The insensitivity of photons to magnetic fields calls for symmetry-based approaches in the design of photonic topological insulators. We experimentally demonstrate the realization of pseudomagnetic fields in deformed photonic crystals, and investigate the resulting photonic Landau-levels and guided topological states. Akin to strained graphene sheets, deformations (i.e., designed strain) in photonic crystals generate synthetic gauge fields for photons, leading to localization at flat bands and new types of topologically protected edge states at the boundaries of strained photonic crystals. We reveal that tailoring these gauge fields via strain-engineering yields new control over light dispersion, localization, and allows broadband minimization of radiation losses.
Topologically tailored photonic crystals offer robust transport of optical states in quantum and classical systems. However, quantifying the robustness of edge states in topologically protected PhCs has remained elusive. In our recent work, we report a rigorous quantitative evaluation of topological photonic edge eigenstates, emulating the quantum valley Hall effect (VPC), and analyze their transport properties in the telecom wavelength range using a phase-resolved near-field optical microscope. Our results demonstrate that the backscattering energy ratio for the VPC is two orders of magnitude smaller compared to that in a conventional W1 waveguide. Such an evaluation opens a pathway for creating quantum photonic networks that can achieve secure and robust communications.
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.
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