Quasi-phase matching of high-harmonics and attosecond pulses in modulated waveguides

We describe theoretically a new technique for quasi-phase-matched generatio n of high harmonics and attosecond pulses in a gas medium, in a high ioniza tion limit. A corrugated hollow-core fiber modulates the intensity of the f undamental pulse along the direction of propagation, resulting in a periodi c modulation of the harmonic emission at wavelengths close to the cutoff. T his leads to an increase of the harmonic yield of up to three orders of mag nitude. At the same time the highest harmonics merge in a broad band that c orresponds to a single attosecond pulse, using 15-fs driving pulses. (C) 20 00 Optical Society of America.