In this paper, we investigate the frequency band-gap features of micromachined air/silicon phononic band structures using layered slanted finger interdigital transducers (SFIT). In order to achieve the applications of phononic crystals on the microelectromechanical system related components, the frequency band-gap widths of surface waves are studied both theoretically and experimentally in micrometer scale phononic crystals. For further integration with the complementary metal-oxide semiconductor processing techniques, silicon is chosen as the base material of the two-dimensional phononic crystals in this study. To cover the frequency band-gap width of the phononic crystal, the wideband SFIT- and the SFIT∕ZnO∕Si-layered structures in the measurement are analyzed and discussed. For layered structures, the dispersive relation is calculated by the effective permittivity approach, and the frequency response of the layered SFIT is then simulated by the coupling-of-modes model. The frequency band-gap width and ...

Frequency band-gap measurement of two-dimensional air/silicon phononic crystals using layered slanted finger interdigital transducers

This paper reviews the interaction between coherently stimulated acoustic phonons in the form of surface acoustic waves with light beams in semiconductor based photonic structures We address the generation of surface acoustic wave modes in these structures as well as the technological aspects related to control of the propagation and spatial distribution of the acoustic fields The microscopic mechanisms responsible for the interaction between light and surface acoustic modes in different structures are then reviewed Particular emphasis is given to the acousto-optical interaction in semiconductor microcavities and its application in photon control These structures exhibit high optical modulation levels under acoustic excitation and are compatible with integrated light sources and detectors

https://iopscience.iop.org/article/10.1088/0034-4885/68/7/R02/pdf

Modulation of photonic structures by surface acoustic waves

We describe an improved two-dimensional optical scanning technique combined with an ultrafast Sagnac interferometer for delayed-probe imaging of surface wave propagation. We demonstrate the operation of this system, which involves the use of a single focusing objective, by monitoring surface acoustic wave propagation on opaque substrates with picosecond temporal and micron lateral resolutions. An improvement in the lateral resolution by a factor of 3 is achieved in comparison with previous setups for similar samples.

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Scanning ultrafast Sagnac interferometry for imaging two-dimensional surface wave propagation

We study theoretically the propagation of lower-order Lamb waves in one-dimensional composite thin plates. The dispersion curves of Lamb modes propagating parallel to the surfaces of the thin plates in the periodic direction are calculated based on the plane wave expansion method. The existence of band gaps for low-order Lamb wave modes is demonstrated in the systems made of alternating strips of tungsten materials and silicon resin. The finite element method is employed to calculate the transmitted power spectra, which is in good agreement with the results by plane wave exposition. A crucial parameter, i.e., the ratio of the plate thickness $(L)$ to the lattice spacing $(D)$, is discussed in detail for the influence of formation of band gaps.

Stopbands for lower-order Lamb waves in one-dimensional composite thin plates

In this paper, we present an analysis of coupling effects in joined parallel phononic crystal waveguides. The finite difference time domain (FDTD) method with periodic boundary condition is adopted to analyze the band gaps and dispersion relation of phononic waveguides. The defect modes of a single phononic waveguide are analyzed and first discussed to serve as a basis for a joined waveguides system. Then, the dispersion relation and displacement field of supermodes of joined waveguides are calculated and discussed. Both displacement pattern and transmission coefficient of the defect modes are calculated. To transfer the power from one waveguide to another, the coupling lengths are evaluated by numerical experiments and can be understood by the concept of beat length. Finally, we analyze an elastic waveguide coupler and demonstrate that the coupler can potentially be employed as a power switch of the acoustic wave.

Analyses of mode coupling in joined parallel phononic crystal waveguides

We introduce a new method for observing the anisotropic propagation of surface acoustic waves on solids. Point source and point detection of ultrasonic waves, scanned as a function of propagation angle, give the group velocities and intensities of Rayleigh and pseudosurface waves. Group-velocity focusing and internal diffraction are observed for Si, and a new Rayleigh mode, induced by water loading of the surface, is discovered. Such effects are predicted for most anisotropic solids.

Surface Acoustic Wave Focusing and Induced Rayleigh Waves.

A new experimental method has been devised that directly determines the group velocities of surface acoustic waves. A point source and a point detector are employed to measure the ultrasonic transmission across a solid surface as a continuous function of the propagation direction. Results for single pulses give the times-of-flight for both Rayleigh surface waves (RSW's) and pseudo-surface-waves (PSW's). Calculations and measurements of the group velocities of the surface waves on silicon show some unanticipated behavior: fluid loading qualitiatively changes the group velocity curves for both RSW and PSW. In particular, the RSW branch gains an additional component which we denote here as an induced Rayleigh wave (IRW). If a wave train is employed in the experiment, the analog of phonon focusing is observed for the ultrasonic waves, modified by “internal-diffraction” effects. Systematic measurements of the wave intensities on silicon as a function of propagation distance are consistent with expected acoustic losses into the surrounding water: the attenuation length of a wave depends on the mode and frequency. A survey of surface-wave images on other crystals is included in this study.

Imaging of surface acoustic waves

A model is introduced to explain our observation of Scholte-like ultrasonic waves traveling at the water-loaded surfaces of solids with periodically varying properties. The observations pertain to two two-dimensional superlattices: a laminated solid of alternating 0.5-mm-thick layers of aluminum and a polymer, and a hexagonal array of polymer rods of lattice spacing 1 mm in an aluminum matrix. The surface waves are generated and detected by line focus acoustic lenses aligned parallel to each other, and separated by varying distances. The acoustic fields of these lenses may be considered a superposition of plain bulk waves with wave normals contained within the angular apertures of the lenses. For homogeneous solids, phase matching constraints do not allow the Scholte wave to be coupled into with an experimental configuration of this type. This is not true for a spatially periodic solid, where coupling between bulk waves and the Scholte surface wave takes place through Umklapp processes involving a change in the wave-vector component parallel to the surface by a reciprocal lattice vector. In the experiments, the source pulse is broadband, extending up to about 6 MHz, whereas the spectrum of the observed Scholte wave is peaked at around 4 and 4.5 MHz formore » the layered solid and hexagonal lattice, respectively. We attribute this to a resonance in the surface response of the solid, possibly associated with a critical point in the dispersion relation of the superlattice. On rotating the solid about its surface normal, the Scholte wave displays dramatic variation in phase arrival time and, to a lesser extent, also group arrival time. This variation is well accounted for by our model. {copyright} {ital 1999} {ital The American Physical Society}« less

Line-focus probe excitation of Scholte acoustic waves at the liquid-loaded surfaces of periodic structures

We study the effect of liquid loading on the characteristics of surface acoustic waves propagating along the (100) surface of silicon. A nonviscous liquid layer of infinite thickness in contact with a solid substrate causes changes in the wave velocity and the attenuation of surface waves along the liquid-solid boundary. More importantly, we examine an additional branch of the Rayleigh surface that is induced by the liquid loading. We also find that the pseudosurface wave, which is restricted only to certain directions on the free surface, extends to all directions on the liquid-loaded (100) surface. Calculating the acoustic Poynting vectors associated with these surface-related acoustic modes, we find that the Rayleigh wave and the induced Rayleigh wave emit energy only into the liquid, whereas the pseudosurface wave emits energy both into the liquid and solid substrate. \textcopyright{} 1996 The American Physical Society.

/pdf/effects-of-liquid-loading-on-surface-acoustic-waves-in-373r57utpy.pdf

Effects of liquid loading on surface acoustic waves in solids.

Acoustic wavefront imaging using point-focus immersion transducers has enabled us to observe propagation of acoustic waves in anisotropic solids. This technique has been useful for studying both bulk-wave and surface-wave propagation in a variety of materials. By using line-focus transducers, we can image surface acoustic wave propagation based on wave-vector, rather than group-velocity, direction. In this paper we apply these techniques to carbon-fiber/epoxy composite materials.

R. E. Vines

Papers

Surface Acoustic Wave Focusing and Induced Rayleigh Waves.

Imaging of surface acoustic waves

Line-focus probe excitation of Scholte acoustic waves at the liquid-loaded surfaces of periodic structures

Effects of liquid loading on surface acoustic waves in solids.

Acoustic wavefront imaging of carbon-fiber/epoxy composites