Thermoreflectance of metal transducers for optical pump-probe studies of thermal properties

We report measurements of the temperature dependence of the optical reflectivity, dR/dT of fifteen metallic elements at a wavelength of λ = 1.03 μm by time-domain thermoreflectance (TDTR); and the thermoreflectance of thin-films of Pt, Ta, Al, Au, SrRuO₃, and LaNiO₃ over the wavelength range 0.4 <λ< 1.6 μm using variable angle spectroscopic ellipsometry. At λ = 1.03 μm, Al, Ta, Re, Ru, have high values of thermoreflectance, dR/dT > 6·10^-5K^-1, and are good choices as optical transducers for TDTR experiments using a Yb:fiber laser oscillator. If low optical reflectivity and the associated high degree of steady-state heating are not a concern, LaNiO₃ provides an exceptionally sensitive thermometer in the infrared; (1/R)(dR/dT) > 2.5·10^-4 K^-1 in the wavelength range 0.85 < λ < 1.3 μm. This compilation of data will assist in the design and interpretation of optical pump-probe studies of thermal properties.