Infrared third-harmonic generation in phase matched CO gas

Resonance-enhanced third-harmonic generation (THG) of CO
2
laser radiation has been studied in CO and NO. It is shown that the coherence length for THG in CO can be increased by adding SF
6
and that phase matching occurs for a CO:SF
6
ratio of 208:1. The pressure dependence of the third-harmonic conversion efficiency in CO-SF
6
mixtures has been measured up to 11 atm, and the results are in good agreement with calculated values for the coherence length and the third-order susceptibility tensor.