CC BY
13LMI-I-17
Ionization-event harmonics: basic mechanisms and characterization of ionization dynamics
A. Husakou1, P. Jürgens2, B. Liewehr3, B. Kruse3, C. Peltz3, W. Engel4, M. Ivanov1,
M. Vrakking2, T. Fennel3, A. Mermillod-Blondin2
1Max Born Institute, Division T, Berlin, Germany
2Max Born Institute, Department A, Berlin, Germany
3University Rostock, Institute of Physics, Rostock, Germany
4Max Born Institute, Department B, Berlin, Germany
The nonlinearity of different materials in the strong pulsed optical field is ubiquitous in modern optics and plays a critical role in key applications such as generation of new frequency components, pulse compression, attosecond science, spectroscopy, metrology, material processing and so on. Up to now, three main background mechanism were considered responsible for the optical nonlinearities in the rarefied media, in particular gases. First of them is the Kerr-type nonlinearity which is based on the anharmonicity of the bound electron motion. Second is highorder harmonics generation by the three-step process. Finally, the Brunel mechanism of the nonlinearity is associated with stepwise increase of the free-electron density during the photoionization and the associated subcycle phase modulation. In this contribution, we investigate additional and novel mechanisms of nonlinearity and harmonic generation which are based on the previously overlooked peculiarities of the photoionization process.
First, we show that the photoionization electron occurs as a free electron at the finite distance (photoionization displacement, PD) from the parent ion, which leads to ultrafast buildup of the polarization and associated light emission. This effect was neglected up to now. The semiclassical theory of the PD harmonic emission is presented, and we connect the obtained expression of the PD to the photoionization energy losses. We analyze the resultant formalism for the harmonic generation and investigate its contribution as a function of the pump parameters such as intensity and wavelength. The regimes where the PD contribution plays a dominant role compared to the three previously known mechanisms are identified. The possibilities to benchmark the above semiclassical theory by solving the time-dependent Schrödinger equation are discussed. Also, the phase of the PD contribution is investigated analytically and numerically, paving the way to detect and characterize this contribution experimentally.
Second, we show that in addition to the PD contribution, the electrons are photoionized with nonzero velocity (photoionization velocity, PV). The corresponding term in the polarization was also up to now completely neglected. Similarly to the PD term, a semiclassical, strong-field theory will be presented for the PV contribution based on a formalism related to the formalism of the three-step mechanism. The dependence of the PV contribution on the pump parameters will be analyzed. The phase of the PV contribution, in contrast to that of the PD contribution, significantly depends on the pump parameters, which on one hand leads to difficulties in experimental detection and characterization, on the other hand, opens new possibilities with respect to phase matching. In summary, in this contribution we discuss and analyze, based on the semiclassical theory, two new fundamental contributions to the optical nonlinearity, which are related to the displacement and initial velocity of an electron after the photoionization.
