Boundary-induced anti-backscattering thin film piezophotonic crystal and acoustic wave transmission device

Abstract

The application discloses a boundary-induced anti-back scattering thin film piezoelectric phononic crystal and an acoustic wave transmission device, wherein a periodic hole array is formed on a piezoelectric film to constitute a phononic crystal with specific lattice symmetry; a Dirac-like degeneracy is formed at a Brillouin zone Γ point in a band structure of the phononic crystal; the phononic crystal has at least one boundary along a specific lattice direction, the boundary is located on a mirror plane symmetry plane of the phononic crystal and satisfies a stress-free free boundary condition; wherein the boundary induces a bulk transmission channel in the phononic crystal, the bulk transmission channel is dominated by an even symmetry mode and penetrates through a crystal aperture to constitute a boundary-induced helical state. The application constructs a helical state transmission channel in the piezoelectric film phononic crystal through the Γ point accidental four-fold degeneracy and a boundary symmetry selection rule, realizes anti-back scattering, wide aperture transducer coupling and low dispersion slow wave transmission, and solves the problems of aperture mismatch, defect sensitivity and low transmission efficiency of a traditional edge state device.

Description

Technical Field

[0001] This invention relates to the field of on-chip phononic devices, and in particular to boundary-induced anti-backscattering thin-film piezoelectric phononic crystals and acoustic wave transmission devices. Background Technology

[0002] On-chip phonons aims to route and process acoustic energy with high reliability and high integration, much like photonics manipulates light. However, a long-standing challenge is how to achieve efficient, wide-aperture excitation and readout of acoustic modes by interdigital transducers (IDTs) while suppressing backscattering. While topologically inspired edge-state schemes can effectively suppress backscattering, their acoustic transmission is typically confined to the vicinity of narrow interfaces, easily leading to a mismatch between aperture and mode, thus limiting transmission throughput, reducing signal fidelity, and weakening material utilization efficiency in the integrated platform. Therefore, the truly ideal solution would be to construct a bulk transmission channel within the crystal, preserving both backscattering resistance and full-aperture mode coupling efficiency.

[0003] Recent macroscopic experiments have shown that specific bulk propagation acoustic channels can be selected through boundary engineering, maintaining disorder resistance similar to topological protection. These studies are mostly based on valley polarization, a quantum valley Hall-like mechanism, and are primarily implemented through external actuation in meter-scale or tabletop systems. They validate the feasibility of boundary-induced bulk transport, but two key questions remain for integrated devices: first, can such internal channels be realized on piezoelectric thin-film platforms and electrically excited, readout, and quantitatively characterized in the radio frequency band? Second, can a helical, quantum spin Hall-like bulk transport channel be selected on demand through boundary conditions, exhibiting pseudo-spin-momentum locking characteristics under time-reversal symmetry?

[0004] Therefore, there is an urgent need for a phononic crystal and acoustic wave transmission device that can achieve internal bulk channel transmission, wide aperture high-efficiency coupling, defect robustness and slow wave modulation within a piezoelectric thin film system. Invention Patent Content

[0005] The purpose of this invention is to provide a boundary-induced anti-backscattering thin-film piezoelectric phononic crystal and acoustic wave transmission device. By using the Γ-point accidental quadruple degeneracy and boundary symmetry selection rules, a helical body state transmission channel is constructed inside the piezoelectric thin-film phononic crystal, realizing anti-backscattering, wide-aperture transducer coupling and low-dispersion slow wave transmission, solving the problems of aperture mismatch, defect sensitivity and low transmission efficiency of traditional edge state devices.

[0006] To achieve the above objectives, the present invention provides the following solution: A boundary-induced anti-backscattering thin-film piezoelectric phononic crystal includes: a piezoelectric thin film, wherein a periodic array of holes is formed on the piezoelectric thin film to form a phononic crystal with specific lattice symmetry; In the band structure of the phononic crystal, there is an accidental fourfold degeneracy allowed by symmetry at the Γ point in the Brillouin zone, forming a Dirac-like degeneracy; The phononic crystal has at least one boundary along a specific lattice direction, the boundary being located on the mirror-symmetric surface of the phononic crystal and satisfying the stress-free free boundary condition. The boundary induces the formation of a bulk transmission channel, dominated by even-symmetric modes, that penetrates the crystal aperture inside the phononic crystal, forming a boundary-induced helical body state. This allows sound waves to propagate along the interior of the phononic crystal body region, achieving wide-aperture coupling compatibility and backscattering suppression.

[0007] Furthermore, the material of the piezoelectric film is lithium niobate, lithium tantalate, aluminum nitride, or gallium nitride; the tangential orientation of the piezoelectric film is Y128°. Preferably, the piezoelectric film is a 300nm thick lithium niobate film with a Y128° tangential orientation. In addition, X-tangential, Z-tangential, etc., can also be used.

[0008] Furthermore, the periodic aperture array is a honeycomb-shaped circular aperture array, whose lattice constant, aperture diameter, and center offset are configured to generate the accidental quadruple degeneracy within the target radio frequency band.

[0009] Furthermore, by continuously changing the position of the boundary in the lattice of the phononic crystal, the parity and even symmetry of the projected energy spectrum of the boundary-induced helical body state are modulated, thereby enabling switching between multiple volume transport channels.

[0010] Furthermore, the operating frequency band of the boundary-induced spiral state is 175–200 MHz, with a relative bandwidth of not less than 7%; the boundary-induced spiral state is dominated by an even-symmetric mode, and the out-of-plane displacement field is... and The periodic evolution of patterns constitutes a spiral pseudo-spin texture.

[0011] The present invention also provides an acoustic wave transmission device, comprising: At least one boundary-induced anti-backscattering thin-film piezoelectric phonon crystal as described above; And at least two interdigital transducers, respectively integrated at the input and output ends of the phononic crystal, for electrically exciting and receiving boundary-induced helical body acoustic waves inside the phononic crystal; The interdigital transducer has a wide aperture that matches the aperture of the bulk transmission channel of the phononic crystal.

[0012] Furthermore, the interdigital transducer is a single-phase unidirectional transducer, including a reflective grating, a ground electrode, and a signal electrode, and is fabricated by electron beam evaporation and lift-off processes to achieve unidirectional excitation of the boundary-induced helical body acoustic wave.

[0013] Furthermore, it also includes an impedance matching layer disposed between the interdigital transducer and the phononic crystal. The impedance matching layer has a gradient aperture structure, and the length, aperture gradient, or material parameters of the impedance matching layer are configured to provide a gradient acoustic impedance within the operating frequency band to reduce interface reflections.

[0014] Furthermore, it also includes a multilayer substrate supporting the piezoelectric film, the multilayer substrate comprising a silicon oxide buried layer, a polycrystalline silicon layer, and an amorphous silicon stack arranged sequentially from top to bottom, for supporting and releasing the piezoelectric film; the middle section of the silicon oxide buried layer is removed by BOE to form a suspended film structure.

[0015] Furthermore, the acoustic wave transmission device exhibits slow wave characteristics within the operating frequency band, and the group velocity of the boundary-induced helical state is lower than the group velocity of the fundamental acoustic wave mode in the piezoelectric film; and the acoustic wave transmission device provides a continuously variable group delay by changing the length of the phonon crystal.

[0016] According to specific embodiments provided by the present invention, the boundary-induced anti-backscattering thin-film piezoelectric phonon crystal and acoustic wave transmission device provided by the present invention disclose the following technical effects: Wide-aperture, high-efficiency coupling: This invention constructs a bulk transport channel inside the piezoelectric thin-film phononic crystal (rather than at the edge) by introducing specific boundary conditions. This bulk channel occupies the entire aperture of the crystal, enabling ultra-efficient coupling with wide-aperture interdigital transducers. This solves the problems of aperture mismatch and limited transport flux caused by mode localization in traditional topological edge-state devices.

[0017] Inherent backscattering resistance: The boundary-induced helical bulk states (BI-HBSs) realized in this invention possess pseudospin-momentum locking characteristics. Experiments and simulations demonstrate that even with wavelength-level defects, this structure can still significantly suppress backscattering, ensuring high-fidelity signal transmission and improving the robustness of the device.

[0018] Slow wave and low dispersion characteristics: The BI-HBSs in this invention exhibit slow wave propagation and low dispersion characteristics within the operating frequency band, enabling larger time delays over shorter physical lengths, providing a new solution for designing compact, high-precision programmable acoustic delay lines and phase modulation devices.

[0019] Chip Integration and Portability: This invention is implemented on a LiNbO3 piezoelectric thin film platform based on standard thin film processing techniques (such as ICP-RIE, electron beam evaporation, lift-off, and sacrificial layer release), operating in the radio frequency band (e.g., 175-200MHz). Its design principle is determined by the symmetry of the system, does not depend on specific materials, and can be easily transferred to other piezoelectric or non-piezoelectric thin film platforms such as LiTaO3, AlN, and GaN. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a top view schematic diagram of the boundary-induced anti-backscattering thin-film piezoelectric phonon crystal of the present invention; Figure 2 This is a schematic cross-sectional view of the boundary-induced anti-backscattering thin-film piezoelectric phonon crystal of the present invention. Figure 3 This is a schematic diagram of BI-HBSs with quadruple degeneracy and boundary selection at point Γ in an embodiment of the present invention. (a) is an SEM image of a honeycomb lattice LiNbO3 phononic crystal, (b) is the calculated band structure showing the quadruple degeneracy and mode distribution at point Γ, (c) is a schematic diagram of a stress-free free boundary set along the y-direction on a mirror-symmetric surface, (d) is the projected band structure showing the internal Dirac-like dispersion dominated by even-symmetric modes, (e) is the out-of-plane displacement field at different phases, showing the evolution of the helical pseudospin, and (f) is a schematic evolution diagram showing the influence of boundary position control on the odd-even symmetry of the projected band. Figure 4 The following are the structure and physical diagram of the monolithically integrated BI-HBS transmission line in the embodiment of the present invention. (a) is a schematic diagram of the device structure: SPUDT, impedance matching section and BI-HBS phononic crystal strip are integrated on a suspended LiNbO3 thin film; (b) is an optical micrograph of the suspended thin film device; (c) is an SEM image of the phononic crystal strip, showing the boundary and impedance matching section details.

[0022] Figure 5 The following are the electrical and near-field characterization results of the BI-HBS transmission line in this embodiment of the invention, wherein (a) is a schematic diagram of the experimental measurement system (laser vibrometer and vector network analyzer VNA); (b) is a comparison of experimental and simulated sound field distribution and reconstructed projected band structure under different aperture widths; and (c) is a comparison of the S-type transmission line with and without PnC. 21The parameter comparison shows a relative bandwidth of approximately 7%.

[0023] Figure 6 To verify the backscattering suppression performance under engineering defects in this embodiment of the invention, (a) and (b) are SEM images of the defective BI-HBS transmission line and the uniform thin film reference line, (c) and (f) are comparisons of the full-field simulated energy flow and out-of-plane displacement field distribution, and (g) and (h) are SEM images of the BI-HBS transmission line with defects and the uniform thin film reference line before and after the introduction of defects. 21 Parameter comparison demonstrates the robustness of BI-HBS; Figure 7 The following describes the performance of the slow-wave BI-HBS delay line in this embodiment of the invention: (a) is a schematic diagram of delay line devices with different transmission lengths (L=220, 330, 440μm); (b) is a graph showing the relationship between group delay and frequency, illustrating linear growth and peak delay; (c) is a graph showing the extracted group velocity and frequency, illustrating the slow-wave effect (600-1100m / s); and (d) is a graph showing the S-wave velocity of delay lines with different lengths. 21 The spectrum shows consistent low-loss transmission.

[0024] Explanation of reference numerals in the attached figures: 1. Interdigital transducer; 2. Impedance matching layer; 3. Phononic crystal; 4. Piezoelectric thin film; 5. Buried silicon oxide layer (SiO2); 6. Poly-Si; 7. α-Si; AA' / BB' are the interfaces between the impedance matching layer and the functional regions of the phononic crystal, respectively. Detailed Implementation

[0025] 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 some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] This invention addresses two key issues concerning integrated devices mentioned in the background section by establishing a boundary symmetry design rule. This rule originates from the accidental quaternary degeneracy, or Dirac-like degeneracy, allowed by symmetry at the Γ point in a two-dimensional LiNbO3 phononic crystal. This invention utilizes a minimum tight-binding model to reveal the relationship between the terminating boundary symmetry and the internal volume channels: when stress-free free boundaries are chosen on a mirror-symmetric surface, the system can select helical boundary-induced bulk states (BI-HBSs), allowing acoustic waves to propagate along the crystal interior rather than being localized at the edges. Based on this design principle, this invention realizes a monolithic integrated device and excites and measures these channels electrically; near-field laser vibrometrics is used to obtain the spatial acoustic field distribution, and two-port RF characterization is used to extract transmission performance, thus enabling verification on a unified chip-level experimental platform.

[0027] The realized BI-HBSs exhibit three key system-level advantages required for on-chip integrated acoustics. First, they significantly suppress backscattering even in the presence of wavelength-scale structural defects, demonstrating the inherent disorder resistance and interface fault tolerance of this internal transmission geometry. Second, because the transmission channels occupy the crystal region aperture, they can achieve ultra-efficient coupling with wide-aperture (transducer longitudinal width), broadband IDTs, thereby alleviating the bottleneck of edge localized modes in terms of transmission throughput and signal fidelity. Third, within the same frequency band, these helical states also exhibit slow wave characteristics and low dispersion transmission behavior, enabling compact and precise group delay modulation. Overall, this invention transforms boundary-selective internal acoustic transmission from a macroscopic physical phenomenon into a quantitatively characterizable thin-film piezoelectric integration platform, and proposes a transferable on-chip robust acoustic transmission design paradigm: Γ-point degeneracy combined with boundary symmetry selection.

[0028] Boundary-Induced Helical Bodies (BI-HBSs): A name for a transport mode, specifically a unique transport mode possessed by the LiNbO3 thin-film phononic crystal designed in this invention. This mode features the ability to couple with a broadband IDT and resistance to back reflection.

[0029] Boundary-induced: The above-mentioned transport state is achieved by designing the boundary position of the phononic crystal.

[0030] Topological boundary state: a transmission mode, which is the object of comparison for the transmission mode designed in this invention. This transmission mode has the characteristics of anti-back reflection and narrow aperture, and cannot be effectively coupled with broadband IDT.

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] like Figure 1 , Figure 2As shown, the present invention provides a boundary-induced anti-backscattering thin-film piezoelectric phononic crystal, comprising: a piezoelectric thin film 4 (preferably a 300nm LiNbO3 thin film), wherein a periodic hole array is formed on the piezoelectric thin film 4 to form a phononic crystal 3 with specific lattice symmetry; In the band structure of the phononic crystal 3, there is an accidental quadruple degeneracy allowed by symmetry at the Γ point in the Brillouin zone, forming a Dirac-like degeneracy; The phononic crystal has at least one boundary along a specific lattice direction, the boundary being located on the mirror-symmetric surface of the phononic crystal and satisfying the stress-free free boundary condition. The boundary induces the formation of a bulk transmission channel, dominated by even-symmetric modes, that penetrates the crystal aperture inside the phononic crystal, forming a boundary-induced helical body state. This allows sound waves to propagate along the interior of the phononic crystal body region, achieving wide-aperture coupling compatibility and backscattering suppression.

[0033] The present invention also provides an acoustic wave transmission device, comprising: At least one boundary-induced anti-backscattering thin-film piezoelectric phonon crystal as described above; And at least two interdigital transducers 1, respectively integrated at the input and output ends of the phononic crystal, for electrically exciting and receiving boundary-induced helical body acoustic waves inside the phononic crystal 3; The interdigital transducer 1 has a wide aperture, which matches the aperture of the bulk transmission channel of the phononic crystal 3. It also includes an impedance matching layer 2, which is disposed between the interdigital transducer 1 and the phononic crystal 3. The impedance matching layer has a gradient aperture structure. The length, aperture gradient, or material parameters of the impedance matching layer 2 are configured to provide a gradient acoustic impedance within the operating frequency band to reduce interface reflection. It also includes a multilayer substrate that supports the piezoelectric film. The multilayer substrate includes a silicon oxide buried layer 5, a polycrystalline silicon layer 6, and an amorphous silicon stack 7 arranged sequentially from top to bottom to support and release the piezoelectric film. The middle section of the silicon oxide buried layer is removed by BOE to form a suspended film structure.

[0034] The interdigitated transducer 1 is a single-phase unidirectional transducer, including a reflective grid, a ground electrode, and a signal electrode. It is fabricated by electron beam evaporation and lift-off processes and is used to realize the unidirectional excitation of the boundary-induced helical body acoustic wave.

[0035] Example 1: Boundary-induced helical structures (BI-HBSs) in suspended LiNbO3 films This invention constructs a Lamb-wave phonon crystal (PnC) by etching a honeycomb array of circular holes on a 300 nm thick Y128° tangential LiNbO3 thin film using ICP-RIE. The structure has a lattice constant of 7.14 μm, a hole diameter of 1.97 μm, and a center offset of b = 2.38 μm. The unpatterned LiNbO3 thin film supports the fundamental frequency A0 Lamb wave branch; while this phonon crystal has C 6v Symmetry, and considering only out-of-plane displacements, forms an accidental quadruple degeneracy at point Γ, permitted by symmetry, i.e., a Dirac-like degeneracy. This degeneracy consists of two even-symmetric... Class pattern and two odd symmetric The class patterns together constitute the irreducible representation. ,like Figure 3 As shown in (a) and (b). The parity labels here indicate the sign changes of the out-of-plane displacement field under mirror reflections about the x-axis and y-axis. For example... Figure 3 As shown in (a), the dashed hexagons mark the structural unit cells. Figure 3 As shown in (b), the calculated band structure reveals that the Γ point exhibits an accidental fourfold degeneracy allowed by symmetry, i.e., a Dirac-like degeneracy. ); Illustration is and Out-of-plane displacement distribution of a Lamb wave-like mode. Here, p and d are standard descriptions of electron spin-orbit modes in physics. It indicates that the space is inversely dual symmetric, the crystal axis is mirror symmetric, and the diagonal mirror is antisymmetric. It indicates that the space is inversely dual symmetric, the crystal axis is anti-symmetrical, and the diagonal is symmetrical. The inversion space is odd symmetric, yz is mirror symmetric, and cz is anti-mirror symmetric. The inversion space is odd symmetric, cz is mirror symmetric, and yz is anti-mirror symmetric.

[0036] To select the acoustic transmission path within the crystal, this invention introduces stress-free free boundaries on a mirror-symmetric surface along the y-direction to truncate the phononic crystal stripes, such as... Figure 3 As shown in (c), a phononic crystal strip has a stress-free free boundary along the y-direction, with the boundary located on a mirror-symmetric surface. This boundary termination method breaks the original even-odd balance constrained by symmetry in the bulk region, while simultaneously satisfying the zero-stress boundary condition at the edge. However, the combination of odd-symmetric modes cannot simultaneously satisfy both mirror phase continuity and stress-free boundary constraints, thus preserving an internal energy band dominated by even-symmetric components throughout the Brillouin zone, such as... Figure 3As shown in (d), the projected band structure exhibits an internal Dirac-like dispersion dominated by even-symmetric modes, selected by the boundaries. The resulting BI-HBSs do not propagate locally along the edges but rather propagate within the phononic crystal; their pseudospin texture manifests during the propagation phase evolution... Class components and The components of the same class undergo periodic interconversion, such as Figure 3 As shown in (e), the out-of-plane displacement field distribution under different phases demonstrates... Class components and Helical pseudospin evolution among class components.

[0037] Furthermore, such as Figure 3 As shown in (f), the projection energy band evolves from even-symmetry dominant to mixed characteristics to odd-symmetry dominant as the boundary position is adjusted. By continuously changing the boundary position, the projection energy spectrum can be adjusted from even-symmetry dominant to mixed characteristics, and finally to odd-symmetry dominant characteristics, thus providing a direct degree of freedom for boundary symmetry adjustment in selecting internal transmission channels.

[0038] Example 2: Monolithically Integrated BI-HBS Transmission Line and its Electrical Characterization To evaluate transducer compatibility, this invention fabricates "RF-acoustic-RF" transmission lines (TLs) that integrate BI-HBS phonon crystal strips between single-phase unidirectional transducers (SPUDTs), such as... Figure 4 (a)-(c). The three-electrode SPUDT consists of a reflective grating, a ground electrode, and a signal electrode. Defined by electron beam evaporation and lift-off processes, it can achieve unidirectional excitation mainly towards the A0Lamb wave branch. Subsequently, the buried oxide layer SiO2 is removed by BOE to release the device, forming a suspended thin film structure. To improve coupling efficiency and reduce interface reflection, this invention introduces a shorter impedance matching section at the phonon crystal entrance.

[0039] This invention employs near-field laser vibrometer technology to characterize the out-of-plane displacement field of a device and reconstruct its projected energy band; simultaneously, it utilizes a two-port vector network analyzer (VNA) to measure S-parameters, such as... Figure 5 (a) For strip structures with different aperture widths (49, 79, 99 μm), the experimentally measured acoustic field distributions all show the existence of a coherent internal mode spanning the entire width of the phonon crystal around 187 MHz; the reconstructed dispersion relation thus matches the theoretical calculation results well, and no additional fitting parameters are required, such as... Figure 5 (b)

[0040] Electrical transmission parameters S of BI-HBS transmission line 21It exhibits a passband of approximately 180–195 MHz, corresponding to a relative bandwidth of approximately 7%; in contrast, unpatterned uniform thin-film transmission lines have a wider transmission range of approximately 180–210 MHz, such as... Figure 5 (c) Although the shaded area in the figure indicates a calculated BI-HBS bandgap range of approximately 175–200 MHz, the high-throughput transmission window observed in actual experiments is narrower. This is mainly attributed to frequency-dependent coupling / excitation efficiency, enhanced mode mixing near the bandgap edges, and losses due to limited device size. Furthermore, the reduced conversion efficiency between the A0 mode and the BI-HBS mode at higher frequencies further contributes to a significant rapid roll-off around 195 MHz near the upper edge of the passband.

[0041] Within the BI-HBS operating band, its insertion loss is comparable to that of a uniform thin-film transmission line, indicating that this structure can achieve efficient wide-aperture mode coupling; while the steep roll-off (>25dB) above approximately 195MHz is consistent with the calculated band edge. This invention introduces a shorter impedance matching layer near the SPUDT–PnC interface and reduces interface reflection and enhances mode coupling through a gradient aperture design. Since this gradient structure is limited to the matching section, it does not significantly change the transmission band of the BI-HBS within the operating frequency window of this invention, but mainly serves to improve transmission throughput. These experimental trends are reproducible on multiple devices and different chips, and all were measured under the same calibration conditions.

[0042] Example 3: Backscattering suppression under engineering defects This invention evaluates backscattering suppression capabilities by introducing designed subwavelength defects into BI-HBS transmission lines and PnC-free reference transmission lines. These defects include apertures or slit structures with controllable size and areal density, such as… Figure 6 (a) and (b). Full-field simulation results show that the BI-HBS strip can still maintain uniform energy flow and good phase coherence after passing through multiple defects, such as... Figure 6 In contrast, uniform thin-film transmission lines exhibit significant energy attenuation and phase disorder, such as (c) and (e); Figure 6 (d) and (f).

[0043] Experimentally, dual-port RF tests showed that after the introduction of defects, the S-band frequency of the BI-HBS transmission line within its operating frequency band (approximately 180–195MHz) was significantly reduced. 21 The transmission remains essentially unchanged; however, uniform thin-film transmission lines exhibit a significant transmission drop within the transducer's operating frequency band (approximately 170–230 MHz), such as... Figure 6 (g) and (h). These results indicate that backscattering suppression is an intrinsic property of boundary-selective internal channels.

[0044] In addition to its robustness to structural defects, the BI-HBS platform can suppress parasitic back reflections and echo artifacts, and supports dispersion engineering and slow-wave transmission. Therefore, this platform promises to improve phase linearity and group delay flatness; furthermore, as explained below, it can also increase delay per unit length, supporting compact delay / phase modulation devices and echo-sensitive sensing applications.

[0045] Example 4: Slow Wave BI-HBS Delay Line Finally, this invention utilizes the slow-wave characteristics of BI-HBSs to realize a compact acoustic delay line. The device aperture is 49 μm, and the center-to-center transmission lengths are L = 220, 330, and 440 μm, respectively. Figure 7 (a) In this study, the ratio of the length of the PnC region to the film thickness can reach up to approximately 1400, demonstrating the platform's potential in high-density time-domain signal processing.

[0046] Experimentally measured group delay Phase dispersion was extracted, and further validated using a time-domain gating method. The results show that... It increases linearly with the transmission length L and remains monotonically and continuously varying within the BI-HBS operating frequency band, with a peak value reaching approximately 400 ns. Figure 7 In (b), this is consistent with the characteristics of weak dispersion transmission.

[0047] The group velocity was further calculated. It is largely independent of device length and remains at 600–1100 ms throughout the entire operating frequency band. -1 Within this range, it is significantly lower than the uniform thin-film transmission line at approximately 1600 ms around 187 MHz. -1 The group velocity. Therefore, BI-HBS delay lines can provide higher delay per unit length, such as Figure 7 (c)

[0048] It is worth noting that the S of three different lengths of BI-HBS delay lines 21 The spectrum is almost uniform within the operating frequency band and is comparable to that of a uniform thin-film transmission line, such as... Figure 7 The value in (d) indicates that the platform can achieve low-loss slow-wave transmission and is suitable for programmable on-chip delay control.

[0049] This invention establishes a method for realizing chip-scale BI-HBSs in suspended LiNbO3 thin films, and verifies it through near-field imaging and microwave electrical testing. This method is based on the accidental quadruple degeneracy at the Γ point, i.e., the boundary symmetry selection rule corresponding to Dirac-like degeneracy, which enables the selection of internal crystal propagation channels without relying on edge-local propagation. This simultaneously achieves backscatter suppression and wide-aperture transducer coupling, and supports low-dispersion slow-wave operation. The device can operate in the RF band, has a passband of approximately 7%, and is compatible with standard thin-film fabrication processes.

[0050] This design framework is not material-dependent, but rather determined by the system's symmetry and mirror parity, thus it can be extended to material platforms such as LiTaO3, AlN, GaN, and Si / SiN / SiO2. Future research directions include bending tolerance testing, phase-resolved detection of helical characteristics, and widening bandwidth and further suppressing dispersion through reverse engineering. Overall, BI-HBSs provide a compact and transferable design for on-chip robust acoustic transmission, while also possessing wide-aperture electrical access capabilities. They can serve as practical components for high-density RF routing, programmable delay / timing control, and photonic-phonon-electronic hybrid integration.

[0051] In summary, this invention experimentally realizes boundary-induced helical bulk states (BI-HBSs) for radio frequency acoustic wave transmission in LiNbO3 thin-film phononic crystals, with an operating frequency of approximately 175–200 MHz. In this system, the accidentally formed Γ-point quadruple degeneracy introduces a boundary symmetry selection rule, thereby generating bulk transmission channels within the crystal. These channels can be efficiently coupled to wide-aperture interdigital transducers, avoiding the edge aperture mismatch problem commonly found in traditional edge-state devices. Near-field vibration measurements and two-port RF S-parameter tests jointly demonstrate that this structure can achieve low-loss propagation and significantly suppress backscattering even in the presence of wavelength-level defects. Furthermore, this helical band structure can also provide slow-wave, low-dispersion acoustic delay and phase modulation capabilities on the chip.

[0052] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A boundary-induced anti-backscattering thin-film piezoelectric phonon crystal, characterized in that, include: A piezoelectric thin film, wherein a periodic array of holes is formed on the piezoelectric thin film, constituting a phononic crystal with specific lattice symmetry; In the band structure of the phononic crystal, there is an accidental quadruple degeneracy allowed by symmetry at the Γ point in the Brillouin zone, forming a Dirac-like degeneracy; The phononic crystal has at least one boundary along a specific lattice direction, the boundary being located on the mirror-symmetric surface of the phononic crystal and satisfying the stress-free free boundary condition. The boundary induces the formation of a bulk transmission channel, dominated by even-symmetric modes, that penetrates the crystal aperture inside the phononic crystal, forming a boundary-induced helical body state. This allows sound waves to propagate along the interior of the phononic crystal body region, achieving wide-aperture coupling compatibility and backscattering suppression.

2. The boundary-induced anti-backscattering thin-film piezoelectric phonon crystal according to claim 1, characterized in that, The piezoelectric film is made of lithium niobate, lithium tantalate, aluminum nitride, or gallium nitride; the tangential direction of the piezoelectric film is Y128°.

3. The boundary-induced anti-backscattering thin-film piezoelectric phonon crystal according to claim 1, characterized in that, The periodic aperture array is a honeycomb-shaped circular aperture array, whose lattice constant, aperture diameter, and center offset are configured to generate the accidental quadruple degeneracy within the target radio frequency band.

4. The boundary-induced anti-backscattering thin-film piezoelectric phonon crystal according to claim 1, characterized in that, By continuously changing the position of the boundary in the phononic crystal lattice, the parity and even symmetry of the projected energy spectrum of the boundary-induced helical body state are modulated, thereby enabling switching between multiple volume transport channels.

5. The boundary-induced anti-backscattering thin-film piezoelectric phonon crystal according to claim 1, characterized in that, The operating frequency band of the boundary-induced spiral state is 175–200 MHz, with a relative bandwidth of not less than 7%; the boundary-induced spiral state is dominated by an even-symmetric mode, and the out-of-plane displacement field is... and The periodic evolution of patterns constitutes a spiral pseudo-spin texture.

6. A sound wave transmission device, characterized in that, include: At least one boundary-induced anti-backscattering thin-film piezoelectric phonon crystal as described in any one of claims 1 to 5; And at least two interdigital transducers, respectively integrated at the input and output ends of the phononic crystal, for electrically exciting and receiving boundary-induced helical body acoustic waves inside the phononic crystal; The interdigital transducer has a wide aperture that matches the aperture of the bulk transmission channel of the phononic crystal.

7. The acoustic wave transmission device according to claim 6, characterized in that, The interdigital transducer is a single-phase unidirectional transducer, including a reflective grating, a ground electrode, and a signal electrode. It is fabricated by electron beam evaporation and lift-off processes and is used to achieve unidirectional excitation of the boundary-induced helical body acoustic wave.

8. The acoustic wave transmission device according to claim 6, characterized in that, It also includes an impedance matching layer disposed between the interdigital transducer and the phononic crystal. The impedance matching layer has a gradient aperture structure, and the length, aperture gradient, or material parameters of the impedance matching layer are configured to provide a gradient acoustic impedance within the operating frequency band to reduce interface reflections.

9. The acoustic wave transmission device according to claim 6, characterized in that, It also includes a multilayer substrate that supports the piezoelectric film. The multilayer substrate includes a silicon oxide buried layer, a polycrystalline silicon layer, and an amorphous silicon stack arranged sequentially from top to bottom to support and release the piezoelectric film. The middle section of the silicon oxide buried layer is removed by BOE to form a suspended film structure.

10. The acoustic wave transmission device according to claim 6, characterized in that, The acoustic wave transmission device exhibits slow wave characteristics within its operating frequency band, and the group velocity of the boundary-induced helical state is lower than the group velocity of the fundamental acoustic wave mode in the piezoelectric film; and the acoustic wave transmission device provides a continuously variable group delay by changing the length of the phonon crystal.

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