**3.8 Polarization controllable metamaterial absorber at terahertz frequency**

Metamaterial Absorbers (MA) is another primary application to tune on the polarization of metamaterial to control the absorption [17]. Over the decades there

#### **Figure 15.**

*(a) The schematic of SNTM system setup. (b–d) Measured THz field intensity in xz-plane and the focal planes from 0.8THz to 1.2THz for the incident wave of (b) LP, (c) LCP and (d) RCP. (e) The relationship of focal length with frequency for LCP and RCP case. ( f) The relationship of PCE with frequency.*

#### **Figure 16.**

*46.9* μ*m × 46.9* μ*m unit cell of the all-dielectric metasurface as follows. Si cube sitting on top of a Si substrate and a thin SiO2 layer with a thickness of t lies between the cube and the substrate. The width W of the cube is 10.7* μ*m, the length L is 38.7* μ*m and thickness t is 80* μ*m.*

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

#### **Figure 17.**

*(a) Photograph of the metalens device based on all-dielectric metasurface; (b) optical microscope image of central area of metalens.*

#### **Figure 18.**

*Targets used in imaging experiments, including letters "H" and "N," and a piece of grating with linewidth of 0.5 mm.*

has been a huge amount of research about Metamaterial Absorbers achieving high absorption and also multi-absorption peaks with split-ring resonator, U-shape, T-shape, hexagon-shape and so on. In this example the polarization controllable dual-band MA is given as follow. The absorber consists of one substrate with two pairs of metallic strips at the top, where two horizontal strips and vertical strips are grouped together to sweep the length to tune on the resonance peaks for different polarizations. Results show that the two perfect absorption peaks could be obtained for x-polarization, and two absorption peaks with an average absorbance of 97.28% can be obtained for y-polarization. The near-field distributions at resonating frequency are also investigated for polarization controllable dual-band absorption. The results show that polarization insensitive dual-band MA can be feasible with the all strips lines having the same length [18–20].

The design details of dual-band polarization controllable MA are shown in **Figure 21**. **Figure 21(a)** and **(b)** show the skeptical side view and the top view of device. The three-layer sandwich structure is utilized to obtain the desired absorption along x and y polarizations, which are marked as A, B, C, and D, respectively. The strips A and B have the length of 130 μm, and strips C and D have the length of 100 μm. The distance between the center of two strips pairs and the unit cell center is labeled as δ1 = 15 μm and δ2 = 15 μm. Simulation results show that the four strips have the same width of *w* = 10 μm, while the lengths along different polarizations are different. This metamaterial-based structure could be very useful for the switching, controlling the broadband absorption for terahertz device.

#### **Figure 19.**

*Imaging results with incidence of a linearly polarized THz wave at wavelength of 118.8* μ*m. Measured images of letters (a) "H" and (b) "N" when the metalens top side faces the targets, and measured images of letters (c) "H" and (d) "N" when the metalens substrate side faces the targets.*

#### **Figure 20.**

*Imaging results with incidence of a linearly polarized THz wave at wavelength of 118.8* μ*m for (a) when the lens metasurface top side faces the grating, and (b) when the metalens substrate side faces the grating.*

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

#### **Figure 21.**

*(a) The structure of dual-band polarization controllable MA absorber; (b) two pairs of metallic strips at the top of structure; (c) absorption curves along the x-polarization and y-polarization; (d) and (f) are the E fields of the two peaks along x-polarization; (e) and (g) are the Ez fields of the two peaks along x-polarization; (h) and (j) are the |E| fields of the two peaks along y-polarization; (i) and (k) are the Ez fields of the two peaks along y-polarization; absorption spectrum with the variations of the l1(l) and* δ*1(m) along x-polarization; absorption spectrum with the variations of the l2 (n) and* δ*2 (o) along y-polarization.*
