A silicon waveguide acousto-optic modulation device based on piezoelectric film

Abstract

The invention discloses a silicon waveguide acousto-optic modulation device based on a piezoelectric film, belonging to the field of integrated optical technology. The device comprises, from bottom to top, a silicon substrate (1), a buried oxide layer (2), a silicon waveguide layer (3), a buffer layer (4), a piezoelectric film (5) and an interdigital transducer (6). Driven by an external electrical signal, the interdigital transducer (6) forms a periodic electric field inside the piezoelectric film (5) and excites an acoustic field through the piezoelectric effect, and finally realizes the interaction between different optical field modes and the acoustic field in the bus waveguide of the silicon waveguide layer (3), and realizes the multiplexing or demultiplexing of different optical field modes through an asymmetric directional coupler. The invention realizes single-sideband acousto-optic modulation in a non-piezoelectric silicon waveguide by electrically driving the piezoelectric film, and can independently regulate the acoustic field and the optical field. In addition, the introduction of the buffer layer (4) solves the problems of large optical transmission loss in the silicon waveguide and deterioration of the crystal growth quality of the piezoelectric film.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated optics, and more specifically, relates to a silicon waveguide acousto-optic modulation device based on a piezoelectric film. Background Art

[0002] In recent years, silicon-based photonics has shown great advantages in integrating passive optical devices, and is expected to become a monolithic integrated optical chip platform that integrates active and passive optical devices, attracting extensive attention from researchers at home and abroad. As the research scope of silicon-based photonics gradually expands, the interaction between the light field and the acoustic field in silicon waveguides has also set off a research boom. The refractive index of the waveguide material is disturbed by the acoustic field, which in turn affects the effective refractive index of the waveguide light field mode, thereby realizing the loading of the information carried by the acoustic field on the light field, which has broad application prospects in the fields of microwave photonics and all-optical signal processing.

[0003] The traditional way to achieve the interaction between the optical field and the acoustic field in the silicon waveguide is to inject strong pump light, so that the strong optical field inside the waveguide generates a self-excited acoustic field due to the electrostrictive and photoelastic effects. However, this self-excited acoustic field is relatively weak and will leak from the silicon waveguide to the silicon dioxide buried oxide layer below. Therefore, it is often necessary to design a suspended silicon waveguide structure or an optomechanical cavity to confine the acoustic field in the waveguide, and it is also necessary to design a very long acoustic-optical interaction region to observe a relatively obvious acoustic-optical interaction phenomenon. This suspended waveguide structure and the long acoustic-optical interaction region are very unfavorable for the large-scale integration of waveguides, greatly limiting their application prospects.

[0004] Piezoelectric film can excite an acoustic field under the action of an applied electric field. The intensity of the excited acoustic field is related to the intensity of the electric field, and is much stronger than the intensity of the acoustic field self-excited by the light field. Since silicon materials do not have piezoelectric properties, in order to achieve the use of applied electric fields to excite the acoustic field in silicon waveguides, it is necessary to grow a layer of piezoelectric film on the silicon waveguide. The piezoelectric film generates an acoustic field under the action of the electric field and then transmits it to the silicon waveguide, thereby realizing the acoustic-optical interaction in the silicon waveguide. However, since silicon materials usually have a large lattice mismatch with piezoelectric materials, growing a piezoelectric film directly on a silicon waveguide will not only deteriorate the quality of the grown piezoelectric film crystal, but will also introduce greater waveguide sidewall scattering losses, resulting in increased light transmission losses in the silicon waveguide. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present invention is to provide a silicon waveguide acousto-optic modulation device based on a piezoelectric film, by introducing a buffer layer between the silicon waveguide and the piezoelectric film, the light field mode is confined in the silicon waveguide and the buffer layer, and the introduction of the buffer layer helps to improve the crystal growth quality of the piezoelectric film, thereby solving the problems of large light transmission loss in the silicon waveguide and deterioration of the crystal growth quality of the piezoelectric film in the silicon waveguide acousto-optic modulation device. In addition, the present invention also preferably further realizes independent regulation of the light field and the sound field by regulating the material and thickness of the buffer layer, the material and thickness of the piezoelectric film, and the structure and tilt angle of the interdigital transducer, thereby ensuring the performance of the silicon waveguide acousto-optic modulation device.

[0006] To achieve the above-mentioned purpose, the present invention provides a silicon waveguide acousto-optic modulation device based on piezoelectric film, which comprises, from bottom to top, a silicon substrate, a buried oxide layer, a silicon waveguide layer, a buffer layer, a piezoelectric film and an interdigital transducer; wherein the silicon waveguide layer comprises a bus waveguide, a first branch waveguide and a second branch waveguide, all of which are ridge waveguides, and the bus waveguide is coupled with the first branch waveguide and the second branch waveguide to multiplex different light field modes in the bus waveguide or demultiplex different light field modes from the bus waveguide; the interdigital transducer is used to form a periodic electric field inside the piezoelectric film under the drive of an external electrical signal, and to excite an acoustic field through the piezoelectric effect, and the excited acoustic field is transmitted to the silicon waveguide layer through the buffer layer, and finally the bus waveguide in the silicon waveguide layer realizes the interaction between the light field and the acoustic field.

[0007] As a further preferred embodiment of the present invention, the material refractive index of the silicon waveguide layer is greater than that of the buried oxide layer and the buffer layer, wherein the bus waveguide is a multimode waveguide with a total cross-sectional thickness of 200 nanometers to 1000 nanometers and a ridge width of 800 nanometers to 2000 nanometers.

[0008] As a further preferred embodiment of the present invention, the structures of the first branch waveguide and the second branch waveguide in the silicon waveguide layer are consistent and are respectively arranged at the input and output ports; the bus waveguide and the first branch waveguide and the second branch waveguide form an asymmetric directional coupler, the fundamental mode effective refractive index of the first branch waveguide and the second branch waveguide is equal to the effective refractive index of a certain high-order mode in the bus waveguide, the coupling zone gap is 100 nanometers to 1000 nanometers, and the coupling zone length is 5 micrometers to 100 micrometers.

[0009] As a further preferred embodiment of the present invention, the material of the buffer layer is silicon nitride or silicon dioxide, and the thickness is more than 100 nanometers.

[0010] As a further preferred embodiment of the present invention, the material used for the piezoelectric film is aluminum scandium nitride, and the thickness is more than 100 nanometers.

[0011] As a further preferred embodiment of the present invention, the metal material used for the IDT is gold, copper or aluminum, with a thickness of 50 nanometers to 1000 nanometers, an output aperture of more than 100 micrometers, a period length equal to the ridge width of the bus waveguide, and the number of periods is more than 2.

[0012] As a further preferred embodiment of the present invention, the interdigital transducer forms a preset angle with the bus waveguide in the silicon waveguide layer, and the size of the angle is determined by the difference in longitudinal propagation constants between the detection light input into the bus waveguide and the sideband light after acousto-optic modulation, and is used to compensate for the wave vector mismatch caused by different light field modes during the acousto-optic modulation process.

[0013] As a further preferred embodiment of the present invention, surface acoustic waves are selected in the acoustic field mode excited by the interdigital transducer and the piezoelectric film to concentrate the energy of the acoustic waves within a thickness range of several acoustic wave wavelengths, so as to maximize the use of the acoustic wave energy in the silicon waveguide layer.

[0014] As a further preferred embodiment of the present invention, the IDT further comprises a disconnected metal grid as a surface acoustic wave reflector for reflecting the surface acoustic wave transmitted in the backward direction to the forward direction.

[0015] The present invention grows a piezoelectric film on top of a non-piezoelectric silicon material, introduces a buffer layer between the silicon waveguide and the piezoelectric film, and undergoes high-temperature annealing and chemical mechanical polishing processes, which can reduce the optical transmission loss of the silicon waveguide and provide a better crystal growth platform for the piezoelectric film. The silicon waveguide acousto-optic modulation device provided by the present invention can achieve independent regulation of the acoustic field and the optical field by electrically driving and exciting a strong acoustic field, and can achieve a strong acoustic-optical interaction under low optical power conditions, thereby obtaining a better acousto-optic modulation effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A three-dimensional model diagram of the silicon waveguide acousto-optic modulation device structure provided by the present invention.

[0017] Figure 2 A local diagram of the silicon waveguide layer of the silicon waveguide acousto-optic modulation device structure provided by the present invention.

[0018] Figure 3 FIG. 4 is a schematic diagram of inter-mode acousto-optic modulation according to an embodiment of the present invention.

[0019] Figure 4 This is a wave vector matching diagram of inter-mode acousto-optic modulation according to an embodiment of the present invention.

[0020] The meanings of the reference numerals in the figure are as follows: 1-silicon substrate, 2-buried oxide layer, 3-silicon waveguide layer, 4-buffer layer, 5-piezoelectric film, 6-interdigital transducer, 7-bus waveguide, 8-first branch waveguide, 9-second branch waveguide. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0022] Example

[0023] The silicon waveguide acousto-optic modulation device provided by the present invention has a three-dimensional structure as shown in FIG. Figure 1 As shown, it includes a silicon substrate 1, a buried oxide layer 2, a silicon waveguide layer 3, a buffer layer 4, a piezoelectric film 5 and an interdigital transducer 6; wherein a partial diagram of the silicon waveguide layer 3 is as shown Figure 2 As shown, it includes a bus waveguide 7, a first branch waveguide 8 and a second branch waveguide 9. The material refractive index of the silicon waveguide layer is greater than that of the buried oxide layer and the buffer layer, which confines the light field mode in the silicon waveguide. The buffer layer can not only protect the side wall of the silicon waveguide and reduce the light transmission loss, but also the smooth surface after high temperature annealing and chemical mechanical polishing provides a more favorable growth environment for the piezoelectric film.

[0024] Driven by an external electrical signal, the periodic structure of the interdigital transducer can form a periodic electric field in the piezoelectric film, and then stimulate the acoustic field through the piezoelectric effect. The excited acoustic field is then transmitted to the silicon waveguide layer through the buffer layer, and finally the acousto-optic modulation is realized in the bus waveguide of the silicon waveguide layer, and the detection light and sideband light of different light field modes are multiplexed or demultiplexed through the asymmetric directional coupler. In the excitation selection of the acoustic field mode, the energy of the surface acoustic wave is concentrated on the chip surface with a thickness of several wavelengths, which can effectively utilize the acoustic field energy to perform acousto-optic modulation in the silicon waveguide layer and reduce the leakage of the acoustic field energy to the bottom substrate.

[0025] In this embodiment, the detection light and the sideband light participating in the acousto-optic modulation are in different light field modes, that is, the detection light is the fundamental mode (TE 00 ), while the sideband light is the first-order mode (TE 01 ), the inter-mode acousto-optic modulation schematic diagram and wave vector matching diagram are shown in Figure 3 and Figure 4As shown. Driven by GHz-level RF signals, the probe light and the sideband light in the bus waveguide are in different light field modes, resulting in a significant difference in their longitudinal propagation constants. To meet the wave vector matching in the inter-mode acousto-optic modulation process, the difference in the longitudinal propagation constants between the two needs to be compensated by the longitudinal wave vector of the surface acoustic wave along the light propagation direction. Therefore, the propagation direction of the surface acoustic wave excited by the interdigital transducer should form a certain angle with the light propagation direction in the bus waveguide, and the size of the angle is determined by the difference in the longitudinal propagation constants between the probe light and the sideband light. After the structure of the interdigital transducer is determined, the longitudinal wave vector of the surface acoustic wave along the light propagation direction is also determined, resulting in only one of the sideband lights being able to meet the wave vector matching condition of the inter-mode acousto-optic modulation, and ultimately achieving single-sideband acousto-optic modulation. Under the condition of wave vector matching, the detection light input in the bus waveguide in the fundamental mode generates sideband light transmitted in the first-order mode after passing through the acousto-optic interaction region, and then the integrated asymmetric directional coupler realizes the mode multiplexing or demultiplexing function, that is, the first-order mode effective refractive index in the bus waveguide is equal to the fundamental mode effective refractive index in the first branch waveguide and the second branch waveguide, so that the first-order mode sideband light in the bus waveguide is coupled to the second branch waveguide for output, while the fundamental mode detection light is still output from the bus waveguide.

[0026] In this embodiment, at a preset light wavelength of 1550 nanometers, the material refractive index of the silicon waveguide layer is 3.48, while the material refractive index of the silicon dioxide buried oxide layer is 1.45, and the material refractive index of the silicon nitride buffer layer is 1.96, so the light field mode can be firmly confined in the silicon waveguide. The total cross-sectional thickness of the bus waveguide, the first branch waveguide, and the second branch waveguide are all 220 nanometers, the etching depth is 70 nanometers, and the ridge widths are 1500 nanometers, 580 nanometers, and 580 nanometers, respectively. The effective refractive index of the first-order mode in the bus waveguide and the fundamental mode in the first branch waveguide and the second branch waveguide are all 2.752, the coupling region gap is 300 nanometers, and the coupling region length is 18 microns. In addition, the effective refractive index of the fundamental mode in the bus waveguide is 2.845, so the difference in the longitudinal propagation constants between the fundamental mode and the first-order mode is 389557 nanometers. -1 The thickness of the silicon dioxide buried oxide layer is 2 microns, the thickness of the silicon nitride buffer layer is 400 nanometers, and the thickness of the aluminum scandium nitride piezoelectric film is 500 nanometers.

[0027] In this embodiment, the materials of the IDT and the disconnected metal grid are both gold, with a thickness of 100 nanometers, a period length of 1500 nanometers, a number of periods of 100 and 50, an output aperture of 300 micrometers, and a preset angle of 5.30 degrees with the bus waveguide. When the RF signal driving frequency of the IDT is around 3.05 GHz or 3.38 GHz, it can excite a strong surface acoustic wave, generate a strong strain in the silicon waveguide, and obtain a better acousto-optic modulation effect.

[0028] The material of the piezoelectric film in the present invention is not limited to aluminum scandium nitride, but can also be aluminum nitride or lead zirconate titanate and other materials, as long as the material has no adverse effect on the light field in the silicon waveguide. The material of the buffer layer in the present invention is not limited to silicon nitride, but can also be other materials that can grow in conjunction with the silicon waveguide. It is best if it can provide a better growth environment for the piezoelectric film. By adjusting the thickness of the buffer layer and the piezoelectric film, the best electromechanical coupling coefficient can be obtained, so that the radio frequency energy can be converted into acoustic wave energy as much as possible, and the acoustic wave energy can be concentrated in the silicon waveguide as much as possible.

[0029] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A silicon waveguide acousto-optic modulation device based on piezoelectric film, characterized in that: The invention comprises, from bottom to top, a silicon substrate (1), a buried oxide layer (2), a silicon waveguide layer (3), a buffer layer (4), a piezoelectric film (5) and an interdigital transducer (6); wherein the silicon waveguide layer (3) comprises a bus waveguide (7), a first branch waveguide (8) and a second branch waveguide (9), all of which are ridge waveguides; the bus waveguide (7) is coupled to the first branch waveguide (8) and the second branch waveguide (9) to multiplex different light field modes in the bus waveguide (7) or demultiplex different light field modes from the bus waveguide (7); the first branch waveguide in the silicon waveguide layer (3) is The structures of the first branch waveguide (8) and the second branch waveguide (9) are consistent and are respectively arranged at the input and output ports; the bus waveguide (7) and the first branch waveguide (8) and the second branch waveguide (9) form an asymmetric directional coupler; the interdigital transducer (6) is used to form a periodic electric field inside the piezoelectric film (5) under the drive of an external electric signal, and to stimulate an acoustic field through the piezoelectric effect; the excited acoustic field is transmitted to the silicon waveguide layer (3) through the buffer layer (4), and finally the bus waveguide (7) in the silicon waveguide layer (3) realizes the interaction between the optical field and the acoustic field.

2. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The material refractive index of the silicon waveguide layer (3) is greater than that of the buried oxide layer (2) and the buffer layer (4), wherein the bus waveguide (7) is a multimode waveguide with a total cross-sectional thickness of 200 nanometers to 1000 nanometers and a ridge width of 800 nanometers to 2000 nanometers.

3. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The fundamental mode effective refractive index of the first branch waveguide (8) and the second branch waveguide (9) is equal to the effective refractive index of a certain high-order mode in the bus waveguide (7), the coupling zone gap is 100 nanometers to 1000 nanometers, and the coupling zone length is 5 micrometers to 100 micrometers.

4. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The material used for the buffer layer (4) is silicon nitride or silicon dioxide, and the thickness is greater than 100 nanometers.

5. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The material used for the piezoelectric film (5) is aluminum scandium nitride, and the thickness is greater than 100 nanometers.

6. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The metal material selected for the interdigital transducer (6) is gold, copper or aluminum, with a thickness of 50 nanometers to 1000 nanometers, an output aperture of more than 100 micrometers, a period length equal to the ridge width of the bus waveguide (7), and a period number of more than 2.

7. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The interdigital transducer (6) forms a preset angle with the bus waveguide (7) in the silicon waveguide layer (3); the size of the angle is determined by the difference between the longitudinal propagation constant of the probe light input into the bus waveguide (7) and the sideband light after acousto-optic modulation, and is used to compensate for the wave vector mismatch caused by different light field modes during the acousto-optic modulation process.

8. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: A surface acoustic wave is selected in the acoustic field mode excited by the interdigital transducer (6) and the piezoelectric film (5), and the energy of the acoustic wave is concentrated within a thickness range of several acoustic wave wavelengths, so as to maximize the use of the acoustic wave energy in the silicon waveguide layer (3).

9. The silicon waveguide acousto-optic modulation device based on piezoelectric film as claimed in claim 1, characterized in that: The interdigital transducer (6) also includes a disconnected metal grid as a surface acoustic wave reflector, which is used to reflect the surface acoustic wave transmitted in the backward direction to the forward direction.

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