*Metalens Antennas in Microwave, Terahertz and Optical Domain Applications DOI: http://dx.doi.org/10.5772/intechopen.99034*

However, those two factors are usually the critical design trade-offs for the designer. As illustrated in **Figure 8(a)**, the presented THz metalens model consists of a subwavelength silicon resonator array with a hyperboloidal phase profile [10]. Due to the subwavelength arrangement and negligible coupling between adjacent resonators, we consider only the electro-magnetic response of one resonator by a period of 80 μm, as shown in **Figure 8(b)**. In order to obtain the sufficient phase range, we first study the transmission of the resonator by FDTD simulator. Here, the incident THz field is assumed to be a plane wave, propagating along z direction with the electric field along x direction. The refractive index n of the silicon resonator is 3.4. The width and the height of the cross resonator are 12 μm and 20 μm, respectively, and these geometries were optimized to excite electric and magnetic eigenmodes at the interested frequency. **Figure 8(c)** and **(d)** show the transmission amplitudes and the corresponding phases as functions of the arm lengths of the cross resonators and frequency. It worth to mention that, by varying the arm length, the phases of the transmitted wave can be tuned to any value in 2π, while the transmission amplitudes is able to keep relatively high at a fixed frequency of 3.11THz. Based on the simulated results, eight different cross resonators with nearly equidistant steps of π∕4 were selected as building atoms of the metasurface, which is able to fully cover 2π phase range.

**Figure 9** shows the setup diagram for the metalens imaging experiment and results. The distance between metalens and object plane could be adjusted along z direction and the measured beam profiles at different distances of z are shown in **Figure 9(b)**. FWHMs of the beam spots at different distances Δz away from the focal plane and normalized experimental and theoretical intensity distributions along the y axis are shown in **Figure 9(d)**. The metalens is fabricated on a SOI substrate composed of the designed group of silicon cross resonators that contain both electric and magnetic dipole modes, which is shown in **Figure 8(a)** and **(b)**. By tweaking on the cross resonators geometrically, the cross-resonance of the two dipoles can be manipulated and the phase of the transmitted wave can be realized over a 2π range with a high transmission rate. With the hyperboloid phase distribution, the laser beam can be focused to a spot at distance of 28 mm with a FWHM of 630 μm. The measured focusing efficiency is 24%, which is relatively higher than the most plasmonic metalens cases. Moreover, this work can also be extended in the design of other planar devices, such as beam deflectors or vortex plates.
