The simplest rout to generating a train of attosecond pulses
  We report on new routes to generating a train of attosecond pulses. The methods are extremely simple and robust; we need to place only a few thin dispersive materials in the optical path. We numerically demonstrate the generation of a train of attosecond pulses with a transform-limited pulse duration of 728 as and a repetition period of 8.03 fs in gaseous parahydrogen. 
Kazumichi Yoshii, John Kiran Anthony, and Masayuki Katsuragawa
physics.optics