Photonic molecule approach to multi-orbital topology

TL;DR

This paper demonstrates how photonic molecules in laser-written waveguide arrays can be engineered to realize topological modes and phase transitions, advancing the design of novel photonic topological states.

Contribution

It introduces a method to create and control topological photonic modes using degeneracies in multi-orbital waveguide systems, revealing multiple topological transitions.

Findings

Observation of topological modes induced by inter-orbital coupling

Identification of multiple topological phase transitions

Control of topological states via lattice spacing and wavelength

Abstract

The concepts of topology provide a powerful tool to tailor the propagation and localization of light. While electromagnetic waves have only two polarization states, engineered degeneracies of photonic modes provide novel opportunities resembling orbital or spin degrees of freedom in condensed matter. Here, we tailor such degeneracies for the array of femtosecond laser written waveguides in the optical range exploiting the idea of photonic molecules -- clusters of strongly coupled waveguides. In our experiments, we observe the emergence of topological modes caused by the inter-orbital coupling and track multiple topological transitions in the system with the change of the lattice spacings and excitation wavelength. This strategy opens an avenue in designing novel types of photonic topological phases and states.