Time-resolved study of third harmonic generation from anisotropically expanding clusters

Time-resolved Third Harmonic Generation (THG) from expanding argon gas clusters has been studied. A 400 nm pump (Ipump ∼ 1 x 1015 W/cm2 ) beam ionizes a gas jet composed of atomic clusters and residual gases. An 800 nm, 100 fs probe then generates third harmonic radiation from expanding clusters with controlled delays. The measured THG is sharply peaked at earlier delays than broad absorption resonances. Simulations show that the nonlinear susceptibility χ (3) of the individual clusters and the THG coherence length of the clustered plasma medium are optimized nearly simultaneously as the pre-heated clusters expand, and both contribute to the observed THG enhancement. We also measured THG anisotropy from expanding clusters. When resovii nantly enhanced, THG becomes temporarily anisotropic – i.e. a probe polarized perpendicular to the pump generates third-harmonic more efficiently than one polarized parallel – thereby characterizing the anisotropy of cluster expansion. By contrast, the linear optical response was isotropic. The physical mechanisms contributing to enhanced THG are scalable to relativistic probe intensity (limited only by pre-pulses in the laser system) and to high-order harmonic generation extending to the soft x-ray regime.