Optical third harmonic generation during femtosecond pulse diffraction in a Bragg grating

We report on a novel phenomenon occurring when ultrashort laser pulses are diffracted in a Bragg grating recorded in a photo-thermo-refractive glass: two intense beams at the third harmonic of the incident laser frequency appear between the direct and the diffracted laser beams. A high conversion efficiency of about 0.04% at an incident intensity of 5 × 1011 W cm−2 in a 1 mm thick grating is attributed to self-phase matching for the third harmonic generation via Cherenkov radiation from the interfering third order polarization of the incident and diffracted beams. It is assumed that this is a general phenomenon for diffraction in a volume grating.