Comparison and real-time monitoring of high-order harmonic generation in different sources

We compare the generation of coherent extreme ultraviolet (xuv) photons in argon from three different high-order harmonic sources: a semi-infinite gas cell, a finite gas cell, and a pulsed valve. We demonstrate photoionization of the background gas by the coherent xuv photons as an alternative to xuv spectroscopy for real-time and in-line monitoring of the high-order harmonics. Using this technique for measuring photocurrent, we show that the gas cells produce 50-100-fold brighter harmonics than the pulsed valve, with an estimated conversion efficiency of 10(-4). The spectral distribution of the harmonics produced in the gas cells peaks at the 27th harmonic compared to the 19th harmonic in the pulsed valve. We attribute this difference to the interplay between phase matching and absorption.