Metasurfaces have attracted large interest in recent years due to their relatively simple fabrication, compact design, and ability to control the wavefront of incident light. Ohmic loss attributed to bulk metal metamaterials are not a primary issue, whereby the meta-atom or plasmonic structure is typically only as thin as a fraction of the operation wavelength. Numerous novel applications have been demonstrated by metasurfaces, including an ultrathin metasurface flat lens, and 3D holography.Here, by combining the freedom of both the structural design and the orientation of split ring resonator antennas, we demonstrate Terahertz metasurfaces that are capable of controlling both the phase and amplitude profiles over a very broad bandwidth at~1THz under linearly-polarised incidence. As an example, we show that these phase-amplitude metasurfaces can be engineered to control the diffraction orders arbitrarily.

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

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.

Metamaterials offer exciting opportunities that enable precise control of light propagation, its intensity and phase by designing an artificial medium of choice. Inducing birefringence via engineered metamolecules presents a fascinating mechanism to manipulate the phase of electromagnetic waves and facilitates the design of polarimetric devices. In this paper, a high‐efficiency, broadband, tunable and flexible quarter‐wave plate based on a multilayer metamaterial is presented. Excellent achromatic π/2 phase retardance with high transmission is observed upon terahertz propagation through the quarter‐wave plate. The calculated Stokes parameter represents the output polarization state numerically, indicating an excellent broadband conversion of linearly polarized light into circularly polarized light. The metamaterial‐based quarter‐wave plate demonstrated in this work could be an important step forward in the development of functional terahertz polarization conversion devices for practical applications.

A toroidal dipole in metasurfaces provides an alternate approach for the excitation of high-Q resonances. In contrast to conventional multipoles, the toroidal dipole interaction strength depends on the time derivative of the surrounding electric field. A characteristic feature of toroidal dipoles is tightly confined loops of oscillating magnetic field that curl around the fictitious arrow of the toroidal dipole vector.