The tunable terahertz (THz) Fano-resonant filter based on hybrid metal-graphene metamaterial was proposed. The optical parameters of metasurface with unit cell in the form of a cross-shaped graphene sheet in the center of a square gold ring were simulated by the finite element method using a surface conductivity model of a graphene monolayer. The narrowband modulation of the transmission by varying the Fermi level of the graphene and the position of graphene cross inside the metal ring was demonstrated. Simulation results were well explained theoretically using a three-coupled oscillator model. The proposed device can be used as a narrowband filter in wireless THz communication systems and sensing applications.

Faraday rotation effect of magnetized hexagonal ferrites BaAl_xFe_12−xO₁₉ ($x=1.4,2.0$ ) with a varied density in the frequency range of 0.1–1.0 THz was investigated using terahertz (THz) time-domain spectroscopy. Broadband polarization rotation up to 20° for 1-mm thick hexaferrite was demonstrated at room temperature. It was shown that the value of Faraday rotation can be effectively controlled by varying hexaferrite sample density as well as Al concentration. From the experimental data, the full permittivity tensor characterization of different hexaferrite samples was extracted to double-check the calculation of Faraday rotation. The obtained results demonstrate prospects for the utilization of BaAl_1.4Fe_10.6O₁₉ hexaferrite with the highest density as a broadband self-biased complete THz isolator with moderate insertion losses below 5 cm^–1 in the range up to 0.4 THz.

The recently developed concept of time-varying metamaterials paves the way for advanced methods of electromagnetic property modulation, such as magnet-free isolators, frequency converters, and other nonreciprocal photonic components. Here, we analyze the metamaterial based on the periodically arranged temporally modulated graphene layers via a steplike graphene Fermi level. The material properties are described by the effective medium theory. The electromagnetic wave temporal scattering phenomenon caused by rapid steplike graphene Fermi level switching is considered. It is demonstrated that the frequency of scattered waves increases as the Fermi level grows rapidly and vice versa. The relation between the amplitudes of forward- and backward-scattered waves is found using the Laplace transform technique. The proposed concept allows us to efficiently generate frequency harmonics under instant switching of the graphene Fermi level at a few meV. Our investigation provides a basement for the realization of temporally modulated multilayer structures based on functional tunable materials as power-efficient frequency converters.

Breaking reciprocity in optical systems is enabling the possibility of implementing devices with unidirectional wave propagation. Currently, there is an active search for suitable material with an efficient nonreciprocal response at the terahertz (THz) frequency range. Ideally, such material should allow low-loss operation without an external magnetic field at room temperature. Gyrotropic hexaferrite ceramic is highly promising for achieving optical isolation in the THz spectral range. Here, we demonstrate a broadband self-biased THz isolator based on magnetized aluminum-substituted barium hexaferrite. We reveal the necessary surface density of the hexaferrite sample for the realization of THz isolation. The proposed device exhibits no less than 30 dB of isolation from 0.2 to 1 THz. Moreover, we show that the gyrotropy properties of hexaferrite ceramics can be reasonably predicted by their magnetic properties. This device is expected to be useful for applications where one-way propagation of broadband THz wave is desired.

In this paper, we discuss the isotropic chiroptical response of two-layered metasurfaces with n -fold dihedral symmetries of resonators ( D n ) in the terahertz (THz) range. The analysis of numerical calculations is based on the homogenization model through effective polarizability approximation. We reveal the impact of resonator symmetry on circular dichroism and polarization anisotropy of the metasurface. In particular, it is shown that an increase in the symmetry order of a metasurface unit cell leads to a proportional resonance red shift in circular dichroism spectra. This research has both fundamental and applied relevance, and may enable the realization of THz devices capable of polarization manipulation or possessing polarization sensitive features.