Experimental Detection of Evanescent Ultrasonic Waves by Bragg Diffraction of Light

Abstract

The presence of evanescent ultrasonic waves diffracted from an acoustic grating has been detected using Bragg diffraction of laser light. The diffraction of light by these evanescent waves (whose concept stems originally from theoretical arguments) confirms their physical existence. The grating used was designed to diffract the sound into two kinds of wave components: (1) a single propagating wave component moving directly away from the grating; and (2) a pair of oppositely directed evanescent-wave components traveling along the back surface of the grating and forming a standing evanescent wave with exponential decay perpendicular to the grating. Such evanescent waves have shorter wavelengths than propagating waves and hence are inherently capable of better resolution if used for imaging. With the proper orientation of the laser beam with respect to the grating, theory predicts that the above wave components will produce diffracted spots of light symmetrically placed about a central axis. The spots due to the evanescent-wave components are found on an axis perpendicular to that of the spots due to the propagating-wave component, and the evanescent-wave spots are further away from the center. Detection and measurement of the positions and the relative intensities of the spots gives information about the wavelengths of the components and about the rate of exponential decay of the evanescent wave.