Transient Cherenkov radiation from an inhomogeneous string excited by an   ultrashort laser pulse at superluminal velocity
  An optical response of one-dimensional string made of dipoles with a periodically varying density excited by a spot of light moving along the string at the superluminal (sub-luminal) velocity is theoretically studied. The Cherenkov radiation in such system is rather unusual, possessing both transient and resonant character. We show that under certain conditions, in addition to the resonant Cherenkov peak another Doppler-like frequency appears in the radiation spectrum. Both linear (small-signal) and nonlinear regimes as well as different string topologies are considered. 
R. Arkhipov, I. Babushkin, M. K. Lebedev, Yu. A. Tolmachev, M. V.   Arkhipov
physics.optics