Optical waveform measurement of femtosecond laser pulses using high-order harmonic generation

Abstract

With the advancement of ultrashort high-power laser technologies, it is now feasible to apply laser pulses having an arbitrary shape. Interaction of such strong laser pulses with atoms, molecules, or solids can reveal ultrafast excitation and ionization dynamics as well as electronic structures of the interacting matter. The precise knowledge of the waveform of driving pulses is required to accurately understand underlying physics of laser-matter interactions. We have explored an optical waveform measurement of femtosecond laser pulses by making use of high harmonic generation (HHG) processes.

This waveform measurement of optical pulses was demonstrated through simulation and experiment using the basic principle of the petahertz optical oscilloscope. By adding weak signal pulses to the generation of isolated single attosecond pulses to perturb the interaction process, we successfully retrieved the waveform of the signal pulses both in simulation and in experiment. The accuracy of measurement was however deteriorated if the signal wavelength was changed. In order to overcome this limitation, a theoretical investigation was carried out to the process of HHG from atoms driven by two femtosecond laser pulses with different intensities. Using the strong-field approximation model, the dependence on wavelength was analyzed and explained in terms of the electron quantum path of HHG. The result from this model was compared with the one obtained by solving the time-dependent $Schr\ddot{o}dinger$ equation. A method improving the accuracy of this technique was presented and applied to show good agreement between the measurement and the input waveform in the ultraviolet range in which the approximation used in the petahertz optical oscilloscope fails. Our fast and efficient waveform characterization method will greatly facilitate the advancement of ultrafast atomic and molecular dynamics by clarifying the temporal structure of interacting pulses.