To confirm the applicability of thermally induced ultrasonic emission from nanocrystalline silicon (nc-Si) devices as radiation pressure generators, the dynamic response has been investigated under a pulse operation mode. The nc-Si emitter is fabricated on a p-type Si wafer by conventional electrochemical anodization with subsequent formation of the surface electrode. Due to the flat nature of the frequency response of this emitter, the device emits an acoustic wave with little distortion under the pulse-drive condition. It is shown that a significant radiation pressure of 34.5 Pa is generated by a concentrated burst-like electrical input, and that a beam located at a distance can be levitated as a result of the mechanical loading effect. This siliconbased emitter is attractive for applications to integrated nano-or micro-electromechanical systems.

Due to a strong quantum confinement effect, the thermal conductivity and heat capacity per unit volume of a nanocrystalline silicon (nc-Si) layer prepared by electrochemical anodization are extremely low when compared to those of single crystal silicon (c-Si). These large differences in the thermal properties between nc-Si and c-Si make it possible to produce an efficient ultrasound emitter device based on thermo-acoustic conversion without any mechanical vibration. In this paper, the fundamental ultrasound characteristics of a fabricated thermally induced nc-Si ultrasound emitter are explained with regard to an application as an ultrasound speaker. Ultrasound generated at the same frequency as the input signal exhibits a flat frequency response over a wide range and is non-directional. This behavior is totally different from that of conventional airborne ultrasound devices such as piezoceramic transducers.

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To confirm the applicability of thermally induced ultrasound emission from nanocrystalline porous silicon (nc-PS) devices as directional sound emitter, the radiation pattern of one-dimensionally arrayed nc-PS device have been investigated. The nc-PS emitter is fabricated on a p-type Si substrate by conventional electrochemical anodization with subsequent formation of the surface electrode. It is shown that the emission pattern of arrayed devices can be controlled due to flat nature of the frequency response of the individual nc-PS devices. Furthermore we report a technique for wave control by phase-shift operation. This PS array enables the development of a directional sound emitter and intense sound source.

Ultrasonic trapping of locomotive organisms was studied by using paramecia of ∼0.2 mm size. An ultrasonic standing wave of ∼3 MHz was generated in a sample cell to trap the paramecia. When the ultrasonic oscillation was turned on, paramecia began to swim along the nodes of the standing wave. Furthermore, when two ultrasonic waves were crossed orthogonally in a thin sample cell, paramecia were trapped at the lattice points of the nodes. The trapping efficiency increased with the ultrasonic power density. The locomotive force of the paramecia was estimated from the threshold power density for trapping.