Metasurfaces based on subwavelength resonators enable novel ways to manipulate the flow of light at optical interfaces. In pursuit of multifunctional or reconfigurable metadevices, efficient tuning of macroscopic performance with little structural/material variation remains a challenge. Here, we put forward the concept of non-Hermitian doping in metasurfaces, showing that an ordinary retroreflector can be switched to asymmetric one by introducing absorptive defects in local regions. The asymmetric absorption performance begins with zero at the Hermitian state, gradually increases under non-Hermitian doping, and reaches the maximum of unity at the exceptional point. This effect is experimentally demonstrated at microwave frequencies via the observation of asymmetric near-field distribution and far-field scattering properties and from a planar metasurface. Furthermore, while importing local gain constituents in conventional retroreflector, it can be tuned to an extremely asymmetric one-side amplifier, extending unidirectional amplification from one-dimensional waveguide to two-dimensional scattering systems. The proposed methodology provides an alternative pathway for engineering electromagnetic metadevices and systems with small perturbations. Introduction
We show that a cylindrical lensing system composed of two metasurfaces with suitably tailored non-Hermitian (i.e., with distributed gain and loss) and nonlocal (i.e., spatially dispersive) properties can perform magnified imaging with reduced aberrations. More specifically, we analytically derive the idealized surface-impedance values that are required for "perfect" magnification and imaging, and elucidate the role and implications of non-Hermiticity and nonlocality in terms of spatial resolution and practical implementation. For a basic demonstration, we explore some proof-of-principle quasi-…
Non-Hermitian degeneracies, also known as exceptional points (EPs), have been the focus of much attention due to their singular eigenvalue surface structure. Nevertheless, as pertaining to a non-Hermitian metasurface platform, the reduction of an eigenspace dimensionality at the EP has been investigated mostly in a passive repetitive manner. Here, we propose an electrical and spectral way of resolving chiral EPs and clarifying the consequences of chiral mode collapsing of a non-Hermitian gated graphene metasurface. More specifically, the measured non-Hermitian Jones matrix in parameter space e…
We show that Maxwell's demon-like nonreciprocity can be supported in a class of non-Hermitian gyrotropic metasurfaces in the linear regime. The proposed metasurface functions as a transmission-only Maxwell's demon operating at a pair of photon energies. Based on multiple scattering theory, we construct a dual-dipole model to explain the underlying mechanism that leads to the antisymmetric nonreciprocal transmission. The results may inspire new designs of compact nonreciprocal devices for photonics.
We propose the concept of one-sided quantum interference based on non-Hermitian metasurfaces.By designing bianisotropic metasurfaces with a non-Hermitian exceptional point, we show that quantum interference can exist only on only one side but not another. This is the quantum inheritance of unidirectional zero reflection in classical optics.The one-side interference can be further manipulated with tailor-made metasurface. With two photons simultaneously entering the metasurface from different sides, the probability for only outputting one photon on the side with reflection can be modified to ze…
Electromagnetic metasurfaces enable the advanced control of surface-wave propagation by spatially tailoring the local surface reactance. Interestingly, tailoring the surface resistance distribution in space provides new, largely unexplored degrees of freedom. Here, we show that suitable spatial modulations of the surface resistance between positive (i.e., loss) and negative (i.e., gain) values can induce peculiar dispersion effects, far beyond a mere compensation. Taking inspiration from the parity-time symmetry concept in quantum physics, we put forward and explore a class of non-Hermitian me…
Active Control of Topological Exceptional Points in Non-Hermitian Metasurfaces
Indian Institute of Technology Bombay (IITB)（印度理工学院孟买分校）；Nanyang Technological University（南洋理工大学）；University of Notre Dame（圣母大学）；Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR)（贾瓦哈拉尔·尼赫鲁高级科学研究中心）
Black phosphorus (BP), a relative new plasmonic two-dimensional (2D) material, offers unique photonic and electronic properties. In this work, we propose a new tunable and broadband ultrathin coherent perfect absorber (CPA) device operating in the terahertz (THz) frequency range. It is based on a bifacial metasurface made of BP patch periodic arrays separated by a thin dielectric layer. Broadband CPA bandwidth is realized due to the ultrathin thickness of the proposed device and the extraordinary properties of BP. In addition, a substantial modulation between CPA and complete transparency is a…
Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as a new ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two s…
Resonant structures in modern nanophotonics are non-Hermitian (leaky and lossy), and support quasinormal modes. Moreover, contemporary cavities frequently include 2D materials to exploit and resonantly enhance their nonlinear properties or provide tunability. Such materials add further modeling complexity due to their infinitesimally thin nature and strong dispersion. Here, a formalism for efficiently analyzing third harmonic generation (THG) in nanoparticles and metasurfaces incorporating 2D materials is proposed. It is based on numerically calculating the quasinormal modes in the nanostructu…
Line waves are recently discovered wave entities that are localized along two directions, and therefore can be viewed as the one-dimensional counterpart of surface waves. These waves can be supported at discontinuities of the surface reactance and/or resistance of low-dimensional materials such as metasurfaces or graphene. Here, a broader class of non-Hermitian surface-impedance junctions is studied that can support coupled line waves, and allows investigating different one-dimensional waveguiding mechanisms in a unified framework. It is theoretically demonstrated that, under parity-time-symme…
