Isolated attosecond pulse generation from different frequency-chirping combined fields

Generation of high-order harmonic spectrum and isolated attosecond pulse (IAP) from frequency-chirping combined field has been investigated. It is found that by adding the middle-chirp form of the laser field, the extension of the harmonic cutoff comes from the middle region of the laser field. However, the intensity of the higher-order spectral continuum is very low. By adding the asymmetric negative chirp form of the laser field, the extension of the harmonic cutoff is attributed to the falling region of the laser field. Moreover, the intensity of the higher-order spectral continuum presents similar value as that produced from the chirp-free pulse. Further, by properly adding a second controlling pulse (i.e., infrared field or ultraviolet pulse) into the fundamental chirped pulse, the intensity of the harmonic spectrum can be enhanced by 2-3 and 1-2 orders of magnitudes for the cases of the middle-chirp form and the asymmetric negative chirp form, respectively. Finally, by using the best laser conditions, a series of isolated attosecond pulses with the durations of 38 as can be produced