Fractional Echoes
  We report the observation of fractional echoes in a double-pulse excited nonlinear system. Unlike standard echoes which appear periodically at delays which are integer multiple of the delay between the two exciting pulses, the fractional echoes appear at rational fractions of this delay. We discuss the mechanism leading to this phenomenon, and provide the first experimental demonstration of fractional echoes by measuring third harmonic generation in a thermal gas of CO2 molecules excited by a pair of femtosecond laser pulses. 
G. Karras, E. Hertz, F. Billard, B. Lavorel, G. Siour, J.-M. Hartmann,   O. Faucher, Erez Gershnabel, Yehiam Prior, and Ilya...
physics.optics physics.atom-ph physics.chem-ph