Backscattering in Slow-Light Valley-Hall Photonic Topological Waveguides

Photonic-crystal waveguides formed at the interface between photonic topological insulators have recently received much attention as they promise robust forward propagation under a variety of disorder classes [1]. An example is time-reversal-symmetric valley-Hall (VH) topological waveguides, which can be implemented in dielectric slabs and free of intrinsic radiative losses. This makes them attractive as contenders for integrated optical waveguides to slow down light without suffering from backscattering at unavoidable fabrication defects [2]. While numerical simulations find VH interface waveguides to outperform conventional waveguides at the same group index for very small disorder levels [3], the failure of modelling real fabrication imperfections, i.e., surface roughness, due to the steep computational cost limits the scope of such results, warranting experimental investigation. We report here on the experimental characterization of the scattering properties of VH waveguides etched into a 220 nm-thick silicon membrane, an example of which is shown in Fig. 1 a. Due to the glide symmetry of the waveguide and the degeneracy it enforces, the structure supports not only the topological mode ensured by the bulk-edge correspondence, but also a topologically trivial mode [4]. To characterize the propagation losses of the two modes we employ the cutback method: we measure the transmittance of a series of photonic circuits containing VH waveguide segments of varying length and fit the transmittance drop with an exponential decay. We observe no significant difference in propagation loss of trivial and topological modes at high group indices (Fig. 1 b) and, by fitting to a simple model, we find backscattering to dominate in both cases [5]. Such backscattering is strong enough to dictate the modal properties of the field, as evidenced by the formation of high-Q spatially localized optical modes observed by far-field imaging techniques. We observe backscattering-induced localization even in waveguides that either contain sharp bends acting as modal filters (Fig. 1 c) or waveguides that sustain only a single topological mode. Hence, even when fabricated using a state-of-the-art silicon photonics manufacturing process [6], as gauged by the losses measured in W1 photonic-crystal waveguides, we find no expression of topological protection against backscattering from realistic fabrication imperfections, casting doubts on the real-world value of slow light in time-reversal symmetric topological waveguides.