Framework for analyzing the thermoreflectance spectra of metal thermal transducers with spectrally tunable time-domain thermoreflectance

The time-domain thermoreflectance (TDTR) technique has been widely used to measure thermal properties. The design and interpretation of the TDTR experiment rely on an in-depth understanding of the thermoreflectance signature for a given metal thermal transducer. Although the TDTR signals of several metal thermal transducers have been experimentally investigated, a practical framework bridging the electronic properties and the thermoreflectance characteristics of metal thermal transducers will be helpful for future studies. Compiling published results and our analysis and tests, in this work, we show a theoretical strategy to determine the thermallyinduced change of reflectance spectra with the electronic properties of metal transducers as the input. As a natural consequence of the proposed framework, we show that the optimal probe photon energy occurs near the interband transition threshold of the metal. To validate our approach, TDTR experiments are performed with Au and Cu as two representative metal thermal transducers in two temporal regimes when electrons and lattices have different temperatures (<10ps) and reach thermal equilibrium (>10ps), respectively. The experimental results show good agreement with the theory. The work fundamentally elucidates the thermally induced optical response of metal thermal transducers and also provides practical guidelines for choosing the appropriate probe photon energy to optimize the TDTR signal for a given metal thermal transducer, which is useful for broadening the adaptability of TDTR to various experimental conditions, materials, and new laser sources.