Differentiating Contributions of Electrons and Phonons to the Thermoreflectance Spectra of Gold

TL;DR

This study combines experimental measurements and theoretical calculations to distinguish the effects of electrons and phonons on gold's thermoreflectance spectra, revealing phonons as the dominant factor across wavelengths.

Contribution

It provides a comprehensive analysis of electron and phonon contributions to gold's thermoreflectance, combining pump/probe experiments with first-principles calculations.

Findings

Phonon temperature primarily influences thermoreflectance of gold.

Electron temperature contributes only about 2% near 480 nm.

Theoretical calculations confirm phonon dominance in thermoreflectance.

Abstract

To better understand the many effects of temperature on the optical properties of metals, we experimentally and theoretically quantify the electron vs. phonon contributions to the thermoreflectance spectra of gold. We perform a series of pump/probe measurements on nanoscale Pt/Au bilayers at wavelengths between 400 and 1000 nm. At all wavelengths, we find that changes in phonon temperature, not electron temperature, are the primary contributor to the thermoreflectance of Au. The thermoreflectance is most sensitive to the electron temperature at wavelength of ~480 nm due to interband transitions between d-states and the Fermi-level. In the near infrared, the electron temperature is responsible for only ~2% of the total thermoreflectance. We also compute the thermoreflectance spectra of Au from first principles. Our calculations further confirm that phonon temperature dominates thermoreflectance of Au. Most of Au's thermoreflectance is due to the effect of the phonon population on electron lifetime.