Impact-ionization mediated self-focusing of long-wavelength infrared pulses in gases.

We numerically investigate the propagation dynamics of intense long-wavelength infrared pulses in a weakly ionized argon gas and show that the pulses undergo self-focusing due to the transverse variations of electron-impact ionization caused by quivering electrons. We demonstrate this plasma-induced self-focusing at a power much lower than the critical power for Kerr self-focusing, and find that it can be controlled by the initial electron density and pulse duration.