**3.7 Imaging with terahertz metalens**

In this example, we try to give imaging metalens device with a simple structure. To design a practical metalens device for THz imaging application, a simple alldielectric structure was proposed to distribute the wavefronts with 2π phase-modulation range. **Figure 16** shows the structure that the metasurface unit cell is a silicon cube sitting on top of a Si substrate, and a SiO2 thin layer is inserted in between. The unit cell size is 46.9 μm × 46.9 μm and the width of the cube W is 10.7 μm, the length L is 38.7 μm, and the thickness t is 80 μm [14–16] (**Figures 17**–**20**). A scanning near-field THz microscope (SNTM) setup in **Figure 15** is built to characterize the focusing effect of the metalens, which is made of THz photoconductive generation and probe detection. The THz field intensity in the x-y and x-z plane from 0.8 to 1.2THz for LP, LCP and RCP waves are shown in **Figure 15**(**b-d**) LCP wave is focused to left of z-axis and RCP wave is focused to the right of z-axis. The experimental results match well with the simulation.

This example gives the experimental result of THz imaging with a linear polarized, all-dielectric metalens. The structure of metalens is quite simple and feasible to a CMOS compatible platform for the practical implementation. The Engraved character gratings on the substrate have been tested and the result indicates that image resolution by this design is close to λ.
