High-efficiency on-chip acousto-optic deflector and method of manufacturing the same

Abstract

The present application relates to the field of integrated photonics on chip, more particularly, to a high-efficiency on-chip acousto-optic deflector and a preparation method thereof. The acousto-optic deflector comprises a lithium niobate-silicon nitride heterostructure arranged on a silicon oxide substrate, and the lithium niobate-silicon nitride heterostructure comprises a lithium niobate thin film and a multimode coupler structure formed by silicon nitride. An interdigital transducer capable of exciting acoustic waves is arranged on the lithium niobate thin film. In the present application, the piezoelectric property of lithium niobate and the acousto-optic effect are utilized, the interdigital transducer and the multimode coupler principle in the photonic waveguide are integrated, and the Bragg diffraction is generated by the periodic structure formed by the photons and the acoustic waves, so as to finally realize deflection. The above-mentioned acousto-optic deflector reduces the size of the on-chip acousto-optic deflector, enhances the interaction between phonons and photons, and improves the efficiency. The lithium niobate and silicon nitride materials used are stable in properties, easy to process, and can be realized in large-scale integration on chip.

Description

Technical Field

[0001] This invention relates to the field of on-chip integrated photonics technology, and more specifically, to a high-efficiency on-chip acousto-optic deflector and its fabrication method. Background Technology

[0002] Acousto-optic devices are multi-physics coupling elements that utilize the interaction between sound waves and light waves, and are indispensable devices in the interaction between microwaves and light waves. Although these acousto-optic devices are all based on structures such as on-chip waveguide electrodes and utilize the principle of interaction between sound waves and light waves, they differ greatly in function, structure, and application. Currently, various acousto-optic devices with different application scenarios and structures have been developed on the market, such as acousto-optic modulators, acousto-optic frequency shifters, acousto-optic deflectors, acousto-optic filters, and acousto-optic Q-switches, which are used to achieve effective control of the intensity, direction, and frequency of light wave transmission. They are physical devices that combine electroacoustic transducers and piezoelectric crystal materials to perform multi-dimensional modulation of the relevant parameters of the incident light wave signal.

[0003] Acousto-optic deflectors are a common and widely used type of acousto-optic device. Unlike modulators that use sound waves to change the intensity of light waves, acousto-optic deflectors utilize the Doppler effect of sound waves to change the propagation direction of light waves. Structurally, acousto-optic deflectors do not have Mach-Zehnder interferometers or resonant cavity structures. Currently, acousto-optic deflectors are divided into bulk wave driven and surface wave driven types. Due to their advantages such as low driving power, small size, high deflection efficiency, and ease of integration, they have broad application and development prospects in fields such as optical communication, laser processing technology, laser ranging technology, optical radar technology, and laser medical technology. Therefore, enhancing the interaction strength between sound waves and light waves on a small scale is an inevitable trend in the future development of on-chip acousto-optic devices. However, current surface acoustic wave deflectors are limited to spatially diffused structures, with planar structures having dimensions in the hundreds of micrometers range, requiring further miniaturization; simultaneously, the device efficiency is not high, necessitating the use of bound-state photonic coupler structures to enhance the coupling efficiency between phonons and photons. In recent years, researchers have experimentally investigated the deflection effect of surface acoustic waves (SAWs) on photons based on different on-chip photonic device structures, such as on-chip deflectors etched from pure lithium niobate and photon bound state mode structures based on lithium niobate-organic materials. However, these methods still suffer from drawbacks such as low deflection efficiency and large structural size. Therefore, achieving efficient acousto-optic deflection remains a significant challenge, making research into novel, high-efficiency on-chip acousto-optic deflectors of paramount practical importance. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a high-efficiency on-chip acousto-optic deflector, which reduces the size of the on-chip acousto-optic deflector, enhances the interaction between phonons and photons, and improves efficiency.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by this invention is: a high-efficiency on-chip acousto-optic deflector, comprising a lithium niobate-silicon nitride heterostructure disposed on a silicon oxide substrate; the lithium niobate-silicon nitride heterostructure includes a lithium niobate thin film and a photonic multimode coupler formed by a silicon nitride structure integrated on the lithium niobate thin film; the lithium niobate thin film is provided with an excitable interdigital transducer for acoustic waves, the interdigital transducer including a plurality of interdigital electrodes. This invention comprehensively utilizes the excellent piezoelectric properties of the lithium niobate thin film and the acousto-induced grating diffraction effect, and places the interdigital transducer on the side of the photonic waveguide coupler to achieve high-efficiency deflection of photons by surface acoustic waves. This invention uses a coupler structure to reduce the structural size of existing acousto-optic deflectors, and makes full use of the coupling of acoustic waves and optical waves for efficient deflection. On the one hand, it solves the drawback of insufficient utilization of acoustic wave energy in traditional acousto-optic deflectors, and on the other hand, it solves the defect of excessively large coupling region size in acousto-optic deflectors.

[0006] In this invention, interdigitated transducers are used to excite surface acoustic waves, and the refractive index of the lithium niobate film is periodically changed to form a Bragg grating in the direction of a certain angle between the light waves, thereby efficiently deflecting the light waves. This reduces the size of the on-chip acousto-optic deflector, enhances the interaction between phonons and photons, and improves efficiency. The lithium niobate and silicon nitride materials used are stable, easy to process, and can be integrated on a large scale on a chip.

[0007] In one embodiment, the lithium niobate-silicon nitride heterostructure is an on-chip structure based on a silicon-silicon dioxide substrate, which does not require etching of lithium niobate, but only requires etching of silicon nitride to define the planar device structure.

[0008] In one embodiment, the lithium niobate-silicon nitride heterostructure includes an input straight waveguide, a multimode coupler, and two tapered output waveguides; both the input straight waveguide and the tapered output waveguides are connected to the multimode coupler.

[0009] In one embodiment, the multi-mode coupler is a polygonal structure.

[0010] In one embodiment, the deflection light exit surface of the multimode coupler is perpendicular to the direction of the light wave vector.

[0011] In one embodiment, the interdigital transducer forms an angle with the input straight waveguide to satisfy the Bragg diffraction formula.

[0012] In one embodiment, the thickness of the lithium niobate film is 100 nm to 1500 nm.

[0013] In one embodiment, the thickness of the silicon nitride is 200nm to 400nm; the width of the silicon nitride is 300nm to 2um; and the operating wavelength is 1000nm to 2000nm.

[0014] In one embodiment, the interdigital transducer is a GS structure with 20 to 80 pairs of interdigits, and the interdigital transducer is capable of exciting Rayleigh surface acoustic waves from 200 MHz to 10 GHz.

[0015] This invention also provides a method for fabricating a high-efficiency on-chip acousto-optic deflector, comprising the following steps:

[0016] S1. A silicon nitride film is deposited on a substrate covered with a lithium niobate film using a thermal evaporation method;

[0017] S2. Positive electron paste is exposed onto a pre-prepared silicon nitride thin film using an electron beam direct writing system. After baking and development, the desired input straight waveguide, tapered output waveguide, and multi-mode coupler patterns can be obtained on the electron paste.

[0018] S3. Using reactive ion etching equipment, dry etching is performed with the pattern obtained on the electron adhesive as a mask, requiring smooth and steep sidewall morphology; etching parameters are set at any time according to the etching effect of the equipment.

[0019] S4. Place the etched substrate into the chamber and use oxygen plasma etching gas to remove the residual electron adhesive on top;

[0020] S5. Positive electron paste is exposed onto the etched substrate using an electron beam direct writing system. After development, the mask pattern of the interdigital transducer can be obtained on the electron paste.

[0021] S6. Deposit gold on the developed and exposed substrate using a thermal evaporation method;

[0022] S7. Immerse the deposited substrate in an organic solution to remove the electron adhesive and the metal film on the substrate surface, thus completing the fabrication of the interdigital transducer.

[0023] Compared with existing technologies, the advantages of this invention are as follows: The high-efficiency on-chip acousto-optic deflector and its fabrication method provided by this invention are based on a lithium niobate-silicon nitride heterostructure, fully utilizing the piezoelectric effect of lithium niobate and the acousto-optic grating diffraction effect. Simultaneously, the use of a multi-mode coupler as the main structure of the deflector significantly enhances the coupling strength of the acousto-optic interaction; and by fully utilizing the photon-binding characteristics of the multi-mode coupler, the device size is significantly reduced. Compared with current technologies, this proposed solution will provide insights for the development of high-efficiency acousto-optic deflectors. Attached Figure Description

[0024] Figure 1This is a schematic diagram of the acousto-optic deflector of the present invention.

[0025] Figure 2 This is a schematic diagram of the acousto-optic deflector of the present invention.

[0026] Figure 3 This is a schematic diagram of the fabrication process of the acousto-optic deflector of the present invention.

[0027] Figure 4 This is a schematic diagram of the test system for the acousto-optic deflector of the present invention.

[0028] Figure 5 This is a light field diagram of an embodiment of the present invention in the absence of sound waves;

[0029] Figure 6 This is a light field diagram of an embodiment of the present invention under the condition of sound waves;

[0030] Figure 7 This is a diagram showing the optical power detected at the deflection output port under different conditions according to an embodiment of the present invention.

[0031] Reference numerals: 1. Input straight waveguide; 2. Multimode coupler; 3. Tapered output waveguide; 4. Interdigital transducer. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The present invention will be described in one embodiment below with reference to specific embodiments. The accompanying drawings are for illustrative purposes only and represent schematic diagrams, not actual pictures, and should not be construed as limiting the present patent. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the actual product size. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0033] In the description of this invention, it should be understood that if terms such as "upper," "lower," "left," and "right" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms describing positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances. In addition, if the embodiments of this invention involve descriptions of "first," "second," etc., such descriptions are only for descriptive purposes and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one of those features. Furthermore, the meaning of "and / or" throughout the text is to include three parallel solutions. Taking "A and / or B" as an example, it includes solution A, or solution B, or a solution that simultaneously satisfies A and B.

[0034] Example 1:

[0035] like Figure 1 As shown, a high-efficiency on-chip acousto-optic deflector includes a lithium niobate-silicon nitride heterostructure disposed on a silicon oxide substrate. The lithium niobate-silicon nitride heterostructure includes a lithium niobate thin film and a photonic multimode coupler 2 formed by a silicon nitride structure integrated on the lithium niobate thin film. An excitable interdigital transducer 4 is provided on the lithium niobate thin film, and the interdigital transducer 4 includes several interdigital electrodes. This invention comprehensively utilizes the excellent piezoelectric properties of the lithium niobate thin film and the acousto-induced grating diffraction effect, placing the interdigital transducer 4 on the side of the photonic waveguide coupler to achieve high-efficiency deflection of photons by surface acoustic waves. This invention uses a coupler structure to reduce the structural size of existing acousto-optic deflectors, fully utilizing the coupling of sound waves and light waves for efficient deflection. On the one hand, it solves the drawback of insufficient utilization of sound wave energy in traditional acousto-optic deflectors; on the other hand, it solves the defect of excessively large coupling region size in acousto-optic deflectors.

[0036] The principle of the acousto-optic deflector: The process of converting microwaves to surface acoustic waves is based on the inverse piezoelectric coupling effect of a lithium niobate thin film. The interdigital transducer 4 converts the input microwave electrical signal into a surface acoustic wave signal from the lithium niobate thin film, causing a periodic mechanical strain field distribution in the lithium niobate film, forming an equivalent Bragg grating. When the light wave in the optical waveguide passes through this periodically strained structure, its direction is deflected. This process is based on the piezoelectric effect and Bragg diffraction effect of the coupling system.

[0037] The lithium niobate-silicon nitride heterostructure is an on-chip structure based on a silicon-silicon dioxide substrate. It does not require etching of lithium niobate; only silicon nitride needs to be etched to define the planar device structure.

[0038] Furthermore, the lithium niobate-silicon nitride heterostructure includes an input straight waveguide 1, a multimode coupler 2, and two tapered output waveguides 3; both the input straight waveguide 1 and the tapered output waveguides 3 are connected to the multimode coupler 2. The multimode coupler 2 has a polygonal structure, and the deflection light exit surface of the multimode coupler 2 is perpendicular to the direction of the light wave vector.

[0039] The interdigital transducer 4 forms a certain angle with the input straight waveguide 1 to satisfy the Bragg diffraction formula.

[0040] like Figure 2 As shown in the deflection principle, the angle between the deflected output waveguide and the input waveguide can be calculated according to the Bragg diffraction formula sinθ=λ2nd; where θ is the angle between the IDT interdigit and the input straight waveguide 1, λ is the wavelength of the light wave, which is 1550nm in this embodiment, n is the equivalent refractive index, and d is the IDT period, which is the wavelength of the excited surface acoustic wave.

[0041] In addition, the thickness of the lithium niobate film is 100nm to 1500nm; the thickness of the silicon nitride is 200nm to 400nm; the width of the silicon nitride is 300nm to 2um, and the operating wavelength is 1000nm to 2000nm; the interdigital transducer 4 is a GS structure with 20 to 80 pairs of interdigits, and the interdigital transducer 4 can achieve the excitation of Rayleigh surface acoustic waves from 200MHz to 10GHz.

[0042] Some existing acousto-optic modulators are based on lithium niobate, but this invention differs significantly from them in principle and structure. Acousto-optic modulators utilize sound waves to alter the refractive index of a single arm of a Mach-Zehnder interferometer, thereby changing the phase of the light wave and modulating the intensity of the resulting interference. This invention, however, is based on the principle of acousto-induced grating diffraction. Light waves strike a grating formed by sound waves, causing diffraction. The direction of the diffracted light is at an angle to the incident light, resulting in deflection. This invention utilizes the direction of first-order diffraction. Structurally, the photonic waveguide structure is crucial in acousto-optic modulators and requires careful design. However, in this invention, the photonic waveguide structure is not the primary structure; the focus is on designing… Figure 1The polygonal multimode coupler 2 described in this invention allows light waves to be completely deflected from below when there are no acoustic waves, while when acoustic waves are excited, most of the energy light waves are deflected from the obliquely upward waveguide. This coupler structure is the first of its kind designed, and the deflector based on this coupler structure is also an innovation of this invention. Previous acousto-optic deflectors used the Bragg diffraction effect of spatial light, resulting in severe scattering and large size. The coupler structure of this invention can confine photons to below 100 micrometers while achieving efficient deflection.

[0043] Example 2

[0044] This embodiment provides a method for fabricating a high-efficiency on-chip acousto-optic deflector, including the following steps:

[0045] S1. A silicon nitride film with a thickness of 200 nm is deposited on a substrate covered with a lithium niobate film using chemical vapor deposition.

[0046] S2. Using an electron beam direct writing system (EBL, Vistec EBPG 5000+), positive electron adhesive (APR6200) is exposed onto a pre-prepared silicon nitride thin film with a thickness of approximately 400 nm. After baking on a hot plate at 130°C for 5 minutes, the desired waveguide and coupler patterns can be obtained on the electron adhesive after development with xylene.

[0047] S3. Using reactive ion etching equipment, dry etching is performed with the pattern obtained on the electron adhesive as a mask, requiring smooth and steep sidewall morphology; etching parameters are set at any time according to the etching effect of the equipment.

[0048] S4. Place the etched substrate into the chamber and use oxygen plasma etching gas to remove the residual electron adhesive on top. The gas flow rate is 50 sccm, the RF power is 20W, and the inductively coupled plasma (ICP) power is 1000W. After the process is completed, the transfer of the pattern on the substrate can be completed.

[0049] S5. Using an electron beam direct writing system (EBL, Vistec EBPG 5000+), positive electron paste (APR6200) is exposed onto the etched substrate; the thickness is about 500nm. After baking on a hot plate at 130℃ for 5 minutes, the mask pattern of the interdigital transducer 4 can be obtained on the electron paste after development with xylene.

[0050] S6. Deposit a gold layer with a thickness of approximately 100 nm on the developed and exposed substrate using a thermal evaporation method;

[0051] S7. Immerse the deposited substrate in an organic solution (such as acetone) to remove the electron adhesive and the metal film on the surface of the substrate, thus completing the fabrication of the interdigital transducer 4.

[0052] Example 3

[0053] As shown in the figure, this is a schematic diagram of the test system corresponding to the present invention. The test system mainly includes: a tunable laser, an optical fiber, a polarization controller, a photodetector, a vector network analyzer, and a spectrometer.

[0054] During device testing, the microwave-to-surface acoustic wave conversion process is based on the inverse piezoelectric coupling effect of the lithium niobate thin film. The interdigital transducer 4 converts the input microwave electrical signal into a surface acoustic wave signal from the lithium niobate thin film, causing a periodic mechanical strain field distribution in the lithium niobate film, forming an equivalent Bragg grating. When the light wave in the optical waveguide passes through the periodically strained structure, its direction is deflected. The deflected photons are output through the upward-sloping waveguide. The output optical signal is converted into an electrical signal by a photodetector. The power of the emitted photons can also be observed using a power meter to assess its efficiency. After passing through a network analyzer, the S21 transmission spectrum of the modulated signal can be obtained. Further inputting the emitted light wave into a spectrometer allows for the evaluation of the conversion and deflection efficiency of the acousto-optic deflector.

[0055] In summary, the high-efficiency acousto-optic deflector and its fabrication method provided by this invention comprehensively utilize the piezoelectric effect and acousto-optic diffraction effect of lithium niobate thin film material, and place the interdigital transducer 4 on the side of the photonic waveguide coupler to achieve high-efficiency deflection of photons by surface acoustic waves. This invention uses a coupler structure to reduce the structural size of existing acousto-optic deflectors, and fully utilizes the coupling of sound waves and light waves for efficient deflection. On the one hand, it solves the drawback of insufficient utilization of sound wave energy in traditional acousto-optic deflectors; on the other hand, it solves the defect of excessively large coupling region size in acousto-optic deflectors. Compared with existing on-chip deflector structures and technologies, this solution will provide ideas for the development of high-efficiency on-chip acousto-optic deflectors. Obviously, the above embodiments of this invention are merely examples to clearly illustrate the invention, and are not intended to limit the implementation of the invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of the claims of this invention.

Claims

1. A high efficiency on-chip acousto-optic deflector characterized by, The lithium niobate-silicon nitride heterostructure includes a lithium niobate-silicon nitride heterostructure arranged on a silicon oxide substrate; the lithium niobate-silicon nitride heterostructure includes a lithium niobate thin film and a photonic multimode coupler (2) formed by a silicon nitride structure integrated on the lithium niobate thin film; the lithium niobate thin film is provided with an interdigital transducer (4) capable of exciting acoustic waves, and the interdigital transducer (4) includes a plurality of interdigital electrodes; the lithium niobate-silicon nitride heterostructure includes an input straight waveguide (1), the multimode coupler (2) and two tapered output waveguides (3); the input straight waveguide (1) and the tapered output waveguides (3) are connected with the multimode coupler (2); the multimode coupler (2) is a polygonal structure; the deflected light exit surface of the multimode coupler (2) is perpendicular to the direction of the optical wave vector; and the interdigital transducer is arranged on the side surface of the multimode coupler.

2. The high-efficiency on-chip acousto-optic deflector according to claim 1, wherein, The lithium niobate-silicon nitride heterostructure is an on-chip structure based on a silicon-silicon dioxide substrate, and etching of the lithium niobate is not needed, and only the silicon nitride needs to be etched to define the planar device structure.

3. The high-efficiency on-chip acousto-optic deflector of claim 1, wherein, The interdigital transducer (4) forms a certain angle with the input straight waveguide (1) to meet the Bragg diffraction formula.

4. The high-efficiency on-chip acousto-optic deflector according to any one of claims 1 to 3, characterized in that, The thickness of the lithium niobate thin film is 100 nm to 1500 nm.

5. The high-efficiency on-chip acousto-optic deflector according to claim 4, wherein, The thickness of the silicon nitride is 200 nm to 400 nm; the width of the silicon nitride is 300 nm to 2000 nm, and the working wavelength is 1000 nm to 2000 nm.

6. The high-efficiency on-chip acousto-optic deflector according to claim 5, wherein, The interdigital transducer (4) can excite a Rayleigh surface acoustic wave of 200 MHz to 10 GHz.

7. A method of fabricating the high efficiency on-chip acousto-optic deflector according to any one of claims 1 to 6, characterized by, The method includes the following steps: S1. A silicon nitride thin film is deposited on a substrate covered with a lithium niobate thin film by using a thermal evaporation method; S2. Positive e-beam resist is exposed on the prepared silicon nitride thin film by using an e-beam direct writing system, and after baking and developing, the patterns of the input straight waveguide (1), the tapered output waveguide (3) and the multimode coupler (2) are obtained on the e-beam resist; S3. Dry etching is performed by using a reactive ion etching device with the patterns obtained on the e-beam resist as a mask, and the side wall morphology is required to be smooth and steep; The etching parameters are set according to the etching effect of the device at any time; S4. The etched substrate is placed in a chamber, and oxygen plasma etching gas is used to remove the residual e-beam resist on the top; S5. Positive e-beam resist is exposed on the etched substrate by using an e-beam direct writing system, and after developing, the mask pattern of the interdigital transducer (4) is obtained on the e-beam resist; S6. Gold is deposited on the developed and exposed substrate by using a thermal evaporation method; S7. The deposited substrate is soaked in an organic solution to remove the e-beam resist on the surface of the substrate and the metal film on the e-beam resist, and the interdigital transducer (4) is completed.

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