Lyman alpha (1216 Å) generation in krypton

We measured the nonlinear susceptibilities of Kr up to fifth order via nonresonant third-harmonic generation (THG) of a frequency-doubled dye laser at 3648 Å. In agreement with theory the optimum intensity conversion efficiency saturates at 10^-5 in pure Kr. The phase matching of the third harmonic and fundamental wave breaks up owing to intensity-dependent changes of refractive indices. Phase matching in high-density Kr/Ar mixtures raised the 3648 Å to 1216 Å third harmonic conversion obtained in pure Kr by more than two orders of magnitude, as predicted by theory but hitherto never experimentally confirmed. With laser intensities of 5 × 10⁹ W/cm² at 3648 Å we measured conversion efficiencies of 1.4 × 10^-4 limited by gas breakdown. Taking into account the Lyman alpha absorption of Kr₂ and KrAr molecules in the high-density regime, a conversion efficiency of 10^-3 is expected from extrapolating the experimental results to Kr/Ar mixtures of up to 3 × 10³ torr. It is limited by self-focusing, assuming the minimum threshold intensity of gas breakdown is 10¹¹ W/cm², as predicted by theory.