The multiband transparency effect in terahertz (THz) domain has intrigued the scientific community due to its significance in developing THz multiband devices. In this article, we have proposed a planar metamaterial geometry comprised of a toroidal split ring resonator (TSRR) flanked by two asymmetric C resonators. The proposed geometry results in multi-band transparency windows in the THz region via strong near field coupling of the toroidal excitation with the dipolar C-resonators of the meta molecule. The geometry displays dominant toroidal excitation as demonstrated by a multipolar analysis of scattered radiation. High Q factor resonances of the metamaterial configuration is reported which can find significance in sensing applications. We report the frequency modulation of transparency windows by changing the separation between TSRR and the C resonators. The numerically simulated findings have been interpreted and validated using an equivalent theoretical model based upon three coupled oscillators system. Such modeling of toroidal resonances may be utilized in future studies on toroidal excitation based EIT responses in metamaterials. Our study has the potential to impact the development of terahertz photonic components useful in building next generation devices.
In this paper, we explore sensing characteristics of a terahertz meta-waveguide consisting of a one-dimensional array of periodically arranged subwavelength scale split-ring resonators (SRRs). The substrate of the meta-waveguide structure has two layers. The lower one is made of metal, whereas the upper layer is made of a dielectric. On the top of it, metallic SRRs are placed. The meta-waveguide is capable of guiding the fundamental as well as higher-order terahertz modes along the designed structures. We analyze the sensing capability of the meta-waveguide by covering it with the thin film analyte. The waveguide transmission properties are observed for varying refractive index of the analyte. Different sensing parameters such as frequency shift, sensitivity, figure of merit (FoM) of resonance modes supported by the meta-waveguide have been studied. The obtained values of sensitivity and FoM indicates better sensing capability of the meta-waveguide rather than meta-designs in the transmission approach. We also employ coupled harmonic oscillator model in order to interpret and validate the numerical observations. The meta-waveguide designs can open alternate avenues for sensing thin film analytes with greater sensitivity.
The toroidal dipole excitation is important for metamaterial research because of its low-loss attribute. In this study, we demonstrate numerically and experimentally, a unique toroidal metasurface that modulates a broad resonance into a sharp mode, independent of the polarization of the incident terahertz radiation, by coupling the inherent toroidal dipole excitation to the lattice mode of the metasurface. The advantage of polarization independence enables the excitation of lattice-coupled toroidal mode for both the orthogonally polarized states of the incident terahertz radiation in the metasurface. The interaction of the two resonances results in the enhancement of the quality factor of the metasurface at the point of resonance matching. The surface current profile as well as multipole analysis of scattered powers by electric, magnetic, and toroidal dipoles confirm the domineering effect of toroidal dipole excitation for both the polarization states of incident terahertz radiation. Such a lattice-matched toroidal excitation-based device has the potential to impact the development of polarization-independent terahertz components for ultrasensitive sensors, lattice-enhanced equipment, and slow light devices for light-matter interaction.
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