Improved signal-to-noise ratio in a passive THz near-field microscope equipped with a helium-free cryostat

We have developed a passive THz s-SNOM (scattering-type scanning near-field optical microscope) equipped with a helium-free cryostat. The mechanical vibration from the compressor is reduced for accurately controlling the probe-sample distance. Besides, the confocal system with a solid immersion lens leads to gather higher signal-to-noise ratio. To demonstrate the capability of the s-SNOM, we measured the thermally excited surface evanescent wave on a Au/SiO2 sample. Achieved SNR of the near-field signal on Au is 5.5 with a 0.1 s/pixel scan rate. These results show that the improved passive s-SNOM not only provides more convenient cooling process but also significantly speeds up the scan rate for the passive THz near-field image.