Kueifu Lai

Topologically protected refraction of robust kink states in valley photonic crystals

A photonic crystal can realize an analogue of a valley Hall insulator, promising more flexibility than in condensed-matter systems to explore these exotic topological states. Recently discovered1,2 valley photonic crystals (VPCs) mimic many of the unusual properties of two-dimensional (2D) gapped valleytronic materials3,4,5,6,7,8,9. Of the utmost interest to optical communications is their ability to support topologically protected chiral edge (kink) states3,4,5,6,7,8,9 at the internal domain wall between two VPCs with opposite valley-Chern indices. Here we experimentally demonstrate valley-polarized kink states with polarization multiplexing in VPCs, designed from a spin-compatible four-band model. When the valley pseudospin is conserved, we show that the kink states exhibit nearly perfect out-coupling efficiency into directional beams, through the intersection between the internal domain wall and the external edge separating the VPCs from ambient space. The out-coupling behaviour remains topologically protected even when we break the spin-like polarization degree of freedom (DOF), by introducing an effective spin–orbit coupling in one of the VPC domains. This also constitutes the first realization of spin–valley locking for topological valley transport.

Experimental Demonstration of Phase Modulation and Motion Sensing Using Graphene-Integrated Metasurfaces

Strong interaction of graphene with light accounts for one of its most remarkable properties: the ability to absorb 2.3% of the incident lights energy within a single atomic layer. Free carrier injection via field-effect gating can dramatically vary the optical properties of graphene, thereby enabling fast graphene-based modulators of the light intensity. However, the very thinness of graphene makes it difficult to modulate the other fundamental property of the light wave: its optical phase. Here we demonstrate that considerable phase control can be achieved by integrating a single-layer graphene (SLG) with a resonant plasmonic metasurface that contains nanoscale gaps. By concentrating the light intensity inside of the nanogaps, the metasurface dramatically increases the coupling of light to the SLG and enables control of the phase of the reflected mid-infrared light by as much as 55° via field-effect gating. We experimentally demonstrate graphene-based phase modulators that maintain the amplitude of the reflected light essentially constant over most of the phase tuning range. Rapid nonmechanical phase modulation enables a new experimental technique, graphene-based laser interferometry, which we use to demonstrate motion detection with nanoscale precision. We also demonstrate that by the judicious choice of a strongly anisotropic metasurface the graphene-controlled phase shift of light can be rendered polarization-dependent. Using the experimentally measured phases for the two orthogonal polarizations, we demonstrate that the polarization state of the reflected light can be by modulated by carrier injection into the SLG. These results pave the way for novel high-speed graphene-based optical devices and sensors such as polarimeters, ellipsometers, and frequency modulators.

Ultrathin gradient nonlinear metasurface with a giant nonlinear response

Gradient metasurfaces have recently been demonstrated to provide control of the phase of scattered fields over subwavelength scales, enabling a broad range of linear optical components in a flat, ultrathin, integrable platform. Additionally, the development of nonlinearmetasurfaces has disrupted conventional nonlinear optical device design by relaxing phase matching constraints, reducing size and dimensionality, and providing record values of localized nonlinear responses. However, extending the “flat optics” paradigm to the nonlinear case faces important challenges, since we are required to simultaneously achieve efficient frequency conversion and sub-diffractive phase control. Here, we experimentally demonstrate continuous phase control of the giant nonlinear second harmonic optical response from metasurfaces tied to intersubband transitions in semiconductor multi-quantum wells, establishing an exciting path toward realizing the vision of flat, nonlinear optics.

Experimental Realization of a Reflections-Free Compact Delay Line Based on a Photonic Topological Insulator.

Photonic topological insulator emulating quantum spin Hall effect is employed to demonstrate the first reflections-free topologically protected delay line. Large broadband time delays of the guided edge modes are measured, compared with narrow-band non-topological ones.

Exciting reflectionless unidirectional edge modes in a reciprocal photonic topological insulator medium

Photonic topological insulators are an interesting class of materials whose photonic band structure can have a band gap in the bulk while supporting topologically protected unidirectional edge modes. Recent studies on bianisotropic metamaterials that emulate the electronic quantum spin Hall effect using its electromagnetic analog are examples of such systems with a relatively simple and elegant design. In this paper, we present a rotating magnetic dipole antenna, composed of two perpendicularly oriented coils, that can efficiently excite the unidirectional topologically protected surface waves in the bianisotropic metawaveguide (BMW) structure recently realized by T. Ma et al. [Phys. Rev. Lett. 114, 127401 (2015)] despite the fact that the BMW medium does not break time-reversal invariance. In addition to achieving a high directivity, the antenna can be tuned continuously to excite reflectionless edge modes in the two opposite directions at various amplitude ratios. We demonstrate its performance through experiments and compare the results to simulation results.

Cherenkov radiation in a surface wave accelerator based on silicon carbide (SWABSiC)

We report on theoretical investigations of Cherenkov-type emission of surface phonon polaritons (SPhPs) by relativistic electron bunches. The polaritons are confined by a planar waveguide comprised of two SiC slabs separated by a vacuum gap. The SPhPs are generated in the reststrahlen band, where the dielectric permittivity of SiC is negative. Two surface modes are analyzed: the accelerating (symmetric) and the deflecting (anti-symmetric) wakes. Both form Cherenkov cones that exhibit rapid spatial oscillations and beats behind the moving charge. Moreover, the accelerating mode forms a reversed Cherenkov radiation cone due the negative group velocity for sufficiently small gaps. The wakefield acceleration of electron bunches inside the structure is also discussed, as well as our recent experimental progress in propagating the electron beam through the structure at the Advanced Test Facility (ATF) that resulted in > 12% beam transmission.

Guiding electromagnetic waves around sharp corners: topologically protected photonic transport in meta-waveguides (Presentation Recording)

Science thrives on analogies, and a considerable number of inventions and discoveries have been made by pursuing an unexpected connection to a very different field of inquiry. For example, photonic crystals have been referred to as “semiconductors of light” because of the far-reaching analogies between electron propagation in a crystal lattice and light propagation in a periodically modulated photonic environment. However, two aspects of electron behavior, its spin and helicity, escaped emulation by photonic systems until recent invention of photonic topological insulators (PTIs). The impetus for these developments in photonics came from the discovery of topologically nontrivial phases in condensed matter physics enabling edge states immune to scattering. The realization of topologically protected transport in photonics would circumvent a fundamental limitation imposed by the wave equation: inability of reflections-free light propagation along sharply bent pathway. Topologically protected electromagnetic states could be used for transporting photons without any scattering, potentially underpinning new revolutionary concepts in applied science and engineering. I will demonstrate that a PTI can be constructed by applying three types of perturbations: (a) finite bianisotropy, (b) gyromagnetic inclusion breaking the time-reversal (T) symmetry, and (c) asymmetric rods breaking the parity (P) symmetry. We will experimentally demonstrate (i) the existence of the full topological bandgap in a bianisotropic, and (ii) the reflectionless nature of wave propagation along the interface between two PTIs with opposite signs of the bianisotropy.