Anomalous ellipticity dependence of high-order harmonic generation

A systematic measurement of ellipticity-dependent high-order harmonic generation has been made in the energy region around the ionization potential of He, Ne, Ar, and Kr, using a 150-fs, 785-nm Ti:sapphire laser. Anomalous ellipticity dependence having a maximum at a nonzero ellipticity was observed at the 11th and 13th harmonics for Ne and at the 7th and 9th for Ar and Kr. The experimental results strongly suggest that the anomalous dependence observed Is due to the near-resonant multiwave mixing process an even number of linearly polarized photons with a single circularly polarized photon in the neutral atomic gases.