[Paper Review] Experimental demonstration of valley-Hall topological photonic crystal at telecommunication wavelengths

Mikhail I. Shalaev, Wiktor Walasik+3 more|arXiv (Cornell University)|Dec 19, 2017

Topological Materials and Phenomena4 citations

TL;DR

This paper experimentally demonstrates valley-Hall topological photonic crystal slab in silicon, fabricated via CMOS-compatible processes, that enables robust, backscattering-immune light propagation at telecommunication wavelengths (1550 nm). The structure uses a C3-symmetric photonic crystal with asymmetric triangular holes to break inversion symmetry, opening a bandgap and enabling topologically protected edge states that transmit light unimpeded around sharp bends.

ABSTRACT

Photonic topological insulators provide unprecedented possibilities to eliminate scattering losses and improve the efficiency of optical communication systems. Despite significant theoretical efforts, the experimental demonstration of an integrated photonic topological insulator operating in the telecommunication regime was still missing. Here, we design, fabricate and characterize a photonic-crystal-based topological structure that exhibits valley-Hall effect. We experimentally demonstrate the propagation of topologically protected edge states in a CMOS-compatible chip operating at telecommunication wavelengths. This contribution is an important step towards integrated topological photonics.

Motivation & Objective

• To realize a topologically protected photonic system operating at telecommunication wavelengths for low-loss optical transmission.
• To design a CMOS-compatible photonic crystal slab that supports valley-Hall effect via structural asymmetry.
• To experimentally verify topological edge states immune to backscattering at sharp defects.
• To demonstrate scattering-free propagation of light in a chip-integrated photonic platform.

Proposed method

• Design a silicon photonic crystal slab with C6 symmetry, then break it to C3 symmetry by using two differently sized triangular air holes per unit cell.
• Fabricate the structure on a silicon-on-insulator (SOI) wafer using wet etching to suspend the membrane, creating an air-silicon-air symmetric structure.
• Use finite-difference time-domain (FDTD) simulations to model band structure and edge state propagation, with effective refractive index for silicon.
• Perform experimental characterization using a femtosecond laser source, pellicle beamsplitter, polarizer, and camera-based imaging to detect edge-state transmission.
• Measure transmittance spectra across three sample types: no-edge, straight interface, and trapezoidal interface, to test topological robustness.
• Compare experimental results with FDTD simulations to validate the existence and protection of edge states.

Experimental results

Research questions

• RQ1 Can a CMOS-compatible photonic crystal slab support valley-Hall topological edge states at telecommunication wavelengths?
• RQ2 Does the topological edge state remain robust when propagating around sharp bends in the interface?
• RQ3 Is the transmission through the interface immune to backscattering from defects that do not induce inter-valley scattering?
• RQ4 Can the valley-Hall effect be experimentally observed in a dielectric photonic crystal at 1550 nm?

Key findings

• The fabricated photonic crystal slab exhibits a bandgap of approximately 1420–1510 nm, confirming the topological phase transition induced by structural asymmetry.
• High transmittance in the bandgap region was observed for samples with an interface between opposite-polarity crystals, confirming the existence of topologically protected edge states.
• Transmittance for straight and trapezoidal interfaces was nearly identical, demonstrating immunity to scattering at sharp bends.
• Numerical simulations using 2D FDTD with effective index approximation showed good agreement with experimental results, validating the topological protection mechanism.
• The low absolute transmittance values were attributed to coupling losses and insufficient PC size, as edge states localized over a width larger than the 21-unit-cell structure.
• This work presents the first experimental demonstration of valley-Hall topological transport in a photonic crystal at telecommunication wavelengths, using a CMOS-compatible platform.