Photonic time crystals assisted by quasi-bound states in the continuum.

Photonic time crystals (PTCs) are characterized by the rapid modulation of the material properties in time, causing a momentum bandgap for light. However, the observation of these bandgaps at optical frequencies remains elusive as the necessary temporal modulation amplitudes to show notable momentum bandgaps are relatively high, inaccessible with available materials. While it has been known that structuring PTCs at the subwavelength scale can improve the bandgap size, we push this concept to the extreme by leveraging the nanophotonic toolbox. Specifically, we demonstrate that structures composed of scatterers supporting quasi-bound states in the continuum can substantially reduce the required modulation amplitudes by enhancing the interaction time between light and time-varying matter. This allows us to observe noticeable momentum bandgaps despite the weak temporal modulation. Our approach bridges the concepts of bound states in the continuum and time-varying metamaterials, paving the way toward realizable PTCs at optical frequencies.