We propose a method to generate an isolated attosecond (as) pulse in combination with a chirped fundamental laser field ($5$ fs, $800$ nm) and a subharmonic laser field ($12$ fs, $1600$ nm). It is shown that, for the case of the chirped parameter $\beta =0.25$, not only is the efficiency of the extended harmonics enhanced, but also an ultrabroad supercontinuum is formed in the second plateau. For the case of $\beta =0.65$, an ultrabroad supercontinuum spectrum with the width of about $1670$ eV can be observed. Furthermore, due to the introduction of chirped pulse, the short quantum trajectory is enhanced, and the long one is suppressed. By superposing a properly selected range of the harmonic spectrum in the continuum region, an intense isolated 38-as pulse is generated.

• Received 1 November 2009

DOI:https://doi.org/10.1103/PhysRevA.82.013411