High quality factor resonances are extremely promising for designing ultra-sensitive refractive index label-free sensors since it allows intense interaction between electromagnetic waves and the analyte material. Metamaterial and plasmonic sensing has recently attracted a lot of attention due to subwavelength confinement of electromagnetic fields in the resonant structures. However, the excitation of high quality factor resonances in these systems has been a challenge. We excite an order of magnitude higher quality factor resonances in planar terahertz metamaterials that we exploit for ultrasensitive sensing. The low-loss quadrupole and Fano resonances with extremely narrow linewidths enable us to measure the minute spectral shift caused due to the smallest change in the refractive index of the surrounding media. We achieve sensitivity levels of 7.75 × 10 3 nm/ RIU with quadrupole and 5.7 × 10 4 nm/RIU with the Fano resonances which could be further enhanced by using thinner substrates. These findings would facilitate the design of

Planar metasurfaces and plasmonic resonators have shown great promise for sensing applications across the electromagnetic domain ranging from the microwaves to the optical frequencies. However, these sensors suffer from lower figure of merit and sensitivity due to the radiative and the non-radiative loss channels in the plasmonic metamaterial systems. We demonstrate a metamaterial absorber based ultrasensitive sensing scheme at the terahertz frequencies with significantly enhanced sensitivity and an order of magnitude higher figure of merit compared to planar metasurfaces. Magnetic and electric resonant field enhancement in the impedance matched absorber cavity enables stronger interaction with the dielectric analyte. This finding opens up opportunities for perfect metamaterial absorbers to be applied as efficient sensors in the finger print region of the electromagnetic spectrum with several organic, explosive, and bio-molecules that have unique spectral signature at the terahertz frequencies.

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

Polarization conveys valuable information for electromagnetic signal processing exhibiting tremendous potential in developing application driven photonic devices. Manipulation of polarization state of an electromagnetic wave has drawn a lot of research interests in many different fields, especially in the terahertz regime. Here, we propose a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces. We experimentally observe a nearly perfect orthogonal polarization conversion with an ultrahigh efficiency, demonstrating a ultrathin terahetz rotator. The Fabry-Perot cavity effect in the tri-layer metasurfaces is attributed to the underlying mechanism of high transmittance and polarization rotation.

By utilizing the vector nature of light as well as the inherent anisotropy of artificial meta-atoms, we investigate parity time symmetry breaking in polarization space using a metasurface with anisotropic absorption, whose building blocks consist of two orthogonally orientated meta-atoms with the same resonant frequency but different loss coefficients. By varying their coupling strength, we directly observe a phase transition in the eigenpolarization states of the system, across which the long axis of the eigenpolarization ellipses experience a sudden rotation of 45°. Despite the lack of rotational symmetry of the metasurface, precisely at the phase transition, known as the exceptional point, the eigenmodes coalesce into a single circularly polarized state. The PT symmetric metasurfaces are experimentally implemented at terahertz frequencies.