Abstract

The efficiency with which VUV radiation can be generated by frequency conversion has been limited by various competing processes to values that are typically of the order of 0.1 - 0.3%. As a result, increases in available VUV pulse energy will have to be made through increases in the pump laser energy. However, the use of higher pump laser power can be accompanied by additional limitations associated with various experimental parameters, especially in the VUV where tight focusing and short interaction lengths are usually used. For example the larger focal spots that must be used with stronger pump lasers to avoid breakdown are necessarily accompanied by longer interaction paths that can make absorption in the nonlinear medium and damage to cell windows more important. We have conducted a series of experiments in third harmonic conversion of XeF laser radiation in Xe to determine the extent to which the additional problems associated with high pump power can be overcome. Our initial experiments, done at a pump energy of 20 to 30 mJ (2 MW peak power) have indicated that the conversion efficiency, which was measured to be of the order of 5 x 10⁻⁴, was limited by a combination of breakdown in the focus and absorption of the VUV radiation in the Xe downstream from the nonlinear interaction. The corresponding pulse energy was measured to be of the order of 10 μJ without phase matching. Further increases in VUV pulse energy can be accomplished with a combination of phase matching and increased pump energy. Our initial measurements with phase matched mixtures of Xe and Ar indicate that the conversion efficiency increases by a factor of about 2 when a 5:1 Xe:Ar mixture is used. Further improvements in phase matching, the use of various gas combinations and their effects on breakdown and absorption limitations will be discussed. Scaling of our current results indicate that the conversion efficiency can be raised from 2 x 10⁻⁴ to 10⁻³ with a corresponding increase in VUV energy to 1 mJ if the pump energy is raised to 250 mJ and the nonlinear medium can be confined to the region of the nonlinear interaction. The usefulness of various gas confinement techniques for the conditions required by our high pump laser power (large apertures, long interaction paths) will be described and the results of conversion efficiency measurements at high pump energy will be reported.

© 1984 Optical Society of America