Toroidal resonances, a new class of electromagnetic excitations, demonstrate exceptional properties as compared to electric and magnetic dipolar resonances. While electric and magnetic dipolar moments have been extensively studied, toroidal dipolar moment is a recent topic of research . The charge separation in materials forms electric moments while magnetic moments form due to circular flowing current. Toroidal dipolar moments form when poloidal currents flow along the arms of a torus, leading to a head-to-tail arrangement of magnetic moments. This results in the formation of a toroidal dipole moment pointing outward along the symmetry axis of the torus. The toroidal excitations are usually masked by electric and magnetic excitations in natural materials. They can however be explored via carefully designed metamaterials. Metamaterials (MMs) are artificially engineered materials consisting of periodically arranged subwavelength resonant unit cells known as meta-atoms . The properties of the MM do not depend on the intrinsic materials out of which it is made but on the structural geometry of the constituent meta-atoms. Carefully designed MM geometries, so as to maximize toroidal excitations, have led to several interesting studies investigating toroidal low loss attributes, active tuning of properties, anapole excitation and switches . Bilayer metamaterials provide an interesting aspect in the study of toroidal excitations . The interaction of near field coupled toroidal resonances has been recently studied and its significance in passive switching from dual to single resonance has been reported. However, the passive switching of such MMs is not practically viable. The dynamic modulation of toroidal excitations in MMs provide a better platform in the study and development of actively tunable toroidal MMs. The advantage of narrow line width in toroidal resonance is utilized in this proposed work where a bilayer metamaterial (MM) sensor has been designed in the terahertz frequency regime (THz). A toroidal MM geometry in single layer is first studied. A second identical MM geometry placed on top of the first layer results in coupling of toroidal excitations, leading to increase in the quality factor (Q) of the resonance. The dynamic switching from an ‘off’ stage to an ‘on’ stage in the bilayer configuration is explored. In this work, we study the dynamic switching of the dual toroidal resonances via the photoexcitation of silicon pads in the capacitive gaps. Such actively tunable switches in bilayer toroidal MM configurations is yet to be explored keenly in the terahertz regime. The study further investigates the sensing capacity of the toroidal resonance. The sensitivity (S) of the toroidal MM is reported. It also explores the sensing of common oils. The ardent study of such toroidal bilayer MMs could provide significant potential in the development of bio-molecular and chemical sensors, switches and modulators.
Audience Take Away:
- The concept of toroidal excitations in metamaterials is a fairly recent topic. The audience will be able to gather the idea of coupling in bilayered toroidal metamaterials for terahertz photonic device applications.
- The idea can be practically executed via traditional photolithography or electron beam, lithography. The idea of high quality factor toroidal resonances can be easily extended to optical and microwave domains. It will help other researchers in designing low loss devices based on toroidal excitations. The small scale terahertz sensor provides the idea of a portable metamaterial device.
- The idea of oil sensing using a toroidal metamaterial sensor is fresh and will help in the design of terahertz based high speed photonic sensors for biochemical and biological sensing applications.
- The audience will receive an idea on the research work going on in India on terahertz photonics and such exchange of ideas will result in healthy research collaborations.