Journal
ACS PHOTONICS
Volume 5, Issue 12, Pages 4880-4887Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsphotonics.8b01045
Keywords
thermoreflectance; electron-phonon scattering; pump-probe; electronic temperature; Drude model
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Funding
- U.S. Department of Defense, Multidisciplinary University Research Initiative through the Army Research Office [W911NF-16-1-0406]
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The temperature-dependent reflectivity of metals is quantified by the thermoreflectance coefficient, which is a material-dependent parameter that depends on the metallic band structure, electron scattering dynamics, and photon wavelength. After short-pulse laser heating, the electronic subsystem in a metal can be driven to temperatures much higher than that of the lattice, which gives rise to unique nonequilibrium electron and phonon scattering dynamics, leading to a hot electron thermoreflectance that is different from the traditionally measured equilibrium coefficient. In this work, we analytically quantify and experimentally measure this hot electron thermoreflectance coefficient through ultrafast pump-probe measurements of thin gold films on silica glass and sapphire substrates. We demonstrate the ability to not only quantify the thermoreflectance during electron phonon nonequilibrium but also validate this coefficient's predicted dependence on the absolute temperature of the electronic subsystem. The approach outlined in this work provides a metrology to further understand and quantify excited-state scattering effects on the dielectric function of metals.
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