Near-field analysis of the thermally excited evanescent waves on GaN

local phenomena of materials including electron motions and lattice vibrations generate ultra-small thermally excited evanescent waves in the long-wavelength infrared range (8-20 μm). To perform a thorough analysis of the evanescent waves, we have developed passive-type scanning near-field optical spectroscopy (SNOS) without using external illuminations. It enables the passive spectral analysis of the thermally excited evanescent waves with a spatial and wavelength resolution of 200 nm and 150 nm, respectively. Here, we performed a near-field measurement of GaN which has a surface phonon polariton resonance at a wavelength of 14.1 μm. We have experimentally verified a long decay length at 14.1 μm, indicating that it enables detecting unique signals due to the surface phenomena.