Effect of surface plasmon on optical detection of picosecond ultrasonic pulses generated in aluminum nanofilms

Photoacoustic and thermoacoustic detection methods, including picosecond ultrasonic laser sonar based on metallic thin films, are widely used in industrial applications for their noninvasiveness. Herein, we present our findings on the phase advance effect of laser-induced picosecond ultrasonic signals in surface plasmon detection in Al nanofilms. Al has been extensively studied as a promising surface plasmon material in the ultraviolet region. Reflection time-resolved spectroscopy was integrated with a Kretschmann configuration to study the optical detection mechanisms with and without meeting the surface plasmon phase-matching condition. Through a comparison of the phase changes in picosecond ultrasonic pulses at different optical detection angles, we attributed the observed phase delay modification to the displacement of the detection region under the surface plasmon phase-matching condition.