Sub-wavelength tight-focusing of terahertz waves by polarization-independent high-numerical-aperture dielectric metalens

A focusing device is one of the key elements for terahertz applications, including homeland security, medicine, industrial inspection, and other fields. Sub-wavelength tight-focusing of terahertz waves is attractive for microscopy and spectroscopy. Flat optical lenses based on metasurfaces have shown potential in diffraction-limit focusing and advantages of ultrathin thickness and lightweight for large-aperture optics. However previously reported THz metalenses suffered from either polarization-dependency or small numerical aperture (NA), which greatly limits their focusing performance. In this paper, to achieve high-NA and polarization-free operation, we proposed a polarization-independent dielectric metasurface with a sub-wavelength period of 0.4 lambda. A planar terahertz lens based on such metasurface was designed for a wavelength of lambda = 118.8 mu m with a focal length of 100 lambda, a radius of 300 lambda, and a high NA of 0.95, which was fabricated with a silicon-on-insulator wafer. The experimental results demonstrate a tight focal spot with sub-wavelength full widths at half-maxima of 0.45 lambda and 0.61 lambda in the x and y directions, respectively, on the focal plane. In the x direction, the size of 0.45. is even smaller than the diffraction limit 0.526 lambda (0.5 lambda/NA). Such a metalens is favorable for sub-wavelength tight-focusing terahertz waves with different polarizations, due to its polarization independence. The metalens has potential applications in THz imaging, spectroscopy, information processing, and communications, among others. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement