Acoustically Controlled Edge States in Dynamic Topological Waveguide Arrays

Most of the physical phenomena, like optical scattering, or electric conductivity, can be explained by investigating the local, or microscopic response of materials and their building blocks. However, starting from the 1970s, several physical effects were identified in condensed matter physics which instead appeared to originate from the non- local, or macroscopic response of the system. This realisation, led to the formulation of the principles of topology in physics.

In particular, in the 2010s several experiments with microwaves and soon after, visible light, led to the development of the field of topological photonics. These first experiments demonstrated that photonic crystals, which would otherwise act like insulators for electromagnetic waves, supported propagation of light along the edges of the structure. Later, the same effects has been explored in photonic crystals undergoing time-modulation of their local response, or equivalently, spatial modulation along the direction of light propagation. These Floquet topological photonic systems promised new ways of achieving active control over the non-local response of optical systems.

In this work we propose a simple photonic Floquet topological system which can be actively switched between a trivially insulating and topologically insulating phase. This is achieved by sending surface acoustic waves (SAW) along the quasi-one-dimensional chain of the photonic waveguides. We use Floquet theory to study the response of this system, both in infinite, and finite chains of waveguides, calculating the topological invariants in the former, and validating the existence of edge states in the latter, by simulating the dynamics of its eigenstates. We also explore the ranges of parameters (like the SAW-modulated couplings between waveguides), which should enable the dynamical switching between trivial and topological phases. We hope that this inspires future work into the active switching between trivially and topologically insulating photonic systems, and encourages its experimental realisation.