  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.
