Direct quantification of topological protection in symmetry-protected photonic edge states at telecom wavelengths

摘要

a SEM of the fabricated structure with two pseudo-colored regions depicting the two lattices VPC1 and VPC2 with opposite valley-Chern invariants. The unit cell with lattice constant a = 503 nm consists of equilateral triangular holes of side lengths d1 = 0.7a and d2 = 0.45a. Scale bar: 500 nm. b Schematic of the near-field scanning optical microscope used to map the in-plane field distribution of the topologically non-trivial PhC edge mode. To facilitate heterodyne-based phase detection, the input beam is split into two branches, labeled signal and reference. An aperture-based near-field probe collects part of the evanescent tail of the in-plane field components while scanning over the crystal at a height of 20 nm and couples the collected light to a single-mode optical fiber. Inset: SEM image of the probe. c Measured normalized amplitude of the in-plane field components at a laser excitation wavelength of λ = 1600 nm over the extent of 165 unit cells, with the scale bar corresponding to 5 μm. Light is launched from a feed waveguide at the left side of the crystal, with the direction indicated by the red arrow. Left inset: zoomed-in view of the detected field amplitude pattern along the domain wall. Right inset: zoomed-in view of the out-coupling flank of the access waveguide