Photothermally modulated ultrasound reflection

Abstract

An intensity modulated laser beam is used to modulate the ultrasound reflection via a thermoelastic mechanism. Modulation frequencies in the range of 5 Hz–2 kHz are used. The ultrasonic reflection is measured in pulse-echo technique using broadband 80 MHz focusing transducers. The effect of the photothermal modulation is detected by lock-in technique after demodulation of the selected interface ultrasound signal using a peak detector and a sample-and-hold stage. Experiments are performed on a fused quartz slab with an embedded Mo sheet. Lack of adhesion between Mo and quartz is detected by the modulated technique with much more contrast than by the reflected ultrasound amplitude. Line scans of the heating laser beam across the ultrasonic beam and measurements as a function of the laser modulation frequency are in accordance with a thermoelastic process as dominant signal generation mechanism.