PASSIVELY DETECTING THERMAL EVANESCENT WAVES FROM ROOM TEMPERATURE OBJECTS

We have developed a scattering-type scanning near-field optical microscope in long-wavelength infrared region (wavelength: λ ~ 14.5 μm) with a highly sensitive detector, named charge sensitive infrared phototransistor. A tungsten near-field probe was fabricated via electrochemical etching and the sample-probe distance was precisely controlled in shear-force mode. By vertically modulating the probe independently of lateral oscillation for shear-force mode, we successfully detected thermal evanescent waves from a 3 μm-pitch Au/GaAs grating without any external illumination. The spatial resolution was experimentally estimated to be better than 150 nm (λ/100) and the signal-to-noise ratio was around 4 with an averaging time of 300 ms. The strong near-field signal from Au was suggested to be ascribed to thermally excited surface plasmons generating evanescent waves at the surface.