Self-Phase Modulation of Light in a Laser-Breakdown Plasma

Self-phase modulation in a ${\mathrm{CO}}_{2}$-laser breakdown plasma is observed to down-shift the wavelength from 10.6 to about 10.0 \ensuremath{\mu}m. This results from the sudden drop in refractive index, from 1 to 0, which accompanies the ionization of the gas. The pressure dependence of this effect in nitrogen is opposite to that in helium.