Comparative study on atomic ionization in bicircular laser fields by length and velocity gauges S-matrix theory**Project supported by the Key Laboratory Project of Computational Physics of National Defense Science and Technology of China (Grant No. 6142A05180401), the National Key Program for S&T Research and Development of China (Grant Nos. 2019YFA0307700 and 2016YFA0401100), and the National Natural Science Foundation of China

Abstract

Ionization of atoms in counter-rotating and co-rotating bicircular laser fields is studied using the S-matrix theory in both length and velocity gauges. We show that for both the bicircular fields, ionization rates are enhanced when the two circularly polarized lights have comparable intensities. In addition, the curves of ionization rate versus the field amplitude ratio of the two colors for counter-rotating and co-rotating fields coincide with each other in the length gauge case at the total laser intensity 5 × 1014 W/cm2, which agrees with the experimental observation. Moreover, the degree of the coincidence between the ionization rate curves of the two bicircular fields decreases with the increasing field amplitude ratio and decreasing total laser intensity. With the help of the ADK theory, the above characteristics of the ionization rate curves can be well interpreted, which is related to the transition from the tunneling to multiphoton ionization mechanism.