Development of a cryogen-free passive near-field microscope

Abstract

Passive THz s-SNOM (scattering-type scanning near-field optical microscope) is a powerful tool, which can reveal the weak spontaneous radiation on a sample surface (e.g., Au, GaAs, or SiC) without external light source [1, 2]. In our passive s-SNOM, an extremely sensitive CSIP (charge-sensitive infrared phototransistor [3]) detector is necessary to be operated at low temperature less than 10 K. To cool down the CSIP more conveniently and to save the helium resource, we introduced a helium-free cryostat to the passive s-SNOM as shown in Fig. 1(a). Mechanical and helium gas dampers are used to attenuate the vibration from the cold head and the compressor [see Fig. 1(a)]. Furthermore, to get a better SNR for higher scan speed, we improved the confocal microscope as follows: (1) The solid immersion lens (SIL) is introduced to seal the CSIP. It can achieve smaller focusing spot on a sensing area of the CSIP to enlarge the number of the collected photon. (2) The metal-mirror type Cassegrain objective (N.A.: 0.4) whose thermal emission is almost zero. Fig. 1(b) shows that the smooth topography (upper panel) indicated that we have overcome the vibration problem. We have successfully observed the passive near-field signal on a Au stripe (lower panel). The SNR is derived to 5 with a scan speed of 100 ms per step.