Spectral features and modeling of high-order harmonics generated by sub-10-fs pulses

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses wa s investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative sp ectral shifts were measured as a function of the position of the gas jet. T he results point out spectral features typical of the quasi-single-cycle ex citation regime. A nonadiabatic three-dimensional numerical model was devel oped, which provides harmonic spectra in remarkable agreement with the expe riments.