Resonant toroidal metasurface as a platform for thin-film and biomaterial sensing.

Toroidal resonances with weak free-space coupling have recently garnered significant research attraction toward the realization of advanced photonic devices. As a natural consequence of weak free-space coupling, toroidal resonances generally possess a high quality factor with low radiative losses. Because of these backgrounds, we have experimentally studied thin-film sensing utilizing toroidal resonance in a subwavelength planar metasurface, whose unit cell consists of near-field coupled asymmetric dual gap split-ring resonators (ASRRs). These ASRRs are placed in a mirrored configuration within the unit cell. The near-field coupled ASRRs support circulating surface currents in both resonators with opposite phases, resulting in excitation of the toroidal mode. In such a way, excited toroidal resonance can support strong light-matter interactions with external materials (analytes to be detected) placed on top of the metasurface. Further, our study reveals a sensitivity of 30 GHz/RIU while sensing AZ4533 photoresist film utilizing the toroidal mode. Such detection of thin films can be highly beneficial for the development of sensing devices for various biomolecules and dielectric materials that can be spin coated or drop casted on metasurfaces. Hence, the toroidal mode is further theoretically explored towards the detection of avian influenza virus subtypes, namely, H5N2 and H9N2. Our study reveals 6 and 9 GHz of frequency redshifts for H5N2 and H9N2, respectively, in comparison to the bare sample. Therefore, this work shows that toroidal metasurfaces can be a useful platform to sense thin films of various materials including biomaterials.

Magnetic wire: transverse magnetism in a one-dimensional plasmonic system.

We experimentally demonstrate magnetic wire in a coupled, cut-wire pair-based metasurface operating at the terahertz frequencies. A dominant transverse magnetic dipole (non-axial circulating conduction current) is excited in one of the plasmonic wires that constitute the coupled system, whereas the other wire remains electric. Despite having large asymmetry-induced strong radiation channels in such a metasurface, non-radiative current distributions are obtained as a direct consequence of interaction between the electric and magnetic wire(s). We demonstrate a versatile platform to transform an electric to a magnetic wire and vice-versa through asymmetry-induced polymorphic hybridization with potential applications in photonic/electrical integrated circuits.

Lattice-induced plasmon hybridization in metamaterials.

We explore an inherent connection between two fundamental concepts of physics-resonance (eigen mode) hybridization and lattice effect in sub-wavelength periodic structures. Our study reveals that coupling with lattice mode is the prime deciding factor to determine the nature, position, and line shape of the hybridized states. Modulating lattice modes can effectively control mode hybridization and tune the relative position of hybridized modes [symmetric (electric), anti-symmetric (magnetic)] without changing any other structural dimensions in subwavelength plasmonic metamaterials. Outcomes of this study can be exploited in designing linear and nonlinear photonic structures toward futuristic meta devices.