Near-field mapping of the edge mode of a topological valley slab waveguide at λ = 1.55 μ m

Abstract

Valley polarized topological states of light allow for robust waveguiding which has been demonstrated for transverse-electric modes in THz and near-infrared parts of the spectrum. As the topological protection relies on guiding the light via a highly structured surface, direct imaging of the photonic modes at sub-unit cell resolution is of high interest but challenging in particular for transverse-magnetic modes. Here, we report mapping the transverse-magnetic modes in a valley photonic crystal waveguide using scattering-type scanning near-field optical microscopy at the optical telecom Cband wavelength. The waveguide based on a triangular air-hole motif with broken inversion symmetry is fabricated from suspended Germanium layer. We observed the launching and guiding of the transverse-magnetic edge mode along the boundary between topologically distinct domains with opposite valley Chern indices. These results are supported by theoretical simulations, and provide insight into the design and use of topological protected states for applications in densely integrated optical telecommunication devices.

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

“…We realize valley-Hall PhCs (VPCs), which rely on the valley degree of freedom linked to the breaking of a specific lattice symmetry 12,[14][15][16] . Similar to the valleyselective polarization caused by spin-orbit coupling in transition metal dichalcogenides 17 , these PhC lattices exhibit a non-vanishing Berry curvature at the K and K′ points of the Brillouin zone 18 .…”

Near‐Field Characterization of Higher‐Order Topological Photonic States at Optical Frequencies

“…[ 29–31 ] The use of scattering scanning near‐field optical microscopy (s‐SNOM) to directly map near‐fields in the optical domain, which was successfully used to study the properties of complex optical media, [ 32,33 ] was recently applied to characterize topologically robust transport in valley photonic crystal waveguides. [ 34 ]…”

“…[29][30][31] The use of scattering scanning near-field optical microscopy (s-SNOM) to directly map near-fields in the optical domain, which was successfully used to study the properties of complex optical media, [32,33] was recently applied to characterize topologically robust transport in valley photonic crystal waveguides. [34] Higher-order topological insulators (HOTIs) represent a new type of topological system, supporting boundary states localized over boundaries, two or more dimensions lower than the dimensionality of the system itself. Interestingly, photonic HOTIs can possess a richer physics than their original condensed matter counterpart, supporting conventional HOTI states based on tight-binding coupling, and a new type of topological HOTI states enabled by long-range interactions.…”

Topological Valley Photonics: Physics and Device Applications

“…At optical frequencies, suspended dielectric VPCs (see Figure 9c) [ 106 ] and SOI‐based VPCs (see Figure 9d) [ 107–110 ] were experimentally realized. In comparison with the valley‐Hall photonic topological insulators based on waveguide arrays, the VPC‐based approach has many advantages, including ultralow metallic losses, large topological bandgap widths (relative bandwidths up to 10%), in‐plane propagation, very small footprints (with unit cells comparable to the operating wavelength), and compatibility with the standard complementary metal–oxide–semiconductor (CMOS) fabrication technique.…”