Supercontinuum Generation in Gases: A High Order Nonlinear Optics Phenomenon

Abstract

The recent development of high power, ultrashort pulse sources has created renewed interest in the interaction between intense laser radiation and free atoms and molecules. Not only is it feasible to apply laser fields that approach, or exceed, the strength of the atomic field as seen by the outer electrons, but it is also possible to apply these fields nonadiabatically using ultrashort pulses. Until now, experiments have been restricted to isolated atoms. However, because the theories of nonlinear optics and multiphoton ionization are so interrelated, we should expect these new phenomena to have optical signatures. In addition to their intrinsic interest, nonlinear optics experiments can add a new perspective for judging emerging theories of high intensity laser processes. Clearly, there is a new class of experiments to be performed using ultrahigh power, ultrashort pulses. We describe an experiment performed in high pressure gases with a 2 psec or 70 fsec 0.6 {mu}m pulse focused to a peak intensity of I {approx lt} 10{sup 13} W/cm{sup 2}. The maximum intensity exceeds that in which multiphoton ionization is observed in longer pulse experiments in Xe using either 1.06 {mu}m or 0.53 {mu}m radiation. It is approximately the intensity at which tunnel ionizationmore » is observed with nanosecond 10 {mu}m pulses in very low pressure Xe. Furthermore, at such intensities in 0.53 {mu}m and 1.06 {mu}m experiments, high energy electrons are observed from Xe.« less