  We report on new electromagnetic modes in space-time modulated
dispersion-engineered media. These modes exhibit unusual dispersion relation,
field profile and scattering properties. They are generated by coupled
codirectional space-time harmonic pairs, and occur in space-time periodic media
whose constituent materials exhibit specific dispersion. Excitation of a slab
of such a medium with subluminal modulation results in periodic transfer of
energy between the incident frequency and a frequency shifted by a multiple of
the modulation frequency, whereas superluminal modulation generates
exponentially growing frequencies. These modes may find applications in optical
mixers, terahertz sources and other optical devices.
