High-order harmonic generation in laser-aligned molecules

We demonstrate high-order harmonic generation in high-density vapors of laser-aligned molecules. Ensembles of aligned ${\mathrm{CS}}_2$ and ${\mathrm{N}}_2$ are formed in the focus of a 300 ps duration laser pulse with sufficient density $(\sim {10}^{17}$ molecules ${\mathrm{cm}}^{\mathrm{-}3}$) to enable efficient high-order harmonic generation by a second, 70-fs, high-intensity laser pulse. We are able to modulate and significantly enhance the harmonic intensity in aligned molecules compared to the randomly oriented case. Our results are explained by considering the influence of an anisotropic dipole phase. Strong support for this interpretation is provided by theoretical results for high-order harmonic generation in the aligned ${\mathrm{H}}_2^{+}$ model system.