How to Measure Hot Electron and Phonon Temperatures with Time Domain Thermoreflectance Spectroscopy?

Résumé fourni par la source

Time domain thermoreflectance (TDTR) is a time-resolved technique aiming at evaluating electron and phonon temperatures. After a few picoseconds, when the thermal equilibrium is reached, the lattice temperature and the electron temperature are equal and an equilibrium thermoreflectance coefficient can be evaluated. In this work, we show that, whatever the probe wavelength, the thermoreflectance signal at equilibrium measured on a 50 nm gold transducer is proportional to the incident pump fluence. The thermoreflectance coefficient can then be identified for each probe wavelength, and lattice temperature variations can be deduced. At short time scales, the electrons can reach much higher temperatures than phonons and the thermoreflectance signal is mainly driven by the electronic contribution. In this nonequilibrium regime, the thermoreflectance signal amplitude is not linear with the incident pump fluence and does not follow the expected electron temperature variation. We have, however, identified a specific probe wavelength (490 nm) for which the electron thermoreflectance coefficient can be estimated. The TDTR technique then becomes finally a self-calibrated electron and phonon ultrafast thermometer.

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