Understanding the Spatiotemporal Resolution of Near-Field Photoacoustics from Nanostructures

TL;DR

This paper investigates the spatiotemporal characteristics of near-field photoacoustic waves generated by nanostructures, revealing how laser parameters and nanoparticle arrangements influence wave profiles and focusing.

Contribution

It provides a detailed analysis of near-field photoacoustic wave shaping and introduces a model for controlling wave focus using anisotropic nanostructure arrangements.

Findings

Near-field photoacoustic amplitude is ~75% stronger along the short axis of a gold nanorod.

The anisotropic distribution converges to an isotropic spherical wave at ~50 nm from the nanorod surface.

Arranging nanospheres anisotropically enables precise near-field photoacoustic focusing within an acoustic wavelength.

Abstract

Understanding the mechanism of photoacoustic generation at the nano-scale is key to developing more efficient photoacoustic devices and agents. Unlike the far-field photoacoustic effect that has been well employed in imaging, the near-field profile leads to a complex wave-tissue interaction but is under-studied. Here we show that the spatiotemporal profile of the near-field photoacoustic waves can be shaped by laser pulses, anisotropy, and nanoparticle(s) spatial arrangement. Using a gold nanorod as an example, we discovered that the near-field photoacoustic amplitude in the short axis is ~75% stronger than the long axis, and the anisotropic spatial distribution converges to an isotropic spherical wave at ~50 nm away from the nanorod's surface. We further extend the model to asymmetric gold nanostructures by arranging isotropic nanospheres anisotropically with broken symmetry to achieve a precisely controlled near-field photoacoustic "focus" largely within an acoustic wavelength.