Extending the strong-field approximation of high-order harmonic generation to polar molecules: gating mechanisms and extension of the harmonic cutoff

Overview & Synopsis

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.