Optimizing high-order harmonic generation in strong fields

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Abstract
The authors present high-order harmonic generation results obtained with different laser systems, a 1 ps 1053 nm Nd-glass laser, a 2 ps 616 nm synchronously pumped dye laser, a 36 ps 1064 nm mode-locked Nd-YAG laser and the second harmonics (308 nm and 532 nm) of the latter two systems. They investigate the influence of the laser pulse width, the excitation wavelength (from the near infrared to the ultraviolet) and the atomic medium on the number of photons produced and on the maximum energy attained. Harmonic generation also depends strongly on the focusing conditions. By using simple arguments and results of numerical calculations in xenon, they show that the conversion efficiency in general follows a simple b3 power law, b denoting the laser confocal parameter, up to a transition regime where the coherence length of the process becomes equal to the medium length. By applying the b3 scaling as a normalization factor, they can then compare experimental results obtained in different focusing geometries. Their experimental data show that the optimization of the photon energy produced is favoured by using long incident wavelengths and light atomic systems with a high ionization energy.