Coherent control schemes for the photoionization of neon and helium in the Extreme Ultraviolet spectral region
Scientific REPORtS |
Coherent control schemes for the photoionization of neon and helium in the Extreme Ultraviolet spectral region
Luca Giannessi
Enrico Allaria
Kevin C. Prince
Carlo Callegari
Giuseppe
Sansone
Kiyoshi Ueda
Toru Morishita
Chien Nan Liu
Alexei N. Grum-Grzhimailo
Elena V. Gryzlova
Nicolas Douguet
Klaus Bartschat
OPEN The seeded Free-Electron Laser (FEL) FERMI is the first source of short-wavelength light possessing the full coherence of optical lasers, together with the extreme power available from FELs. FERMI provides longitudinally coherent radiation in the Extreme Ultraviolet and soft x-ray spectral regions, and therefore opens up wide new fields of investigation in physics. We first propose experiments exploiting this property to provide coherent control of the photoionization of neon and helium, carry out numerical calculations to find optimum experimental parameters, and then describe how these experiments may be realized. The approach uses bichromatic illumination of a target and measurement of the products of the interaction, analogous to previous Brumer-Shapiro-type experiments in the optical spectral range. We describe operational schemes for the FERMI FEL, and simulate the conditions necessary to produce light at the fundamental and second or third harmonic frequencies, and to control the phase with respect to the fundamental. We conclude that a quantitative description of the phenomena is extremely challenging for present state-of-the-art theoretical and computational methods, and further development is necessary. Furthermore, the intensity available may already be excessive for the experiments proposed on helium. Perspectives for further development are discussed.
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Soon after the invention of optical lasers, harmonic-generation systems were constructed, which allowed the
tripling and doubling of the fundamental frequency. These harmonics are generated coherently and so have a fixed
phase relationship with respect to the (co-propagating) fundamental. This property was exploited to perform
experiments where the phase relationship was of prime importance, i.e., experiments belonging to the two-color,
coherent-control class1,2. In such experiments, the fundamental typically generates a multiphoton process, while
the harmonic gives rise to a single-photon process. Thus the fundamental is usually required to be far more
intense than the harmonic to create measurable interference.
The principal motivation for this paper is the need for fresh approaches to implement coherent control
experiments in the extreme ultraviolet (XUV) and soft x-ray region. We recently demonstrated such a scheme3, but there
are many more that could be pursued. Using our previous work as a starting point, we propose additional
experiments, and we also analyze to what extent such experiments could be supported by theory. As is often the case in
developing fields, the parameters (here the laser intensities and pulse lengths, as well as the target states involved)
that are most convenient for experiment are not necessarily the same as those that can be handled by current
theoretical methods. In many cases, including the results reported in3, the optimal parameters in experiment and
theory are widely divergent, and hence it is necessary to pave a path along which they can meet in the near future.
One group of experiments pioneered by Brumer, Shapiro and others4,5 is based on the use of the fundamental
and its third harmonic. In this approach, typically a resonance is simultaneously excited by the third harmonic in
a one-photon process, and by the more intense fundamental via a three-photon process. The two matrix elements
for the excitations must have comparable amplitude for this scheme to function, because otherwise the
interference is weak. By varying the relative phase of the light pulses, which determines the relative phase of the matrix
elements, the outcome of the resonance excitation is controlled. Examples include the product distribution after
dissociation4, the angular distribution of emitted electrons, the cross section, or other observables such as the line
shape of the transition6–9. The process is non-linear because the excitation by the first harmonic depends on the
third-order susceptibility of the target10,11.
A second group of experiments, established by Elliott and co-workers12,13, as well as Baranova and
co-workers14–16, involves interference between phase-locked fundamental and second harmonic radiation. Once
again, the fundamental is far more intense than the second harmonic, but there are two differences with respect
to the previous case. Firstly, only the angular distribution of the signal varies and not the total cross section12.
Secondly, the symmetry is broken, and as a result the photoelectron angular distributions are asymmetric. For
this situation to occur, it is a necessary (but not a sufficient) condition that the optical response is anharmonic, so
again this effect has been (...truncated)