Production and characterization of a fully ionized He plasma channel

We report guiding of intense (I=1.3±0.7×1017 W/cm2) 80 fs laser pulses with negligible spectral distortion through 1.5-cm-long preformed helium plasma channels. Channels were formed by axicon-focused laser pulses of either 0.3 J energy, 100 ps duration, after preionizing a 200–700 Torr backfill of He gas to ne∼1016 cm−3 with a pulsed electrical discharge; or 0.6–1.1 J energy, 400 ps duration, which required neither preionization nor intentional impurities for seeding. Transverse interferometry showed that He was fully ionized on the channel axis in both cases. Identical femtosecond pulses suffered substantial ionization-induced blueshifts after propagating through Ar and Ne channels of similar dimensions.