A scaling law of photoionization in few-cycle limit

Using a nonperturbative QED theory of above-threshold ionization (ATI), the photoionization of atoms in few-cycle laser pulses is studied and the ionization rate is obtained analytically. Our study shows that the ionization rates in a pair of opposite directions are not always equal to each other which is termed as inversion asymmetry and that the asymmetry degree varies with the CE phase, the pulse duration, the peak intensity of laser pulse, and the kinetic energy of photoelectrons. The scaling law explains why similar phenomena appear in few-cycle pulses of various carrier-wave frequencies.