Abstract
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.
Highlights
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 Free Electron Lasers (FELs)
The main focus of the paper was on the effect of undesired second harmonic radiation, which is produced in FELs at the level of about 1% or lower of the fundamental intensity
We have studied the dynamics of this process by solving the Maxwell-Lorentz coupled equations governing the system, with the code PERSEO45,46, designed to study the longitudinal dynamics of a FEL
Summary
We have presented a detailed description of how a seeded Free-Electron Laser can be used to perform some coherent-control experiments using the fundamental and second and third harmonics. Two kinds of experiment have been proposed: first plus second harmonic, for which we have previously published initial experimental results, and first plus third harmonic, which we intend to pursue in the future This technique opens the way to coherent control with short-wavelength multicolor experiments, in which the amplitude, wavelength, and phase can be controlled. As illustrated for the case of helium, state-of-the-art calculations are prohibitively expensive in terms of computing time for the current pulse length available at FERMI. This may be remedied by more powerful computers, or by accelerator physics developments leading to pulses of shorter duration. We foresee that the methods developed here at short wavelengths will in the future be applied to solid-state samples to control the outcome of light-matter interactions
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