Collisionless self-focusing of CO210 µm P(20) laser light in SF6

We develop a simple intuitive picture of the vibration-rotation structure of the SF6molecule such that the molecular susceptibility responsible for self-focusing can be calculated. We treat the propagation dynamics by generalizing the standard steady-state Gaussian propagation equations to include the important effect of absorption in the wings of the spatial profile. By calibrating the model to absorption data at CO210 μm we find good agreement with beam waist data at the same wavelength. Absorption in the wings is dominant at low laser fluences, and the real part of the susceptibility is responsible for the defocusing-to-focusing turnover in the beam waist near 100 mJ/cm², consistent with the interpretation of Nowak and Ham [6].