Hai-Xu Cui

Asymmetric transmission for linearly polarized electromagnetic radiation

Metamaterials have shown to support the intriguing phenomenon of asymmetric electromagnetic transmission in the opposite propagation directions, for both circular and linear polarizations. In the present article, we propose a criterion on the relationship among the elements of transmission matrix, which allows asymmetrical transmission for linearly polarized electromagnetic radiation only while the reciprocal transmission for circularly one. Asymmetric hybridized metamaterials are shown to satisfy this criterion. The influence from the rotation of the sample around the radiation propagation direction is discussed. A special structure design is proposed, and its characteristics are analyzed by using numerical simulation.

Angular diffraction of an optical vortex induced by the Gouy phase

We investigate the diffraction of an optical vortex, when half of the beam is blocked by a knife edge. The Gouy phase is shown to influence the diffraction pattern strongly, in particular leading to the emergence of distorted and deformed diffraction fringes in the azimuthal direction. In analogy with single-slit diffraction, this phenomenon is analyzed in terms of angular diffraction within the azimuthal degree of freedom, by considering the uncertainty principle for the angular position and the angular momentum. Numerical simulations based on the fast-Fourier algorithm and the angular momentum expansion fit well with the experimental results.

Fano–Feshbach resonance in structural symmetry broken metamaterials

Metamaterials support optically excitable dark-plasmon modes featured by antisymmetric surface current oscillations, which can be explained by Fano-type resonance and can be tailored by controlling the embedded structural geometry. In this article, we numerically investigate the Fano-type resonance in complex metamaterials, and demonstrate the presence of Fano–Feshbach resonances due to the interaction between two Fano-type resonances in the overlapping region, implemented by breaking and tuning the symmetric properties of the resonant metallic element. Features of the resonance are discussed. This work shows that the domain of dark-plasmon mode based metamaterial system supports rich physics and can lead to various potential applications.

Circular polarizer via selective excitation of photonic angular momentum states in metamaterials

We propose a scheme in realizing a compact circular polarizer by using a metamaterial with a properly designed input meta-surface. This scheme is based on the selective excitation of photonic angular momentum states in a coaxial element array by the symmetry-broken input meta-surface. The topological charge m of the excited photonic angular momentum state in the coaxial element determines the handedness of the transmitted light via the orbit-spin conversion of the angular momentum of light. A net circular-polarization generation efficiency of 64.5% for a linearly polarized incidence is demonstrated.

Exceptional points in extraordinary optical transmission through dual subwavelength metallic gratings

Complex metamaterials with multiple optical resonances in constituent elements possess many similarities with open quantum systems that can be described by non-Hermitian Hamiltonian. By analogy with a two-state open quantum system, we show that a classic analogue of exceptional points can be observed in the transmission spectra of dual subwavelength metallic gratings. Anti-crossing (crossing) between the two branches λ(R) of extraordinary optical transmission, with crossing (anti-crossing) of the corresponding widths Γ(R), is observed in the parameter space spanned by the lateral displacement L and the angle of incidence φ0. Exchanges of field patterns and phases, and the variation of field profile when circling the exceptional point are discussed. This work highlights the potential to transfer the concepts and applications from open quantum systems to optical metamaterials.

Slow light from sharp dispersion by exciting dark photonic angular momentum states.

A photonic angular momentum state (PAMS) with a topological charge of m≠±1 is dipole forbidden at all polarizations of free-space incidence due to the existence of a unique helical phase. We show that by indirectly exciting dark PAMSs through coupling with a bright resonant element, a sharply variant transmission behavior and strong dispersion can be achieved. This behavior can subsequently be utilized in slow light. A metamaterial design, in which a group index n(g) greater than 500 can be achieved, is present.

Nonreciprocal optical diffraction by a single layer of gyromagnetic cylinders

We study the diffraction of optical waves by a single layer of gyromagnetic cylinders. We show that a nonvanishing rotating dipole momentum is excited in a single gyromagnetic cylinder because of the classic analog of the Zeeman effect on photonic angular momentum states (PAMSs). Consequently, different collective dipole modes are excited in a gyromagnetic cylinder array at opposite incident angles. Nonreciprocal optical diffraction effects can be observed, where the transmission and reflection coefficients depend on the sign of the incident angle. A novel phenomenon of nonreciprocal negative directional transmission is demonstrated and numerically analyzed. This work highlights the potential of PAMSs in manipulating the propagation of optical waves for various applications.

Manipulation of dark photonic angular momentum states via magneto-optical effect for tunable slow-light performance

We propose a novel scheme in realizing tunable slow-light performance by manipulating dark photonic angular momentum states (PAMSs) in metamaterials via the magneto-optical effect. We show that by applying a static magnetic field B, some pairs of sharp transmission dips can be observed in the background transparency window of a complex metamaterial design. Each pair of transmission dips are related to the excitation of dark PAMSs with opposite topological charges -m and +m, with a lifted degeneracy due to the classic analogue of Zeeman effect. Nonreciprocal characteristics can be observed in the distributions of field amplitude and transverse energy flux. The performance of slow light, including the group index ng, its abnormal feature, the associated strong absorption and the dependence with B are also discussed.

Guided modes in magneto-optical waveguides and the role in resonant transmission

Magneto-optical (MO) effect can break the reciprocal propagation of an optical wave along a MO-metal interface. We show that this nonreciprocal property also influences the guided modes in metal-MO-metal waveguides. Especially, the field profiles of the guided modes are neither symmetric nor anti-symmetric, but asymmetric. We then study the resonant optical transmission through a thin metal film with subwavelength MO slits. Magnetic field changes the transmission spectra of the structure, and a MO-induced transparent window is open, where the MO medium becomes extremely anisotropic. The guided-mode mediated high transmission is associated with an asymmetric field distribution and a circling energy flux.

Study on photonic angular momentum states in coaxial magneto-optical waveguides

By rigorously solving Maxwells equations, we develop a full-wave electromagnetic theory for the study of photonic angular momentum states (PAMSs) in coaxial magneto-optical (MO) waveguides. Paying attention to a metal-MO-metal coaxial configuration, we show that the dispersion curves of the originally degenerated PAMSs experience a splitting, which are determined by the off-diagonal permittivity tensor element of the MO medium. We emphasize that this broken degeneracy in dispersion relation is accompanied by modified distributions of field component and transverse energy flux. A qualitative analysis about the connection between the split dispersion behavior and the field distribution is provided. Potential applications are discussed.