Tuning photoacoustics with nanotransducers via Thermal Boundary Resistance and Laser Pulse Duration

TL;DR

This study theoretically explores how thermal boundary resistance and laser pulse duration influence photoacoustic wave generation in water-immersed gold nanocylinders, revealing conditions favoring thermophone or mechanophone mechanisms for efficient nanotransducers.

Contribution

It provides a detailed theoretical analysis of the roles of TBR and laser pulse duration in photoacoustic mechanisms, highlighting design principles for nanotransducers.

Findings

Thermophone dominates with ns pulses in gold-water systems.

Mechanophone dominates with ps pulses in gold-water systems.

Graphene-functionalized interfaces favor mechanophone across pulse durations.

Abstract

The photoacoustic effect in liquids, generated by metal nanoparticles excited with short laser pulses, offers high contrast imaging and promising medical treatment techniques. Understanding the role of the thermal boundary resistance (TBR) and the laser pulse duration in the generation mechanism of acoustic waves is essential to implement efficient photoacoustic nanotransducers. This work theoretically investigates, for the paradigmatic case of water-immersed gold nanocylinders, the role of the TBR and of laser pulse duration in the competition between the launching mechanisms: the thermophone and the mechanophone. In the thermophone, the nanoparticle acts as a nanoheater and the wave is launched by water thermal expansion. In the mechanophone, the nanoparticle directly acts as a nanopiston. Specifically, for a gold-water interface, the thermophone prevails under ns light pulse irradiation, while the mechanophone dominates shortening the pulse to the 10 ps regime. For a graphene-functionalized gold-water interface, instead, the mechanophone dominates over the entire range of explored laser pulse durations. Results point to high-TBR, liquid-immersed nanoparticles as potentially efficient photoacoustic nanogenerators, with the advantage of keeping the liquid environment temperature unaltered.