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 featureoffers new insights into the strong field ionization process and also promisesopportunities 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 recentcooperativework from the Ultrafast optics group and Dr. Ingo Barth’s group inMax Planck Institute of Microstructure Physics, the new phenomenon ofresonant enhancement effectis concerned. They found that the resonant enhancement effect is alsohelicity sensitive.Enhancement of ionization is found in the deepmultiphoton ionization regime when the helicity of the laser field is oppositeto that of theelectron, while this enhancement is suppressed when thehelicities are the same.Theiranalysis showsthat theionization enhancement is attributed to the multiphoton resonant transitionsto excited statesprior toionization.And the helicity sensitivity is due to the different transition probabilitiesfor different excitation-ionizationchannelsrestricted by the conservation of both energy and angular momentum, when the electronsabsorbphotons fromleft or right circularly polarized laser fieldsrespectively. 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 underGrants No. 11234004, No. 11404123 and No. 61275126, the 973 Program of China underGrant No. 2011CB808103, and the Natural Science Foundation of Hubei Province underGrant No. 2014CFB174.
