Broadband frequency translation by space–time interface with weak permittivity temporal change

Abstract

Breaking spatial and temporal homogeneities simultaneously incurs the combination of wavenumber and frequency translations. In this work, broadband frequency translations with both redshifts and blueshifts triggered by a single photonic space–time interface (PSTI) with weak temporal change of permittivity across which a homogeneous medium suddenly becomes a one-dimensional photonic crystal is proposed. Mode conversions induced by the PSTI are analyzed, according to which the frequency translation amplitudes are independent of the change of permittivity and the initial frequency but are given by the product of the phase speed of the homogeneous medium and the spatial modulation frequency of the photonic crystal. Hence, a static field can be partially converted into dynamic fields by imposing the PSTI. Our findings pave the way for the study of PSTIs and provide a new scheme to realize broadband frequency translations.