Ultrafast topological transitions driven by permittivity modulation in non-Hermitian multilayers

Ultrafast modulation of epsilon-near-zero (ENZ) media enables dynamic control of non-Hermitian photonic systems. We demonstrate that an ITO/SiO 2 /Ag multilayer supporting hybrid ENZ–plasmon modes undergoes pronounced temporal reshaping when described by a time-dependent Drude–Lorentz permittivity and analyzed using rigorous coupled-wave analysis. Picosecond carrier relaxation induces avoided crossings, transient spectral gaps, and mode hybridization, while reflection-phase singularities traverse the angle–wavelength plane and produce discrete changes in the winding number. The results establish ENZ-assisted temporal modulation as a compact platform for reconfigurable non-Hermitian photonics.