Topological cascade laser for frequency comb generation in $\mathcal{P}\mathcal{T}$-symmetric structures

The cascade of resonant topological structures with $\mathcal{P}\mathcal{T}$-symmetry breaking is shown to emit laser light with a frequency-comb spectrum. We consider optically active topological Aubry-Andr'e-Harper lattices supporting edge-modes at regularly spaced frequencies. When the amplified resonances in the $\mathcal{P}\mathcal{T}$-broken regime match the edge modes of the topological gratings, we predict the emission of discrete laser lines. A proper design enables to engineer the spectral features for specific applications. The robustness of the topological protection makes the system very well suited for a novel generation of compact frequency comb emitters for spectroscopy, metrology, and quantum information.