Strong interaction between terahertz (THz) and matter is a topic of paramount importance, considering continuously enhanced interest in THz photonics as well as condensed matter physics, which can lead to the observation of many linear and nonlinear phenomena in the THz regime. Here, we demonstrate a unique and novel metamaterial-based technique of strong THz matter interaction towards thin film sensing, where the analyte is sensed in between the stacked metasurfaces forming the Fano cavity. Sub-wavelength structures typically overcome the diffraction limit of any optical system, which also possess very high confinement of electromagnetic energy. Fano resonance possesses a sharp asymmetric line shape, low radiation loss and large tuning capability. In addition to them, the material under test is placed in between the metasurfaces to utilize the substantial energy confinement leading to strong light matter interaction, a scheme never explored before. By intelligently exploiting the above characteristics, we have demonstrated a novel way to detect both the refractive index (dielectric constant), thickness and loss factor of the material under test when placed between the array of the meta-resonators forming the Fano metamaterials. Our study revealed that the sensitivity and figure of merit (FOM) are strikingly different for dipole and Fano modes. A maximum sensitivity of >1 THz RIU −1 (1.76 × 10 5 nm RIU −1 ) and FOM of around 14.05 are achieved at the Fano mode. Additionally, our sensor shows better performance with decreasing spacer thickness (lesser the material, more the sensitivity). Moreover, the proposed device is passive towards typical ambience temperature variation, and is highly compact because of its stacked configuration. The demonstrated device can be extremely beneficial towards realizing ultra-sensitive meta-sensors and other miniaturized THz meta-photonic devices, and bio-chemical sensing where strong light matter interaction is mandatory.

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.

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.

Fano resonances in metamaterials attract intense attention due to their sharp asymmetric spectral feature and low radiative loss channel. Typically, Fano resonances are excited in metamaterials with broken structural symmetry in a planar configuration. Herein, the excitation of Fano resonance in geometrically symmetric metamaterial structure operating in the strong, deep sub‐wavelength coupling regime at terahertz frequencies is experimentally demonstrated. The structure consists of vertically stacked symmetric resonator array separated by an ultrathin polyimide spacer. Such meta‐structures can strongly support highly localized, intense electromagnetic field confined inside the sub‐wavelength volumes. It is further shown that the resonance linewidth and the field strengths can be engineered by tailoring the near‐field coupling through the ultrathin spacer layer. Such exotic features of perfectly symmetric Fano resonators provide an ideal playing ground for strong light–matter coupling, resulting in realization of ultrasensitive sensors, slow‐light devices, and energy‐efficient ultrafast terahertz modulators.

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