Multiphoton Physics at Short Wavelengths
Argonne National Laboratory
In order to describe nonlinear optical processes, knowledge of the average laser pulse shape and energy is insufficient. This makes the experimental determination of multiphoton absorption cross sections using a self-amplified spontaneous emission (SASE) free-electron laser a challenging problem. Radiation pulses generated in the SASE process are chaotic, i.e., the time evolution of the field amplitude and phase during a pulse changes from shot to shot in a random manner. As an example, I discuss a multiphoton experiment carried out at the DESY Tesla Test Facility, at a photon energy of 13 eV. The so-far incomplete characterization of the SASE-FEL pulses makes the comparison of the Hamburg experiment with theoretical calculations rather difficult. It may be anticipated that the interpretation of multiphoton experiments with a seeded free-electron laser will be easier.