Two-photon resonantly-enhanced negative nonlinear refractive index in xenon at 248 nm

We report measurements and ab initio calculations of a large negative nonlinear refractive index in xenon for linearly-polarized KrF laser light, and describe a simple experiment that demonstrates xenon compensation of both self-focusing and self-phase modulation. This effect arises because KrF laser frequencies lie just above a two-photon resonance with the Xe 6p[ 1 2]₀ state at 249.6 nm. The negative nonlinear index resulted in self-defocusing of 10 ps linearly-polarized 248.4 nm pulses in a 4.6 m cell at Xe pressures up to 1000 Torr. Numerical simulations of this defocusing at pressures up to 266 Torr yield a nonlinear index n₂ = - (2.06 ± 0.14) × 10^-14 esu for 760 Torr and 300 K, but the simulations fail to adequately reproduce the shapes of the measured profiles at pressures significantly above 266 Torr. In the compensation experiment, the cell was first filled up to 1260 Torr with pure carbon dioxide, whose large positive n₂ resulted in self-focusing of the beam. A small amount of Xe was then added to the cell and allowed to mix with the CO₂. Less than 130 Torr of Xe completely compensated the 1260 Torr of CO₂, allowing both the spectrum and beam profile to be recovered. Numerical simulations of the self-focusing measurements yield a nonlinear index of (1.60 ± 0.07) × 10^-15 esu for CO₂, and the compensation experiment gives n₂(Xe) = - (1.82 ± 0.08) × 10^-14esu at 760 Torr, 300 K, and 248.4 nm. The combined defocusing and compensation measurements give n₂(Xe)=-(1.97±0.23)×10^-14 esu, in good agreement with our ab initio calculation of - (1.54 ± 0.30) × 10^-14 esu.