Photonic time crystals assisted by structural resonances

Photonic time crystals (PTCs) are formed by modulating the refracting index of a medium as a periodic function of time.The frequency of this modulation compares with the oscillation frequency of light.PTCs exhibit momentum bandgaps as their characteristic feature.While the conventional bandgaps in spatial photonic crystals appear with respect to the frequency , momentum bandgaps in PTCs appear with respect to the wavevector k.Inside a momentum bandgap, there exist eigenmodes with complex frequencies.Such complex frequency eigenmodes lead to the amplification of the incident electromagnetic radiation that gets coupled to the momentum bandgap.