Realizing a terahertz far-field sub-diffraction optical needle with sub-wavelength concentric ring structure array.

The terahertz (THz) lens is an essential and strategic element of THz optical systems, while a conventional THz lens cannot even reach high resolution due to the diffraction limit. Optical super-oscillation paves a way to generate sub-diffraction hotspots in the far field, and demonstrates the capacity for resolution improvement of microscopic imaging in the visible range. However, there are few demonstrations of THz lenses for focusing hotspots or needles based on super-oscillation. We propose and experimentally demonstrate a far-field sub-diffraction focusing planar lens, consisting of a sub-wavelength concentric ring structure array, for a wavelength of 118.8 μm with focal length 420λ and radius 160λ. Utilizing the silicon-etching process, a sub-diffraction focusing lens is fabricated. The experimental results show that the planar lens can generate a sub-diffraction needle with length 19.7λ in the focal region along the optic axis. Moreover, the smallest focal spot, with a transverse size of 1.212λ, is smaller than the diffraction limit of 1.476λ. The proposed sub-diffraction optical needle planar lens can substitute for its traditional counterpart, and it has great potential in super-resolution tomography THz imaging systems.