Near-field thermal imaging of optically excited gold nanostructures: scaling principles for collective heating with heat dissipation into the surrounding medium

Abstract

In this paper, we introduce a new optical temperature and thermal imaging technique combining near-field microscopy and Er³⁺ photoluminescence thermometry. The tip aperture of 120 nm limits the spatial resolution of near-field thermal imaging. We use the technique with two different approaches towards local temperature measurement and thermal imaging. In the first approach, gold nanostructures on top of Al_0.94Ga_0.06N thin film embedded with Er³⁺ ions are optically excited through the SNOM tip with 532 nm CW laser to generate thermal images that have a Gaussian thermal profile because heating and probing are done through a single tip aperture. In the second approach, nanostructures on top of thermal sensor film of AlGaN : Er³⁺ ions deposited on a transparent sapphire substrate are excited with 532 nm CW laser through the substrate with a large spot size (FWHM ∼10 μm) and Er³⁺ emission from the film is collected in transmission mode through the SNOM tip. We use this approach to measure steady-state thermal profiles from optically excited different sized clusters made from 40 nm gold nanoparticles. This approach yields steady-state thermal profiles that have inverse distance temperature decay away from the cluster and we find that the maximum temperature change and temperature decay length into the surrounding medium (r_½) scales with cluster radius.