**3.5 Tunable terahertz metalens on graphene**

Dynamic tuning is the highly desired feature for a few applications, such as 5G MIMO antenna, imaging radar, free space communication and LiDAR. **Figure 12** shows the stacked graphene metasurface structure proposed in [13]. In this case, the dielectric layers and graphene ribbons are stacked sequentially and the Ag layer is inserted as the rear reflector. The Fermi levels can be realized by adjusting the gate voltages accordingly. **Figure 12(b)** gives a cross-sectional view of a unit cell in this structure. The graphene ribbons period is P = 5 μm and the width is W = 2.9 μm, respectively. The thicknesses are d2 = 8 μm, d1 = 13.8 μm for two dielectric layers and the refractive index is *n* = 1.45. The Ag layer is d3 = 2 μm and could be treated as the perfect reflector. Due to the reflection from the graphene ribbons and the Ag layer, Fabry–Perot cavity is formed, which is mostly used to boost the resonance between the graphene and the incident light. The incident wave will be chosen to be controlled by one of layers. This feature enables that two-layer graphene structure can effectively control the light by tuning Fermi levels of graphene ribbons from each layer.

Electron beam evaporation technique is used to deposit the Ag film to the substrate and it also is used as the electrodes. Another SiO2 layer is deposited on the Ag film by plasma enhanced chemical vapor deposition (PECVD). The graphene layer is transferred to the SiO2 layer and graphene nano-ribbon pattern is etched and formed by electron-beam lithography, which is also another electrode structure shown in **Figure 12(b)**. At last, by repeating the above progress, the stacked graphene metasurface can be implemented (**Figure 13**).

#### **Figure 12.**

*(a) Schematic of the stacked graphene plasmonic metasurface. (b) the cross-sectional view of the stacked graphene plasmonic metasurface. P is the grating period, and W is the widths of graphene strips. d1, d2 and d3 are the thickness of dielectric layer and Ag layer, respectively.*

**Figure 13.**

*Intensity distributions of reflective focusing waves from the same metasurface designed under the normal incidences at (a) 3.5THz, (b) 7.0THz. The intensity distributions along the (c) X direction and (d) Z direction at different frequencies.*

Normally, the interaction of graphene layer on light is relatively weak since graphene layer only contains single carbon atom layer. It is an issue for most graphene– based devices. In the proposed structure, the stacked graphene structure takes the advantage of Fabry-Perot resonance to achieve multi-band functionality. At one resonant frequency peak, the graphene ribbon layer works as a strong coupler, and other graphene layers are almost transparent. Therefore, in this case, two layers of graphene ribbons can separately tune on optical resonance at different frequencies and there is not strong interference with each other, which is actually extremely difficult to implement by the stacked metal structure. Therefore, the appropriate resonant frequencies should be determined carefully to separate the Fermi levels of the different resonances to isolate the interaction between graphene ribbon layers, since the resonant frequencies of graphene ribbon layers can be independently tuned by Fermi levels in 0-1 eV.

#### **3.6 Liquid crystal tunable terahertz metalens**

In this part we give an example of liquid crystal (LC) tunable terahertz lens with spin-selected focusing property [5]. The spin state of LC could be controlled by the external voltage and this case shows how LC is designed and fabricated to function as focus tuning device. The decomposed structure of the device is illustrated in **Figure 14(a)**. Top and bottom substrates are both 800-μm-thick fused silica. With ultrasonically cleaning process, the substrates are transferred with grapheme thin layer and the alignment layer sulfonic azo dye is spin coated onto the grapheme layer. Then the substrates are assembled and separated by Mylar spacer 250 μm away. The dynamic micro-lithography technique using the digital micro-mirror device is introduced to control the spatial distribution of LC directors to implement the desired phase distribution shown in **Figure 14(c)**. Finally, implemented LC orientation profile shown in **Figure 14(b)** agrees well with the design target in **Figure 14(c)** after an LC NJU-LDn-4 is injected with the birefringence over 0.3 from 0.5THz to 2.0THz (**Figure 15**).

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

**Figure 14.**

*(a) The schematic of the LC spin-selected flat lens. (b) the photo of the metalens with crossed polarizers incidence in yellow. Scale bar: 1 mm. (c) the designed phase profile. Enlarged part shows a 6 × 6 pixel array, which is composed of lattice I and II with the periodicity p = 152* μ*m. (d) the focusing effects of mode I and mode II.*
