Magnetotransport, the magnetic field induced electron transport phenomenon through metals and semiconductors, has proven to be a useful technique to tailor the properties of electromagnetic radiation upon interaction with suitable materials and structures. Very recently, magnetotransport has become an important tool to dynamically tailor terahertz (THz) radiation and response of different optical devices. Based on these backgrounds, optically thin, subwavelength superlattice (with alternate arrangement of four metallic films: two non-magnetic aluminum [Al] and two ferromagnetic nickel [Ni] films, having thickness of each film as 10 nm) metasurfaces are demonstrated, consisting of periodic arrays of asymmetric cut-wire pair metasurfaces, which have a unique ability to exhibit both frequency and intensity modulation at its hybridized resonances when low-intensity magnetic fields are applied. Such dynamic tuning characteristics are attributed to the combined effects of spin-dependent THz magnetotransport in superlattice films, near-field electromagnetic coupling between the resonators, and lattice mode coupling. Such THz metasurface is further employed for non-contact detection of external magnetic fields in the range of 0-30 mT, while operating at the optically thin regime. The demonstrated scheme can further be extended to realize THz magneto-spectroscopy toward devising state-of-the-art photonic and magnetic technologies.

Tunable slow light systems have gained much interests recently due to their efficient control of strong light-matter interactions as well as their huge potential for realizing tunable device applications. Here, a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators. Optical pump-power dependent THz transmissions through the metasurface samples are studied using the optical pump terahertz probe technique. Under various photoexcitations, the EIT spectra undergo significant modulations in terms of its resonance line shapes (amplitude and intensity contrast) leading to dynamic tailoring of the slow light characteristics. Group delay and delay bandwidth product (DBP) values are modulated from 0.915 ps to 0.42 ps and 0.059 to 0.025 as the pump fluence increases from 0 to 62.5 〖nJ cm〗^(-2). This results in tunable slow THz light with group velocities ranging from 2.18×〖10〗^5 〖m s〗^(-1) to 4.76×〖10〗^5 〖m s〗^(-1), almost 54% change in group velocity. The observed tuning is attributed to the photo-induced modifications of the optoelectronic properties of the substrate layer. The demonstrated slow light scheme can provide opportunities for realizing dynamically tunable slow light devices, delay lines, and other ultrafast devices for 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.

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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