Ultimate-resolution thermal spectroscopy in time domain thermoreflectance (TDTR)

Time domain thermoreflectance is a very well-suited technique for thermophysical property measurements. The heterodyne or asynchronous optical sampling versions of the technique allow transient temperature measurements from hundreds of femtoseconds up to tens of nanoseconds. The corresponding spectral material response expands from 100MHz up to 10THz. However, the ultimate bandwidth of the technique is not limited by optical sampling but by the presence of the metal transducer deposited on top of the material of interest. In this work, we investigated the thermal transparency of several metal transducer thin films. We implemented and solved numerically the two-temperature model for a set of Al, Au, Ag, Cu, Cr, and Pt thin (50 and 150 nm) layers. This numerical study showed that Al thin films are better suited for spectroscopic lattice temperature measurements up to 10GHz, while noble metals like Au allow the measurement of hot carrier dynamics up to 5THz.