A multiband tunable metamaterial terahertz absorber based on VO2 material is introduced and studied. The absorber consists of gold planes and periodic metallic materials. The first layer is gold plane, the second layer is polyimide, the third layer is silicon film, and the fourth layer is super surface. From the simulation results, we can see that the phase transition occurs when the temperature of VO2 is greater than or equal to 68°C. After the phase transition, there are three absorption peaks, and the absorption rates are more than 99% at 2.53, 5.7, and 8.67 THz, realizing perfect absorption. When the temperature of VO2 is less than 68°C, the maximum absorption rate is 30.4%, which fails to meet the absorption requirements. By increasing the thickness of polyimide, the absorption spectrum moves slightly to the low frequency band to achieve redshift. Compared with most of the proposed multiband absorbers, the metamaterial absorber with adjustable absorption spectrum has better application prospects. VO2 is a phase change material with insulator metal phase transition characteristics and reversible process. By using the phase transition characteristics of VO2, the absorber has a switching function and is conducive to the flexible regulation of the absorption spectrum.
A metamaterial- and graphene-based broadband terahertz (THz) electromagnetic wave absorber that is composed of a stack of patterned gold film layer, patterned graphene layer, polyimide layer, and silicon layers was studied in detail. Our study is based on numerical simulations and electromagnetic absorption level higher than 90% over 2.09 THz band was demonstrated. The absorption frequency band significantly depends on the thickness of the polyimide layer and red shifts in parallel with its thickness. The major role of the added patterned graphene layer is to introduce frequency tunability property to the almost perfect absorption. As the Fermi level of graphene changed from 0 to 0.6 eV, the center frequency of the absorption band blueshifts from 6.19 to 8.15 THz. We also investigated incoming beam angle of incidence dependence and polarization sensitivity of the absorption. Metamaterial-based tunable absorbers prospected to be utilized in high-performance THz devices as a performance boosting critical element.
A linear cavity all normal dispersion Yb-doped fiber laser based on the reflection volume grating and the SESAM(Semiconductor Saturable Absorber Mirror) has been demonstrated. Stable wavelength continuous tuning passively mode-locked laser pulse is obtained at room temperature with the repetition frequency of 16.52MHz. The spectral bandwidth of passively mode-locked pulse is 0.33nm at the central wavelength of 1032nm with the maximum average output power of 10.3mW and the monopulse energy of 0.64nJ. The central wavelength of the mode-locked pulse is tuned in the range of 1011.8~1050.7nm with the tuning range of 38.9nm by rotating the volume grating in taking advantage of its dispersion and wavelength selection characteristic. The fiber laser can be used as the optical source in DWDM/OTDM communication system or OCT system due to its wavelength tuning characteristic.
A liquid crystal (LC)-modulated tunable filter is theoretically studied, consisting of an Au nanorod array and an Au film separated by a dielectric indium tin oxide (ITO) glass layer. It is well established that this system can achieve double absorptive peaks resulting from the strong plasmonic coupling between the plasmon-induced transparency effect of the nanorod array and the cavity mode of the Au microcavity structure. The positions of the double absorptive peaks can be tuned dynamically based on the electro-optic effect of LC. The simulation results reveal that a band range of 160 nm has been confirmed at near-infrared wavelengths by altering the driven voltage of LC from 0 to 8 V. The proposed structure is able to filter the two peaks with the reflection coefficients <5 % . Compared with the existing tunable filter, it has many advantages, such as continuous tunability, low tuning voltage, and great degree of tunability.
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