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LiNbO3 (LN) or LiTaO3 (LT) thin plates bonded to quartz are novel types of layered substrates for temperature-compensated surface acoustic wave (TCSAW) devices. In SAW resonators with Al electrodes arranged on LT/quartz and LN/quartz substrates, improved temperature behavior due to opposite signs of temperature coefficients of frequency (TCF) in quartz substrates and LT or LN plates can be combined with high ${Q}$ -factors if the quartz orientation is optimized. The quartz orientation areas that enabled high ${Q}$ -factors were deduced from analyzing the quartz anisotropy and the characteristics of shear horizontally (SH) polarized waves were calculated in the optimal orientations as functions of the quartz cut angle and plate thickness. The simulation results illustrated by contour plots of the wave characteristics revealed the presence of zero lines on the plots demonstrating TCFs at resonant (TCFR) and antiresonant (TCFA) frequencies in both layered structures. The strong quartz anisotropy explained anomalous temperature behavior of SAW resonators and the existence of LT/quartz structures with TCFA = TCFR facilitating the design of SAW devices with improved thermal stability in the passband. In the LT/quartz structures, two TCFs vanished simultaneously at certain plate thickness and quartz orientation, while in the LN/quartz zero lines did not intersect but low TCF = −(10–20) ppm/°C combined with electromechanical coupling exceeding 18% were obtained numerically in some structures. The effect of inverting the propagation direction or cut angle in one of the combined materials on the wave characteristics was discussed and numerically estimated.

A configuration of a laser polarisation modulator based on isotropic acousto-optic interaction is proposed. Polarisation control is performed by a 2D acousto-optic cell based on one monoclinic single crystal. A possibility of implementing this modulator on the basis of a KY(WO4)2 crystal is demonstrated.

Layered structures widely used today as substrates for surface acoustic wave (SAW) devices are characterized by asymmetry of wave propagation, which manifests itself as variation of SAW or leaky SAW characteristics and admittance function of SAW resonators built on these substrates with inversion of propagation direction or interchange of top/bottom surfaces in one of combined materials. In this paper, matrix formalism known as an effective tool for theoretical and numerical investigation of acoustic wave propagation in multilayered structures is used to explain the existence of asymmetry in the behavior of acoustic waves. This phenomenon is illustrated by two examples of layered structures using different types of acoustic waves and previously reported as promising substrates for SAW devices with improved performance.

Multilayered substrates for Surface Acoustic Wave (SAW) devices are able to combine SAW characteristics that cannot coexist in a single crystal substrate and, thus, meet the strong requirements of the new class of SAW devices developed for the next generations of communication systems. Recently, high performance resonators arranged on LiTaO3/quartz bonded wafers and utilizing shear horizontally polarized acoustic waves were reported. Leaky SAWs with quasi-longitudinal polarization propagate faster and can facilitate fabrication of high frequency SAW devices but generally leak strongly into the substrate. This paper describes how the LiTaO3/quartz structure can be optimized to allow longitudinal SAWs to propagate without attenuation. Due to the symmetry consideration, which is supplemented by a rigorous numerical simulation of the admittance functions of SAW resonators and an accurate extraction of the propagation losses, the found optimal LiTaO3 and quartz orientations with the optimized LiTaO3 thickness ensure the propagation of acoustic waves with a velocity exceeding 5400 m/s and an electromechanical coupling of 6.8% in resonators with Q factors up to 10,000. The optimal LT/quartz structures with plate thicknesses varying between 0.32 and 0.68 wavelengths can be employed in SAW resonators operating at high frequencies, up to 5 GHz. The existence of numerous orientations in quartz supporting the propagation of non-attenuated longitudinal SAWs is explained based on the concept of exceptional bulk waves, which is a part of SAW theory.

Batteryless, wireless, and packageless acoustic wave sensors are particularly desirable for harsh high-temperature environments. In this letter, an acoustic wave sensor based on a lithium niobate (Y + 128° cut, abbreviated LN-Y128) substrate with a buried platinum interdigital transducer (IDT) in an aluminum nitride (AlN) overlayer is investigated. Previously, it was demonstrated theoretically that due to the specific properties of LN-Y128, Rayleigh-type guided waves can propagate at the AlN/IDT(Pt)/LN-Y128 interface. Here, this structure is, for the first time, studied experimentally, including the growth and properties of the AlN layer onto irregular platinum IDTs. Both Shear Horizontal and Rayleigh-type waves have been identified after the AlN deposition and the velocities are consistent with the fitted SDA-FEM-SDA (a combination of finite element modeling with spectral domain analysis) simulations. Electrical measurements with a surface perturbation and temperature measurements show that the AlN/IDT(Pt)/LN-Y128 bilayer structure is promising as a packageless high-temperature sensor.

We demonstrate that anisotropy of Fresnel diffraction in a birefringent medium can be quantitatively characterized by a planar tensor. Eigenvectors of the tensor correspond to directions of minimum and maximum beam divergence. Zero eigenvalues exist for the fast eigenmode near the optic axis of biaxial crystals and correspond to autocollimated beam propagation. Applications of the beam diffraction tensor are demonstrated for analysis of noncritical phase matching in acousto-optics and three-wave mixing., Comment: 8 pages, 9 figures

A new, to the best of our knowledge, type of acousto-optic Q-switch was developed using slow shear acoustic mode in potassium yttrium tungstate (KYW) crystal. Two Q-switch configurations were created: one for vertical and one for horizontal light polarization, both providing over 50% diffraction efficiency at a wavelength of 2.1 μm and an RF driving power below 8 W. The laser-induced damage threshold of the KYW crystal was found to equal 650 MW/cm2. Operation of a nanosecond periodically pulsed Ho:YAG laser emitting 15 mJ pulses at 2.1 μm with the KYW Q-switch is reported.

The high acoustic wave velocities required for high-frequency SAW devices can be combined with strong piezoelectric coupling and low propagation losses if a thin plate of LiNbO3 (LN) is bonded to a high-velocity sapphire substrate. This paper presents a numerical investigation of acoustic modes propagating in an LN/sapphire substrate under a periodic Al grating, demonstrating the occurrence of longitudinal leaky SAWs (LLSAWs) with a unique combination of high velocities exceeding 10 000 m/s, an electromechanical coupling coefficient of 4.5%, and negligible leakage into the substrate. The leakage was suppressed by optimizing the LN plate and Al electrode thicknesses. Compared with conventional SAW substrates, the LN/sapphire substrate offers 2- to 2.5-fold higher frequencies for periodic electrode structures and enables the production of 6-GHz devices by standard photolithographic processes while the required LN plate and Al electrode thicknesses remain compatible with existing wafer bonding technologies. In the optimal LN/sapphire structure, LLSAW behaves as a perfect Rayleigh SAW unperturbed by interactions with spurious modes in a wide range of frequencies.

We designed a new configuration of acousto-optic spatial light modulator based on biaxial crystal KY(WO4)2 (KYW). This material has proved to be a good candidate to fill the gap between paratellurite having high acousto- optic efficiency and quartz having low efficiency but high laser-induced damage threshold. The modulator uses isotropic diffraction by a slow quasi-shear bulk acoustic wave propagating in the autocollimation direction. This ensures good compromise between acousto-optic figure of merit, which is only 30% less than in z-cut longitudinal- wave paratellurite, and high laser-induced damage threshold. The prototype modulator has the spatial resolution of 250 with the central frequency of 100 MHz and the aperture of 20 mm. The designed spatial light modulator is aimed at high-power ultrashort laser pulse shaping applications in near and middle infrared.

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.

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.

Acoustic wave propagation in single crystals, metamaterials and composite structures is a basic mechanism in acoustic, acousto-electronic and acousto-optic devices. Acoustic anisotropy of crystals provides a variety of device performances and application fields, but its role in pre-estimation of achievable device characteristics and location of crystal orientations with desired properties is often underestimated. A geometrical image of acoustic anisotropy can be an important tool in design of devices based on wave propagation in single crystals or combinations of anisotropic materials. We propose a fast and robust method for survey and visualization of acoustic anisotropy based on calculation of the eigenvalues of bulk acoustic wave (BAW) diffraction tensor (curvature of the slowness surface). The stereographic projection of these eigenvalues clearly reveals singular directions of BAW propagation (acoustic axes) in anisotropic media and areas of fast or slow variation of wave velocities. The method is illustrated by application to three crystals of different symmetry used in different types of acoustic devices: paratellurite, lithium niobate, and potassium gadolinium tungstate. The specific features of acoustic anisotropy are discussed for each crystal in terms of their potential application in devices. In addition, we demonstrate that visualization of acoustic anisotropy of lithium niobate helps to find orientations supporting propagation of high-velocity surface acoustic waves., 12 pages, preprint submitted to EPJ Plus

Multilayered substrates, including thin plates of LiTaO3 (LT) bonded to silicon (Si) or glass wafers either directly or via intermediate layers, were numerically investigated as potential materials for surface acoustic wave (SAW) resonators with high quality (Q)-factor required for the next generation of mobile communication systems. The propagation velocities and electromechanical coupling coefficients of shear horizontally (SH) polarized acoustic waves were estimated as functions of LT orientation and thicknesses of all layers in LT/Si, LT/SiO2/Si, LT/glass, LT/AlN/glass and structures with several pairs of SiO2/AlN layers between LT and the glass wafer. Optimal combinations of cut angle, LT and electrode thicknesses, as well as the number and thicknesses of intermediate layers, required for the construction of resonators with improved performance were observed for each analyzed structure. In the resonators employing LT/Si and LT/SiO2/Si structures with 30°YX - 48°YX LT cuts, high electromechanical coupling k2, reaching 11.6%, can be combined with high velocities up to 4000 m/s, zero TCF at the resonant frequency and Q-factors that are considerably higher than in the LSAW filters using regular LT substrates. To understand the loss mechanisms that limit resonator Q-factors in LT/glass, mechanical displacements that accompany wave propagation in multilayered structures were visualized. Investigation of the nature of acoustic modes and their transformations with number and thicknesses of the layers revealed that the low-velocity glass wafer can be used as a supporting substrate if an intermediate AlN layer or alternating pairs of low- and high-velocity layers, for example SiO2/AlN, are introduced between LT and the glass wafer.

Bonded wafers combining a thin LiNbO3 plate with a quartz substrate were studied numerically as potential substrate materials for high frequency wideband SAW devices. Longitudinal leaky waves propagating with velocities up to 7000 m/s were investigated, and the multilayered structure was optimized to combine high velocity, high electromechanical coupling and negligible propagation losses. In the optimized structures non-attenuated waves can propagate with velocities V=5500-6200 m/s and coupling up to 18%. Examples of simulated admittances of SAW resonators demonstrate that in some optimal LiNbO 3 /quartz structures high quality factors Q>5000 can be achieved simultaneously at resonant and antiresonant frequencies if duty factor of electrode structure varies between 0.48 and 0.65. Due to sufficiently large optimal plate and electrode thicknesses, 40% and 5% of a wavelength, respectively, the found structures can be employed in SAW resonators operating at high frequencies, up to 5 GHz.

International audience; Multilayered structures based on wide bandgap nitride piezoelectric thin films are very attractive for high-temperature surface acoustic waves (SAW) sensor applications. In this respect, scandium aluminium nitride (ScAlN) films are of particular interest as they combine enhanced piezoelectric properties and slower acoustic waves velocities when the Sc content steadily increases up to 40%. This property offers the possibility to combine slow ScAlN films on fast substrates like sapphire, to generate higher-order SAW modes which often show a better electromechanical coupling coefficient k 2 compared to zero-order modes. In this letter, we show that low-attenuated longitudinal SAW can be generated in ScxAl1-xN/Sapphire structure, for x parameter varying in a large range. This theoretical result is then confirmed by the experimental investigation of SAW resonators based on highly-textured (002) Sc0.09Al0.91N films sputtered on c-cut sapphire substrates. It is finally shown that the use of electrodes based on metals with high density can lead to SAW structures offering a unique combination between a large bandgap over 5 eV, a k 2 value beyond 1% and a high SAW velocity near 10000 m/s.

We designed a high-efficient acousto-optic Q-switch based on a potassium yttrium tungstate crystal (KY(WO 4 ) 2 ), or KYW. Isotropic acousto-optic interaction along N m N g plane of dielectric axes of the crystal was used. The Q-switch operated at the wavelength of 2.1 μ m of a Ho:YAG laser with the diffraction efficiency over 60% at the RF driving power of 20 W. No laser damage of the crystal with antireflection coatings was observed at 12 J/cm 2 fluence for nanosecond pulses.

The formalism of planar diffraction tensor was applied to the analysis of anisotropy of bulk acoustic wave diffraction and to build a full map of anisotropic diffractional coefficients for three bulk acoustic wave modes propagating in lithium niobate. For arbitrary propagation direction the diffractional coefficients derived allow estimation of ultrasonic beam divergence in far-field. Analysis of obtained data revealed that the maxima of acousto-optic figure of merit for anisotropic diffraction in the YZ plane correspond to moderate diffractional spreading of the beams exceeding isotropic diffraction 2-3 times.

Scandium aluminum nitride (ScxAl1-xN) films are currently intensively studied for surface acoustic waves (SAW) filters and sensors applications, because of the excellent tradeoff they present between high SAW velocity, large piezoelectric properties and wide bandgap for the intermediate compositions with an Sc content between 10 and 20%. In this paper, the growth of Sc0.09Al0.91N and Sc0.18Al0.82N films on sapphire substrates by sputtering method is investigated. The plasma parameters were optimized, according to the film composition, in order to obtain highly-oriented films. X-ray diffraction rocking-curve measurements show a full width at half maximum below 1.5&deg, Moreover, high-resolution transmission electron microscopy investigations reveal the epitaxial nature of the growth. Electrical characterizations of the Sc0.09Al0.91N/sapphire-based SAW devices show three identified modes. Numerical investigations demonstrate that the intermediate compositions between 10 and 20% of scandium allow for the achievement of SAW devices with an electromechanical coupling coefficient up to 2%, provided the film is combined with electrodes constituted by a metal with a high density.

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.

Bonded wafers combining thin plate of LiTa03 with quartz substrate were recently reported as a new material for high performance TC SAW resonators. To suppress losses and improve Q-factors of SAW resonators, wave characteristics were studied numerically as functions of LT and quartz orientations, as well as LT and Cu electrode thicknesses. In addition to propagation losses, coupling k2 and TCF were extracted from admittance functions. SH waves can propagate with negligible attenuation in a wide range of angles θ if the quartz substrate with Euler angles ( $(0^{\circ},\theta, 90^{\circ}$ )is combined with YX cut of LT. Maximum coupling (about 12.5%)can be achieved for 30°YX LT. In LT bonded to quartz ( $(0^{\circ},\theta, 90^{\circ}$ ), leakage of SH wave is generally strong but vanishes in ST cut. TCF improves in LT/ST, X+ψ when ψ approaches 90°. Propagation losses can be also suppressed for high velocity leaky SAWs if LT and quartz orientations with Euler angles ( $(0^{\circ},\theta, 90^{\circ}$ )are combined and optimized properly. An example of such wave is presented, with a velocity of 5400 m/s and coupling k2=6.64%.

Multilayered substrates, including thin plates of LiTaO

Quasi-collinear type of acousto-optic (AO) diffraction by bulk acoustic waves (BAW) is widely used for high-resolution tunable optical filters and dispersive delay lines for ultrashort laser pulse shaping. Most often, quasi-collinear paratellurite (TeO 2 ) AO filters are used due to high AO figure of merit of this material (V. Molchanov et al. Quant. Electron. 39:353, 2009). Lithium niobate (LN, LiNbO 3 ) collinear filters along Y-axis are known to provide a better spectral resolution or a shorter response time. A demand for higher spectral performance of AO filters is caused by progress in ultrafast laser technology and adaptive spectroscopy. The performance of a LN filter can be improved using quasicollinear diffraction. We aimed at design of a quasi-collinear AO filter in LN with maximum diffraction efficiency.

Multilayered structures with LT plates bonded to silicon or glass substrates via AlN, isotropic SiO 2 or their combinations, were theoretically investigated as potential materials for resonators with high Q-factors required for the 5th generation of mobile communication systems. Characteristics of SH-waves propagating under Al grating in such structures, including electromechanical coupling and propagation losses, were extracted from admittance functions simulated with SDA-FEM-SDA software. An upgraded version of the software was developed to visualize the waves in multilayered structures and understand the loss mechanisms and their dependence on the supporting substrate material, as well as on the number, type and thicknesses of the intermediate layers. The methods of loss reduction required to improve the Q-factors of acoustic wave resonators are discussed.

Acoustic wave characteristics are numerically investigated and compared in AlN/ZnO/diamond and Al 2 O 3 /GaN/sapphire with Pt electrodes. These structures have been previously reported as candidates for application in packageless sensors. They include a low-velocity guiding piezoelectric layer sandwiched between two high-velocity materials. The anisotropy of the combined materials, their symmetry and the ratio between bulk acoustic wave velocities in the sagittal plane are compared to find the selection criterion for optimal combination of materials enabling high coupling of the useful mode and suppression of parasitic modes. Acoustic fields have been visualized for the reported structures and for Si 3 N 4 /GaN/sapphire, an alternative structure suggested to be used in packageless sensors, to understand the correlation between the acoustic fields and the wave characteristics.

Surface Acoustic Wave sensors are a promising enabling technology to achieve temperature monitoring in extreme conditions, and especially at very high temperature, above 900°C. Applications in the air and space, chemical, steel, and automotive industries are notably expected. Although many of the technical challenges posed by the development of these sensors have been tackled and solved over the years, there is still no SAW sensor able to withstand very high temperatures for a long time on the market (yet). A possible alternative could be to combine a SAW transponder with a thermocouple ('SAW Hybrid'). The thermocouple would be read wirelessly using a SAW transponder, itself located at a place nearby where the temperature doesn't exceed 350°C. The feasibility of this concept was recently demonstrated, using a 400MHz device. However, the obtained sensitivity was very low, and an optimized Design is needed, to increase it. It is notably expected that a higher operating frequency will significantly improve the sensitivity. In this paper, we present an optimal Design of the SAW Hybrid AlN/LN multilayered structure, for operation at 2.45GHz, with buried Pt electrodes.

Dispersion of Lamb waves propagating in AlN plates with a periodic Al grating on the top surface and an Al electrode on the bottom surface is investigated using the numerical technique SDA-FEM-SDA, which combines finite element modeling (FEM) analysis of the electrode region with spectral-domain analysis (SDA) of the adjacent multi-layered half-spaces. Characteristics of zero-order and higher-order Lamb waves are presented as functions of plate thickness and spectral frequency, which varies in the first Brillouin zone. The structures of typical Lamb waves are examined via visualization of the instantaneous displacement fields in the AlN plate confined between the grating and the bottom electrode. The mechanism of building hybrid modes, which arise from intermode coupling between the counter-propagating Lamb waves of different symmetry and order, is illustrated by two examples of modes propagating with wavelengths λ = 3p and λ = 4p, where p is the pitch of the grating.

Orientations of langasite (LGS) with Euler angles (0°, μ, 90°) are potentially useful for applications in wireless SAW sensors because of wide variation of their temperature coefficient of frequency (TCF), between -50 and +50 ppm/°C, combined with high values of electromechanical coupling coefficient, especially if thick electrodes are utilized. The nature and characteristics of shear-horizontally polarized SAWs (SHSAWs) propagating in these orientations are investigated using a combination of numerical techniques: analysis of the BAW slowness surface and its effect on the SH-SAW structure; visualization of displacements that follow wave propagation in the grating; calculation of admittance of infinite periodic gratings; and analysis of SAW dispersion in the gratings, including possible interaction between SH-SAWs and parasitic Rayleigh SAWs. The characteristics of SH-SAWs propagating in symmetric cuts (velocities, localization depth, electromechanical coupling, etc.) are supplemented by analysis of propagation directions deviating from the YZ-plane toward the x-axis of LGS, for the temperature compensated cut with Euler angles (0°, 22°, 90°) and one more orientation for comparison. This extended analysis reveals that in actual devices, the anisotropy may be a reason for strong interaction between SH-SAWs and parasitic Rayleigh SAWs and cause degradation of the device's performance.

The combination of experimental and numerical techniques previously developed for analysis of spurious BAWs in delay lines based on YX-LGS was applied to LGS cut with Euler angles (0°, 138.5°, 26.7°) used in high-temperature SAW sensors. The ratio between the amplitudes and delay times of SAW propagating at the frequency ƒ=3·ƒ 0 and the undesired BAW overlapping with this SAW was studied in order to suppress the BAW and use the SAW overtone for sensing purposes.

The mechanism of SAW transformation with variation of film thickness is investigated in a piezoelectric substrate with a metal grating overlaid by a dielectric film, via simulation and visualization of the acoustic fields. By way of example, two orientations of lithium niobate substrates are analyzed, YX-LN and 128°YX-LN, with a Cu grating and an isotropic silica glass overlay. The motions, which follow the wave propagation in the sagittal plane, are visualized within two periods of the grating, with added contour plots showing the shear horizontal displacements. The continuous transformation of the wave's nature is investigated for each wave propagating in the analyzed material structures when the film thickness is increased from zero to a few wavelengths. The examples of the SAW transformation into boundary waves and into plate modes of different polarization have been found and investigated. The behavior of the SAW characteristics in the grating is correlated with transformation of the wave structure with increasing overlay thickness.

The characteristics of plate modes propagating under Pt grating in rotated Y-cuts of LGS were simulated as functions of the cut angle and plate thickness. Quasi-symmetric and quasi-anti-symmetric modes, which can be excited in rotated YX-cuts, and SH-polarized modes propagating in rotated Y-cuts with propagation direction normal to the X-axis demonstrated high electromechanical coupling up to 2.7%, in some orientations combined with zero TCF. The variation of the mode characteristics with plate thickness was investigated in YX and YZ cuts as the typical examples of the two cut families. The effect of metal film at the plate bottom on electromechanical coupling of different modes was investigated.

The measured characteristics of a test SAW device built on YX cut of LGS were analyzed in the time-frequency domain, using spectrogram representation. Especially, spurious modes were detected and investigated. A combination of numerical techniques was applied to the analysis of the observed spurious modes, including simulation of the admittance of a Pt grating on an LGS plate of finite thickness and visualization of the wave structure at different frequencies. The influence of the anisotropy on the frequency-time ratio was taken into account for each of three obliquely propagating bulk waves via detailed numerical investigation of the BAW slowness surface. The main spurious BAW modes propagating parallel to the surface or at certain tilt angles and reflected from the bottom were identified. The frequency-time dependences predicted via analysis of BAW anisotropy showed good agreement with experimental results.

There is a need for wireless sensors able to operate in the intermediate temperature range (ITR) between 300 °C and 600 °C. Surface acoustic wave (SAW) sensors are promising candidates to solve this issue. However, existing SAW sensors most often fail in the ITR, due to the quick degradation of the sensor housing in extreme conditions. A promising way to circumvent the issue is to use “package-less” devices, where the acoustic waves are guided in a multilayered structure where they are intrinsically protected from adverse environmental effects. We present here an innovative multilayered structure that fulfills all the basic requirements, to achieve a wireless and “package-less” SAW Sensor for the ITR. The structure is made of a thin AlN layer deposited on top of a Y + 128°LN substrate and equipped with buried Pt electrodes. Numerical simulations of the acoustic waves propagating in SAW resonators built on this structure reveal the existence of a useful Rayleigh-type SAW that propagates at the AlN/LN inter...

Rigorous analysis of plate modes propagating in LiTaO 3 plates was performed using the numerical technique SDA-FEM-SDA. The characteristics of zero- and first-order plate modes were calculated in rotated YX cuts of LT as functions of cut angle and plate thickness. Orientations providing high electromechanical coupling (up to 19%), zero TCF at resonance and high velocities (up to 19,000 m/s) were found and investigated. The wave motions following the propagation of some modes are visualized. The possibility of enhancement of some modes by deposition of metal film on the plate bottom surface and the sensitivity of higher-order modes to variation of plate thickness are discussed.

A numerical technique developed for simulation of admittance of a finite-aperture resonator is applied to three LGS orientations: YX-cut, SAW cut with Euler angles (0°, 138.5°, 26.7°) and STW cut, Euler angles (0°, 22°, 90°), with a periodic platinum grating. The degradation of a resonator performance is observed for each cut when a resonator aperture decreases from 100 to 20 wavelengths. The ripples of the admittance function are analyzed in correlation with the slowness surface varying with the electrode thickness. Some specific features of each analyzed orientation, such as strong anisotropy or interaction with parasitic modes, and their impact on the resonator performance are discussed. The minimum acceptable aperture is determined for each cut and the possibility of decreasing it without further degradation of admittance, by variation of electrode thickness, is analyzed.

The results of a theoretical and experimental investigation of the SAW propagation characteristics in an optimal region of langasite defined by the Euler angles /spl phi/ from -15/spl deg/ to +10/spl deg/, /spl theta/ from 120/spl deg/ to 165/spl deg/, and /spl psi/ from 20/spl deg/ to 45/spl deg/ are presented. Based on temperature coefficients of the elastic constants derived from experimental data, some optimal orientations of langasite characterized by high electromechanical coupling factor (k/sup 2/), zero power flow angle (PFA) and low or zero temperature coefficient of frequency (TCF) were found. The SAW velocity in the region of interest is highly anisotropic; this results in a significant amount of diffraction, which must be taken into account in the search for orientations useful for SAW devices. An orientation having simultaneously zero PFA, zero TCF, negligible diffraction, and relatively high piezoelectric coupling has been found and verified experimentally. The experimental results are in excellent agreement with the calculated SAW characteristics. The frequency response of a SAW device fabricated on the optimal cut of langasite is presented and demonstrates that high performance SAW filters can be realized on this optimal cut of langasite.

The surface acoustic wave (SAW) propagation properties of zinc oxide (ZnO) films on silicon carbide (SiC) have been theoretically and experimentally characterized in the film thickness-to-acoustic wavelength ratio range up to 0.12. The experimental characterization of the SAW propagation properties was performed with a linear array of interdigital transducer (IDT) structures. The measurements characterized the velocity and propagation loss of two surface modes, a generalized SAW (GSAW) mode with velocities between 6000 and 7000 m/s, and a high velocity Pseudo-SAW (HVPSAW) mode with velocities between 8500 and 12 500 m/s. The experimentally determined characteristics of the two waves have been compared with the results of calculations based on published data for SiC and ZnO. Simulation of wave characteristics was performed with various values of the elastic constant C/sub 13/, which is absent in the published set of material constants for SiC, within the interval permitted by the requirement of positive elastic energy in a hexagonal crystal. The best agreement between the measured and calculated propagation losses of the HVPSAW has been obtained for C/sub 13/ near zero. Although for the GSAW mode the calculated velocity dispersion has been found nearly insensitive to the value of C/sub 13/ and consistent with the experimental data, for the HVPSAW, some disagreement between measured and calculated velocities, which increased with ZnO film thickness, has been observed for any C/sub 13/ value. Theoretical analysis of HVPSAW has revealed the existence of a previously unknown high velocity SAW (HVSAW). The displacement components of this wave have been analyzed as functions of depth and confirmed its pure surface, one-partial character.

Acoustooptic (AO) light dispersive delay lines (LDDL) are widely used in high-power ultrashort laser pulse amplification chains for pulse shaping. The geometry of AO interaction of LDDL is determined mainly by the acoustic beam propagation and photoelastic properties of the crystal. Propagation of bulk acoustic wave (BAW) beams depends on acoustic anisotropy of the crystal and is characterized by the planar tensor of diffraction, which can be considered as an extension of anisotropy parameter for 3D-space. Two nonzero eigenvalues of the tensor are compared for pure shear BAWs propagating in the symmetry planes of TeO2 and LiNbO3. The found combinations of BAW characteristics, including high velocity, low diffraction, and sufficiently large acousto-optic figure of merit, make lithium niobate more preferable for application in LDDL with rapid scan repetition frequency up to 100 kHz compared to TeO2.

International audience; Passive high-temperature sensors are a most promising area of use for SAW devices. Langasite (La3Ga5SiO14; LGS) has been identified as promising piezoelectric material to meet high-temperature SAW challenges. Because it is necessary to know the material behavior for an accurate device design, the frequency-temperature behavior of Rayleigh SAW (R-SAW) and shear-horizontal SAW (SH-SAW) LGS cuts is investigated on delay line and resonator test structures up to 700°C by RF characterization. In the range of the 434-MHz ISM band, the (0°,22°,90°) SH-SAW cut shows thermal behavior similar to the (0°,138.5°,26.7°) R-SAW cut. Associated with the (0°,22°,31°) cut, in which SAWs present mixed types of polarization, the (0°,22°,90°) SH-SAW orientation might allow differential measurements on a single substrate. In the temperature range of 400 to 500°C, delay line test devices using the SH-SAW cut show a considerable drop of signal. Theoretical analysis indicates that this newly described behavior might be a result of anisotropy effects in this cut, occurring in case of any slight misorientation of electrode alignment.

The exceptional wave theory developed in the last two decades is applied to trigonal crystals, widely used in surface acoustic wave devices: quartz, lithium niobate, and lithium tantalate. For quartz all crystallographic orientations where one of bulk acoustic waves satisfies the stress‐free mechanical boundary conditions, have been found and plotted on the stereographic projection of a unit wave normal sphere as ‘‘exceptional wave lines.’’ The possibility of existence and the main features of pure shear, quasishear, and quasilongitudinal exceptional waves are examined in three cut families with Euler angles (0°,θ,0°), (90°,90°,ψ), and (0°,θ,90°). These orientations are analyzed both without and with piezoelectric effect by means of analytical and numerical techniques. The obligatory existence of undamped surface skimming bulk waves or leaky waves is proved for selected cuts of trigonal crystals, such as a well‐known 41°‐rotated Y cut of lithium niobate. The behavior of these waves is compared in three crystals. A simple relation between elastic moduli of trigonal crystals with point symmetry of 32 or 3m has been derived, which allows the existence of a longitudinal type of exceptional waves. Such waves have been discovered in quartz. They are similar to those found earlier in lithium tetraborate and in both crystals they give rise to piezoelectrically coupled high velocity leaky waves with small attenuation.

Characteristics of lower symmetric and antisymmetric modes s0 and a0 (velocities, electromechanical coupling coefficients, TCF) were numerically investigated in Lamb wave resonators using AlN and AlN/SiO2 membranes. Two configurations of AlN/SiO2 structure were compared, with SiO2 film located either at the bottom or on top of AlN. All characteristics were calculated as functions of SiO2 film thickness with normalized AlN thickness varying between 0.1λ and 0.5λ. The influence of uniform Al electrode at the bottom of AlN on the Lamb wave characteristics has been studied. For symmetric mode s0 it was found that SiO2 film thickness, which is required for zero TCF, decreases from 60% to 25% of AlN thickness when the latter grows from 0.1λ to 0.5λ. The electromechanical coupling of s0 mode in the temperature compensated structure depends on the presence of metal film at the bottom of AlN and drops with poor conductivity of this film. The behavior of the reflection coefficient per period of the grating reveals the crucial difference between the analyzed configurations.

Unknown LH type leaky and non-leaky waves were theoretically found and examined in lithium tetraborate. The waves have very promising parameters: high velocity, strong piezoelectric coupling, zero power flow angle and small temperature coefficients of delay. The theory of exceptional waves proved to be very useful to investigate the acoustic anisotropy and to search for the cuts where leaky and non-leaky SAW can exist.

We report on a surface-bulk hybrid acoustic mode, which exists within a limited frequency interval in some piezoelectric substrates within a periodic metal grating. These hybrid modes result from the coupling between a bulk wave and a counter-propagating surface (or leaky) wave. An example of such a hybrid mode, which propagates in 36°-YX cut of lithium tantalate, is described. The hybrid mode gives rise to an additional branch of dispersion relation. The Bragg reflection stop band occurs due to phase match between surface and bulk waves. The lower edge of this stop band manifests itself by a resonance of harmonic admittance, being a reason of undesired resonances in resonator filters.

The numerical technique based on a previously developed rational approximation of harmonic admittance of a periodic grating was applied to analysis of SAW behavior in platinum grating on langasite cut with Euler angles (0°, 138.5°, 26.6°). The approximation is able to take into account interaction between surface and bulk waves or between two SAW modes. SAW dispersion was calculated at different values of electrode thickness varying between 1% and 4% of wavelength. It was found that with increasing Pt thickness, SAW behavior in the grating is strongly affected by interaction between two SAW modes propagating in the same orientation. An additional stopband, which results from this interaction, occurs at certain detuning from synchronous reflection condition and can cause spurious resonances of the admittance function. Interaction between two SAW modes is also responsible for anomalously slow growth of reflectivity with increasing platinum thickness.

Since 70-ies, when the first delay lines and filters employing surface acoustic waves (SAW) were designed and fabricated, the use of SAW devices in special and commercial applications has expanded rapidly and the range of their working parameters was extended significantly (Hashimoto, 2000; Ruppel, 2001, 2002). In the last decade, their wide application in communication systems, cellular phones and base stations, wireless temperature and gas sensors has placed new requirements to SAW devices, such as very high operating frequencies (up to 10 GHz), low insertion loss, about 1 dB, high power durability, stable parameters at high temperatures etc. The main element of a SAW device is a piezoelectric substrate with an interdigital transducer (IDT) used for generation and detection of SAW in the substrate. The number of single crystals utilized as substrates in SAW devices did not increase substantially since 70ies because a new material must satisfy the list of strict requirements to be applied in commercial SAW devices: sufficiently strong piezoelectric effect, low or moderate variation of SAW velocity with temperature, low cost of as-grown large size crystals for fabrication of 4-inch wafers, long-term power durability, well developed and non-expensive fabrication process for SAW devices etc. Today only few single crystals are utilized as substrates in SAW devices: lithium niobate, LiNbO3 (LN), lithium tantalate, LiTaO3 (LT), quartz, SiO2, lithium tetraborate, Li2B4O7 (LBO), langasite, La3Ga5SiO14 (LGS) and some crystals of LGS group (LGT, LGN etc.) with similar properties. The SAW velocities in these single crystals do not exceed 4000 m/s, which limit the highest operating frequencies of SAW devices by 2.5-3 GHz because of limitations imposed by the line-resolution technology of IDT fabrication. The minimum achievable insertion loss and maximum bandwidth of SAW devices depend on the electromechanical coupling coefficient, which can be evaluated for SAW as k2≈2ΔV/V, where ΔV is the difference between SAW velocities on free and electrically shorted surfaces. The largest values of k2 can be obtained in some orientations of LN and LT. Ferroelectric properties of these materials are responsible for a strong piezoelectric effect. As a result, k2 reaches 5.7% in LN and 1.2% in LT, for SAW. For leaky SAW (LSAW) propagating in rotated Y-cuts of both crystals, the coupling is higher and can exceed 20% for LN and 5% for LT. However, LSAW attenuates because of its leakage into the bulk waves when it propagates along the crystal surface. As a

The numerical technique recently developed for simulation of SAW dispersion in periodic gratings was applied to langasite cut with Euler angles (0°,138.5°,26.6°) and platinum grating, when electrode thickness varies between 1% and 6% λ (SAW wavelength). Analysis has revealed that with increasing Pt thickness, the dispersion appears to be strongly affected by interaction between two SAW modes. It results in additional stopband, which occurs at certain detuning from synchronous reflection condition and manifests itself by additional resonances of admittance function if such detuning occurs in SAW resonator. The anomalous character of dispersion is not adequately described by the known COM models and can explain the slow growth of reflectivity with increasing electrode thickness.