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

Masaaki Tsubouchi,Hiromichi Hoshina,Masaya Nagai,Goro Isoyama

doi:10.1038/s41598-020-75337-6

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. in Sci Rep 10:9008, 2020).

Highlights

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse
When laser light with short pulse duration is strongly absorbed by liquid or chromophores dissolved in liquid, the energy of the light is instantaneously confined in a small volume
The energy is released as a pressure wave into the liquid via the thermoelastic effect, when the stress confinement condition, τ αvs ≪ 1, is satisfied, where τ is the laser pulse duration, α is the absorption coefficient of liquid, and vs is the speed of sound in liquid[9]

Summary

Introduction

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. 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. In previous methods of pressure wave generation, it was necessary to introduce an absorber or to produce the plasma in water or tissues, which are invasive processes with serious risk of damage to materials and biological tissues The strong absorption of the THz laser light with a picosecond pulse duration induces a rapid and local pressure increase followed by effective photoacoustic wave generation without requiring any additional absorber. The characteristics of the THz photoacoustic wave are investigated by observing the spatiotemporal evolution

Methods

Results

Conclusion