Temporal measurement of hot-electron relaxation in a phonon-cooled metal island

We report temporal measurements of the dynamic electronic temperature and the electron-phonon thermal relaxation rate in a micron-scale metal island, with an electronic heat capacity of order 1 fJ/K (C~10⁷kB). We employed a superconductor--insulator--normal-metal tunnel junction, embedded in a radio-frequency resonator, as a fast (~20MHz) thermometer. A resistive heater coupled to the island allowed us to pulse the electronic temperature well above the phonon temperature. Using this device, we have determined the thermal relaxation rate of a hot-electron population in a thin normal-metal film with a measurement bandwidth that exceeds the low-temperature thermal bandwidth.