Terahertz (THz) wave imaging has potential features for the 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 a lightweight, ultrathin, and easy integration. Although many THz metasurface lenses have been proposed for the wave focusing application, 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 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 full phase from 0 to 2π, and is independent with 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 0.55λ, a focal length of 660 µm (4λ), radius of 4.678 mm (28.35 λ), and numerical aperture of 0.99 that can work at 1.82 THz (wavelength (λ) of 165 µm). The designed metalens with the near-unity numerical aperture achieves a high spatial resolution of 89 µm (0.54λ), which can produce high-quality images. It means that the proposed metalens can resolve the microscale features separated by sub-wavelength distance (< 90 µm). Therefore, the suggested metalens can be served 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 application.INDEX TERMS High numerical aperture, high spatial resolution, metasurface lens, metalens, terahertz
