In order to demonstrate the absorption characteristics of the proposed structure, the numerical simulation results are as follows the transverse magnetic (TM)wave and the transverse electric(TE)wave have been studied and the simulation result is presented in Fig. 2. It’s clear that have the similar absorption in TE and TM mode. We can also see that achieve about 97% of the absorption between 0.458 THz and 1.1492 THz. Considering the periodic structure of sub-wavelength and then the electromagnetic wave incident from free-space to the structure, its equivalent impedanceZ can be expressed as[33]:
(2)
With perfectly normalized impedance, the real part of Z is close to 1 and the imaginary part of the impedance is close to 0 in the range of 0.458 THz − 1.1492THz, thus resulting in minimal reflection and very high wideband absorption. To further illustrate the absorption mechanism, the distribution of electric and magnetic fields are studied at the resonant frequencies of 0.9569 THz and 1.124 THz, which absorption values are at quite high peak. The electrical field and magnetic field direction for the top layer as shown in Fig. 2. Obviously, it can be seen that VO2 films display the different resonant frequencies, and exist coupling of different sizes in a plane, and multiple absorption bands overlap to expand the absorption band.
We also demonstrate the absorption capability of the broadband absorber that the corresponding simulation of a cross-shaped unit structure is shown in Fig. 4. It can be seen that the absorption spectrum of more than 80% have two resonant ranges, which from 0.312THz to 1.226THz. By reducing the size of the top pattern, from Fig. 4(b) to (d),the bandwidth of the absorption is also reduced.
Owing to the properties of the VO2 transition from insulating to metallic states at high temperature, the tunable absorption characteristics of a broadband absorber can be realized and we set the conductivity variable from 500 S/m to 1×105 S/m. Figure 5 shows the absorption spectrum of VO2 with different electrical conductivity under normal incidence. The results indicate that with the electrical conductivity of cruciform patches 2 and 4 changes, the corresponding absorption spectrum changes accordingly. It can be seen from that the position of the bandwidth is almost constant and the intensity of the absorption spectrum changes significantly and the absorption is above 97% in the low THz frequency range. Specially, with the conductivity decreases, the absorption decreases gradually at 0.95 THz. Since the imaginary parts of VO2 mainly affects the loss. Meanwhile, VO2 is in conductor state, broadband tuning can also be achieved by adjusting the electric field. We give the effective dielectric constant of aligned nematic LC and the arrangement angle of it [34].
(3)
When the orientation of the LC molecules is θ = 0°, the direction of the electric field is perpendicular to the LC molecules. As liquid crystals are anisotropic medium and the electric field becomes higher, its molecules direction tend to be oriented in the z direction. Through calculating the direction of the LC under 0°, 30°, 45°, 60°, 90° corresponding to the neff = 1.58, 1.62, 1.67, 1.72, 1.77. Figure 6 shows that the absorption spectrum of tunable bandwidth is achieved as bias voltage is applied to the LC cell.
In Fig. 6,it was shown that broadband absorption as the LC molecule is reorientated from 0° to 90°, which can be attributed to the high absorptivity and adjust within a relatively small range. To validate the effect of the thickness of the VO2 on absorption. It is obvious from the Fig. 7 that the absorption of the broadband absorber gradually decreases with the thickness of the VO2 increases. The absorption reaches an optimum at h1 = 0.08 µm. We also provide additional performance validation of the proposed absorber under different polarization angles in TM mode, incident TE waves. It can be seen from Fig. 8 that the absorption level is nearly unchanged with the increasing polarized angle, while the resonant frequency range is changed. Thus, we providing a characteristic of broadband absorber with flexible tunability, it maintains about 95% absorption above from 0.6 THz to 1.1 THz over relatively wide angles.