Strong-field ionization driven by circularly polarized laser fields from orbitals with nonzero angular momentum becomes a hot topic recently, since it was demonstrated by I. Barth et al that the ionization rate is sensitive to the sign of the magnetic quantum number m. This new feature offers new insights into the strong field ionization process and also promises opportunities for many novel applications. The previous works on this field mainly paid attention on the ionization rates and offset angles in the photoelectron momentum distribution with different relative helicities between the laser field and the electron, and the roles of excited states were not discussed.

In the recent cooperative work from the Ultrafast optics group and Dr. Ingo Barth’s group in Max Planck Institute of Microstructure Physics, the new phenomenon of resonant enhancement effect is concerned. They found that the resonant enhancement effect is also helicity sensitive. Enhancement of ionization is found in the deep multiphoton ionization regime when the helicity of the laser field is opposite to that of the electron, while this enhancement is suppressed when the helicities are the same. Their analysis shows that the ionization enhancement is attributed to the multiphoton resonant transitions to excited states prior to ionization. And the helicity sensitivity is due to the different transition probabilities for different excitation-ionization channels restricted by the conservation of both energy and angular momentum, when the electrons absorb photons from left or right circularly polarized laser fields respectively. This work shows the important role of excited states and reveals the more abundant physics in the process of strong-field ionization from orbitals with internal angular momentum.

This work is published on Opt. Express Vol. 24, No. 4, 4196 (2016). This work was supported by the National Natural Science Foundation of China under Grants No. 11234004, No. 11404123 and No. 61275126, the 973 Program of China under Grant No. 2011CB808103, and the Natural Science Foundation of Hubei Province under Grant No. 2014CFB174.