Abstract

High-order harmonic generation is normally characterized by the appearance of a comb of well-defined peaks in the spectrum at odd intervals of the fundamental photon energy. For some experiments, such as time-resolved absorption spectroscopy, a continuum source will be extremely useful. For a sub-10 fs driving pulse, a continuum in the harmonic emission at higher orders has been predicted and observed, the merging of the peaks into a quasi-continuum is not expected to occur below harmonic 135 for a 25 fs driving pulse¹. Ionization blue-shifting of the fundamental can also lead to broadening of the harmonic spectrum. A quasi-continuum, with an energy ratio between peaks and valleys of 10:1, was reported recently¹, using ~100 mJ in a 25 fs pulse focused into a gas jet. Here, using <0.005 of the input energy of that work, we demonstrate the generation of a true, coherent continuum (25-38 nm) by combining the fundamental and its second harmonic in a gas-filled capillary. Pressure tuning allows control over the spectral characteristics of the harmonic light, from discrete peaks, to continuum, to near-elimination of the harmonic light.

© 1999 Optical Society of America