Study Of Broad And Tunable Terahertz Electromagnetic Absorber Based On Metamaterial Structures

Recently, metamaterial(MM) absorbers at microwave, light wave, especially terahertz(THz) bands have attracted much attention due to the advantages such as high absorption, ultrathin thickness, frequency selectivity, and design flexibility.[ Theoretically, a unity absorptivity can be achieved in such MM absorber devices with a thickness of typically λ/20(λ is the work wavelength), which is especially important in the THz band since such a strong and ultrathin absorbing material is difficult to find in nature. After the first demonstration of an MM absorber at microwave frequency by Landy et al., great efforts are made to develop THz MM absorbers and to improve the angular and polarization performance. However, most of the above mentioned absorbers run in a relatively narrow band, which limits the applications such as stealth technology or energy harvesting. Broadband THz MM absorbers are highly required. What’s more, all of the abovementioned sandwich structure absorbers can only act as passive THz devices. Nowadays, the increasingly complex electromagnetic applications require actively tunable absorbers, the absorption can be actively tuned at a fixed frequency when the electromagnetic environment changes. But to date, few of them have been reported and demonstrated. Therefore, designing and fabricating an actively tunable THz absorber is still a challenge.Firstly, we design and experimentally demonstrate a broadband metamaterial absorber in the terahertz band based on a periodic array of aluminum(Al) squares with two different sizes. A thin silicon dioxide(Si O2) film rather than conventional polyimide(PI) layer is used as dielectric spacer to separate Al squares and platinum(Pt) ground plane in our design, which significantly improves the design precision and the feasibility of the device fabrication. Then, we present an ultra-broadband polarization-independent terahertz metamaterial absorber made of circular truncated cone metamaterial. Absorbtivity of higher than 92.3% at normal incidence is obtained in a wide range of frequencies from 2 to 10 THz. We employ an isotropic metamaterial cell which consists of alternating layers of Au metal and Si O2 dielectric spacer. The absorption spectra of the THz MA are calculated using the finite-difference time domain(FDTD) method within the CST Microwave Studio 2009 at the frequency of 0-10 THz. Lastly, we experimentally demonstrate a tunable terahertz metamaterial absorber with a compound dielectric layer based on the conventional sandwich structure absorbers. The absorber is composed of periodic arrays of gold(Au) squares separated from a continuous platinum(Pt) film by a compound dielectric layer, which consists of an insulator-metal phase transition of vanadium dioxide(VO2) thin film and a buffer film of silicon dioxide(Si O2). The absorption of the absorber can be actively tuned when the temperature of VO2 changes from 25 to 70℃. The maximum modulation depth of this absorber can approach to 70% in terahertz regime. The structural simplicity and large tuning range are promising for stealth technology, thermal radiation, spectrum detection and electromagnetic protection, etc.