Abstract

An experiment is described where the supercontinuum generated in CO₂ gas at high pressure (1-40 atm) by 1-ps long pulses at 593 nm was studied with a 1.3 × 10° W typical power. CO₂ was chosen because it has a low threshold for supercontinuum generation¹ and, as a molecular gas, has a higher threshold for optical breakdown than rare gases. While changing the gas pressure and the laser power, we observed self-focusing (SF) and optical breakdown (OB). To our knowledge, the observation of breakdown during supercontinuum generation in gases has never been reported before. The measurement of the critical power for SF allowed us to estimate the nonlinear refractive index n₂ for CO₂ to be 1.6 × 10⁻¹⁶ cm³ erg⁻¹atm⁻¹ at the laser wavelength. Multiphoton and avalanche ionization are the two possible processes leading to OB. Experimental observations indicate that avalanche ionization dominates in our experiment.² Assuming that this process occurs, the laser intensity I after filamentation can be evaluated and equals 2 × 10¹⁴ W/cm². It corresponds to a 9-μm-diameter interaction volume, which is typical for filaments.

© 1992 Optical Society of America