Accessing dual toroidal modes in terahertz plasmonic metasurfaces through polarization-sensitive resonance hybridization

Abstract

Plasmonic metasurfaces have been quite a fascinating framework to invoke transformation of incident electromagnetic waves in recent times. Oftentimes, the building block of these metasurfaces (unit cells) consists of two or more meta-resonators. As a consequence, near-field coupling amongst these constituents may occur depending upon the spatial and spectral separation of the individual elements (meta-resonators). In such coupled structures resonance mode-hybridization can help in explaining the formation and energy re-distribution among the resonance modes. However, the coupling of these plasmonic modes is extremely sensitive to incident probe beam polarization and offers ample scope to harness newer physics. A qualitative understanding of the same can be attained through mode-hybridization phenomena. In this context, here, we have proposed a multi-element metastructure unit cell consisting of split ring and dipole resonators aiming to explore the intricate effects of the polarization dependency of these hybridized modes. Therefore, multi-resonator systems with varied inter-resonator spacings (sp= 3.0, 5.0 and 7.0 μm) are fabricated and characterized in the terahertz domain, showing a decrement in the frequency detuning (δ) by 30% (approx.) for a particular polarization orientation of THz probe beam. However, no such detuning is observed for the other polarization. Further, as an outcome of the strong near-field coupling, the emergence of dual toroidal modes is observed. Excitation of toroidal modes demands thoughtful mode engineering to amplify the response of these otherwise feeble modes. Such modes are capable of strongly confining electromagnetic fields due to higher Quality (Q-) factor. Our experimental studies have shown significant signature of the presence of these modes in the Terahertz (THz) domain, backed up with rigorous numerical investigations along with multipole analysis. The calculated multipole decomposition demonstrates stronger scattering amplitude enhancements (~ 7 times) at both the toroidal modes compared to off-resonant values. Such dual toroidal resonances are capable of superior field confinements as compared to single toroidal mode, and therefore, can potentially serve as an ideal testbed in developing next-generation multi-mode bio-sensors as well as realization of high Q-factor lasing cavities, electromagnetically induced transparency (EIT), non-radiating anapole modes, novel ultrafast switching, and several other applications.

High-quality factor Quasi-BIC mode via selective symmetry-breaking approach in a terahertz metasurface

“…However, as structural symmetry is broken, the dark mode couples to the continuum with a net reduced dipole moment and manifests as a sharp Fano line shape resonance. The coupling between two resonators depends on the spatial and respective spectral position [25,26]. To further understand the mechanism of coupling in the proposed design, we have simulated the near-field distributions of the electric field, as shown in figure 4.…”

Active dual-control electromagnetically induced transparency analog in metal- vanadium dioxide-photosensitive silicon hybrid metamaterial

“…Additionally, it holds crucial importance for advancing the development of slow-light switch devices within this domain. According to existing research findings, the parameters specified above can be successfully implemented in practical applications [30]. We utilize finite-difference time-domain (FDTD) method to evaluate how the proposed EIT metamaterial couples.…”

“…The distance s between the center of HS and unit cell in horizontal direction is set to 5 μm. The thicknesses of VO 2 , aluminum, photosensitive silicon, and SiO 2 are all 5 μm.According to existing research findings, the parameters specified above can be successfully implemented in practical applications[30].…”