cycle of 50% in the propagation direction. When the target second harmonic waveguide mode is TM 30 In the mode, the two-dimensional ferroelectric domain structure includes four transverse domain regions in the transverse direction perpendicular to the transmission direction: positive domain, negative domain, positive domain, and negative domain, with a period of 3.0 μm and a duty cycle of 50% in the transmission direction. The polarization directions of adjacent transverse domain regions are opposite, so that the spatial distribution of the nonlinear polarization wave generated by the two-dimensional ferroelectric domain structure in the transverse direction matches the transverse electric field distribution of the target second harmonic guided mode, thereby realizing the fundamental mode to TM 20 Pattern or TM 30 Selective nonlinear conversion of second harmonic guided modes; The cross-sectional geometry of the Z-cut thin-film lithium niobate waveguide is as follows: the thickness h1 of the Z-cut thin-film lithium niobate layer is 300-700 nm, the waveguide top width w is 2-3 μm, the waveguide etching depth h2 is 300-500 nm, the waveguide sidewall tilt angle is 65-85°, the silicon dioxide layer thickness is 1.9-3.0 μm, and the silicon substrate layer thickness is 500-550 μm.

2. The on-chip nonlinear guided wave mode conversion device according to claim 1, characterized in that, The target second harmonic waveguide mode is TM. 20 The polarization electrode structure consists of three sets of comb-shaped electrodes arranged alternately, with the electrode width being 1 / 4 of the polarization period.

3. The on-chip nonlinear guided wave mode conversion device according to claim 1, characterized in that, The target second harmonic waveguide mode is TM. 30 The polarization electrode structure consists of four sets of comb-shaped electrodes arranged alternately, with the electrode width being 1 / 4 of the polarization period.

4. A method for fabricating an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering as described in any one of claims 1 to 3, characterized in that, include: S1: Prepare overlay marks on the surface of Z-cut thin-film lithium niobate samples; S2: Based on the overlay markings, a polarized electrode structure is prepared on the surface of the Z-cut thin film lithium niobate sample; S3: Apply an electric field to the Z-cut thin film lithium niobate sample with a polarized electrode structure to cause ferroelectric domain inversion in the thin film lithium niobate sample in the polarized electrode covered area; remove the polarized electrode structure to obtain the ferroelectric domain inversion structure; S4: Based on the overlay marks, a waveguide shape profile is prepared on the region with the ferroelectric domain inversion structure of the Z-cut thin film lithium niobate sample, and the waveguide structure is etched out to further obtain a Z-cut thin film lithium niobate waveguide with a two-dimensional ferroelectric domain structure. S5: Polish the end face of the prepared Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

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

6. The method for fabricating an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 4, characterized in that, The method described in S2 for preparing a polarized electrode structure on the surface of a Z-cut thin-film lithium niobate sample is as follows: according to the overlay marks, the shape contour of the polarized electrode is prepared on the surface of the Z-cut thin-film lithium niobate sample by electron beam lithography, and a nickel metal electrode is deposited on the lithographic polarized electrode contour by electron beam evaporation; the photoresist is removed by N-methylpyrrolidone solution to obtain the polarized electrode structure.

7. The method for fabricating an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 4, characterized in that, The method described in S3 for obtaining the ferroelectric domain inversion structure is as follows: A Z-cut thin-film lithium niobate sample with a polarized electrode structure is placed on a copper sheet on a high-temperature hot stage. The polarized electrode is connected to the positive terminal of the polarization circuit, and the copper sheet is connected to the negative terminal of the polarization circuit. An electric field of 100-400V is applied to cause the Z-cut thin-film lithium niobate in the polarized electrode-covered area to undergo ferroelectric domain inversion. The polarized electrode is removed using dilute hydrochloric acid to obtain the ferroelectric domain inversion structure.

8. The method for fabricating an on-chip nonlinear waveguide mode conversion device based on ferroelectric domain engineering according to claim 4, characterized in that, The method described in S4 for obtaining a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure is as follows: based on the overlay markings, the waveguide shape profile is prepared on the domain inversion structure region of the Z-cut thin-film lithium niobate sample using electron beam lithography; the waveguide structure is etched on the surface of the Z-cut thin-film lithium niobate sample by ion beam etching; the photoresist is removed using N-methylpyrrolidone solution and ultrasonically treated in a water bath to obtain a Z-cut thin-film lithium niobate waveguide with a two-dimensional ferroelectric domain structure.

Images