Fig. 2: Conceptual realization method of time-varying medium with resonance frequency modulated.
From: Expanding momentum bandgaps in photonic time crystals through resonances
a, Illustration of the temporal modulation of the intrinsic resonance frequency of a bulk material. The modulation can be described by a time-varying effective spring constant κ(t). b, Illustration of the temporal modulation of the resonance frequency of a metamaterial consisting of meta-atoms. The modulation can be achieved in various ways, including modulating the permittivity ϵr or permeability μr of the material from which the meta-atoms are made, the size of the meta-atoms R, or the metamaterial periodicity d.
