Jia-Hong Sun

Focusing of the lowest antisymmetric Lamb wave in a gradient-index phononic crystal plate

In this letter, we numerically demonstrate focusing of the lowest antisymmetric Lamb wave in a gradient-index phononic crystal (PC) silicon plate and its application as a beam-width compressor for compressing Lamb wave into a stubbed phononic tungsten/silicon plate waveguide. The results show that beam width of the lowest antisymmetric Lamb wave in the PC thin plate can be compressed efficiently and fitted into tungsten/silicon PC plate waveguide over a wide range of frequency.

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, Blochs 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.

Utilization of phononic-crystal reflective gratings in a layered surface acoustic wave device

In this paper, a design that combines two-port surface acoustic wave (SAW) devices and phononic crystals (PCs) acting as reflected gratings is demonstrated. Finite-difference time-domain method is used to analyze SAWs encountering the PC and optimize the design. A layered ZnO/Si SAW device and a square lattice PC composing of cylindrical holes on silicon were fabricated. With the PC of 15-layer cylinders, experimental insertion loss shows a 7 dB improvement at 212 MHz at central frequency. In addition, the size of gratings is reduced significantly as compared to the traditional gratings with hundreds of metal strips.

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 two-port ZnO/silicon Lamb wave resonator using phononic crystals

We present numerical and experimental studies on a two-port ZnO/silicon Lamb wave resonator using two-dimensional phononic-crystal (PC) gratings. Band gaps and reflections of Lamb waves in PC structures were analyzed using the finite element method. Constructive interference was obtained with suitable reflective distance between wave sources and the PC. The simulated result was utilized to optimize the design of Lamb wave resonators. On the experimental side, a layered ZnO/Si Lamb wave resonator with square-lattice PC reflective gratings was fabricated. The measurement showed that with 15-row PCs, the resonator Q factor can be up to 2269 at 158.15 MHz resonant frequency.

Design of a highly magnified directional acoustic source based on the resonant cavity of two-dimensional phononic crystals

The authors report a design of a highly magnified directional acoustic source based on the planar resonant cavity of two-dimensional phononic crystals. The authors demonstrate that the order of the resonant mode and the reflective ratio of the double phononic crystal slab are the key factors to the magnified ratio of the directional acoustic amplifier. With properly designed mode and cavity width of the asymmetric phononic structure, the optimal magnified amplitude can be achieved by more than 86.5 times in comparison with the amplitude of the original line source freely radiating in water.

Acoustic beamwidth compressor using gradient-index phononic crystals

We report a novel approach to effectively couple acoustic energy into a two-dimensional phononic-crystal waveguide by an acoustic beamwidth compressor using the concept of a gradient-index phononic crystal (GRIN PC). The GRIN PC-based beamwidth compressor is composed of a square array of solid scatterers embedded in epoxy. By gradually modulating the density and elastic modulus of the scatterers along the direction transverse to the phononic propagation, the beamwidth compressor can efficiently compress the wide acoustic beam to the scale of the phononic-crystal waveguide. This acoustic beamwidth compressor is investigated through a finite-difference time-domain method. A beam-size conversion ratio of 6.5?:?1 and a transmission efficiency of up to 90% is obtained over the working frequency range of the phononic-crystal waveguide. Potential applications for this device include acoustic biosensors and signal processors.

Three-dimensional phononic nanocrystal composed of ordered quantum dots

We demonstrated a nanoscaled artificial phononic crystal composed of three-dimensionally ordered quantum dots (QDs) with functional acoustic properties. Femtosecond ultrasonic technique is used to investigate the lattice dynamics of this phononic nanocrystal. The measurement results indicate that three-dimensional ordering and uniformity of the QDs are important factors influencing the observed acoustic resonance at the forbidden bands. For well-arranged QDs, noticeable features of the phononic band gap and the associated phonon cavity mode can be found, while this nanocrystal also serves as an effective acoustic medium, or acoustic meta material, for low-frequency acoustic phonons.

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.

Directional enhanced acoustic radiation caused by a point cavity in a finite-size two-dimensional phononic crystal

In this letter, we present results of a study on the directional enhanced radiation generated by a point resonant cavity operating at the band-gap frequency of a two-dimensional finite-size phononic crystal. We find that the directive radiation for operating frequency within the band gap comes from the direction-dependent transmittance property of the square-lattice phononic crystal. The directive-radiation characteristic can be capitalized on enhancing the emission from the phononic crystals by coupling the source with point-defect modes, and the enhanced directive radiation patterns with main lobe width less than 12.9° are demonstrated.