Propagation of intense and short circularly polarized pulses in a molecular gas: From multiphoton ionization to nonlinear macroscopic effects

We present a detailed analysis of the propagation dynamics of short and intense circularly polarized pulses in an aligned diatomic gas. Compared to linearly polarized intense pulses, high harmonic generation (HHG) and the coherent generation of attosecond pulses in the intense-circular-polarization case are a new research area. More specifically, we numerically study the propagation of intense and short circularly polarized pulses in the one-electron ${\mathrm{H}}_{2}{}^{+}$ molecular gas, using a micro-macro Maxwell\char21{}Schr\"odinger model. In this model, the macroscopic polarization is computed from the solution of a large number of time-dependent Schr\"odinger equations, the source of dipole moments, and using a trace operator. We focus on the intensity and the phase of harmonics generated in the ${\mathrm{H}}_{2}{}^{+}$ gas as a function of the pulse-propagation distance. We show that short coherent circularly polarized pulses of same helicity can be generated in the molecular gas as a result of cooperative phase-matching effects.