Polarisation states of high harmonic generation from aligned molecules

Abstract

We present detailed theoretical calculations based on the recently developed quantitative rescattering theory (QRS) for polarisation and ellipticity of high-order harmonics from aligned N2 and O2 generated with mid-infrared linearly polarised laser pulses. Within the QRS, the two polarisation states of high-order harmonics are due to the high-energy photons emitted with polarisation parallel or perpendicular to the direction of the same returning electron wave packet in the photo-recombination process. The complex transition dipoles responsible for these two components have, in general, difference phases, which results in the ellipticity of emitted harmonics. Our results show clear target-species dependent polarisation states. In particular, we found strong harmonic ellipticity in aligned N2, but weak ellipticity in aligned O2 for harmonics below 100 eV. We further show the factorisation of the macroscopic high harmonic generation under typical experimental conditions. This implies that results of polarisation measurements are quite robust to the macroscopic phase matching conditions.