Exceptional-point-based flat spectrum generation in frequency lattices

We theoretically investigate the generation of flat-top frequency combs by the exceptional point (EP) in synthetic frequency dimensions. Two coupled ring resonators under amplitude modulation simulate an anti--parity-time $(APT)$ symmetric Su-Schrieffer-Heeger frequency lattice. At the phase transition point for the $APT$ symmetry being completely broken, the bands become real and linearly cross at the EP. We find that when the incident light contains a component along the generalized eigenvector at the EP, a flat spectrum with a uniformly increasing width emerges. Further, we prove that the speed of the spectrum broadening and the spectral amplitude can be controlled flexibly via amplitude modulations. The numerical simulations are performed based on temporal coupled-mode theory. The study exemplifies the novel approach that combines non-Hermiticity and synthetic dimensions to generate flat spectra, which may enable new strategies in spectral shaping and flat-top microcomb design.