Unidirectional reflectionless propagation in a non-ideal parity-time metasurface based on far field coupling

A liquid crystal based non-Hermitian metasurface for broadband full-Stokes polarization detection

The ability to detect the polarization information of light is often crucial for various applications in optical systems. However, conventional polarization-sensitive photodetectors struggle to simultaneously achieve a wide band coverage and high-precision detection, severely hindering the development of polarization detectors. In this study, a reflective metasurface with full-Stokes detection capabilities over a wide range is proposed. It integrates four linear polarization filters and two circular polarization filters operating in the near-infrared region. By dynamically adjusting the refractive index of the liquid crystal covering the detector surface, high performance full-Stokes parameter detection can be achieved between 730-770 nm with detection error below 0.07. Therefore, this study provides a design approach for the potential application of Stokes polarization detection over a broadband spectrum.

Nonreciprocal reflection based on asymmetric graphene metasurfaces

We propose a scheme to achieve controllable nonreciprocal behavior in asymmetric graphene metasurfaces composed of a continuous graphene sheet and a poly crystalline silicon slab with periodic grooves of varying depths on each side. The proposed structure exhibits completely asymmetric reflection in opposite directions in the near-infrared range, which is attributed to the pronounced structural asymmetry and its accompanying nonlinear effects. The obtained nonreciprocal reflection ratio, reaching an impressive value of 21.27 dB, combined with a minimal insertion loss of just -0.76 dB, highlights the remarkable level of nonreciprocal efficiency achieved by this design compared to others in its category. More importantly, the proposed design can achieve dynamic tunability by controlling the incident field intensity and the graphene Fermi level. Our design highlights a potential means for creating miniaturized and integratable nonreciprocal optical components in reflection mode, which can promote the development of the integrated isolators, optical logic circuits, and bias-free nonreciprocal photonics.

Full-space and multi-polarization holograms realized by a non-Hermitian bilayer metasurface

Multilayer metasurfaces break the mirror symmetry along the path of light propagation, thereby increasing the potential for light manipulation. Herein, a paradigm is proposed that building a non-Hermitian bilayer metasurface, which is composed of two identical, orthogonally oriented, chiral J-shaped Au structures in each layer, allows exceptional points (EPs) to exist in full-space. Specifically, in the reflected half-space that adheres to mirror symmetry, the circularly polarized eigenstates coalesce at the EP, while in the transmission half-space, where mirror symmetry is broken, the linearly polarized eigenstates converge at the EP. By considering the intrinsic property of topologically protected 2π-phase accumulation encircling both EPs, we investigated full-space holography through using circularly polarized light (in reflection half-space) and linearly polarized light (in transmission half-space).

Study of a High-Index Dielectric Non-Hermitian Metasurface and Its Application in Holograms

We demonstrate that a high-index dielectric Si metasurface with a designed chiral unit structure possesses an exceptional point (EP) when it is described by a non-Hermitian Hamiltonian associated with the transmission matrix. By encircling any path in the parameter space around the EP, topologically protected 2π-phase accumulation occurs. These typical non-Hermitian properties are ascribed to complex scattering phenomena related to the coupling between electric and magnetic dipolar modes from the high-index dielectric Si metasurface. The topologically guaranteed entire 2π-phase accumulation and chiral distinction around the EP open up many promising possibilities in nanophotonic device designing; for instance, phase-only and polarization multiplexing holograms are realized in this work.

Unidirectional Invisibility in PT-Symmetric Cantor Photonic Crystals

In this paper, we investigate the nonreciprocity of reflection in parity-time−symmetric (PT-symmetric) Cantor photonic crystals (PCs). Two one-dimensional PCs abiding by the Cantor sequence are PT-symmetric about the center. The PT symmetry and defect cavities in Cantor PCs can induce optical fractal states which are transmission modes. Subsequently, the left and right reflectionless states are located on both sides of a transmission peak. The invisible effect depends on the incident direction and the invisible wavelength can be modulated by the gain–loss factor. This study has potential applications in tunable optical reflectors and invisible cloaks.

Dual-frequency unidirectional reflectionless propagation in a non-Hermitian graphene plasmonic waveguide-cavity coupling system

We theoretically investigate a controllable dual-frequency unidirectional reflectionlessness at exceptional points by applying external voltage in a graphene plasmonic waveguide system. The system consists of a graphene waveguide and two end-coupled resonators. COMSOL simulation results show that the reflection of edge fundamental graphene surface plasmon polaritons mode for forward (backward) incidence is near to zero at frequency 24.418 THz (20.865 THz), while that for backward (forward) incidence is 24.71% (22.945%), respectively. In addition, the non-Hermitian scattering matrix is proposed to verify the existence of double exceptional points, and the tunable unidirectional reflectionless phenomenon is also achieved by changing the Fermi level (E_f) of graphene.

Theoretical investigation of a controlled unidirectional reflectionlessness by applying external voltage in an electro-optical plasmonic waveguide system

We theoretically investigate an controlled unidirectional reflectionlessness and near perfect absorption by applying external voltage in an electro-optical plasmonic waveguide system based on near-field coupling between two resonators. The system consists of two resonators side coupled to a metal-dielectric-metal plasmonic waveguide. Based on the numerical simulation, when external voltage is U = 7.4 V, the reflections for forward and backward directions are close to 0 and 0.82 at frequency 144.18 THz, while the reflections for forward and backward directions are close to 0.81 and 0 at frequency 150.86 THz when external voltage is U = 1.5 V. And the high absorption for forward (backward) direction is ∼0.97 (∼0.99) at frequency 144.18 THz (150.86 THz).

Manipulating the critical gain level of spectral singularity in active hybridized metamaterials

In this paper, we investigate the spectral singularity in an active hybridized metamaterial, which manifests itself as ultra-high transmission and reflection at the same frequency in the far-field. A transmission line combined with lumped element model is utilized to describe the proposed active metamaterial. With this model, we reveal that the critical gain level for triggering the spectral singularity is related to the coupling strength between different components of the system. Through optimizing the coupling coefficients between different components, we demonstrate the spectral singularity of the proposed structure at very low gain level, which can bring active metamaterials systems a step closer to their practical implementation. Furthermore, we demonstrate rapid switching between two spectral singularities at different frequencies in the same structure by adding or reducing small amount of gain. The exotic properties of the proposed sub-wavelength structure promise applications in switching, sensing, spaser and nonlinear optics areas.

Highly-dispersive unidirectional reflectionless phenomenon based on high-order plasmon resonance in metamaterials

In this work, we design a structure of metamaterials that consists of double sliver-ring resonators, in which highly-dispersive unidirectional reflectionlessness and absorption are achieved based on high-order plasmon resonance. Reflections of +z and -z directions at 461.34 THz (456.68 THz) are ∼0 (0.82) and ∼0.85 (0) when the distance d=222.9 nm (259.8 nm), respectively. High absorption of ∼0.97 and the quality factor of ∼435 can be obtained in the loss metal structure at room temperature. What's more, unidirectional reflectionlessness is investigated at low temperature.

Dual-band unidirectional reflectionlessness at exceptional points in a plasmonic waveguide system based on near-field coupling between two resonators

Dual-band unidirectional reflectionlessness at exceptional points is investigated theoretically in a non-Hermitian plasmonic waveguide system, based on near-field coupling by using only two resonators. The system consists of a metal-insulator-metal waveguide end-coupled to two nanohole resonators. The reflectivity for the forward (backward) direction is ∼0 (∼0) at frequency 205.20 THz (194.56 THz), while for the backward (forward) direction it is ∼0.76 (∼0.78). Moreover, the quality factors of the dual-band unidirectional reflectionlessness for forward and backward directions can reach ∼132 and ∼137, respectively.

Dual-Band Unidirectional Reflectionless Propagation in Metamaterial Based on Two Circular-Hole Resonators

Dual-band unidirectional reflectionless propagation at two exceptional points is investigated in metamaterial, which is composed of only two gold resonators with circular holes, by simply manipulating the angle of incident wave and distance between two resonators. Furthermore, the dual-band unidirectional reflectionless propagation can be realized in the wide ranges of incident angle from 0∘ to 50∘ and distance from 255 nm to 355 nm between two resonators. In addition, our scheme is insensitive to polarization of incident wave due to the circular-hole structure of the resonators.

Unidirectional reflectionless phenomena in a non-Hermitian quantum system of quantum dots coupled to a plasmonic waveguide

Unidirectional reflectionless phenomena are investigated theoretically in a non-Hermitian quantum system composed of several quantum dots and a plasmonic waveguide. By adjusting the phase shifts between quantum dots, single- and dual-band unidirectional reflectionlessnesses are realized at exceptional points based on two and three quantum dots coupled to a plasmonic waveguide, respectively. In addition, single- and dual-band unidirectional perfect absorptions with high quality factors are obtained at the vicinity of exceptional points.

Enhanced asymmetric transmissions attributed to the cavity coupling hybrid resonance in a continuous omega-shaped metamaterial

In this paper, the infinite-length metallic bar is folded to a continuous omega-shaped resonator and then arranged as a bi-layer metamaterial, which presents a hybrid resonance and a Fabry-Perot-like cavity mode. The asymmetric transmission (AT) for linearly polarized light is powerfully enhanced at a near-infrared regime by strongly coupling the hybrid resonance to the cavity, with the maximum value of the high-efficiency AT effect reaching 0.8 at around 1364 nm. At this near-infrared band, such a high-efficiency AT effect has never been realized previously by a bi-layer metamaterial. More importantly, we demonstrate that our design is robust to the misalignments, which greatly decreases the difficulties in sample fabrications. Accordingly, the proposed omega-shaped metamaterial provides potential applications in designing polarization filters, polarization switches, and other nano-devices.

Dual-band unidirectional reflectionless phenomena in an ultracompact non-Hermitian plasmonic waveguide system based on near-field coupling

Dual-band unidirectional reflectionlessness and coherent perfect absorption (CPA) are demonstrated in a non-Hermitian plasmonic waveguide system based on near-field coupling between a single resonator and the resonant modes of two resonators showing an electromagnetically induced-transparency-like (EIT-like) effect. The non-Hermitian plasmonic system consists of three metal-insulator-metal (MIM) resonators coupled to a MIM plasmonic waveguide.