Acousto-optic modulator based on heterogeneous integration of lead zirconate titanate and chalcogenide glass

Abstract

The invention discloses an acousto-optic modulator based on heterogeneous integration of PZT and chalcogenide glass, and belongs to the technical field of integrated photoelectronics. The modulator sequentially comprises a silicon substrate, a silicon dioxide lower cladding layer and a lead zirconate titanate (PZT) piezoelectric film layer from bottom to top, a chalcogenide glass optical waveguide is heterogeneously integrated on the PZT layer, and the optical waveguide is patterned into a Mach-Zehnder interferometer (MZI) structure. An interdigital transducer (IDT) made of aluminum (Al) is arranged between two arms of the MZI, and air grooves which extend to a lower cladding layer or a substrate are etched in the outer sides of the two arms respectively. The advantage of high piezoelectric coefficient of PZT and the advantage of high photoelasticity of chalcogenide glass are combined through heterogeneous integration. Aluminum is used as an IDT electrode material, so that the mass loading effect of the electrode on the surface acoustic wave is effectively reduced; acoustic resonant cavities are constructed through air grooves in the outer sides of the two arms, sound wave energy is limited in an optical waveguide area, and the acousto-optic interaction is remarkably enhanced. The structure realizes optical signal modulation with low driving voltage, low loss and high efficiency, and is suitable for an integrated optical path system of intermediate infrared and communication wavebands.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of integrated optoelectronics and microwave photonics, and particularly relates to a Mach-Zehnder interferometer (MZI) type acousto-optic modulator integrated with a piezoelectric film and a chalcogenide glass waveguide. BACKGROUND

[0002] With the rapid development of optical communication, optical interconnection and optical sensing technology, high-performance optical modulators have become the core devices in photonic integrated circuits (PICs). The current mainstream modulation techniques mainly include thermal optical modulation, electro-optical modulation and acousto-optical modulation.

[0003] Limitations of existing materials: Although traditional silicon-based photonic devices have mature processes, the silicon material itself lacks linear electro-optic effect (Pockels effect) and has weak acousto-optic interaction. Lithium niobate (LiNbO3) has excellent electro-optic and piezoelectric properties, but it is difficult to process and is difficult to be compatible with CMOS technology.

[0004] Advantages and disadvantages of PZT materials: Lead zirconate titanate (PZT) is a strong ferroelectric material with a very high piezoelectric coefficient (much higher than lithium niobate), which is an ideal material for exciting surface acoustic waves (SAW). However, the optical propagation loss of PZT thin film is usually high (2-5 dB / cm), and the refractive index regulation cannot be directly realized by low-loss waveguide.

[0005] Advantages of chalcogenide glass: Chalcogenide glass has a very high Kerr nonlinear coefficient and excellent acousto-optic figure of merit, and has wide spectral transmittance in the mid-infrared waveband, which is an ideal optical waveguide material.

[0006] Structural defects of existing acousto-optic devices: acoustic energy dissipation: In traditional acousto-optic modulators, the sound wave usually spreads to the surrounding after being excited, resulting in low acoustic energy density acting on the optical waveguide and low modulation efficiency.

[0007] Mass loading effect: Traditional interdigital transducers (IDT) often use heavy metal materials such as gold (Au) or platinum (Pt). Because these metals have high density, they can significantly change the propagation speed of the acoustic wave under the electrode, and produce strong acoustic wave reflection at the electrode edge, resulting in increased acoustic wave propagation loss and reduced high-frequency response performance of the device.

[0008] Therefore, there is an urgent need for a new modulator structure that can combine the high piezoelectricity of PZT with the high acousto-optic properties of chalcogenide glass, while solving the problems of acoustic energy dissipation and electrode mass loading. SUMMARY

[0009] The technical problem to be solved by the present invention is to overcome the problems of low acousto-optic modulation efficiency, poor acoustic wave confinement ability and severe electrode mass loading effect in the prior art, and to provide an acousto-optic modulator based on heterogeneous integration of lead zirconate titanate and chalcogenide glass. The technical solution of the present invention is as follows: an acousto-optic modulator based on heterogeneous integration of lead zirconate titanate and chalcogenide glass, comprising a substrate, a lower cladding layer, and a piezoelectric thin film layer stacked from bottom to top; a chalcogenide glass optical waveguide layer is formed above the piezoelectric thin film layer, the optical waveguide layer being patterned into a Mach-Zehnder interferometer (MZI) structure, including an input waveguide, a beam splitter, a first modulation arm, a second modulation arm, a beam combiner, and an output waveguide; an interdigital transducer (IDT) is disposed on the surface of the piezoelectric thin film layer, the interdigital transducer being located in the region between the first modulation arm and the second modulation arm; characterized in that: the electrode material of the interdigital transducer is aluminum (Al); at least one air slot is etched on the outer side of the first modulation arm and the outer side of the second modulation arm, the air slot extending parallel to the modulation arm, and the etching depth penetrating the piezoelectric thin film layer.

[0010] Furthermore, the substrate is a silicon (Si) substrate, the lower cladding layer is a silicon dioxide (SiO2) layer, and the piezoelectric thin film layer is a polycrystalline or monocrystalline lead zirconate titanate (PZT) thin film.

[0011] Furthermore, the chalcogenide glass material is selected from one of As2S3, As2Se3, or Ge-Sb-S based glass, and is used to transmit optical signals in the near-infrared or mid-infrared band.

[0012] Furthermore, the aluminum (Al) interdigital transducer is configured to excite surface acoustic waves (SAWs), which are perpendicular to the propagation direction of the optical waveguide and form an acousto-optic interaction in the waveguide region.

[0013] Furthermore, the two air slots and the central region together constitute an acoustic Fabry-Perot resonant cavity, used to confine sound wave energy within the region containing the two modulation arms. The beneficial effect of this invention is that it combines the ultra-high piezoelectric coefficient of PZT (for efficient sound wave generation) and the extremely high acousto-optic coefficient of chalcogenide glass (for efficient optical signal modulation), achieving device performance of "1+1>2". Aluminum (Al, density approximately 2.7 g / cm³) is used. 3 It replaces traditional gold (Au, density approximately 19.3 g / cm³). 3Aluminum is used as the electrode for the IDT. The acoustic impedance of aluminum is a better match for PZT, and its extremely light weight significantly reduces reflection and scattering losses of sound waves passing through the electrode area, improving the sound wave transmission efficiency at high frequencies. By etching deep grooves (air grooves) on the outer sides of the MZI arms, the huge acoustic impedance mismatch between air and the solid medium is utilized to construct a totally internally reflected acoustic boundary. This causes sound waves to reflect back and forth between the two air grooves, forming standing waves or enhancing traveling waves, significantly increasing the acoustic energy density at the optical waveguide, thereby reducing the driving voltage. The IDT is placed in the middle of the two arms, and the generated sound waves propagate simultaneously to the left and right. It can be designed to apply anti-phase refractive index modulation (push-pull operating mode) to the two arms, thereby doubling the modulation efficiency. Attached Figure Description

[0014] Figure 1 This is a top view of the overall structure of the acousto-optic modulator based on heterogeneous integration of PZT and chalcogenide glass provided in an embodiment of the present invention. Figure 2 yes Figure 1 A schematic diagram of the cross-sectional structure along line A-A'. In the figure, 101-silicon substrate; 102-silicon dioxide lower cladding layer; 103-PZT piezoelectric thin film layer; 104a-first modulation arm; 104b-second modulation arm; 105-aluminum interdigital transducer; 106-air slot. Detailed Implementation

[0015] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0016] Implementation Examples

[0017] like Figure 2 As shown in the cross-sectional view, the acousto-optic modulator of this embodiment adopts a multilayer heterogeneous integrated structure. The bottom layer is a silicon substrate 101, which is used to provide mechanical support. A silicon dioxide (SiO2) lower cladding layer 102 with a thickness of about 2-3 μm is deposited on the silicon substrate 101. This layer not only serves as an optical lower cladding layer, but also prevents acoustic waves from leaking into the substrate.

[0018] A PZT piezoelectric thin film layer 103 is deposited on the lower cladding layer 102. Preferably, the PZT thin film is a polycrystalline thin film prepared by sol-gel method or a single-crystal thin film bonded to wafers, with a thickness between 500 nm and 1000 nm, and is polarized to have a piezoelectric effect.

[0019] like Figure 1 As shown in the top view, a chalcogenide glass waveguide 104 is fabricated on the surface of the PZT piezoelectric thin film layer 103. This waveguide material is selected from high acousto-optic glass such as As2S3 or Ge-Sb-S, forming the optical path of a Mach-Zehnder interferometer (MZI). The optical path includes an input terminal, a Y-type beam splitter, two parallel first modulation arms 104a and second modulation arms 104b, a Y-type beam combiner, and an output terminal.

[0020] An interdigital transducer (IDT) 105 is fabricated in the central region between the two modulation arms (104a and 104b). Material selection: Aluminum (Al) is specifically chosen as the electrode material for the IDT in this invention. Principle explanation: [The following text appears to be a continuation of the previous sentence and is left untranslated.] Figure 2 It can be seen that the aluminum electrode layer is very thin and has a low density (approximately 2.7 g / cm³). 3 Compared to gold (approximately 19.3 g / cm³), 3 The better acoustic impedance matching between aluminum and PZT substrate can significantly reduce the mass loading effect when sound waves pass through the electrode area, thereby reducing sound wave reflection loss.

[0021] like Figure 1 and Figure 2 As shown, elongated air grooves 106 are etched on the outer side (i.e., above) of the first modulation arm 104a and on the outer side (i.e., below) of the second modulation arm 104b. Positional relationship: The two air grooves 106 are arranged parallel to the two MZI arms in the middle, sandwiching the "IDT-waveguide" region in between. Depth characteristics: as shown... Figure 2 As shown, the etching depth of the air slot 106 completely penetrates the PZT piezoelectric thin film layer 103 and extends into the interior of the silicon dioxide underlayer 102. Working mechanism: Due to the significant acoustic impedance difference (acoustic impedance mismatch) between the PZT material (solid) and the air slot (gas), when the surface acoustic wave generated by the IDT propagates outward and encounters the air slot 106, total internal reflection occurs. This prevents the sound wave from dissipating through the air slot and instead reflects it back to the waveguide region. Therefore, the two air slots 106 effectively form an acoustic cavity, tightly confining the acoustic energy to the central region containing the first modulation arm 104a and the second modulation arm 104b, significantly enhancing the sound field intensity at the optical waveguide.

[0022] When an radio frequency signal is applied to the aluminum interdigital transducer 105, the inverse piezoelectric effect based on the PZT layer will... Figure 2 Surface acoustic waves are generated within the cross-sectional plane shown. These waves propagate from the center outwards to the left and right, acting on the chalcogenide glass waveguides 104a and 104b, respectively. Due to the acousto-optic effect, the refractive index of the glass material changes periodically, thereby altering the propagation phase of the light waves in the two arms. Through the interference of the MZI structure, the light intensity is ultimately modulated.

[0023] Thanks to the confinement effect of the air slot 106 and the low loss characteristics of the aluminum electrode 105, the structure of this embodiment can achieve the same modulation depth with lower RF power.

[0024] 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, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An acousto-optic modulator based on heterogeneous integration of lead zirconate titanate and a chalcogenide glass, characterized in that, include: The stacked structure, from bottom to top, includes: a silicon (Si) substrate, a silicon dioxide (SiO2) cladding layer, and a lead zirconate titanate (PZT) piezoelectric thin film layer; an optical waveguide layer, formed on one side or above the PZT piezoelectric thin film layer, made of chalcogenide glass, patterned on a plane as a Mach-Zehnder interferometer (MZI) structure, comprising an input waveguide, a Y-type beam splitter, a first modulation arm, a second modulation arm, a Y-type beam combiner, and an output waveguide; an interdigital transducer (IDT), made of aluminum (Al), directly deposited on the surface of the PZT piezoelectric thin film layer; and an acoustic isolation trench structure consisting of two air trenches etched on the device surface. The system comprises (Trenches); wherein the interdigital transducer is disposed in the central region between the first modulation arm and the second modulation arm, for generating surface acoustic waves (SAW) that propagate to both sides; wherein the two air slots are located on the outer side of the first modulation arm and the outer side of the second modulation arm, respectively, and extend parallel to the modulation arm; the interdigital transducer, the first modulation arm, the second modulation arm, and the two air slots together constitute an acoustic resonant cavity structure.

2. The acousto-optic modulator of claim 1, wherein, The PZT piezoelectric thin film layer is a polycrystalline thin film deposited on the SiO2 under-cladding layer by a sol-gel method or magnetron sputtering method, or a single-crystal thin film transferred to the SiO2 under-cladding layer by wafer bonding technology. The PZT thin film has a preferred oriented lattice structure to maximize the in-plane piezoelectric coefficient, thereby improving the excitation efficiency of surface acoustic waves.

3. The acousto-optic modulator of claim 1, wherein, The chalcogenide glass material has a high photo-elastic coefficient and a high acousto-optic figure of merit; the chalcogenide glass is selected from one of As2S3, As2Se3, Ge 23 Sb7S 70 or Ge-As-Se-Te glass; the optical waveguide layer has a thickness ranging from 200 nm to 800 nm and a width ranging from 500 nm to 3 μm to support single-mode optical transmission.

4. The acousto-optic modulator of claim 1, wherein, Regarding the electrode design of the interdigital transducer (IDT): The thickness of the aluminum (Al) electrode is controlled between 50 nm and 300 nm; compared with gold (Au) or platinum (Pt) electrodes, the aluminum electrode has lower acoustic impedance and mass density to minimize the mass loading effect, thereby minimizing the reflection loss and velocity dispersion of surface acoustic waves as they pass through the electrode region.

5. The acousto-optic modulator of claim 1, wherein, Regarding the aforementioned acoustic isolation groove structure (air groove), wherein: The etching depth of the air slot penetrates the chalcogenide glass layer and extends into the interior of the PZT piezoelectric thin film layer, or completely penetrates the PZT layer to reach the SiO2 under-cladding interface; the width of the air slot is greater than one-quarter of the wavelength of the surface acoustic wave to form an effective acoustic wave reflecting surface; the two air slots form the boundary of the acoustic Fabry-Perot resonator, confining the acoustic energy to the central region containing the two modulation arms and the IDT, thereby forming a standing wave or enhanced traveling wave acoustic field at the optical waveguide.

6. The acousto-optic modulator of claim 1, wherein, The modulator is configured in a push-pull modulation mode; the acoustic-induced refractive index change (Δn1) generated by the interdigital transducer at the first modulation arm and the acoustic-induced refractive index change (Δn2) generated by the interdigital transducer at the second modulation arm have a phase difference, which is achieved by adjusting the lateral distance from the center of the IDT to the two optical waveguides, so that the optical signals of the two arms produce the maximum interference cancellation or reinforcement when combined.

7. The acousto-optic modulator of claim 1, wherein, A very thin dielectric buffer layer (such as Al2O3 or Si3N4) with a thickness less than 50 nm is further arranged between the PZT piezoelectric film layer and the chalcogenide glass optical waveguide layer, which is used to prevent chemical diffusion reaction between the chalcogenide glass and the PZT in the high-temperature annealing process, while not significantly hindering the transmission of acoustic waves.

8. The optoelectronic feedback loop-based electric domain nonlinear enhanced chaotic secure communication system according to claim 3 and claim 5, wherein the parameters of the Mach-Zehnder interferometer are set to be the same.

9. The acousto-optic modulator of claim 1, wherein, The design frequency f of the interdigital transducer is matched with the effective acousto-optic interaction length of the optical waveguide mode; the interdigital transducer adopts a single-finger structure or a split-finger geometry to suppress internal acoustic reflection.

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