Preparation method of lithium niobate film waveguide frequency doubling device based on ferroelectric domain regulation and control

Abstract

The invention discloses a preparation method of a lithium niobate film waveguide frequency doubling device based on ferroelectric domain regulation and control. The method comprises the following steps of: spin-coating photoresist on the surface of a lithium niobate film, transferring a pattern by utilizing a photoetching technology, coating a film, stripping to form an electrode, applying high-voltage electric pulse to the electrode, and controlling the size of an electric field in the lithium niobate film and a reversal domain growth process through the material and the thickness of an insulating layer in the electrode, thereby controlling the reversal depth of a ferroelectric domain. And finally, spin-coating the photoresist again, transferring the pattern by photoetching, and performing dry etching to obtain the frequency doubling waveguide device. The frequency multiplication waveguide device preparation method provided by the invention has the characteristics of simple processing method, high device preparation efficiency and low loss, and meets the actual requirements in the fields of nonlinear optics, integrated optics and the like.

Description

Technical Field

[0001] The invention belongs to the field of nonlinear optics, and in particular relates to a method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology. Background Art

[0002] Lithium niobate materials have many advantages in the field of optics, such as high refractive index, wide transparent window, strong electro-optical effect, large nonlinear coefficient, and good piezoelectric performance. They are widely used in optical devices, especially integrated photonic devices. Waveguides made by etching lithium niobate thin film materials have strong light restriction and can manufacture devices with both high performance and high integration. With the improvement of micro-nano processing technology, the thickness of common lithium niobate films can reach several hundred nanometers, and can ensure a certain degree of integrity and uniformity. Therefore, the research on integrated photonic devices based on lithium niobate films has developed rapidly.

[0003] Optical second-order nonlinear effects are of great value in many applications such as frequency conversion, photon pair generation, and supercontinuum generation. The intensity of the nonlinear effect produced by light in a material depends on the intensity of the light field and the nonlinear coefficient of the material. Lithium niobate itself has the characteristics of a high second-order nonlinear coefficient. At the same time, lithium niobate thin film materials can provide strong light field confinement and improve the local light field intensity. Therefore, the lithium niobate thin film platform has become an ideal platform for people to realize second-order nonlinear effects. In order to achieve efficient nonlinear effects, it is necessary to achieve phase matching between the pump frequency light and the target frequency light. Quasi-phase matching is a common phase matching technology. At present, it is usually achieved by applying high-voltage pulses to lithium niobate materials for periodic polarization to produce periodically reversed nonlinear coefficients. However, this phase matching technology has low processing robustness and a relatively complex process. A slight deviation in size may cause the matching wavelength to drift and reduce the conversion efficiency. Inter-mode phase matching adjusts the waveguide dispersion to achieve phase matching between different order modes of the pump frequency light and the target frequency light. This phase matching technology is relatively simple and has high process robustness, but it has problems such as high transmission loss of high-order modes and low mode overlap. Some related literatures have proposed a semi-nonlinear waveguide solution, which makes a waveguide with heterogeneous integration of titanium dioxide material and lithium niobate film without second-order nonlinear coefficients, introduces the asymmetry of nonlinear coefficient distribution, and avoids the reduction of mode overlap integral caused by the antisymmetric distribution of high-order mode electric field. However, in this case, the light field of the zero nonlinear part in the waveguide does not participate in the nonlinear effect, resulting in low overall conversion efficiency. Some related literatures have also proposed a semi-nonlinear waveguide with continuous domain bound state, which avoids the tedious steps such as etching and deposition for preparing heterogeneous integrated waveguides, but due to the problem of light field limitation, the nonlinear conversion efficiency has not been improved. In addition, some literatures have proposed that it is possible to go a step further on the basis of semi-nonlinear waveguides, bond two layers of lithium niobate films and etch waveguides, so that the reverse nonlinear coefficient will replace the zero nonlinear part of the semi-nonlinear waveguide, effectively improving the conversion efficiency. However, the cost is that the manufacturing process is cumbersome, requiring the lithium niobate film wafer to be reversely aligned, thermally bonded, wet-etched to remove the substrate layer and buffer layer, and finally dry-etched to form a double-layer thin film lithium niobate waveguide. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention proposes a method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation, which can use ferroelectric domain regulation technology to control the inversion depth of ferroelectric domains inside the lithium niobate film, thereby realizing a waveguide frequency doubling device with high conversion efficiency, low loss and easy preparation.

[0005] In the present invention, the inter-mode phase matching between the transverse electric fundamental mode of the fundamental frequency optical signal and the transverse electric high-order mode of the frequency-doubled optical signal can be achieved through the waveguide size design. On this basis, through the ferroelectric domain control technology, an insulating layer is introduced when making the polarization electrode, and the polarization voltage is continuously optimized in the polarization step to control the polarization electric field size and the inversion domain growth process inside the lithium niobate film, so that the direction of the lithium niobate ferroelectric domain is reversed at a certain depth, thereby introducing an asymmetric second-order nonlinear coefficient, eliminating the influence of the antisymmetric distribution of the electric field of the high-order mode of the frequency-doubled optical signal on the mode overlap integral, and finally obtaining a high-efficiency, low-loss lithium niobate film waveguide frequency doubling device.

[0006] The present invention proposes a method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation, comprising the following steps: Step (1): firstly, clean the surface of the lithium niobate film with an organic solvent, spin-coat an ultraviolet photoresist, obtain a mask required for coating by an ultraviolet photolithography method, sequentially grow an insulating layer, a connecting layer, and a metal layer film by an electron beam evaporation method, and form an electrode on the surface of the lithium niobate film by a stripping method.

[0007] Step (2), for the surface electrode of the lithium niobate film, use ferroelectric domain control technology, apply high-voltage electric pulse polarization, control the internal electric field size and the inversion domain growth process of the lithium niobate film through the material and thickness of the electrode insulation layer as required, and reverse the direction of the ferroelectric domain of the lithium niobate film at a corresponding depth close to the film surface, thereby forming a double-layer lithium niobate film structure with opposite ferroelectric domain directions.

[0008] Step (3), using an organic solvent to clean the surface of the lithium niobate film treated in step (2), spin-coating electron beam photoresist, obtaining a waveguide pattern mask by electron beam or ultraviolet exposure, and transferring it to the lithium niobate film by dry etching to form a ridge waveguide structure.

[0009] Step (4), using an inorganic solvent to clean the ridge waveguide, remove residual glue and surface contaminants, and grind and polish the end face of the ridge waveguide to obtain a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology.

[0010] In one embodiment, the process of cleaning the surface of the lithium niobate film in step (1) is: using acetone and isopropyl alcohol for ultrasonic cleaning to remove organic and inorganic particles, and finally cleaning with deionized water and drying with nitrogen.

[0011] In one embodiment, the mask layer used in the electron beam evaporation is a negative photoresist.

[0012] In one embodiment, the insulating layer material is aluminum oxide Al2O3 or silicon dioxide SiO2, with a thickness of 50-150 nanometers; the connecting layer material is titanium Ti or chromium Cr, with a thickness of 10-20 nanometers; the metal layer material is gold Au, with a thickness of 50-500 nanometers.

[0013] In one embodiment, the voltage used for the polarization is in the range of 300 volts to 800 volts, and the thickness of the lithium niobate at which the direction of the ferroelectric domains on the surface of the film is reversed is in the range of 100 nanometers to 600 nanometers.

[0014] In one embodiment, the process of cleaning the surface of the lithium niobate film in step (3) is: soaking and cleaning with acetone and isopropyl alcohol to remove organic and inorganic particles, and finally cleaning with deionized water and drying with nitrogen.

[0015] In one embodiment, the mask layer used for the dry etching is a negative photoresist.

[0016] In one embodiment, the dry etching method is inductively coupled plasma etching, using argon as the etching gas.

[0017] In one embodiment, the process of removing residual glue and surface contaminants is: preparing a solution using concentrated sulfuric acid and hydrogen peroxide solution, then immersing the chip therein for 3 to 12 hours, and finally rinsing with deionized water and drying with nitrogen.

[0018] The beneficial effects of the present invention are as follows: (1) By utilizing ferroelectric domain control technology, an asymmetric nonlinear coefficient can be introduced into the lithium niobate thin film waveguide, eliminating the influence of the antisymmetric distribution of the electric field in the high-order mode of the target frequency light on the mode overlap integral, greatly improving the nonlinear conversion efficiency.

[0019] (2) The ferroelectric domain control technology is used to prepare a lithium niobate thin film waveguide frequency doubling device based on inter-mode phase matching. First, it can avoid the complex process steps and high micro-nano processing level required for preparing a precise periodically polarized ferroelectric domain structure when using quasi-phase matching technology, thereby having a larger process tolerance; secondly, it can avoid the complex steps such as wafer alignment, thermal bonding, and wet etching required for double-layer lithium niobate thin film waveguides, thereby reducing the processing complexity of the device.

[0020] (3) The dry-etched lithium niobate thin film ridge waveguide device can effectively enhance the confinement of the light field compared to the lithium niobate bulk material optical waveguide or the continuous domain bound state waveguide, thereby improving the nonlinear conversion efficiency and integration of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a process flow chart for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology in an embodiment.

[0022] Figure 2 It is a schematic diagram of the application process of the ferroelectric domain control technology in the embodiment.

[0023] Among them: (a) is a schematic diagram of the direction and size of the polarization electric field inside the lithium niobate film when the ferroelectric domain control technology is applied; (b) is a schematic diagram of the direction of the ferroelectric domain inside the lithium niobate film after etching the ridge waveguide.

[0024] Figure 3 Schematic diagram of the mode of application of lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology in mode phase matching technology in the embodiment.

[0025] Among them: (a) is the fundamental mode distribution diagram of the fundamental frequency light transverse electric mode in the waveguide cross section; (b) is the high-order mode distribution diagram of the frequency-doubled light transverse electric mode in the waveguide cross section.

[0026] Figure 4 This is a diagram showing the simulation results of the normalized conversion efficiency of the lithium niobate thin film waveguide frequency doubling device based on the ferroelectric domain control technology in the embodiment.

[0027] Figure 5 This is a graph showing the experimental results of the normalized conversion efficiency of the lithium niobate thin film waveguide frequency doubling device based on the ferroelectric domain control technology in the embodiment. DETAILED DESCRIPTION

[0028] The present invention is further described below in conjunction with the accompanying drawings and an embodiment of a method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology. To make the accompanying drawings clearer, the dimensions of the components in the drawings are not drawn completely in accordance with the actual scale.

[0029] The specific embodiments of the present invention are as follows: like Figure 1 As shown, a method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology comprises the following steps: (1) For an x-cut lithium niobate film with a length of 1.5 cm, a width of 1.2 cm, a silicon substrate layer thickness of 0.5 mm, a silicon dioxide buffer layer thickness of 2 μm, and a lithium niobate film thickness of 600 nm, acetone and isopropanol were used for ultrasonic cleaning to remove organic and inorganic particles, and then the film was cleaned with deionized water and dried with nitrogen.

[0030] (2) Spin-coat a 1 μm thick UV photoresist on the surface of the lithium niobate film, and use a mask alignment photolithography machine to transfer the electrode pattern on the mask to the surface of the lithium niobate film to form a UV photoresist mask.

[0031] (3) Using electron beam evaporation coating equipment, a 50-nanometer-thick aluminum oxide film, a 15-nanometer-thick chromium film, and a 150-nanometer-thick gold film are deposited on the sample surface from bottom to top.

[0032] (4) The coated sample was immersed in acetone for 3 hours, and the coating portion covered by the UV photoresist mask was peeled off to obtain the electrode structure on the surface of the lithium niobate film.

[0033] (5) Using a signal generator, a high-voltage amplifier, an oscilloscope and a probe, the surface electrodes of the lithium niobate film are loaded with multiple continuous 400-volt electric pulses for polarization, thereby obtaining a lithium niobate film with 300 nanometers of lithium niobate iron domains reversed in direction on the surface.

[0034] (6) The treated lithium niobate film is soaked and cleaned with acetone and isopropyl alcohol to remove organic and inorganic particles, and then cleaned with deionized water and dried with nitrogen.

[0035] (7) Spin-coat a 500-nanometer-thick electron beam photoresist on the surface of the lithium niobate film, and use an electron beam exposure machine to transfer the waveguide pattern to the surface of the lithium niobate film to form an electron beam photoresist mask.

[0036] (8) The sample is placed in an inductively coupled plasma etcher for dry etching to obtain a lithium niobate thin film ridge waveguide structure with an etching depth of 400 nanometers.

[0037] (9) The etched sample was immersed in a solution prepared by concentrated sulfuric acid and 30% hydrogen peroxide aqueous solution in a ratio of 3:1 for 3 h to remove the remaining residual glue and surface contaminants, and then washed with deionized water and dried with nitrogen.

[0038] (10) The two ends of the sample waveguide are cleaved and polished to obtain high-quality end faces.

[0039] (11) Perform end-face coupling on the waveguide and test the frequency doubling conversion performance, and record the results.

[0040] In this embodiment, the bottom layer of the lithium niobate thin film wafer used is a silicon substrate layer, the substrate layer is a silicon dioxide buffer layer, the buffer layer is a lithium niobate thin film, and the film surface is prepared into a pair of polarized electrodes by evaporation and stripping methods. Figure 2 As shown in (a), when the ferroelectric domain control technology is applied, positive high voltage and grounding are applied to the surface electrodes of the lithium niobate film respectively, and the direction of the electric field formed is opposite to the direction of the ferroelectric domain of the lithium niobate film. The insulating layer in the polarization electrode can control the size of the polarization electric field inside the lithium niobate film, and reduce the domain wall leakage current, thereby improving the uniformity of the growth of the inversion domain, reducing the lateral expansion speed of the inversion domain, and forming a structure in which the ferroelectric domain direction is layered and reversed up and down inside the lithium niobate film. By making additional test pieces, scanning the polarization voltage and etching at different depths, and using a piezoelectric force microscope for characterization, the ferroelectric domain reversal depth under the corresponding voltage can be observed and determined.

[0041] like Figure 2As shown in (b), after dry etching the ridge waveguide, the cross section of the lithium niobate film waveguide has a ferroelectric domain direction corresponding to the depth of the polarization electric field. Figure 3 As shown in the figure, the waveguide cross-section mode distribution diagram of the fundamental frequency light transverse electric fundamental mode and the frequency doubling light transverse electric high-order mode used in the lithium niobate thin film waveguide frequency doubling device in the embodiment when the mode phase matching technology is applied, and the theoretical models such as the wave equation and the coupled mode equation of the frequency doubling process are applied, after designing the waveguide size and the ferroelectric domain inversion depth, the frequency doubling normalized conversion efficiency simulation curve of the lithium niobate thin film waveguide frequency doubling device in the embodiment can be obtained, as shown in FIG. Figure 4 shown.

[0042] Through the ferroelectric domain control technology, different ferroelectric domain directions are introduced in the depth direction of the lithium niobate film, which is equivalent to an asymmetric second-order nonlinear coefficient, thus eliminating Figure 3 The effect of the antisymmetric distribution of the electric field in the high-order mode of the mid-frequency light on the mode overlap integral when there is no ferroelectric domain regulation can significantly improve the nonlinear conversion efficiency. The performance of the lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation technology prepared in the embodiment was tested, and the test results of the normalized conversion efficiency of the frequency doubling changing with the wavelength of the fundamental frequency light were finally obtained as follows: Figure 5 As shown, the highest normalized conversion efficiency is 7000% / W / cm 2 , showing a good agreement with the simulation results.

[0043] In addition, according to the test results, the calculated fundamental frequency optical coupling loss and transmission loss are both low. The above simulation and test results of the embodiment show that the lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology and its preparation method proposed by the present invention can produce a lithium niobate thin film waveguide frequency doubling device with high conversion efficiency, low loss and simple process.

[0044] The above embodiments are used to illustrate the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation. It is characterized in that The following steps are involved: Step (1), firstly, using an organic solvent to clean the surface of the lithium niobate film, spin-coating an ultraviolet photoresist, obtaining a mask required for coating by an ultraviolet photolithography method, sequentially growing an insulating layer, a connecting layer, and a metal layer film by an electron beam evaporation method, and forming an electrode on the surface of the lithium niobate film by a stripping method; Step (2), using ferroelectric domain control technology for the surface electrode of the lithium niobate film, applying high voltage electric pulse polarization, controlling the internal electric field size and the inversion domain growth process of the lithium niobate film by using the material and thickness of the electrode insulation layer as required, and reversing the direction of the ferroelectric domain of the lithium niobate film at a corresponding depth close to the film surface, thereby forming a double-layer lithium niobate film structure with opposite ferroelectric domain directions; Step (3), using an organic solvent to clean the surface of the lithium niobate film treated in step (2), spin-coating an electron beam photoresist, obtaining a waveguide pattern mask by electron beam or ultraviolet exposure, and transferring the mask to the lithium niobate film by dry etching to form a ridge waveguide structure; Step (4), using an inorganic solvent to clean the ridge waveguide, remove residual glue and surface contaminants, and grind and polish the end face of the ridge waveguide to obtain a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain control technology.

2. A method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The process of cleaning the surface of the lithium niobate film in step (1) is: using acetone and isopropanol for ultrasonic cleaning to remove organic and inorganic particles, and finally cleaning with deionized water and drying with nitrogen.

3. The method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The mask layer used for electron beam evaporation is negative photoresist.

4. The method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The insulating layer material is aluminum oxide Al2O3 or silicon dioxide SiO2, with a thickness of 50-150 nanometers; the connecting layer material is titanium Ti or chromium Cr, with a thickness of 10-20 nanometers; the metal layer material is gold Au, with a thickness of 50-500 nanometers.

5. The method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The voltage range used for the polarization is 300 volts to 800 volts, and the thickness of the lithium niobate in which the ferroelectric domain direction is reversed on the surface of the film is 100 nanometers to 600 nanometers.

6. The method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The process of cleaning the surface of the lithium niobate film in step (3) is: soaking and cleaning with acetone and isopropanol to remove organic and inorganic particles, and finally cleaning with deionized water and drying with nitrogen.

7. The method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The mask layer used for the dry etching is a negative photoresist.

8. A method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1 or 7, It is characterized in that The dry etching method is inductively coupled plasma etching, using argon as etching gas.

9. The method for preparing a lithium niobate thin film waveguide frequency doubling device based on ferroelectric domain regulation according to claim 1, It is characterized in that The process of removing residual glue and surface pollutants is: using concentrated sulfuric acid and hydrogen peroxide aqueous solution to prepare a solution, then immersing the chip in the solution for 3 to 12 hours, and finally rinsing with deionized water and drying with nitrogen.

Images