Schemes for generation of isolated attosecond pulses of pure circular polarization

We propose and analyze two schemes capable of generating isolated attosecond pulses of pure circular polarization, based on results of numerical simulations. Both schemes utilize the generation of circularly polarized high-order-harmonics by crossing two circularly polarized counter-rotating pulses in a noncollinear geometry. Our results show that in this setup isolation of a single attosecond pulse can be achieved either by restricting the driver pulse duration to a few cycles or by temporally delaying the two crossed driver pulses. We further propose to compensate the temporal walk-off between the pulses across the focal spot and increasing the conversion efficiency by using angular spatial chirp to provide perfectly matched pulse fronts. The isolation of pure circularly polarized attosecond pulses, along with the opportunity to select their central energy and helicity in the noncollinear technique, opens new perspectives from which to study ultrafast dynamics in chiral systems and magnetic materials.The authors acknowledge Luis Plaja for valuable discussions. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA Grant Agreement No. 328334. C.H.-G. and I.J.S. acknowledge support from Junta de Castilla y León (Project SA116U13, UIC016) and MINECO (Grants No. FIS2013-44174-P and No. FIS2015-71933-REDT). A.J.-B. was supported by grants from the U.S. National Science Foundation (Grants No. PHY-1125844 and No. PHY-1068706). D.H. was supported via a grant from the Department of Energy. M.M.M., H.C.K., C.G.D., and A.B. acknowledge support by a MURI grant from Air Force Office of Scientific Research under Award Number FA9550-16-1- 0121. This work utilized the Janus supercomputer, which is supported by the U.S. National Science Foundation (Grant No. CNS-0821794) and the University of Colorado Boulder