Longitudinal Leaky Surface Acoustic Wave Research Articles

Analysis of propagation and resonance properties of longitudinal leaky surface acoustic wave on LiNbO3/SiC bonded structure

Abstract The propagation and resonance properties of longitudinal leaky surface acoustic waves (LLSAW) on a bonded structure comprising an X-cut LiNbO3 (LN) thin plate and a 4H-SiC support substrate are theoretically investigated. The strong LLSAW responses with high Q factors were obtained at the LN thin-plate thickness h where the LLSAW phase velocity was slower than the bulk shear wave of 4H-SiC of 7126 m/s, and a fractional bandwidth (FBW) of 9–10% was obtained for the normalized Al film thickness by wavelength h Al/λ = 0.06–0.07 and h/λ = 0.30–0.40. Moreover, even at h/λ with a faster phase velocity than the bulk shear wave of 4H-SiC, the strong LLSAW responses without spurious response owing to the LLSAW higher-order mode were obtained. Finally, h Al/λ = 0.031 and h/λ = 0.19 were extracted to obtain a phase velocity of 7800 m/s, high Q factors, and FBW of 7.6%.

Resonance analysis of longitudinal leaky surface acoustic wave third harmonic on LiNbO3/quartz bonded structure

For HF operation of surface acoustic wave (SAW) devices, the resonance properties and temperature coefficient of frequency (TCF) of the longitudinal leaky SAW (LLSAW) third harmonic on a bonded structure comprising a LiNbO3 (LN) thin plate and quartz support substrate were simulated using a finite element method. The theoretical results showed that, by utilizing the bonded structure, the third harmonic can be excited more strongly than when using a single LN substrate. The TCF of the LLSAW fundamental wave had larger absolute values than that of a typical SAW device.

Ordered creation and motion of skyrmions with surface acoustic wave

Magnetic skyrmions with a well-defined spin texture have shown unprecedented potential for various spintronic applications owning to their topologically non-trivial and quasiparticle properties. To put skyrmions into practical technology, efficient manipulation, especially the inhibition of skyrmion Hall effect (SkHE) has been intensively pursued. In spite of the recent progress made on reducing SkHE in several substituted systems, such as ferrimagnets and synthetic antiferromagnets, the organized creation and current driven motion of skyrmions with negligible SkHE in ferromagnets remain challenging. Here, by embedding the [Co/Pd] multilayer into a surface acoustic wave (SAW) delay line where the longitudinal leaky SAW is excited to provide both the strain and thermal effect, we experimentally realized the ordered generation of magnetic skyrmions. The resultant current-induced skyrmions movement with negligible SkHE was observed, which can be attributed to the energy redistribution of the system during the excitation of SAW. Our findings open up an unprecedentedly new perspective for manipulating topological solitons, which could possibly trigger the future discoveries in skyrmionics and spin acousto-electronics.

Phonon–magnon conversion using longitudinal leaky surface acoustic waves through magnetoelastic coupling

Surface acoustic waves (SAWs) can transmit magnetization oscillation inside magnetoelastic films in the form of magnetoacoustic waves. If the frequency and wavenumber of SAWs match those of spin waves, magnon excitation can be observed. In this work, we studied the phonon–magnon conversion excited by longitudinal leaky SAWs, which possess the same dominant strain component like Rayleigh-type SAWs but a much higher phase velocity. The measured transmission power absorptions of both SAW modes due to spin wave resonance follow a linear frequency dependence and exhibit a fourfold symmetry. We demonstrate that longitudinal leaky SAWs can serve as a very effective means to excite stronger phonon–magnon coupling than Rayleigh SAWs at a lower wave number.

Analysis of longitudinal leaky surface acoustic waves on piezoelectric thin plates bonded to diamond substrate

Using the finite element method, we analyzed the resonance properties of a longitudinal leaky surface acoustic wave (LLSAW) on the structure of a piezoelectric LiTaO3 (LT) or LiNbO3 (LN) thin plate bonded to a diamond support substrate. When the plate thickness was 0.3 wavelength or larger, the particle displacements of the LLSAW were substantially concentrated near the surface, and the resonance properties were significantly improved compared to those of the single LT or LN. However, spurious responses due to other SAW modes also emerged. Regarding the LN thin plate, we obtained a phase velocity and fractional bandwidth of approximately 7300 m s−1 and 9.4%, respectively. Furthermore, a structure was proposed to suppress spurious responses in which the piezoelectric thin plate is divided into two layers with different Euler angles. The spurious responses were significantly suppressed after dividing the thin plate, while the main response of the LLSAW was maintained.

A near spurious-free 6 GHz LLSAW resonator with large electromechanical coupling on X-cut LiNbO3/SiC bilayer substrate

The fast development of the fifth-generation (5G) wireless systems and substantial growth of data usage have imposed stringent requirements for high-frequency and wideband radio frequency devices. Here, it is reported on a longitudinal leaky surface acoustic wave (LLSAW) mode acoustic resonator with a large electromechanical coupling factor (kt2), high operating frequency, and efficient spurious suppression. Through systematical finite element method simulations, available design spaces such as supporting substrate, propagation angle, and lithium niobate (LN) thickness have been fully investigated with the aim of stimulating the intended LLSAW and suppressing spurious modes concurrently. Optimization results reveal that the LLSAW mode wave propagating in X-35°Y LN/SiC piezoelectric-on-insulator (POI) bilayer structure possesses a large kt2 without significant interference from other spurious modes. To verify the theoretical analyses, LLSAW resonators were fabricated and exhibited a near spurious-free response with the operating frequency over 6 GHz, and kt2 as large as 22.7%. This work demonstrates a high-performance LLSAW resonator on the POI platform with a simple prototype as well as potentially providing a high-frequency filtering solution for 5G applications in the 6-GHz spectrum.

Analysis of longitudinal leaky surface acoustic waves on bonded structures consisting of similar and dissimilar materials

The propagation and resonance properties of longitudinal leaky surface acoustic waves on structures consisting of a LiTaO3 (LT) thin plate bonded to a quartz (Qz) similar-material bonded structure were investigated theoretically. It was found by surface acoustic wave (SAW) propagation analysis that a small attenuation can be obtained by combining an LT thin plate and a Qz similar-material bonded structure with appropriate Euler angles. Furthermore, regarding SAW resonance analysis, resonances with admittance ratios exceeding 50 dB and Q factors exceeding 1000 were obtained in the LT/Qz/Qz bonded structure. Such a small propagation loss and high Q can be obtained with a larger LT plate thickness than in the case of the LT/Qz bonded structure.

Analysis of surface acoustic wave resonance properties on piezoelectric substrates with periodic voids

The resonance properties of leaky surface acoustic waves (LSAWs) and longitudinal LSAWs (LLSAWs) on bonded structures consisting of a LiNbO3 (LN) thin plate and a support substrate, with periodic voids exhibiting a rectangular cross-section, were simulated using the finite element method. The voids were formed below the electrode at the boundary between the LN thin plate and support substrate and presented the same pitch and period as the electrode. LN, quartz, glass, or Si were used as support substrates. Simulations revealed that by introducing periodic voids, resonance properties similar to those of the SH0-mode plate and S0-mode Lamb waves can be obtained. The values of the simulated fractional bandwidths of approximately 13 and 18% were found for LSAW and LLSAW, respectively, when the ratio of the width of the void to the pitch of the electrode was 0.7.

Analysis of longitudinal leaky surface acoustic waves on quartz thin plate bonded to similar-material substrate

The propagation and resonance properties of longitudinal leaky surface acoustic waves (LLSAWs) on bonded structures consisting of a quartz (Qz) thin plate and a Qz support substrate with different Euler angles were investigated theoretically. By using both an X-cut Qz thin plate and a Qz support substrate with optimal Euler angles, we obtained LLSAWs with a larger coupling factor, a smaller attenuation, and a lower temperature coefficient of frequency than those on a single Qz substrate. Furthermore, from the resonance properties simulated by the finite element method, the bonded structures were found to exhibit a large admittance ratio and a high quality factor, which could not be obtained when using a single Qz substrate; the bandwidth, however, was as small as 0.016%–0.086%.

Analysis of longitudinal leaky surface acoustic waves on LiNbO3/amorphous layer/quartz structure

Recently, bonded structures comprising a quartz substrate and a LiNbO3 (LN) or LiTaO3 thin-plate have been attracting attention as a means of obtaining enhanced properties for surface acoustic wave (SAW) devices. It was reported that the bonding strength was extremely improved by utilizing an amorphous thin film as a middle layer. In this research, the propagation and resonance characteristics of longitudinal leaky SAWs on an amorphous layer inserted to the boundary between an X-cut 36°Y-propagating LN (X36°Y-LN) and X35°Y-quartz were investigated. For the metallized surface of an X36°Y-LN/Al2O3/X35°Y-quartz structure, an attenuation of 0.0001 dB /λ was obtained at an LT thin-plate thickness of 0.072λ (λ: wavelength), which was better than that of an X36°Y-LN/X35°Y-quartz structure. Utilizing a finite element method, we found that the admittance ratio and Q factor for the X36°Y-LN/Al2O3/X35°Y-quartz model were improved owing to the reduction of the leakage due to the shear-vertical particle displacement.

Ultrahigh-Frequency Surface Acoustic Wave Sensors with Giant Mass-Loading Effects on Electrodes.

Surface acoustic wave (SAW) devices are widely used for physical, chemical, and biological sensing applications, and their sensing mechanisms are generally based on frequency changes due to mass-loading effects at the acoustic wave propagation area between two interdigitated transducers (IDTs). In this paper, a new sensing mechanism has been proposed based on a significantly enhanced mass-loading effect generated directly on Au IDT electrodes, which enables significantly enhanced sensitivity, compared with that of conventional SAW devices. The fabricated ultrahigh-frequency SAW devices show a significant mass-loading effect on the electrodes. When the Au-electrode thickness increased from 12 to 25 nm, the Rayleigh mode resonant frequency decreased from 7.77 to 5.93 GHz, while that of the higher longitudinal leaky SAW decreased from 11.87 to 9.83 GHz. The corresponding mass sensitivity of 7309 MHz·mm2·μg-1 (Rayleigh mode) is ∼8.9 × 1011 times larger than that of a conventional quartz crystal balance (with a frequency of 5 MHz) and ∼1000 times higher than that of conventional SAW devices (with a frequency of 978 MHz). Trinitrotoluene concentration as low as 4.4 × 10-9 M (mol·L-1) can be detected using the fabricated SAW sensor, proving its giant mass-loading effect and ultrahigh sensitivity.

Analysis of leaky surface acoustic wave characteristics propagating on high piezoelectric ScAlN film/high velocity quartz substrate

Recently, it has been reported that leaky surface acoustic waves (LSAWs) and longitudinal LSAWs on piezoelectric LiNbO3 and LiTaO3 thin plates bonded to high velocity quartz substrates have high phase velocity, high coupling, and low attenuation. These lead to high-frequency operation, a wide band-width, and a high Q factor for next generation SAW filters. Moreover, we theoretically demonstrated that longitudinal LSAWs on high piezoelectric ScAlN films/quartz substrates also possesses high phase velocity, high coupling, and low attenuation. In this paper, LSAW characteristics on ScAlN films/quartz substrates were investigated theoretically. When the Euler angles and the film thickness of the ScAlN film and quartz substrate were optimized for LSAW, the LSAW resonators with (0° 90° 72°) Sc0.4Al0.6N film/(0° 128.5° 0°) quartz were found to have a high Q factor of ∼21 610 and a high coupling of ∼5.8%.

Advanced Substrate Material for SAW Devices Combining LiNbO₃ and Langasite.

A layered structure combining a thin plate of lithium niobate (LN) with a langasite (LGS) substrate was numerically investigated as an advanced substrate material for surface acoustic wave (SAW) devices with improved characteristics, including capability of operating at high temperatures. The longitudinal leaky SAWs (LLSAWs) propagating with negligible attenuation and high velocities up to 6400 m/s were found in LN/LGS using an optimization technique previously applied to the LN/quartz structure. Compared with quartz, the use of LGS as a substrate imposes less constrains on bonding at elevated temperatures for fabrication of a layered structure. Due to the specific acoustic anisotropy of LGS crystals, low-attenuated high-velocity waves exist in a wide range of LGS orientations, including the commercially available SAW cut, when LGS substrate is combined with an LN plate. In the found optimal structures, the electromechanical coupling increased from 5.5% to 14.5%, with increasing LN thickness, making LN/LGS substrate suitable for application in SAW devices with a widely varying bandwidth. In addition, low shear bulk wave velocities in LGS provide a wide spurious-free frequency range in resonators using LLSAW.

Optimization of LiNbO3/Quartz Substrate for High-Frequency Wideband SAW Devices Using Longitudinal Leaky Waves.

Multilayered structures can successfully replace single crystal substrates in high-performance surface acoustic wave (SAW) devices. A combination of high propagation velocity and strong piezoelectric coupling is required for acoustic modes used in wideband high-frequency SAW devices. These characteristics can be achieved simultaneously in SAW devices arranged on a lithium niobate (LN) thin plate bonded to quartz and using longitudinal leaky SAW (LLSAW) if the orientations of both the crystals and the plate thickness are optimized. This article describes an optimization procedure developed to find low-attenuated LLSAWs in layered structures that can be used in LN/quartz applications. The symmetry consideration of two crystals was followed by the optimization of the LN orientation to achieve the largest electromechanical coupling and by an analysis of quartz anisotropy as a crucial factor of the existence of nonattenuated LLSAWs. Finally, the quartz orientation and LN thickness were optimized by a rigorous numerical simulation of resonator admittances and by the extraction of LLSAW attenuation at resonant and antiresonant frequencies. The discovered structures enable the propagation of LLSAWs with velocities in the range of 5400-6000 m/s, electromechanical coupling up to 18%, and negligible attenuation. High Q -factors can be achieved at resonance and antiresonance by the variation of the duty factor in the same layered structure. The specific behavior of the LLSAW attenuation with a variation of the LN thickness and quartz cut angle is illustrated by contour plots for each of the optimal structures discovered.

Acousto-optic modulators driven by longitudinal leaky surface acoustic waves on LiNbO3 thin-plate bonded structures

To obtain an acousto-optic modulator (AOM) driven by a longitudinal leaky surface acoustic wave (LLSAW) with a low driving voltage and a high diffraction efficiency, proton-exchanged planar optical waveguide-type AOMs with structures consisting of an X-cut 36°Y-propagating LiNbO3 (LN) thin-plate bonded to a AT-cut 45°X-propagating quartz or c-plane sapphire (Al2O3) substrate were fabricated and their diffraction properties evaluated. The maximum diffraction efficiency in the zeroth transverse electric (TE0) mode optical guided wave with an optical wavelength of 0.633 μm and the driving voltage required by the LLSAWs of the AOMs were 91% and 5.1 VRMS for LN/quartz and 60% and 11 VRMS for LN/Al2O3, respectively. The diffraction properties of the LLSAWs on the bonded structures were improved compared with those on a single LN substrate.

Longitudinal leaky surface acoustic wave with low attenuation on LiTaO3 or LiNbO3 thin plate bonded to quartz substrate

To obtain a bonded structure with low attenuation for longitudinal leaky surface acoustic waves (LLSAWs), the propagation and resonance properties on a LiTaO3 (LT) or LiNbO3 thin plate bonded to an X-cut quartz substrate were theoretically analyzed. The attenuation of an X-cut 31°Y-propagating LT (X31°Y-LT)/X32°Y-quartz (X32°Y-Q) was calculated to be 0.0005 dB/λ at the normalized LT thin plate thickness h/λ = 0.062 (λ: wavelength) and was lower than that on an X31°Y-LT/AT45°X-Q. Using a finite element method, for the X31°Y-LT/X32°Y-Q, the admittance ratio and Q factor were improved to 120 dB and 53 400 from 62 dB and 1000 for the X31°Y-LT/AT45°X-Q, respectively. Then, the propagation and resonance properties were measured. For the X31°Y-LT/X32°Y-Q, the measured electromechanical coupling factor (K2) and Q factor increased to 5.6% and 280 from 1.8% and 32 for the single LT, respectively. The temperature coefficient of frequency of the LLSAW was measured to be −26.2 ppm °C−1.

Analysis of longitudinal leaky surface acoustic wave propagation characteristics on a piezoelectric ScAlN layer/sapphire or quartz substrate

Longitudinal leaky surface acoustic wave (LLSAW) modes with a high phase velocity are suitable for high-frequency SAW filter applications. However, their attenuation is higher than that of other SAW modes, causing a small Q factor. On the other hand, a low attenuation and high coupling factor K2 in LLSAWs on a piezoelectric thin plate/high-velocity sapphire or quartz substrate have been demonstrated theoretically and experimentally. We expected that LLSAWs propagating on a highly piezoelectric ScAlN layer/sapphire or quartz substrate also have a high phase velocity, high K2 and low attenuation. In this study, the characteristics of LLSAWs on ScAlN layer/sapphire or quartz substrates were theoretically investigated. The cut angle and SAW propagation direction of quartz substrates strongly affect the attenuation of LLSAWs on the layered structure. We found that a LLSAW resonator with (0° 90° 90°) Sc0.4Al0.6N layer/X-cut 30° Y propagation quartz has a high Q factor and high K2, which is adequate for SAW filter applications.

3.5 GHz longitudinal leaky surface acoustic wave resonator using a multilayered waveguide structure for high acoustic energy confinement

In this paper, the use of a structure comprising a thin LiNbO3 plate and a multilayered acoustic mirror composed of SiO2 and Pt for high-performance longitudinal leaky surface acoustic wave (LLSAW) device is proposed. The mirror is expected to offer a much higher reflectivity than that composed of SiO2 and AlN, which the authors proposed previously. The field distribution of these structures is calculated by using a finite element method. It is shown that the acoustic wave energy of the proposed structure is well confined in the vicinity of the top surface, and that leakage to the substrate is reduced. A one-port resonator is fabricated on the structure and its performance characteristics are evaluated. Owing to a high phase velocity of 6,035 m/s, which is about 1.5 times higher than that of conventional SAWs, a large impedance ratio of 71 dB was achieved at 3.5 GHz in addition to a large fractional bandwidth of 9.5%.

Acousto-optic Bragg diffraction using longitudinal leaky surface acoustic wave

In this study, to increase the amount of frequency shift by adopting a longitudinal leaky surface acoustic wave (LLSAW) with a higher phase velocity than a Rayleigh-type SAW to a waveguide-type acousto-optic Bragg deflector, a Ti-diffused optical waveguide was fabricated on an X-cut 36°Y-propagating LiNbO3 substrate with a high electromechanical coupling factor for the LLSAW and its diffraction properties were evaluated. The diffraction of the guided light by the LLSAW was observed for the first time. The diffraction efficiency in the TE0 mode was approximately 50% for an optical wavelength of 0.633 µm.

Surface acoustic waves in semi-insulating Fe-doped GaN films grown by hydride vapor phase epitaxy

The propagation properties of surface acoustic waves (SAWs) in semi-insulating Fe-doped GaN films grown on sapphire substrates by hydride vapor phase epitaxy are investigated. Compared with native n-type GaN, Fe-doped GaN exhibits a higher electromechanical coupling coefficient due to its high electrical resistivity. In addition, guided longitudinal leaky surface acoustic wave (LLSAW) was observed experimentally with a very high phase velocity (about 7890 m/s), and this mode was verified by numerical simulations. The small propagation attenuation of LLSAW along liquid/solid interfaces was demonstrated in glycerol solutions, which implies the potential applications in high-frequency chemical sensing.