Topological protection in photonic crystals

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.