Abstract-Tunable mantle cloaking utilizing graphene metasurface for terahertz sensing applications

Zahra Hamzavi-Zarghani, Alireza Yahaghi, Ladislau Matekovits,  Ali Farmani

 (a) Structure of graphene strips, (b) A dielectric cylinder coated by graphene strips.
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-24-34824

Design of sensors which are able to probe electromagnetic radiation with larger cross section and at the same time with having negligible perturbation in measurement has attracted significant attention. For this purpose, scattering-cancellation sensors or cloaking sensors are introduced. However, tunable cloaking sensors are very challenging. In this regards, here, a metasurface based on graphene strips is proposed to cloak a dielectric cylinder under illumination of TEz and TMz polarized incident waves in terahertz range. According to the in plane effective surface impedance tensor for the considered metasurface and the required surface impedance for achieving invisibility under TE and TM polarized impinging waves, the geometrical parameters of the covering structure and characteristics of graphene are obtained. Numerical simulations show radar cross section reduction for both TE and TM polarizations. Furthermore, the introduced metasurface is able to cloak the cylinder for incoming waves with circular polarization. In addition, it is shown that by properly adjusting the chemical potential of graphene, the required surface impedance to have cloaking for the two polarizations in other frequencies can also be achieved, which results in a tunable dual polarized cloaking. The proposed structure provides 2-11 dB reduction in scattering strength relative to the uncloaked configuration for 0.3eV variation of graphene chemical potential.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Designing chirped aperiodically poled structures for high-energy single-cycle terahertz generation

Alireza Yahaghi, Koustuban Ravi, Arya Fallahi, and Franz Kärtner

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-34-3-590&origin=search

We introduce a collinear scheme for the highly efficient generation of broadband single or few-cycle high-power terahertz (THz) pulses using optical rectification. For this purpose, two concepts are introduced and thoroughly analyzed. The first concept is the generation of chirped broadband terahertz pulses using chirped aperiodically poled electro-optic crystals. The second concept involves the compression of the terahertz pulses extracted from the aperiodically poled structure using a chirped mirror. An illustrative design employing cryogenically cooled aperiodically poled lithium niobate crystals and the appropriate chirped mirrors is presented. It is shown that the presented design allows the optical pulse to be re-used in subsequent generation stages, resulting in optical-to-THz conversion efficiencies in excess of 5% for terahertz radiation centered around ∼0.3  THz$\sim 0.3\mathrm{THz}$. Analytic solutions and numerical calculations are presented. In order to design a crystal with the optimum conversion efficiency, we take advantage of binary optimization techniques. This work paves the way for the generation of broadband terahertz radiation with several mJ of pulse energy using large-area aperiodically poled structures.