All-dielectric metalens for terahertz wave imaging.

Terahertz wave imaging offers promising properties for non-destructive testing applications in the areas of homeland security, medicine, and industrial inspection. However, conventional optical lenses are heavy and bulky and difficult to integrate. An all-dielectric metasurface provides an attractive way to realize a planar lens of light weight that is ultrathin and offers ease of integration. Terahertz lenses based on various metasurfaces have been studied, especially for the application of wave focusing, while there are few experimental demonstrations of terahertz wave imaging lenses based on an all-dielectric metasurface. In the present work, we propose a metalens based on an all-dielectric metasurface with a sub-wavelength unit size of 0.39λ for terahertz wave imaging and experimentally demonstrate its performance in focusing and imaging. A large numerical aperture metalens was fabricated with a focal length of 300λ, radius of 300λ, and numerical aperture of 0.707. The experimental results show that the lens can focus THz waves with an incident angle up to 48°. More importantly, clear terahertz wave images of different objects were obtained for both different cases of forward- and inverse-incident directions, which demonstrate the reversibility of the metalens for imaging. Such a metalens provides a way for realization of all-planar-lens THz imaging system, and might find application in terahertz wave imaging, information processing, microscopy, and others.

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.

The Fabrication of Large-Area, Uniform Graphene Nanomeshes for High-Speed, Room-Temperature Direct Terahertz Detection.

In recent years, graphene nanomesh (GNM), a material with high flexibility and tunable electronic properties, has attracted considerable attention from researchers due to its wide applications in the fields of nanoscience and nanotechnology. Herein, we have processed large-area, uniform arrays of rectangular graphene nanomesh (r-GNM) and circular graphene nanomesh (c-GNM) with different neck widths by electron beam lithography (EBL). The electronic properties of those high-quality GNM samples have been characterized systematically. Electrical measurements illustrated that top-gated field effect transistors with different neck widths of the GNM possessed different Ion/Ioff ratios. In particular, the devices based on r-GNM with a neck width of 30 nm were found to possess the largest Ion/Ioff ratio of ~ 100, and the band gap of the r-GNM was estimated to be 0.23 eV, which, to the best of authors' knowledge, is the highest value for graphene ribbons or a GNM with a neck width under 30 nm. Furthermore, the terahertz response of large-area r-GNM devices based on the photoconductive effect was estimated to be 10 mA/W at room temperature. We also explored the practical application of terahertz imaging, showing that the devices can be used in a feasible setting with a response time < 20 ms; this enables accurate and fast imaging of macroscopic samples.