Level repulsion of GHz phononic surface waves in quartz substrate with finite-depth holes.

This paper presents numerical and experimental results on the level repulsion of gigahertz surface acoustic waves in an air/ST-cut quartz phononic structure with finite-depth holes. The colorful dispersion with the parameter of the in-plane (sagittal plane) ratio of polarization was adopted to determine the Rayleigh wave bandgap induced by the level repulsion. The results of numerical analyses showed that the frequency and width of the bandgap induced by the level repulsion strongly depend on the geometry of the air holes in the phononic structure. In the experiment, a pair of slanted interdigital transducers with frequency in the gigahertz range was designed and fabricated to generate and receive broadband Rayleigh waves, whereas the reactive ion etching process with electron-beam lithography was used to fabricate submicrometer phononic structures. The measured results of the bandgap induced by the level repulsion agreed favorably with the numerical prediction.

Focusing and waveguiding of Lamb waves in micro-fabricated piezoelectric phononic plates.

This paper presents results on the numerical and experimental studies of focusing and waveguiding of the lowest anti-symmetric Lamb wave in micro-fabricated piezoelectric phononic plates. The phononic structure was based on an AT-cut quartz plate and consisted of a gradient-index phononic crystal (GRIN PC) lens and a linear phononic plate waveguide. The band structures of the square-latticed AT-cut quartz phononic crystal plates with different filling ratios were analyzed using the finite element method. The design of a GRIN PC plate lens which is attached with a linear phononic plate waveguide is proposed. In designing the waveguide, propagation modes in square-latticed PC plates with different waveguide widths were studied and the results were served for the experimental design. In the micro-fabrication, deep reactive ion etching (Deep-RIE) process with a laboratory-made etcher was utilized to fabricate both the GRIN PC plate lens and the linear phononic waveguide on an 80 µm thick AT-cut quartz plate. Interdigital transducers were fabricated directly on the quartz plate to generate the lowest anti-symmetric Lamb waves. A vibro-meter was used to detect the wave fields and the measured results on the focusing and waveguiding of the piezoelectric GRIN PC lens and waveguide are in good accordance with the numerical predictions. The results of this study may serve as a basis for developing an active micro plate lens and related devices.

Phononic plate waves.

In the past two decades, phononic crystals (PCs) which consist of periodically arranged media have attracted considerable interest because of the existence of complete frequency band gaps and maneuverable band structures. Recently, Lamb waves in thin plates with PC structures have started to receive increasing attention for their potential applications in filters, resonators, and waveguides. This paper presents a review of recent works related to phononic plate waves which have recently been published by the authors and coworkers. Theoretical and experimental studies of Lamb waves in 2-D PC plate structures are covered. On the theoretical side, analyses of Lamb waves in 2-D PC plates using the plane wave expansion (PWE) method, finite-difference time-domain (FDTD) method, and finite-element (FE) method are addressed. These methods were applied to study the complete band gaps of Lamb waves, characteristics of the propagating and localized wave modes, and behavior of anomalous refraction, called negative refraction, in the PC plates. The theoretical analyses demonstrated the effects of PC-based negative refraction, lens, waveguides, and resonant cavities. We also discuss the influences of geometrical parameters on the guiding and resonance efficiency and on the frequencies of waveguide and cavity modes. On the experimental side, the design and fabrication of a silicon-based Lamb wave resonator which utilizes PC plates as reflective gratings to form the resonant cavity are discussed. The measured results showed significant improvement of the insertion losses and quality factors of the resonators when the PCs were applied.

A passive wireless hydrogen surface acoustic wave sensor based on Pt-coated ZnO nanorods.

Using a passive wireless sensor to detect hydrogen can reach the goals of reducing cost and increasing the lifetime since the sensor can work without batteries. In this paper, a passive wireless hydrogen SAW sensor operating at room temperature has been achieved by combining a SAW tag and a resistive hydrogen sensor. The SAW tag is fabricated on a 128 degrees YX-LiNbO(3) substrate and its central frequency is 433 MHz. The resistive hydrogen sensor with the Pt-coated ZnO nanorods as the sensing film has the advantages of high stability, good repeatability and simple fabrication. The ZnO nanorods are synthesized by using the aqueous solution method and the Pt coating is employed as a catalyst for the hydrogen detection. The property of the resistive hydrogen sensor is examined before combining with the SAW tag. Results show that the resistance changes caused by the variations of relative humidity and temperature are negligible. Finally, the hydrogen SAW sensor is configured and measured wirelessly for various hydrogen concentrations at room temperature. The difference of the relative return loss caused by the hydrogen concentration variation is obvious and recognizable. All responses show that the proposed hydrogen sensor not only has good repeatability and high sensitivity but is capable of passive wireless detection.

A room temperature surface acoustic wave hydrogen sensor with Pt coated ZnO nanorods.

A surface acoustic wave (SAW) sensor with Pt coated ZnO nanorods as the selective layer has been investigated for hydrogen detection. The SAW sensor was fabricated based on a 128 degrees YX-LiNbO(3) substrate with a operating frequency of 145 MHz. A dual delay line configuration was adopted to eliminate external environmental fluctuations. The Pt coated ZnO nanorods were chosen as a selective layer due to their high surface-to-volume ratio, large penetration depth, and fast charge diffusion rate. The ZnO nanorods were synthesized by an aqueous solution method and coated with the noble metal Pt as a catalyst. Finally, the SAW sensor responses to humidity and hydrogen were tested. Results show that the sensor is not sensitive to humidity; moreover, the frequency shift for a hydrogen concentration variation of 6000 ppm is 26 kHz while operating at room temperature. It can be concluded that the Pt coated ZnO nanorod based SAW hydrogen sensor exhibits fast response, good sensitivity and short-term repeatability. It is worth noting that not only is the sensor sensitive enough to operate at room temperature, but also it can avoid the influence of humidity.

A ZnO nanorod-based SAW oscillator system for ultraviolet detection.

A high-precision ultraviolet (UV) detector combining ZnO nanostructure and a dual delay line surface acoustic wave (SAW) oscillator system is presented. The UV detector is made of ZnO nanorods on a 128 degrees YX-LiNbO(3)-based two-port SAW oscillator. The ZnO nanorod synthesized by chemical solution method is used as a UV sensing material. The center frequency of the SAW device is at 145 MHz. A dual delay line SAW oscillator system was constructed to eliminate external environmental fluctuations. Under illumination of a UV source consisting of an Xe lamp and a monochromator, frequency shifts of the UV detector were measured. A maximum frequency shift of over 40 kHz was observed under 365 nm illumination for several on-off cycles, indicating the ZnO nanorod-based detector was sensitive to UV light and with good repeatability. Moreover, frequency shifts reached a value of 19 kHz after 365 nm was turned on for 10 s, which implies a real-time high-sensitivity UV sensor was successfully fabricated. Results show a ZnO nanostructure-based SAW oscillator system is a promising candidate for a real-time, fast-response, high-precision UV detector.

A Lamb wave source based on the resonant cavity of phononic-crystal plates.

In this paper, we propose a Lamb wave source that is based on the resonant cavity of a phononic-crystal plate. The phononic-crystal plate is composed of tungsten cylinders that form square lattices in a silicon plate, and the resonant cavity is created by arranging defects inside the periodic structure. The dispersion, transmission, and displacement of Lamb waves are analyzed by the finite-difference time-domain (FDTD) method. The eigenmodes inside the cavities of the phononic-crystal plate are identified as resonant modes. The fundamental and higher order resonant modes, which vary with the length of cavities, are calculated. By exciting the specific resonant mode in an asymmetric cavity, the 232.40 MHz flexural Lamb wave has a magnified amplitude of 78 times larger than the normal one. Thus, the cavity on the tungsten/silicon phononic-crystal plate may serve as a source element in a microscale acoustic wave device.

Calculations of Lamb wave band gaps and dispersions for piezoelectric phononic plates using mindlin's theory-based plane wave expansion method.

Based on Mindlin's piezoelectric plate theory and the plane wave expansion method, a formulation is proposed to study the frequency band gaps and dispersion relations of the lower-order Lamb waves in two-dimensional piezoelectric phononic plates. The method is applied to analyze the phononic plates composed of solid-solid and airsolid constituents with square and triangular lattices, respectively. Factors that influence the opening and width of the complete Lamb wave gaps are identified and discussed. For solid/solid phononic plates, it is suggested that the filling material be chosen with larger mass density, proper stiffness, and weak anisotropic factor embedded in a soft matrix in order to obtain wider complete band gaps of the lower-order Lamb waves. By comparing to the calculated results without considering the piezoelectricity, the influences of piezoelectric effect on Lamb waves are analyzed as well. On the other hand, for air/solid phononic plates, a background material itself with proper anisotropy and a high filling fraction of air may favor the opening of the complete Lamb wave gaps.

Bleustein-Gulyaev-Shimizu surface acoustic waves in two-dimensional piezoelectric phononic crystals.

In this paper, we present a study on the existence of Bleustein-Gulyaev-Shimizu piezoelectric surface acoustic waves in a two-dimensional piezoelectric phononic crystal (zinc oxide, ZnO, and cadmium-sulfide, CdS) using the plane wave expansion method. In the configuration of ZnO (100)/CdS(100) phononic crystal, the calculated results show that this type of surface waves has higher acoustic wave velocities, high electromechanical coupling coefficients, and larger band gap width than those of the Rayleigh surface waves and pseudosurface waves. In addition, we find that the folded modes of the Bleustein-Gulyaev-Shimizu surface waves have higher coupling coefficients.

Three-dimensional phononic band gap calculations using the FDTD method and a PC cluster system.

This paper aims at studying the band gap phenomena of three-dimensional phononic crystals using the finite difference time domain (FDTD) method and a PC cluster system. In the paper, Bloch's theorem is applied to the wave equation and to the boundary conditions of the periodic structure. We calculate the variations of displacements and take discrete Fourier transform to acquire the resonances of the structures. Then, the dispersion relations of the bulk acoustic wave can be obtained and the band gaps are predicted accordingly. On the other hand, because of larger data calculation in three-dimensional phononic crystals, we introduce the PC cluster system and parallel FDTD programs written with respect to the architecture of a PC cluster system. Finally, we discuss the numerical calculation of two-dimensional and three-dimensional phononic crystals consisting of steel/epoxy and draw conclusions regarding the band gap phenomena between these phononic crystals.

Analysis and design of focused interdigital transducers.

Focused interdigital transducers (FIDTs) can generate surface acoustic wave (SAW) with high intensity and high beamwidth compression ratio. Owing to these features, they are very suitable to be used as the sources of microacoustic channels or waveguides in the near future. The focusing properties of FIDTs are dominated solely by their geometric shapes. Therefore, to obtain optimal performance, it is essential to analyze the FIDTs with a variety of geometric shapes. However, among the existing studies concerning the diffraction of FIDTs, a detailed analysis and design of FIDTs is still in paucity. In this paper, we adopted the exact angular spectrum of plane wave theory (ASoW) to calculate the amplitude fields of FIDTs on Y-Z lithium niobate (LiNbO3) with the shape as a concentric circular arc and the concentric wave surface. Based on the calculation results, we discussed the variations of the amplitude fields induced by changing number of pairs, degree of arc, and geometric focal length. In addition, the focusing properties of FIDTs on the (100)-oriented GaAs substrate were also analyzed and discussed. We also summarized the guiderules for designing a FIDT via four important factors. It is worth noting that the results of this study provide an important basis for designing various FIDTs to fit the desired applications.

Temperature effect on the bandgaps of surface and bulk acoustic waves in two-dimensional phononic crystals.

In this paper, we analyzed the temperature effect on two-dimensional phononic crystals. Bandgap variations of both of the bulk modes and surface modes due to changing of temperature in an air/quartz band structure from 0 to 50 degrees C were calculated and discussed. The results show that the elastic bandgaps can be enlarged or reduced by adjusting the temperature of the band structure. The temperature effects potentially can be used for fine-tuning of the phononic bandgap frequency.

Exact analysis of dispersive SAW devices on ZnO/diamond/si-layered structures.

In this paper, a formulation for calculating the effective permittivity of a piezoelectric layered SAW structure is given, and the exact frequency response of ZnO/diamond/Si-layered SAW is calculated. The effective permittivity and phase velocity dispersion of a ZnO/diamond/Si-layered half space are calculated and discussed. The frequency response of an unapodized SAW transducer is calculated, and the center frequency shift caused by the velocity dispersion is explained. In addition, the electromechanical coupling coefficients of the ZnO/diamond/Si -layered half space based on two different formulas are calculated and discussed. Finally, based on the results of the study, we propose an exact analysis for modeling the layered SAW device. The advantage of using the effective permittivity method is that, not only the null frequency bandwidth, but also the center frequency shift and insertion loss can be evaluated.