  The high harmonic generation from atoms in the combination of chirped laser
pulse and static field is theoretically investigated. For the first time, we
explore a further physical mechanism of the significant extension of high
harmonic generation cutoff based on three-step model. It is shown that the
cutoff is substantially extended due to the asymmetry of the combined field. If
appropriate parameters are chosen, the cutoff of high harmonic generation can
reach Ip+42Up. Furthermore, an ultrabroad super-continuum spectrum can be
generated. When the phases are properly compensated for, an isolated 9
attosecond pulse can be obtained.
