Plane photoacoustic wave generation in liquid water using irradiation of terahertz pulses

TL;DR

This study demonstrates the generation of a plane photoacoustic wave in liquid water using terahertz laser pulses, enabling deep, non-invasive acoustic wave propagation with potential industrial and biological applications.

Contribution

It introduces a novel method of generating plane photoacoustic waves in water using THz pulses, avoiding traditional absorbers or plasma, and achieves deep, non-invasive wave propagation.

Findings

Plane photoacoustic wave generated in water with THz pulses

Wave propagates with a plane wavefront over significant depth

Low radiant exposure of <1 mJ/cm2 used for wave generation

Abstract

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. This is in contrast with conventional methods using absorbers or plasma generation using near-infrared laser light. The photoacoustic wave generation and plane wave propagation are observed using a system with a THz free-electron laser and shadowgraph imaging. The plane photoacoustic wave is generated by incident THz light with a small radiant exposure of <1 mJ/cm2 and delivered 600 times deeper than the penetration depth of THz light for water. The THz-light-induced plane photoacoustic wave offers great advantages to non-invasive operations for industrial and biological applications as demonstrated in our previous report (Yamazaki et al, Sci. Rep. 846295 (2020)).