Tunable defect modes in temporal Fabry–Pérot cavities based on photonic time crystals

Photonic time crystals (PTCs) are realized by periodically modulating the electromagnetic (EM) properties of time-varying media, leading to novel wave–matter interactions. Previous studies of defect modes in PTCs have not addressed the effect of defect position in multi-defect configurations. Here, we investigate a temporal analog of the Fabry–Pérot cavity (FPC), enabling enhanced control and engineering of bandgap properties. The proposed temporal FPC, composed of two inversion-symmetric PTCs enclosing a central resonant cavity, exhibits stationary defect modes at a target momentum and its odd multiples. Furthermore, we demonstrate that a sequence of defect modes with quasi-linearly tunable positions can be achieved. Theoretical predictions obtained from the temporal transfer matrix method (TTMM) show excellent agreement with finite-difference time-domain (FDTD) simulations, corroborating our analysis. The proposed temporal FPC expands the controllability of momentum bandgaps and holds significant potential for applications such as tunable lasers and multi-channel optical filters.