Photothermalization and Hot Electron Dynamics in the Steady State

Significant recent interest in plasmonic nanomaterials is based on the ability to use the strong resonant absorption to produce large transient populations of photoexcited non-equilibrium hot carriers that can then be employed in novel classes of photochemical reactions and more general optoelectronic detection schemes and power cycles. In this Feature Article, we outline nanoscale design features that allow for systematic control over photothermalization in plasmonic materials, connecting the microscopic mechanism of absorption, photoexcitation, relaxation, and thermal emission with the electronic temperature and lattice temperature of a metal during steady state illumination. Further, we show how anti-Stokes Raman spectroscopy can provide a quantitative measure of the energy distribution of the hot electrons and the surrounding lattice temperature, as well as indicate the electron-phonon coupling constant of hot electrons, all under optical conditions relevant to emerging hot electron devices, i.e., relatively low fluence, continuous wave (CW) excitation. A major insight from our experiments is the presence of a sustained subpopulation of hot electrons at an elevated temperature in comparison with the majority of the conduction electrons in the metal. In conjunction, we show what features of nanoscopic geometries give rise to the largest population and longest-lived hot electrons, as required for the goals of optimizing electron dynamics in developing applications of plasmonic hot electrons.