Topological photonics leverages the principles of topological insulators to create robust light propagation modes immune to disorder and defects. Recent breakthroughs include the experimental realization of topological edge states in photonic crystals with a group index exceeding 10^3, enabling ultra-slow light propagation. These systems exhibit unidirectional transport with transmission efficiencies above 99%, even in the presence of sharp bends or imperfections. Such robustness is quantified by the Chern number, a topological invariant that ensures the existence of edge states in these systems.
Non-Hermitian photonics explores systems with gain and loss, leading to exotic phenomena like parity-time (PT) symmetry breaking. In 2023, researchers demonstrated PT-symmetric lasers with threshold gains as low as 0.1 cm^-1, achieving lasing efficiencies of over 80%. These systems exhibit exceptional points (EPs), where eigenvalues and eigenvectors coalesce, enabling ultra-sensitive sensors with detection limits down to 10^-9 refractive index units (RIU). Such sensitivity surpasses traditional plasmonic sensors by two orders of magnitude.
The integration of non-Hermitian physics with topological photonics has led to the discovery of hybrid modes with unprecedented properties. For instance, hybrid topological-non-Hermitian waveguides exhibit lossless propagation over distances exceeding 1 km, despite intrinsic material losses of 1 dB/cm. These modes are characterized by complex Berry phases, which govern their unique dispersion relations and stability against perturbations. Such systems are promising for long-haul optical communication and quantum information processing.
Recent advancements in fabrication techniques have enabled the realization of three-dimensional (3D) topological photonic crystals operating at visible wavelengths. These structures exhibit Weyl points—topological singularities in momentum space—with group velocities as high as 0.8c (c being the speed of light). The ability to engineer these points in real materials opens avenues for compact optical isolators and circulators with insertion losses below 0.5 dB.
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