  Thermionic converters generate electricity from thermal energy in a power
cycle based on vacuum emission of electrons. While thermodynamically efficient,
practical implementations are limited by the extreme temperatures required for
electron emission (> 1500 K). Here, we show how metal nanostructures that
support resonant plasmonic absorption enable an alternative strategy. High
electronic temperatures required for efficient vacuum emission can be
maintained during steady-state optical absorption while the lattice temperature
remains within the range of thermal stability, below 600 K. We have also
developed an optical thermometry technique based on anti-Stokes Raman
spectroscopy that confirms these unique electron dynamics. Thermionic devices
constructed from plasmonic absorbers show performance that can out-compete
other strategies of concentrated solar power conversion in terms of efficiency
and thermal stability.
