Experimental demonstration of an ultra-broadband subwavelength resolution probe from microwave to terahertz regime.

An ultra-broadband subwavelength resolution probe that consists of a Teflon rod and six metallic strips is developed for the near-field imaging system. The slit between two metallic strips maintains quasi-TEM modes, avoiding the problem of low coupling efficiency caused by the cutoff effect. The numerical calculations visualize the process of energy compression into a 0.047λ diameter spot with great field enhancement at the taper apex, and the probe holds subwavelength focusing behavior from 10 GHz to 0.25 THz. Although limited by the fabrication, the resolution of 0.16 and 0.25λ are still experimentally demonstrated at 14 GHz and 0.1 THz. The properties of easy fabrication and no cutoff frequency would lower the threshold of a high-resolution near-field imaging system.