Summary
The vigorous research on low-loss photonic devices has brought significance to a new kind of electromagnetic excitation, known as toroidal resonances. Toroidal excitation, possessing high-quality factor and narrow linewidth of the resonances, has found profound applications in metamaterial (MM) devices. By the coupling of toroidal dipolar resonance to traditional electric/magnetic resonances, a metamaterial analogue of electromagnetically induced transparency effect (EIT) has been developed. Toroidal induced EIT has demonstrated intriguing properties including steep linear dispersion in transparency windows, often leading to elevated group refractive index in the material. This review summarizes the brief history and properties of the toroidal resonance, its identification in metamaterials, and their applications. Further, numerous theoretical and experimental demonstrations of single and multiband EIT effects in toroidal-dipole-based metamaterials and its applications are discussed. The study of toroidal-based EIT has numerous potential applications in the development of biomolecular sensing, slow light systems, switches, and refractive index sensing.
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 examine the plasmon-induced transparency (PIT) effect in a parallel-plate waveguide comprising two similar pyramidal-shaped grooves. One of the grooves is filled with air, while the other is filled with a dielectric material whose refractive index can be varied. The resonant frequencies corresponding to the air and dielectric grooves in the proposed configuration result in the transparency window, which can be modulated with the refractive index of the dielectric material. The approach provides flexibility to control the transparency effect in a waveguide configuration without changing the physical dimensions. We examined field profiles in the transparency region to clearly depict the PIT effect. We have employed an analytical model based upon the three-level plasmonic model to validate our numerical findings. Further, we examined the switching and tunability of the transparency effect by including silicon layers between the grooves, whose conductivity can be varied. The tunable response in the PIT effect in terahertz waveguides can be significant in the construction of terahertz waveguide components.
We discuss the excitation of dual toroidal dipolar resonances in a bilayer terahertz metamaterial configuration and examine their near field coupling induced modulation. The study is focused on the interaction and modulation between toroidal resonances excited in two layers of a bilayer system. The rotation of the symmetric circular split ring of the top layer resonator with respect to the bottom one, causes the dual resonances to modulate and ultimately switching into a single toroidal resonance. The strong near field coupled modulation is observed when both the resonator layers are placed in close proximity. A Lagrangian approach is suggested to understand the underlying mechanism of the coupled toroidal resonances. The increase in strength of the toroidal dipolar resonance on adding two layers is suggested based upon the quality factors of the resonances. Such a study enables the design of toroidal photonics devices with high quality factors and improved light–matter interaction.
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