Thermoreflectance of metal transducers for time-domain thermoreflectance

Abstract

We report measurements of the temperature dependence of the optical reflectivity, i.e., the thermoreflectance dR/dT, of 18 metallic elements at two laser wavelengths commonly used in ultrafast pump-probe experiments, 1.55 μm and 785 nm. The thermoreflectance is determined using time-domain thermoreflectance combined with measurements of the laser power and spot size and comparisons between the data and quantitative modeling of the temperature evolution at the surface of the sample. At a laser wavelength of 1.55 μm, four elements within this set of samples, Nb, Re, Ta, and V, have dR/dT comparable to or larger than 0.6×10−4 K−1. At a laser wavelength of 785 nm, the highest thermoreflectance is found in Al and Ta, dR/dT≈2.1×10−4 K−1 and 2.2×10−4 K−1, respectively. Alloying Au with 5% Pd increases the optical absorption by a factor of 3 and the thermoreflectance by a factor of 2.