Generation of circularly polarized attosecond pulses by intense ultrashort laser pulses from extended asymmetric molecular ions

Abstract

We present a method for generation of single circularly polarized attosecond pulses in extended asymmetric HHe${}^{2+}$ molecular ions. By employing an intense ultrashort circularly polarized laser pulse with intensity $4.0\ifmmode\times\else\texttimes\fi{}{10}^{14}$ W/cm${}^{2}$, wavelength 400 nm, and duration 10 optical cycles, molecular high-order-harmonic generation (MHOHG) spectra with multiple plateaus exhibit characters of circular polarization. Using a classical laser-induced collision model, double collisions of continuum electrons first with neighboring ions and then second with parent ions are presented at a particular internuclear distance and confirmed from numerical solutions of a time-dependent Schr\"odinger equation. We analyze the MHOHG spectra with a Gabor time window and find that, due to the asymmetry of HHe${}^{2+}$, a single collision trajectory of continuum electrons with ions can produce circularly polarized harmonics, leading to single circularly polarized attosecond pulses for specific internuclear distances.