Wavelength-dependent photothermal imaging probes nanoscale temperature differences among sub-diffraction coupled plasmonic nanorods

TL;DR

This paper introduces an optical thermometry method using wavelength-dependent photothermal imaging to measure and control nanoscale temperature differences in coupled plasmonic nanorods, revealing new ways to manipulate thermal profiles at the nanoscale.

Contribution

It demonstrates a novel all-optical thermometry technique that exploits wavelength-dependent plasmon modes to actively create and measure nanoscale thermal gradients.

Findings

Photothermal microscopy encodes wavelength-dependent temperature profiles.

Hybrid plasmon modes can be optically dark and excited at specific beam positions.

Thermal gradients within nanorod trimers can be controlled via plasmonic mode excitation.

Abstract

While the thermal and electromagnetic properties of plasmonic nanostructures are well understood, nanoscale thermometry still presents an experimental and theoretical challenge. Plasmonic structures can confine electromagnetic energy at the nanoscale, resulting in local, inhomogeneous, controllable heating. But reading out the temperature with nanoscale precision using optical techniques poses a difficult challenge. Here we report on the optical thermometry of individual gold nanorod trimers that exhibit multiple wavelength-dependent plasmon modes resulting in measurably different local temperature distributions. Specifically, we demonstrate how photothermal microscopy encodes different wavelength-dependent temperature profiles in the asymmetry of the photothermal image point spread function. These point spread function asymmetries are interpreted through companion numerical simulations of the photothermal images to reveal how differing thermal gradients within the nanorod trimer can be controlled by exciting its hybridized plasmonic modes. We also find that hybrid plasmon modes that are optically dark can be excited by our focused laser beam illumination geometry at certain beam positions, thereby providing an additional route to modify thermal profiles at the nanoscale beyond wide-field illumination. Taken together these findings demonstrate an all-optical thermometry technique to actively create and measure thermal gradients at the nanoscale below the diffraction limit.