Control of the high-order harmonics cutoff and attosecond pulse generation through the combination of a chirped fundamental laser and a subharmonic laser field

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.