Supercontinuum generation in CO₂ gas accompanied by optical breakdown

An experiment on the supercontinuum generation in high pressure CO₂ (1-40 atm) by weakly focused 1 ps pulses at 593 nm is described. While changing the gas pressure and the laser power, the authors observed self-focusing and optical breakdown in the gas. They studied the evolution of the supercontinuum spectral width and efficiency after self-focusing. Their results show that: (i) the spectral width of the supercontinuum spectrum before optical breakdown can be explained in terms of self-phase modulation by only taking into account the cubic non-linearity of the gas and the increased laser intensity due to self-focusing; (ii) the plasma contribution to the supercontinuum generation was, in their experiment, a self-limiting type, owing to the shortening of the interaction length as the electron density increased.