Abstract
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 $(C\ensuremath{\sim}{10}^{7}{k}_{B}).$ We employed a superconductor--insulator--normal-metal tunnel junction, embedded in a radio-frequency resonator, as a fast $(\ensuremath{\sim}20\mathrm{MHz})$ 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.
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