Generation of unipolar pulses in a circular Raman-active medium excited   by few-cycle optical pulses
  We study theoretically a new possibility of unipolar pulses generation in Raman-active medium excited by a series of few-cycle optical pulses. We consider the case when the Raman-active particles are uniformly distributed along the circle, and demonstrate a possibility to obtain a unipolar rectangular video pulses with an arbitrarily long duration, ranging from a minimum value equal to the natural period of the low frequency vibrations in the Raman-active medium. 
R.M. Arkhipov, M.V. Arkhipov, I. Babushkin, A.V. Pakhomov, Yu.A.   Tolmachev, N.N. Rosanov
physics.optics