Numerical Design of 0.99 Numerical Aperture Planar Metalens for High Spatial Resolution Terahertz Imaging.

Terahertz (THz) wave imaging has potential features for medicine, such as cancer detection. Nevertheless, traditional lenses are heavy, bulky, low spatial resolution, and difficult to integrate. Recently, metasurface has emerged as a compelling approach for achieving lightweight, ultrathin, and easy integration. Although many THz metasurface lenses have been proposed for the wave-focusing applications, there are a few THz metalenses that are polarization insensitive and have a high spatial resolution for THz imaging. In this paper, an ultra-thin, planar, polarization-independent, and high numerical aperture metalens has been proposed using double split-ring resonator (DSRR) structures for high-resolution THz wave focusing. The combination method of the propagation phase (adjusting the diameter of the outer ring of the DSRR) and the geometric phase (changing the slit of the DSRR) is applied to build the unit cell library including eight DSRRs, which can cover the full phase from 0 to $2\pi $ and is independent with the polarization of the incident wave. By arranging the DSRRs into the concentric rings on a thin substrate, an ultrathin, polarization-insensitive, and high numerical aperture metalens is designed with a thickness of $0.55\lambda $ , a focal length of $660~\mu \text{m}$ ( $4\lambda$ ), radius of 4.678 mm ( $28.35~\lambda$ ), and numerical aperture of 0.99 that can work at 1.82 THz (wavelength ( $\lambda$ ) of $165~\mu \text{m}$ ). The designed metalens with a near-unity numerical aperture achieves a high spatial resolution of $89~\mu \text{m}$ ( $0.54\lambda$ ), which can produce high-quality images. It means that the proposed metalens can resolve the microscale features separated by a sub-wavelength distance ( $ < 90~\mu \text{m}$ ). Therefore, the suggested metalens can serve as an objective lens for the miniaturized microscopy, opening a new avenue for microscopic THz imaging and showing potential usage in tiny THz imaging systems for cancer detection applications.