COMMUNICATIONlinewidth accompanied with an extremely small resonance intensity. Typically, in most of the Fano resonant plasmonic and metamaterial systems, the quality factor declines exponentially with the increase in the resonance intensity. Thus, it becomes very important to investigate the tradeoff between the quality factor and the intensity of Fano resonances. Terahertz is a perfect regime to study this tradeoff behavior due to the ease of fabrication and the precise control that could be exercised in designing metamaterial samples with extremely small variation in the geometry of the chosen meta-atoms. Terahertz split-ring resonators (SRRs) with dual split capacitive gaps that consist of two unequal metallic wires form an asymmetric resonator that have been demonstrated in the recent past to be excellent candidates in exciting the Fano resonance with ultrahigh quality factor ( Q factor). [ 2,5 ] Such a high Q factor design can overcome the radiative loss to a large extent due to the strong confi nement of photons in the resonators. [ 25 ] The Fano resonances have also been demonstrated to be potential candidates for designing ultrasensitive sensors. [ 4,26 ] Strong confi nement of energy in such systems occur due to the antiparallel oscillating currents in the metasurface array that minimizes the radiative losses if arranged in a large periodic lattice. Therefore, weak coupling of the current mode to the free space occurs at Fano resonance once the intrinsic symmetry of the unit cell is broken, which actually breaks the resonance equilibrium in the adjacent arms. Such a weak free space coupling enables long decay time and has been argued to be an excellent cavity to realize metasurfacebased fl at lasing spaser. [ 27 ] However, the ultrahigh Q factor is obtained at the expense of the Fano resonance intensity which makes it challenging to effi ciently harness this low-loss resonance feature at subwavelength scales. The high Q resonance at low intensities also presents the diffi culty in measuring the Fano resonance with low resolution and low signal-to-noise ratio systems. Therefore, it is extremely important to excite a rather high Q resonance that has strong intensity in the transmission spectra in order to exploit these resonances for several photonic applications.In this work, we address the problem of optimizing the Q factor and the resonance intensity of the Fano resonances by probing the Figure of Merit (FoM) that we defi ne here as the product of quality factor and the resonance intensity. In order to thoroughly study the factors that determine the behavior of Fano resonances, we investigated the infl uence of structural confi guration on Fano resonance with geometrically symmetric and asymmetric SRRs through detailed experiments and simulations. The asymmetry parameter in the Fano resonator is defi ned as 1 2 1 2

A broad range of dynamic metasurfaces has been developed for manipulating the intensity, phase and wavefront of electromagnetic radiation from microwaves to optical frequencies. However, most of these metasurfaces operate in single-input–output state. Here, we experimentally demonstrate a reconfigurable MEMS Fano resonant metasurface possessing multiple-input–output (MIO) states that performs logic operations with two independently controlled electrical inputs and an optical readout at terahertz frequencies. The far-field behaviour of Fano resonance exhibits XOR and XNOR operations, while the near-field resonant confinement enables the NAND operation. The MIO configuration resembling hysteresis-type closed-loop behaviour is realized through inducing electromechanically tuneable out-of-plane anisotropy in the near-field coupling of constituent resonator structures. The XOR metamaterial gate possesses potential applications in cryptographically secured terahertz wireless communication networks. Furthermore, the MIO features could lay the foundation for the realization of programmable and randomly accessible metamaterials with enhanced electro-optical performance across terahertz, infrared and optical frequencies.

Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

Fano resonances offer exciting features in enhancing the non-linearity and sensing capabilities in metamaterial systems. An active photoswitching of Fano resonances in a terahertz metadevice at low optical pump powers is demonstrated, which signifies the extreme sensitivity of the high-quality-factor resonant electric field to the external light illumination.

Localized electromagnetic excitation in the form of toroidal dipoles has recently been observed in metamaterial systems. The origin of the toroidal dipole lies in the currents flowing on the surface of a torus. Thus, the exotic toroidal excitations play an important role in determining the optical properties of a system. Toroidal dipoles also contribute towards enabling high quality factor subwavelength resonances in metamaterial systems which could be an excellent platform for probing the light matter interaction. Here, we demonstrate sensing with toroidal resonance in a two-dimensional terahertz metamaterial in which a pair of mirrored asymmetric Fano resonators possesses anti-aligned magnetic moments at an electromagnetic resonance that gives rise to a toroidal dipole. Our proof of concept demonstration opens up an avenue to explore the interaction of matter with toroidal multipoles that could have strong applications in the sensing of dielectrics and biomolecules.