Browsing by Author "Roumi, Bita"

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Single-Frequency Into Dual-Frequency Absorption Switch Based on a One-Dimensional Photonic Crystal Containing Graphene and Vanadium Dioxide Layers
(Elsevier, 2023) Roumi, Bita; Abdi-Ghaleh, Reza; Akkus, Harun
A terahertz absorber based on a one-dimensional photonic crystal (1DPC) containing VO2 and graphene sheet that connects single-frequency into dual-frequency absorption by temperature control is proposed in this paper. The structure consists of a graphene sheet on the Si layer, a Bragg reflector, and a VO2 layer at the end. At 300 K, the VO2 layer is in its insulator phase, and the structure behaves as a single-frequency absorber with 95% ab-sorptivity at 0.472 THz (f1 mode). When temperature varies to 350 K, the VO2 transfer to the metallic phase and the proposed structure can be utilized as a dual-frequency absorber with 98% and 99% absorptivity at 0.472 THz and 0.523 THz (f2 mode), respectively. The physical reason for the absorption modes is explained by the electric field distribution and impedance-matching technique. The frequency position of the absorption modes, and their absorptivity can be tuned by adjusting the Fermi energy of graphene and the period numbers of 1DPC. Also, the first mode absorption shows high absorptivity over a wide incident angle range for TE and TM polarization at both temperatures; in contrast, the second mode absorption at 350 K vanishes over 25of incidence angle. The proposed optical absorber would have potential applications in THz biosensors, filters, and emitters.
Thermally Switchable Terahertz Absorber Based on a Vo2-Included One-Dimensional Photonic Crystal
(Springer Heidelberg, 2023) Roumi, Bita; Erzen, Mehmet; Abdi-Ghaleh, Reza; Akkus, Harun; Zhou, Yuanguo; Prajapati, Yogendra Kumar; Maurya, Jitendra Bahadur
Here, a thermally switching absorber based on a one-dimensional photonic crystal containing a phase change material is proposed, which operates in the terahertz range. Vanadium dioxide (VO2) is utilized as the phase-change material in the structure, which shows semiconductor-to-metal transition with varying temperatures. The frequency of switching is regulated in such a way that according to the VO2 thickness, the absorption band displays switching properties from low to high frequencies and vice versa, and also from narrow to broadband absorption at the same frequency when the temperature increases from 300 to 350 K. The absorptivity in both bands is obtained at over 90%. Field distribution profile and the impedance matching technique elucidate the physical mechanism of absorption peaks. At 300 K, maximum absorption is realized by localizing the intensity at the defect layer, and at 350 K, the Tamm state excitation makes it possible to achieve perfect absorption. Also, relative impedance matching of the structure at the peak frequencies with vacuum impedance explains high absorption. Finally, the effects of incidence angle and polarization of light that influence the absorption peaks are analyzed. According to the results, the proposed absorber, despite showing switching features between two bands, also can be adjusted by incident angle for both TE and TM polarizations. This work may have potential applications in designing terahertz switches, filters, and sensors.