Pressure Dependent Thermoreflectance Spectroscopy Induced by Interband Transitions in Metallic Nano-Film

Utilizing high-pressure to modulate optical properties, such as thermoreflectance (dR/dT), over a wide range has received much attention. Nevertheless, how the pressure exerts on the complex dielectric constant and finally on dR/dT remains elusive. Here, we perform a thoroughly experimental and theoretical investigation on dR/dT of Al nano-film from 0 to 25 GPa. The dR/dT values exhibit a sine-like pressure-dependence, with the zero-crossing appearing at around 6 GPa. These special phenomena are well explained from electron transition viewpoints. The first-principles calculations show that the energy difference of parallel bands is enlarged from 1.45 to 2 eV, thereby increasing the threshold for electron transitions. The lifted threshold changes optical absorption rates of Al and the density of states of electrons involving interband transitions, finally resulting in that dR/dT exhibits such a pressure-dependent behavior. Our findings provide a deep insight on pressure-induced electronic transitions and photon-electron interactions in metals.