We theoretically demonstrate stacked dipole resonators (broadside near-field coupling configuration) based multilayer metasurfaces separated by a vanadium dioxide film to achieve stronger field confinement in the spacer (VO2) region. Under relatively intense terahertz excitation (20 Vm-1) assisted by larger area electric field confinement, insulator to metal transition (IMT) in VO2 spacer is realized resulting in frequency (Dipole mode) and amplitude (Fano mode) tunable metasurfaces. Enhancement in probing THz field triggers much stronger field confinement (107 Vm-1) inside the spacer layer leading to increased VO2 conductivity (responsible for IMT) through the Poole-Frankel effect. Such broadside coupled IMT based terahertz metamaterials can help in realizing active meta devices for THz domain.
Resonance excitation of surface plasmons in sub-wavelength periodic apertures (popularly known as hole arrays) is typically decided by its lattice configurations and the constituent material characteristics. Therefore, the excitation frequency of surface plasmon resonances (SPRs) in hole arrays is not easy to alter without modifying these basic structural parameters. However, we experimentally demonstrate modulation of SPR frequency by carefully incorporating an additional hole of similar geometry. By suitably modifying the relative positions between the holes inside the unit cell (fixed lattice parameters), we have tailored the SPR excitation frequency. Predominantly, we attribute such frequency detuning to near-field Coulomb interactions in between the holes that can modify the effective permittivity of the hole arrays, hence SPR characteristics. In totality, our experiments demonstrate a 7.6% shift in the SPR frequency. Further, all the experimental findings are explained through elaborate electromagnetic simulations that helped to acquire deeper physical insights related to the SPR excitation. We believe such near-field effect-based resonance tuning can find potential applications in realizing SPR-based sensors, tunable filters, and tunable non-linear devices operating in the terahertz (THz) domain.
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.
Bright mode resonances are not well-acknowledged for inducing mode hybridizations. However, we demonstrate multiple bright resonators coupled through electromagnetic fields can induce resonance mode hybridizations. Although one of the hybridized modes shows parallel magnetic moments but the other mode demonstrates anti-parallel magnetic moments leading to magnetic toroidal resonances. Normally excitation of toroidal modes demands complex structures and/or bright-dark mode interactions. However, in this work, we employ solely bright resonators to excite toroidal modes. Unlike bright-dark mode coupling, exclusive bright mode resonance coupling enables larger free space energy merging into the metasystem leading to stronger energy confinement in the metasurface array.
Most of the compelling phenomena pertaining to plasmonic metamaterials revolve around the associated odd and even order resonances. However, excitation of odd and even order modes is polarization sensitive, particularly in the case of well-accepted split-ring resonator based terahertz (THz) plasmonic metasurfaces. Such a drawback limits the practical applications of plasmonic metasurfaces across the electromagnetic spectrum. In this context, we experimentally demonstrate multi-split-ring resonator based THz metasurfaces capable of simultaneously sustaining odd and even order resonances when the polarization of the probe beam is altered through 90°. We believe this work should be beneficial in realizing polarization-independent switches and frequency selective surfaces.
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