Guiding of Intense Laser Pulses in Efficient End-pumped Plasma Channels Generated by Self-guiding in Ar and H2 Clusters

We demonstrate that self-guiding of intense short pulses in clustered gases can be utilized to generate long plasma channels, which upon expansion form waveguides suitable for propagation of laser pulses at high intensity. This scheme has several advantages over waveguide-generation in non-clustered gases. The absorption of energy by the target depends on the size of the clusters and not on the average density of the gas, which allows greater control of the density encountered by the guided pulse. In particular, electron densities less than 1018 cm-3 are feasible. Moreover, since clusters absorb sub-picosecond pulses very efficiently, channel generation by an auxiliary long-pulse laser is no longer necessary and a considerably simpler setup suffices. The problem of taper at the channel entrance, an old bugbear of side-pumped waveguides in gases, is shown to be significantly reduced. Evidence will be presented of waveguide generation in gases of argon and hydrogen clusters, using different cryogenic sources. A slit source is used for argon, and waveguides < 1 cm could be formed, in which laser pulses with intensity > 1017 Wcm-2 were guided. The results of a propagation code suggest that even longer channels are well within experimental reach. Argon, however, has the disadvantage that a super-intense pulse would likely produce further ionization, and hence suffer ionization induced defocusing. Hydrogen clusters, which can easily be fully ionized, were formed using a more efficient conical nozzle cooled to 90 K, limiting maximum waveguide lengths to < 3 mm. Though these channels are short, there is no obvious reason why a longer target would not allow longer waveguides to be generated, and the experiments demonstrate the utility of this novel scheme