Rui-Peng Guo

Unidirectional Excitation of Radiative-Loss-Free Surface Plasmon Polaritons in PT -Symmetric Systems

We investigate the excitation and propagation of surface plasmon polaritons (SPPs) at a geometrically flat metal-dielectric interface with a parity-time (PT) symmetric modulation on the permittivity ϵ(x) of the dielectric medium. We show that two striking effects can be simultaneously achieved thanks to the nonreciprocal nature of the Bloch modes in the system. First, SPPs can be unidirectionally excited when light is normally incident on the interface. Secondly, the backscattering of SPPs into the far field is suppressed, producing a radiative-loss-free effect on the unidirectional SPPs. As a result, the lifetime and propagation distance of SPPs can be significantly improved. These results show that PT symmetry can be employed as a new approach to designing transformative nanoscale optical devices, such as low-loss plasmonic routers and isolators for efficient optical computation, communication, and information processing.

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.

Coherent-trapped helical mode in parity-time symmetric metamaterials

Coaxial optical subwavelength elements support helical modes Lm with different topological indexes m. Here we propose to couple the two bright L±1 modes with the dark one L0 via a parity-time (PT) symmetric perturbation. We show that the cascading coupled configuration is similar to a three-level atomic system, and supports a special hybridized mode Lc via a classic analog of coherent-population-trapping effect. Resonant frequency of Lc is independent of the PT-symmetric perturbation. Populations in L±1 can be manipulated by tuning the PT-symmetric perturbation, and no population is trapped in L0. Since the L±1 modes are associated with optical waves of opposite circular polarizations, the polarization of transmitted wave is independent of the polarization of incidence but solely determined by the PT-symmetric perturbation. Such an effect can be utilized to manipulate the polarization state of light. Numerical simulation in a well-designed coaxial metamaterial verifies our analysis.

Optical spin-sensitive Zitterbewegung in bianisotropic metamaterials

We present a theoretical analysis on optical spin-sensitive Zitterbewegung (ZB) in metamaterials. By developing some formulas about the dispersions and eigenstates of optical modes we show that spin-sensitive ZB can be obtained in a bianisotropic metamaterial with a proper coupling between the electric and magnetic responses. A close analogue of the developed analytical results with these of Dirac equation is proposed. Numerical simulation proves the existence of ZB on the refracted optical beam along a direction determined by the optical spin of incidence. Furthermore, we show that when the incident optical field is linearly polarized, although ZB on field intensity does not exist, the optical spin possesses an interesting spatial split and trembling phenomena. Significance of this investigation is discussed.

Anomalous amplified and bound-state-like optical transmissions via unidirectional interaction in parity-time symmetric metamaterials

We develop a coupled-mode theory on the optical transmission in parity-time ( PT) symmetric coaxial metamaterials. Modeled by coupled lossy Lorentzian oscillators, the theory provides a good fit to numerical full-wave simulation. In the scenario of unidirectional coupling, two polarization-sensitive anomalies are obtained: an amplified transmission and an ultra-narrow one analogous to bound states in continuum. We argue that these phenomena are associated with either a unidirectional-field-transfer process or an indirect unidirectional-field-trapping resonance. The broadening effect is shown to determine the magnitude and polarization of the transmission. Our theory and analysis provide a deep understanding on the importance of PT symmetry and dark helical modes and would contribute to applications such as light storage, field amplification, and even lasing.

Localization and oscillation of optical beams in Moiré lattices

We study the propagation of optical beams in two-dimensional Moiré lattices, and demonstrate position-dependent beam dynamics when a quasi-Bragg condition is satisfied. We show that when the optical beam is incident to a peak of the lattice envelop, an optical Zitterbewegung is obtained. If the optical beam is incident to a node of the envelop, a field localization effect takes place. The localized beam oscillates with a much larger spatial period than that of the optical Zitterbewegung. Variation of the oscillation period versus the split in periods is discussed. The position-dependent beam dynamics are explained by the excitation of proper bandedge eigenmodes of the Moiré lattice, and can be engineered via tuning the periods of the two superimposed Bragg lattices.

A photonic analog of Möbius strips using coupled optical ring resonators

A Mobius strip has an intriguing topological property in that it only has one non-orientable side. Here we propose to utilize coupled optical ring resonators (ORRs) to simulate the topological effect of Mobius strips. This scheme is based on the fact that the counter-clockwise mode in an ORR only couples to the clockwise mode of an adjacent ORR. We show that if an odd number of ORRs form a closed loop, after a round trip the handedness of the excited mode does not return to the initial one. Only after a double round trip does the mode come back to its initial state. Such a kind of Mobius-type coupling topology can be observed from the strong backward reflection in a common bus that provides the initial excitation. Eigenmodes, reflection and transmission spectra, and field distributions are calculated and analyzed. We also study the situation without Mobius-type coupling. The difference between these two categories is discussed. COMSOL simulations verify our analysis. The importance of this investigation and potential applications are briefly discussed.

Oblique-incidence-induced phase transition in parity-time symmetric optical bilayers

We investigate the scattering of optical waves from one-dimensional parity-time () symmetric structures of arbitrary complexity. We prove that it is feasible to observe the transition from an unbroken -symmetric phase to a broken one when the angle of incidence increases. Analytical formulae are developed that emphasize the key roles played by the interference between the exponential increase and decayed fields in the gain and loss media, and the phase shifts when reflected/transmitted from the gain-loss boundary. A specific layer thickness is proposed. Distributions of field and energy flux under both phases are studied. This investigation shows that we can observe optical -symmetric phase transition in a single structure that obeys the causality principle.

Study on Zeeman-split spoof surface plasmon polaritons by use of spin-sensitive enhanced electromagnetic transmission

Structured metal surfaces could support spoof surface plasmon polaritons (SPPs), the dispersion of which is determined by the cutoff condition of guided modes in the nanostructures. We show that we can achieve split spoof SPPs by breaking the degeneracy of guided helical modes in concentric nanostructures via the classic analogue of the Zeeman effect. This split effect is shown to be observable from the spectra of enhanced electromagnetic transmission. Spin-sensitive enhanced electromagnetic transmission and the associated characteristics of field are investigated. Transmission branches versus parallel wavevector can be satisfactorily fitted by using the dispersion of spoof SPPs.

Role of photonic angular momentum states in nonreciprocal diffraction from magneto-optical cylinder arrays

Optical eigenstates in a concentrically symmetric resonator are photonic angular momentum states (PAMSs) with quantized optical orbital angular momentums (OAMs). Nonreciprocal optical phenomena can be obtained if we lift the degeneracy of PAMSs. In this article, we provide a comprehensive study of nonreciprocal optical diffraction of various orders from a magneto-optical cylinder array. We show that nonreciprocal diffraction can be obtained only for these nonzero orders. Role of PAMSs, the excitation of which is sensitive to the directions of incidence, applied magnetic field, and arrangement of the cylinders, are studied. Some interesting phenomena such as a dispersionless quasi-omnidirectional nonreciprocal diffraction and spikes associated with high-OAM PAMSs are present and discussed.