Ta and Au(Pd) alloy metal film transducers for time-domain thermoreflectance at high pressures

We studied the pressure dependence of the thermoreflectance and piezo-optical coefficients of metal film transducers-Al, Ta, and Au(Pd) alloy (≈5 at. %Pd)-at a laser wavelength, 785 nm, commonly used in time-domain thermoreflectance (TDTR) and picosecond acoustics experiments. Al has exceptionally high thermoreflectance at ambient pressure, dR/dT ≈ 1.3 × 10^-4 K^-1, but its applicability at high temperatures is limited by the low melting temperature. The thermoreflectance of Al also has an undesirable zero-crossing near 6 GPa. The thermoreflectance values of Ta and Au(Pd) are comparable to that of Al at ambient conditions but independent of pressure in the pressure range 0 ≤ P ≤ 10 GPa. Ta and Au(Pd) thin film transducers also show strong picosecond acoustic echoes at all pressures in this range. We conclude that Ta and Au(Pd) metal film transducers can replace Al in TDTR experiments and thereby facilitate the extension of TDTR methods to high pressures and temperatures.