Extremely Narrow Band in Moiré Photonic Time Crystal

The moiré superlattice has attracted growing interest in the electromagnetic and optical communities. Here, we extend this concept to time-varying photonic systems by superposing two binary modulations on the refractive index with different modulation periods, i.e., the moiré photonic time crystal (PTC). Such a moiré PTC leads to extremely narrow bands in momentum space that support temporal localized modes, exhibiting periodically self-reconstructing pulses in the time domain. We investigate the tunability of the band structure of the moiré PTC and the temporal localization behavior, which can be greatly manipulated by varying the temporal modulation parameters. Moreover, we explore the Floquet mode-locking mechanism in the moiré PTC, which points toward potential applications in mode-locked lasers with a tunable time width of the generated pulses. The modulation-induced extremely narrow band also offers intriguing opportunities in exceptional-point-enhanced sensing. Our Letter brings the concept of moiré patterns to the field of PTCs, and unveils new possibilities in wave manipulations with time-varying systems.