Xiang Ni

Robust reconfigurable electromagnetic pathways within a photonic topological insulator

The discovery of topological photonic states has revolutionized our understanding of electromagnetic propagation and scattering. Endowed with topological robustness, photonic edge modes are not reflected from structural imperfections and disordered regions. Here we demonstrate robust propagation along reconfigurable pathways defined by synthetic gauge fields within a topological photonic metacrystal. The flow of microwave radiation in helical edge modes following arbitrary contours of the synthetic gauge field between bianisotropic metacrystal domains is unimpeded. This is demonstrated in measurements of the spectrum of transmission and time delay along the topological domain walls. These results provide a framework for freely steering electromagnetic radiation within photonic structures.

Three-Dimensional All-Dielectric Photonic Topological Insulator

The theoretical study of a 3D photonic topological metacrystal based on an all-dielectric metamaterial platform shows robust propagation of surface states along 2D domain walls, making it a promising solution for photonics applications. The proposed metacrystal design might also open the way for the observation of elusive fundamental physical phenomena.

Spin- and valley-polarized one-way Klein tunneling in photonic topological insulators

We demonstrate a robust pseudospin- and valley-polarized one-way Klein tunneling and topological edge states. Recent advances in condensed matter physics have shown that the spin degree of freedom of electrons can be efficiently exploited in the emergent field of spintronics, offering unique opportunities for efficient data transfer, computing, and storage (1–3). These concepts have been inspiring analogous approaches in photonics, where the manipulation of an artificially engineered pseudospin degree of freedom can be enabled by synthetic gauge fields acting on light (4–6). The ability to control these degrees of freedom significantly expands the landscape of available optical responses, which may revolutionize optical computing and the basic means of controlling light in photonic devices across the entire electromagnetic spectrum. We demonstrate a new class of photonic systems, described by effective Hamiltonians in which competing synthetic gauge fields, engineered in pseudospin, chirality/sublattice, and valley subspaces, result in bandgap opening at one of the valleys, whereas the other valley exhibits Dirac-like conical dispersion. We show that this effective response has marked implications on photon transport, among which are as follows: (i) a robust pseudospin- and valley-polarized one-way Klein tunneling and (ii) topological edge states that coexist within the Dirac continuum for opposite valley and pseudospin polarizations. These phenomena offer new ways to control light in photonics, in particular, for on-chip optical isolation, filtering, and wave-division multiplexing by selective action on their pseudospin and valley degrees of freedom.

Far-field probing of leaky topological states in all-dielectric metasurfaces

Topological phase transitions in condensed matter systems give rise to exotic states of matter such as topological insulators, superconductors, and superfluids. Photonic topological systems open a whole new realm of research and technological opportunities, exhibiting a number of important distinctions from their condensed matter counterparts. Photonic modes can leak into free space, which makes it possible to probe topological photonic phases by spectroscopic means via Fano resonances. Based on this idea, we develop a technique to retrieve the topological properties of all-dielectric metasurfaces from the measured far-field scattering characteristics. Collected angle-resolved spectra provide the momentum-dependent frequencies and lifetimes of the photonic modes that enable the retrieval of the effective Hamiltonian and extraction of the topological invariant. Our results demonstrate how the topological states of open non-Hermitian systems can be explored via far-field measurements, thus paving a way to the design of metasurfaces with unique scattering characteristics controlled via topological effects.Topological modes in photonics systems are not completely confined to the structure but can leak into free space. Here, Gorlach et al. exploit these leaky modes to probe the topological properties of a dielectric metasurface from far-field scattering measurements.

Pseudo-spin–valley coupled edge states in a photonic topological insulator

Pseudo-spin and valley degrees of freedom engineered in photonic analogues of topological insulators provide potential approaches to optical encoding and robust signal transport. Here we observe a ballistic edge state whose spin–valley indices are locked to the direction of propagation along the interface between a valley photonic crystal and a metacrystal emulating the quantum spin–Hall effect. We demonstrate the inhibition of inter-valley scattering at a Y-junction formed at the interfaces between photonic topological insulators carrying different spin–valley Chern numbers. These results open up the possibility of using the valley degree of freedom to control the flow of optical signals in 2D structures.Valleys in the photonic band structure provide an additional degree of freedom to engineer topological photonic structures and devices. Here, Kang et al. demonstrate that inter-valley scattering is inhibited at a Y-junction between three sections with different valley topology.

Spectroscopy of topological photonic states in dielectric metasurfaces

We investigate photonic topological states supported by the metasurface based on the triangular lattice of hexamers. It is demonstrated that the transmittance and reflectance spectra recorded for such structure allow one to extract the information about the topological properties of the metasurface photonic modes and the topological invariant in particular. Our method thus provides a technique complementary to the analysis of edge states.

Edge modes of 2D photonic crystals with and without PT symmetry(Conference Presentation)

Bulk spectrum and edge modes of 2D photonic crystals with parity and time-reversal symmetries broken in a different way are investigated. It is shown that for specific values of parameter of the symmetry reduction the bulk modes exhibit a peculiar one-way Dirac-like dispersion. The domain wall formed by two crystals with the symmetry reduction parameter reversed is shown to exhibit an edge mode which coexists with the one-way bulk Dirac regime. In addition, we demonstrate that parity-time symmetric interfaces between photonic crystals with gain and loss support a new class of dissipation-less surface modes.