Probing surface evanescent waves with an ultra-high sensitive terahertz near-field microscope

Abstract

Compared to the visible range, nano-scale material research in the infrared or terahertz wavelength is seriously limited due to a lack of useful microscope which can overcome diffraction limit and achieve spatial resolution down to the nanometers. For this purpose, we constructed a new terahertz scattering-type scanning near-field optical microscope (THz s-SNOM, Fig. 1(a)). By equipping an ultra-high sensitive detector (15 µm) in a well-designed confocal microscope system, and modulating a sharp tungsten tip close to sample surface, thermally excited near-field evanescent waves of room temperature objects can be picked up (without external illumination), therefore we term the system as a passive type THz s-SNOM. Fig. 1(b) shows a SiO 2 sample covered with sub-wavelength Au patterns (SEM image), which can be passively imaged by our microscope (Fig. 1(c)). The estimated spatial resolution is ∼ 50 nm, which is already far beyond the diffraction limit (λ/300). Various other samples (sub-wavelength plasmonic structures, semiconductor nano-devices, carbon nanotubes, et al.) have been studied, and many interesting new phenomenon has been revealed. The introduced THz s-SNOM can be a powerful tool for various material research.