Generation of high-order harmonic continuum supporting single attosecond pulse in argon driven by intense 7 fs laser pulse

High-order harmonic continuum in the cutoff region is demonstrated with an argon gas cell driven by 7 fs/0.4 mJ ultrashort intense laser pulses. It is found that the spectral structure, modulation depth and continuum bandwidth vary greatly when the carrier envelope phase (CEP) of driving laser pulse is stabilized at different value. At some CEP value, a smooth continuous spectrum with modulation depth less than 17% and 10 eV continuum bandwidth is achieved, supporting a transform-limited single 500 as pulse in time domain.