Elastic properties of proton exchanged lithium niobate

A method is demonstrated for determination of elastic constants of anisotropic layers on arbitrarily anisotropic and piezoelectric substrates only using the easily measured velocity of surface acoustic waves. By means of a detailed theoretical analysis it is shown that by use of the presented method, the elastic stiffness constants and propagation properties of any nonpiezoelectric isotropic, cubic, or even trigonal layer can be determined. The method is applied to proton-exchanged lithium niobate (PE:LiNbO(3)). Complete measurements of dispersion characteristics of Rayleigh waves on Y -cut PE:LiNbO(3) and calculated values of all elastic stiffness constants of the proton-exchanged film are reported.     

Shear-horizontal acoustic waves in piezoelectric plates bordered with conductive liquid

The influence of a conductive liquid on the characteristics of shear-horizontal acoustic waves of zeroth order (SH0 mode) propagating in thin piezoelectric plates of lithium tantalate, lithium niobate, and potassium niobate was investigated. Experimental results obtained for SH0 mode devices fabricated on lithium niobate plates are found to be in good agreement with theory. The data presented in this paper is useful for a proper design of various acoustic wave sensors operating in contact with conductive liquids.     

Tunable acoustic resonator on a photoinduced grating in lithium niobate

It is experimentally demonstrated that photoinduced gratings in lithium niobate single crystals can be used as tunable high-frequency resonators for the surface and bulk acoustic waves. The resonance frequency can be tuned within broad limits by rewriting gratings with interfering laser beams.     

The power flow angle of acoustic waves in thin piezoelectric plates

The curves of slowness and power flow angle (PFA) of quasi-antisymmetric (A0) and quasi-symmetric (S0) Lamb waves as well as quasi-shear-horizontal (SH0) acoustic waves in thin plates of lithium niobate and potassium niobate of X-,Y-, and Z-cuts for various propagation directions and the influence of electrical shorting of one plate surface on these curves and PFA have been theoretically investigated. It has been found that the group velocity of such waves does not coincide with the phase velocity for the most directions of propagation. It has been also shown that S0 and SH0 wave are characterized by record high values of PFA and its change due to electrical shorting of the plate surface in comparison with surface and bulk acoustic waves in the same material. The most interesting results have been verified by experiment. As a whole, the results obtained may be useful for development of various devices for signal processing, for example, electrically controlled acoustic switchers.     

Acoustic loss mechanisms in leaky SAW resonators on lithiumtantalate

Discusses acoustic losses in synchronous leaky surface acoustic wave (LSAW) resonators on rotated Y-cut lithium tantalate (LiTaO₃ ) substrates. Laser probe measurements and theoretical models are employed to identify and characterize the radiation of leaky waves into the busbars of the resonator and the excitation of bulk acoustic waves. Escaping LSAWs lead to a significant increase in the conductance, typically occurring in the vicinity of the resonance and in the stopband, but they do not explain the experimentally observed deterioration of the electrical response at the antiresonance. At frequencies above the stopband, the generation of fast shear bulk acoustic waves is the dominant loss mechanism     

Thin-film characterization using a scanning laser acoustic microscope with surface acoustic waves

The use of surface acoustic waves in a scanning laser acoustic microscope for the characterization of the mechanical or acoustic properties of thin films deposited on piezoelectric substrates is demonstrated. Quantitative measurements of mass loading effects of 5000-A-thick tungsten films deposited on lithium niobate substrates were obtained using 100-MHz surface acoustic waves. No information about the tungsten film could be obtained using 100-MHz compressional waves. Methods of generating surface waves on nonpiezoelectric materials so that this technique could be used on arbitrary substrates are discussed.     

Hybrid acousto-optic Fourier processor for imaging spatially inhomogeneous acoustic fields

A new method is proposed for imaging spatially inhomogeneous acoustic fields. The approach is based on the Fourier transform of a coherent light field formed as a result of the Bragg diffraction of a probing laser beam on an ensemble of quasi-plane acoustic waves in a lithium niobate crystal. These waves appear as a result of the transformation of an imaged acoustic field by a spherical acoustic lens. The proposed method has been experimentally verified using model multielement acoustic sources.     

Phases of the SAW reflection and transmission coefficients for short reflectors on 128 degree LiNbO3

We study numerically the phase of surface acoustic waves reflected by or transmitted through short reflectors comprising only 1-3 aluminium electrodes on 128 degree YX-cut lithium niobate (LiNbO3). The electrodes have a finite thickness, and they are either open-circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted surface acoustic wave (SAW). Using artificial enhancement of time resolution in conjunction with the fast Fourier transform (FFT) and time-gating, calculation of the Y-parameters in a relatively wide frequency range allows us to determine the phase of the reflection and transmission coefficients.     

Side radiation of Rayleigh waves from synchronous SAW resonators

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.     

LiNbO3和LiTaO3晶体电子结构、平面声波和光折射特性

本研究基于密度泛函理论的第一性原理超软赝势平面方法计算了LiNbO₃和LiTaO₃的晶格参数、电子结构和弹性常数, 并利用Christoffel方程研究了二者平面声波特征。结果表明: 两者的理论计算晶格参数和弹性常数与实验值接近, 禁带宽度分别为3.78和3.98 eV, 导带底和价带顶主要由O-2p和Nb-4d(Ta-5d)态电子贡献。化学键理论揭示Li和Nb(Ta)与O原子之间有两种成键类型。 电荷布局分析结果显示有两种相应的重叠布居数, Nb(Ta)-O键呈现强共价键作用, 并且Nb-O(Ta-O)键长小于Li-O键长。LiNbO₃和LiTaO₃晶体平面声波有两支横波和一支纵波, 纵波速度大于横波速度, 在xy平面呈现六重对称性, 在xz和yz平面各向异性程度强于xy平面, 沿[001]、晶向上两支横波振动速度相等。最后利用模守恒赝势(Norm-conserving)计算了介电常数和静态折射率, 计算表明LiNbO₃晶体的折射性能和非寻常光(e光)离散程度均强于LiTaO₃晶体。     

Effect of metallization on the power flow angle of SH0 waves in thin piezoelectric plates

The anisotropy of the angle between the phase and group velocity (power flow angle, PFA) as well as the influence of electrical boundary conditions on this angle have been studied for shear-horizontal fundamental (SH₀) acoustic waves in thin piezoelectric plates of lithium niobate (LNO) and potassium niobate (KNO). Both LNO and KNO crystals possess orientations at which the PFA for SH₀ waves reaches large values (17° and 48°, respectively). It is established that metallization of the piezoelectric plate surface can significantly change PFA values, in particular, by 18° and 55° for Y-X + 25° cut plates of LNO and KNO, respectively.     

Reflection of plate acoustic waves produced by a periodic array of mechanical load strips or grooves

The reflection of fundamental acoustic waves propagating in a thin piezoelectric plate by a periodic array of conducting strips of finite thickness or grooves has been theoretically and experimentally investigated. The analysis has shown that electrical shorting and mass loading affect the relationship of neighboring region impedances in a contrary manner. In some cases, these effects are comparable, and there exists a certain strip thickness for each piezoactive fundamental plate mode at which the reflection coefficient can become zero. A high efficiency of grooved reflector for plate acoustic waves was theoretically revealed. Experimental results for mass loading and grooved reflections, which have been obtained for an SH/sub 0/ wave propagating in the Y-X lithium niobate plate, are in a good agreement with the theory. They show a high efficiency of such reflectors and confirm the validity of using a model based on an equivalent circuit for the analysis of their operation. Investigations indicate that nearly 100% reflection of the SH/sub 0/ wave in the lithium niobate plate can be obtained with the use of a mass loading reflector containing 10 silver strips of thickness d/h=0.08 or a grooved reflector containing eight grooves of depth d/h=0.25. Here h is the plate thickness and d is the reflector thickness or depth.     

Monolithic bulk shear-wave acousto-optic tunable filter

We demonstrate a monolithic bulk shear-wave acousto-optic tunable filter combining a piezoelectric transducer array and the acoustic interaction medium in a single crystal. An X-propagating acoustic longitudinal wave is excited in the "crossed-field" scheme by an RF-E/sub y/-field in a chirped acoustic superlattice formed by domain-inversion in lithium niobate (LiNbO/sub 3/). The acoustic longitudinal wave is efficiently (97.5%) converted at a mechanically free boundary into a Y-propagating acoustic slow-shear wave that couples collinearly propagating e- and o-polarized optical waves. A relative conversion efficiency of 80%/W was measured at 980 nm.     

New method of change in temperature coefficient delay of acoustic waves in thin piezoelectric plates

As is well-known, the development of high-effective and thermostable acoustic devices assumes using the acoustic waves with high coefficient of electromechanical coupling (K2) and low temperature coefficient of delay (TCD). At present, it also is well-known that fundamental shear horizontal (SH0) acoustic waves in thin piezoelectric plates possess by significantly more electromechanical coupling compared to surface acoustic waves (SAW) in the same material. However, although the value of TCD of SH0 waves is insignificantly less than for SAW, this is not enough for development of thermostable devices. This paper suggests a new way of decreasing TCD of SH0 waves in piezoelectric plates at a high level of electromechanical coupling. This way assumes to use the structure containing the piezoelectric plate and liquid with the special dependence of permittivity on temperature. Theoretical and experimental investigation showed that, for SH0 wave in YX LiNbO3 plate at hf = 700 m/s (h = plate thickness, f = wave frequency) the presence of butyl acetate can decrease the value of TCD by six times at K2 = 30%. In a whole the obtained results open the wide prospect of using SH0 wave in thin piezoelectric plate for development of high effective and thermo-stable acoustic devices.     

Minimum-loss short reflectors on 128 degrees LiNbO3

We consider the interaction of surface acoustic waves (SAWs) with short electrode gratings encompassing only few electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The qualifications of the reflectors are evaluated by comparing the part of incident SAW energy scattered by the structure into the bulk to the energy reflected back as a SAW.     

Acoustic vibrations in integrated electro-optic modulators on substrates of lithium niobate

The processes of the excitation of acoustic vibrations and their influence on the optical signals in integrated electro-optic modulators on substrates of lithium niobate have been investigated. The resonant frequencies of the excitation of substrate vibrational modes have been determined. It has been shown that acoustic vibrations can make a significant contribution to the modulator transfer characteristic at frequencies up to several hundred megahertz, which should be taken into account when using the modulators at low frequencies, for example, as a part of fiber-optic sensors.     

Propagation properties of longitudinal leaky surface waves on lithium tetraborate

Theoretical and experimental results of longitudinal leaky surface waves with a higher phase velocity than that of ordinary leaky surface waves and a low propagation loss on lithium tetraborate (LBO) are investigated in detail. They propagate along the surface with a phase velocity close to that of longitudinal bulk wave, slightly radiating two kinds of shear bulk waves (or one shear bulk wave in the case that one of two shear wave terms is uncoupled) into the solid. Most surface components of the mode consist of a longitudinal wave term and an electromagnetic wave term. The detailed propagation properties of the longitudinal leaky surface waves on LBO with the Euler angles (phi, theta, 90 degrees ) are investigated theoretically and experimentally. The (011) cut of LBO was found to be desirable for higher frequency SAW devices. One of the reasons why that mode on LBO has a low propagation loss is also discussed.     

Tunable acoustic resonator on a periodic domain structure

The possibility of using photoinduced domain structures formed in lithium niobate crystals as a basis for tunable high-frequency spatial resonators of surface and bulk acoustic waves is demonstrated.     

Suppression of the leaky SAW attenuation with heavy mechanical loading

We discuss effects on the propagation of surface acoustic waves (SAW) due to heavy mass loading on Y-cut lithium niobate and lithium tantalate substrates. An abrupt reduction in the leaky-SAW (LSAW) attenuation is observed in the measured admittance of a long resonator test structure on 64 degrees -YX-cut lithium niobate for aluminum electrodes of thickness h/lambda(0) beyond 9-10%. This experimental fact is explained theoretically as the slowing down of the leaky wave below the velocity of the slow shear surface-skimming bulk wave (SSBW), such that energy dissipation into bulk-wave emission becomes inhibited. An infinite transducer structure is modeled using the periodic Green's function and the boundary-element method (BEM); the computed theoretical properties well explain for the experimental findings. The model is further employed to quantify the leaky surface-wave attenuation characteristics as functions of the crystal-cut angle and the thickness of the electrodes. The resonance and antiresonance frequencies and the corresponding Q values are investigated to facilitate the selection of crystal cuts and electrode thicknesses. The transformation of the leaky SAW into a SAW-type nonleaky wave is also predicted to occur for gold electrodes, with considerably thinner finger structures.