We propose a half wave plate in the terahertz region to realize broadband linear polarization conversion in the reflection mode through anisotropic metamaterials. The structure of this design is composed of two pairs of patches in one unit cell that can rotate the polarization direction of a linearly polarized incident wave for an angle of 2β in a broadband frequency range. This work concentrates mainly on numerical simulations by using the standard finite difference time domain method. The efficiency of the half wave plate is higher than 50%, and PCR is higher than 85% in a broadband frequency range for cross polarization conversion. In the operating frequency band, three polarization conversion peaks can be found with the conversion efficiency nearly 100%. A more outstanding performance can be achieved by using low-loss substrate. The in-depth physical mechanism is revealed by further analysis. Besides, the expressions referring to theoretical decomposed electric field are deduced.

In order to design optical metamaterials to surpass the limitation of unnaturally high refractive index in visible range, we propose the nanocylinders nanostructure with hexagonal close-packed arrangement to precisely control the electric and magnetic resonances, resulting in the manipulation of the effective index of metamaterials. By drastically increasing the strong capacitive coupling between nanocylinders and decreasing the diamagnetic effect with core-shell nanostructure, the unnaturally high refractive index of designed metamaterials can be achieved to be 10.5 in visible region. The diamagnetic effect of solid nanocylinders can be significantly suppressed by using the design strategy of hollow nanocylinders with higher order mode resonances, leading to great enhancement of the effective refractive index in broadband visible region. Expanding the refractive index into an unnaturally high positive region in visible range can complete the spectrum of attainable refractive index, and the high refractive index metamaterials can provide more design flexibility for nanophotonics.

Based on the principle of super-symmetric lens with quadratic phase gradient transformation, combined with the principle of digital coding of metasurface, we propose a wide-angle coded metalens for focusing control in two-dimensional space. This metalens achieves focus shift in the x-direction by changing the oblique incidence angle of the incident wave, and focus control in the y-direction by combining with the convolution principle of the digitally coded metasurface to achieve flexible control of light focusing in the two-dimensional plane. The metasurface unit is mainly composed of three-layer of metal structure and two layers of medium, and the transmission phase is obtained by changing the middle layer of metal structure, which in turn obtains the required phase distribution of the metalens. The design of the metalens realizes the function of the lens with a large viewing angle at the x-polarized incidence, and realizes two-dimensional focus control. Experimentally, we prepared the designed coding metalens and tested the focus control function of the wide-angle coding metalens. The experimental results are in good agreement with the design results.