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

Valley-polarized topological states of light allow for robust waveguiding, which has been demonstrated for transverse-electric modes in THz and near-infrared regions of the spectrum. As the topological protection relies on guiding the light via a highly structured surface, direct imaging of the photonic modes at subunit cell resolution is of high interest but challenging, particularly 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 C-band wavelength. The waveguide based on a triangular air-hole motif with broken inversion symmetry is fabricated from the 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.