Researchers at the Max Planck Institute (Heidelberg, Germany) and the University of Hannover (Hannover, Germany) have used a Ti:sapphire oscillator with an extended cavity to generate 50 fs, 0.5 J pulses at a 6 MHz repetition rate to ionize helium. The standard z configuration of the Ti:sapphire oscillator was extended by a multiple-pass Herriott cell in which the laser mode was reflected back and forth in a circular pattern between two concave mirrors, effecting a unity transformation, which made laser alignment and modelocking much easier. The oscillator was pumped by a continuous wave, 15 W, frequency-doubled Yb:YAG laser.
With 30% output coupling, stable modelocking at net positive intracavity dispersion was achieved, delivering 3.5 W of average power. Temporal compression and focusing of the laser pulses close to the diffraction limit was employed to achieve intensities exceeding 1014 W/cm2. As a consequence, the ionization of helium, a nonlinear process in which at least 17 photons must be absorbed simultaneously, could be demonstrated. A surprisingly high ionization rate of 1 kHz was achieved, leading the researchers to believe that the laser configuration will open a new window for investigations of multiphoton laser-atom and laser-molecule interactions at “low” intensities (1011 to 1012 W/cm2). Contact Sebastian Dewald at [email protected].