Generation of ultrasonic waves from a layered photoacoustic source

Abstract

ACOUSTIC pulses are used extensively for imaging, materials characterization and non-destructive testing. Here we describe a method for producing trains of square-wave photoacoustic pulses. Our device comprises an alternating series of light-absorbing and transparent fluid layers. When this layered structure is irradiated with an amplitude-modulated laser, constructive and destructive interference of the resulting photoacoustic waves generates pressure waves at specific resonance frequencies, the bandwidths of which are determined by the number of layers and their acoustic impedances. When operated at the resonance condition, a device consisting of a large number of layers produces ultrasonic waves that are highly directional in space. A structure excited by a single short pulse from a Nd:YAG laser generates a train of acoustic square waves, the pulsewidth of each being determined by the transit of sound across the absorbing layer.