Effects of pulse duration on self-focusing of ultra-short lasers in underdense plasmas

An experiment investigating laser self-focusing in underdense plasmas is presented. It was shown experimentally that the critical power for relativistic self-focusing P-c is not the only relevant parameter, in particular when the laser pulse duration is comparable to plasma particle motion times: omega(p)(-1) for electrons and omega(pi)(-1) for ions. Using time resolved shadowgraphy, it was demonstrated that: (i) a pulse does not relativistically self-focus if its duration is too short compared to omega(p)(-1), even in the case where the power is greater than P-c. This is due to defocusing by the longitudinal wake which is generated by the laser pulse itself. (ii) For pulses longer than omega(pi)(-1), self-focusing can occur even for powers lower than P-c. This is due to the radial expansion of ions, creating a channel whose effect combines with relativistic focusing and helps the pulse to self-focus. (C) 2002 American Institute of Physics.