Thermal near-field spectroscopic analysis on dielectrics

Material surface is covered with thermally evanescent waves (wavelength: $8\sim 20\mu \mathrm{m}$ at room temperature) because any material contains positive/negative charges due mainly to their electron movement and lattice vibration. Probing such thermal evanescent waves with nanoscale spatial resolution can visualize local dynamics of thermal equilibrium and non-equilibrium phenomena. Recently we have developed a thermal near-field optical microscope (SNOM), which probes thermal evanescent waves without any external illumination. In this study, we focus on the spectroscopic analysis of thermally excited evanescent waves on dielectrics. First, we developed a passive spectroscopic SNOM based on gratings and then probed spontaneous evanescent waves on GaN and A1N at the wavelength of around 11 and 14 micrometers. Finally, we found that the thermally excited evanescent waves inside Reststrahlen band should be generated mainly by phonon polaritons.