Effects of phase-matching on high-order harmonic generation from aligned N2

We have examined high-order harmonic generation from transiently aligned room-temperature N-2 molecules in a hollow-core waveguide. We identify alignment-dependent propagation effects which can mask the single-molecule response. In particular, we observe an intensity-dependent reversal in the harmonic efficiency from parallel to perpendicular alignment. The reversal can be attributed to the alignment dependence of the tunneling ionization rate and the concomitant modification of the phase-matching conditions due to the changes in the free-electron density in the fiber.