High-Order Harmonic Generation in Aligned Molecules

We demonstrate alignment control in high density vapors of $\mathrm{CS}{}_2$, hexane, and $\mathrm{N}{}_2$ $(\sim {10}^{17}$ molecules $\mathrm{cm}{}^{-3\mathrm{}})$ using the electric field of a 300 ps duration laser pulse. This was sufficient to study for the first time the molecular orientation dependence of high-order harmonic generation using a second, 70 fs duration laser pulse of intensity $\sim 5\times {10}^{14}\mathrm{W}\mathrm{cm}{}^{-2\mathrm{}}$. We were able to modulate and significantly enhance the harmonic intensity in aligned molecules compared to the randomly oriented case. Our results are consistent with the existence of an anisotropic dipole phase.