Bringing topological physics from condensed matter to the optical domain offers unprecedented prospects in the control of light. Recently, the photonic analogue of the quantum spin Hall effect (QSHE) was proposed in 2D photonic crystal (PhC) structures featuring an interface between two topological distinct domains. Photonic spin-orbit coupling, mediated by the specific lattice symmetries, results in the emergence of helical edge states, guided along the interface in a protected manner. We fabricate and study topological PhC cavities emulating the QSHE that are coupled to the radiation continuum and perform imaging and Fourier spectroscopy in the far field to characterize their properties. We examine the robustness of cavity spectra and intrinsic loss against varying cavity size and shape, and demonstrate pseudo-spin conserved coupling between topological waveguides and cavities. The reliance on only passive media render such components promising building blocks for on-chip devices.
Topological states of light can be induced in nanophotonic systems by encoding spin or valley degrees of freedom in the electromagnetic vector field. We study topological light propagation and storage in waveguides and cavities in two-dimensional photonic crystals at telecom wavelengths, directly imaging their propagation and band structure in experiment. Through phase- and polarization-resolved measurement of the states' electromagnetic fields, we reveal their origin in photonic spin-orbit coupling. Our quantitative measurement techniques allow us to test the level of topological protection in these systems, which rely on spatial symmetries to achieve topological robustnes. We study topological protection of backreflection at sharp corners and defects and discuss the merits of these principles in realistic nanophotonic devices.
Leveraging topological protection in the photonic domain could lead to new ways to transport information on-chip, potentially increasing its robustness to scattering at disorder. We realize a photonic analogue of topological insulators based on the quantum spin Hall effect in symmetry-broken photonic crystals. We directly observe the propagation of topological edge states at telecom wavelengths in a silicon-on-insulator platform. Analyzing their properties through their far-field radiation allows characterizing their inherent spin, dispersion, and propagation. We reveal that the radiation of the topological states carries a signature of their origin in photonic spin-orbit coupling, linking the unidirectional propagation of two states with opposite pseudospin to circular far-field polarization. Polarimetric Fourier spectroscopy allows mapping the edge state dispersion and characterize their quality factors. The positive and negative group velocity modes can be selectively excited with circular polarization of opposite handedness. Moreover, we detect a small gap at the edge state crossing that is related to spin-spin scattering, inherent to the symmetry breaking at the edge, and a defining difference between photonic and electronic topological insulators. We image edge state propagation in real-space microscopy, and show how they can be routed at sharp waveguide junctions, attesting to their topologically protected nature. Thus, we observe the unique nature of topologically protected light transport in photonic crystals, through a technique that holds great promise for developing novel topological systems for various applications, including integrated photonic components, quantum optical interfaces, and nanoscale lasing.
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