Topologically protected frequency control of broadband signals in dynamically modulated waveguide arrays

We theoretically propose a synthetic frequency dimension scheme to control the spectrum of a light beam propagating through an array of evanescently coupled waveguides modulated in time by a propagating sound wave via the acousto-optical effect. Configurations are identified where the emerging two-dimensional synthetic space-frequency lattice displays a nontrivial topological band structure. The corresponding chiral edge states can be exploited to manipulate the frequency spectrum of an incident beam in a robust way. In contrast to previous works, our proposal is not based on discrete high-Q cavity modes and can be applied to the manipulation of broadband signals with arbitrary spectra.