Frequency domain photoacoustics using intensity-modulated laser sources

Abstract

In this paper, we present an overview of research in the area of narrow-bandwidth generation and detection of ultrasound using a technique referred to as frequency domain photoacoustics. An intensity-modulated continuous wave laser is used for narrowband ultrasound generation, and an interferometer coupled to a radio frequency lock-in amplifier is used for detection. Excellent sensitivity is achieved using sinusoidal modulation of the excitation laser over long time scales, thereby focusing the acoustic energy at the excitation frequency. We describe an experimental approach for the direct detection of narrowband signals at the ultrasound excitation frequency, and a superheterodyne technique in which the signal is optically down-converted to a low and fixed intermediate frequency prior to detection. Two approaches for materials characterisation using frequency domain photoacoustic measurements are presented. In the first approach, the modulation frequency of the excitation laser is scanned over the bandwidth of interest, and a transient sample response is constructed from the frequency-domain data. Ultrasound arrivals that are separated in the time domain can then be time gated for further analysis. In the second approach, the modulation frequency of the excitation laser is fixed, but the source-to-receiver distance is varied. The spatial frequencies of the ultrasound modes generated by the laser are detected by analysing the spatial variation of the phase, allowing for the velocity of each mode at a given frequency to be determined. Experimental measurements in thin films, plates and nanomechanical structures are presented.

Keywords:

• photoacoustics
• narrowband
• resonance