. Extending the strong-field approximation of high-order harmonic generation to polar molecules: gating mechanisms and extension of the harmonic cutoff. Journal of Physics B: Atomic, Molecular and Optical Physics, IOP Publishing, 2010, 43 (15) Extending the strong-field approximation of high-order harmonic generation to polar molecules: Gating mechanisms and extension of the harmonic cutoff Adam Etches and Lars Bojer MadsenLundbeck Foundation Theoretical Center for Quantum System Research, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark Abstract. Polar molecules such as CO are interesting target systems for highorder harmonic generation (HHG) as they can be oriented with current laser techniques, thus allowing the study of systems without inversion symmetry. However, the asymmetry of the molecule also means that the molecular orbitals are Stark shifted in energy due to their interaction with the driving laser. We extend the strong-field approximation of HHG by incorporating the Stark shift into the Lewenstein model, and discuss its impact on two different gating mechanisms in CO. In system-induced gating an oriented target molecule serves as a gate by selecting every other half-cycle due to an increased (decreased) ionization rate. In field-induced gating the waveform of the driving laser is tailored such that the harmonic emission from an aligned molecule is damped (enhanced) every other half-cycle. We show that the Stark shift weakens the strength of system-induced gating, and also determines the relative contribution from opposite orientations in field-induced gating. Finally, we propose a novel scheme for extending the high-order harmonic cutoff by letting the two gating mechanisms counteract each other, thus allowing for a higher laser intensity without increased ionization of the target gas.

A recent paper reported elliptically polarized high-order harmonics from aligned N 2 using a linearly polarized driving field [X. Zhou et al., Phys. Rev. Lett. 102, 073902 (2009)]. This observation cannot be explained in the standard treatment of the Lewenstein model and has been ascribed to many-electron effects or the influence of the Coulomb force on the continuum electron. We show that nonvanishing ellipticity naturally appears within the Lewenstein model when using a multicenter stationary-phase method for treating the dynamics of the continuum electron. The reason for this is the appearance of additional contributions, which can be interpreted as quantum orbits in which the active electron is ionized at one atomic center within the molecule and recombines at another. The associated exchange harmonics are responsible for the nonvanishing ellipticity and result from a correlation between the ionization site and the recombination site in high-order harmonic generation.

We investigate the effect on high-order harmonic generation of the distortion of molecular orbitals by the driving laser field. Calculations for high-order harmonic generation including orbital distortion are performed for N2. Our results allow us to suggest that field-distortion is the reason why the two-center interference minimum has never been observed experimentally in N2. We propose experimental parameters which should allow an observation of the two-center interference minimum.