Multiphoton Processes

Angular Momentum Distributions in Rydberg States

We have studied the excitation to Rydberg states by a short, strong laser pulse. The parity of the populated angular momentum states is found to agree with the selection rules for multiphoton resonant absorption at low intensities, if the pulse length is not too short. In contrast, the parity effect cannot be observed for ultrashort pulses as well as for long pulses at high intensities. Signatures of the population in the excited states via the line emissions from the populated np states after the end of the pulse exhibiting the parity effect have been identified.

J. Venzke et al., Phys. Rev. A 98, 043434 (2018).

PRE 25th Anniversary Milestone

In 2018 the journal Physical Review E celebrated its 25th anniversary by highlighting a series of papers that made important contributions to their field. One milestone article for each calendar year since 1993 has been selected by the members of the Editorial Board, in collaboration with the journal's editors. For 2006, our work in collaboration with See Leang Chin and his group on the plasma density inside a femtosecond laser filament in air is featured as milestone article.

F. Theberge et al., Phys. Rev. E 74, 036406 (2006).

featured by Physical Review E as 25th Anniversary Milestone

Breakdown of Power Series Expansion

For many materials the polarization induced by an external field is found to be proportional to the field itself. In weak fields the electric susceptibility as a measure of this proportionality can be expanded in a power series. We have shown that the perturbative power series expansion breaks down, as indicated by the ratio of successive terms in the expansion (see Figure), at about 1x1013 W/cm2. In the same intensity regime there occurs the transition from perturbative to non-perturbative interaction in low-order harmonic generation. At higher intensities ab-initio theory that accounts for the nonperturbative interaction between the atom and the external field can be used to resolve the change of the susceptibility as a function of time over the duration of the pulse.

A. Spott et al., Phys. Rev. A 90, 013426 (2014); 91, 023402 (2015); 96, 053404 (2017).

HHG Control via Angular Spatial Chirp

We have studied the use of a pulse with angular chirp at the focal plane in high-order harmonic generation. Results of our numerical simulations show that each harmonic is emitted with an angular chirp which scales inversely with the harmonic order. This leads to a control of the separation of the harmonics in two dimensions and the temporal periodicity of the harmonic pulse trains - in collaboration with Chip Durfee (Colorado School of Mines).

C. Hernandez-Garcia et al., Phys. Rev. A 93, 023825 (2016)

Past Projects

Imaging of Molecular Dissociation
We studied how electron rearrangement during dissociation of molecules as well as the expansion of clusters can be imaged via strong-field processes.
A. Jaron-Becker, IEEE Journ. Select. Topics Quant. Electron. 18, 105 (2012)
A. Picon et al., Phys. Rev. A 83, 013414 (2011)
V. Strelkov et al., Phys. Rev. Lett. 107, 113901 (2011)
W. Li et al., Proc. Nat. Acad. Sciences U.S.A. 107, 20219 (2010)

From Intensity Clamping to Tunable Pulses
We studied high-harmonic generation as a tool to image molecular structure on an ultrafast time scale. Theoretical predictions showed how the radius of a fullerene (C20 to C180) can be retrieved from the position of characteristic interference minima in the spectrum.
M.F. Ciappina et al., Phys. Rev. A 76, 063406 (2007); 78, 063405 (2008)

Imaging of Fullerenes
We studied high-harmonic generation as a tool to image molecular structure on an ultrafast time scale. Theoretical predictions showed how the radius of a fullerene (C20 to C180) can be retrieved from the position of characteristic interference minima in the spectrum.
M.F. Ciappina et al., Phys. Rev. A 76, 063406 (2007); 78, 063405 (2008)