Critical power for self-focussing of a femtosecond laser pulse in helium

The critical power for self-focussing of a femtosecond laser pulse in helium has been measured using the moving focus method. The experimental value is Pcritexp(1 atm) ∼268 GW. Using this value, the nonlinear refractive index is inferred to be n2exp(1atm) ∼ 3.6 × 10-2110-21 cm2/W. In addition, the plots of the electron densities versus energy and pressure have also been used to determine the critical power of helium, based on the intensity clamping of the filamentation process. The value agrees well with the one by the moving focus method.