Non-Hermitian multi-cavity reflective optical limiters engineered via exceptional points of degeneracy

A non-Hermitian multi-cavity platform based on a 1D photonic crystal multilayer stack is demonstrated to simultaneously manifest angular filtering and reflective optical limiting via the manipulation of exceptional points of degeneracy (EPDs). Mapping an N = 4 coupled-cavity network onto an alternating ZnS–cryolite thin-film sequence stabilizes the system at a permanent EPD within the low-power, normal-incidence regime, yielding a high-transmission, maximally flat Butterworth filter profile. Oblique illumination breaks the degeneracy, providing angular filtering characterized by a narrow 19° peak-transmission cone. Under high-power laser irradiation, non-uniform internal field profiles trigger a thermal detuning cascade across K = 3 active nonlinear ZnS cavities. This spatial perturbation shifts the refractive indices, breaks the effective Hamiltonian matrix symmetry, and accelerates the decay of transmittance above a limiting threshold of 0.7 MW/cm2, driving a sharp transition into a protective, highly reflective state. Under concurrent high-power oblique incidence, these geometric and nonlinear mechanisms act as complementary pathways, narrowing the transmission cone to ensure robust, wide-angle optical protection. Engineering a non-Hermitian degeneracy within distributed nonlinear cavities establishes a rigorous paradigm for passive optoelectronic protection, lens-free fluorescence imaging, and optical isolation.