Low terahertz-band scanning near-field microscope with 155-nm resolution

Highlights: • We undertook a low THz-band near-field microscope based on a continuous wave solid state source operating at 110 GHz, with a spatial resolution of 155 nm, which was about 1/20000 of the incident wavelength, approaching the theoretical limit of about 100 nm. • We can extend the all solid state design of the s-SNOM fairly easily to the THz band based on the same technical approach. • This work may provide an excellent instrument to realize the dream of nano-imaging in the long wavelength band, and pave the way for interesting interdisciplinary research works in biology and material science. We report on the design and implementation of a scattering-type scanning near-field microscope working in the low terahertz-band under ambient conditions for nanoscopic investigations of physical properties and characteristics at sample surfaces and interfaces in the microwave and millimeter wave bands. Employing a nano-tip that oscillates vertically at a frequency Ω as the antenna, and a subharmonic mixer as the receiver, and corresponding demodulation algorithms, the back-scattered light carrying tip-sample interaction information is effectively extracted, while excluding almost all of the background noises. The amplitude and phase images constructed from signals demodulated at various harmonics (nΩ, n = 1 - 4) are obtained while scanning an Au-Si step structure with the newly developed microscope, and a resolution of 155 nm (~λ/20,000) has been demonstrated at the fourth harmonic frequency (4Ω) working at 110 GHz, with signal-to-noise ratio (SNR) equal to 44.4 dB on the Au surface and 36.2 dB on the Si surface, demonstrating the power of this new instrument for micro/nano-resolution studies in the millimeter wave band.