Attosecond polarization modulation of x-ray radiation in a free-electron laser

A new method to generate short wavelength free-electron laser output with modulated polarization at attosecond timescales is presented. Simulations demonstrate polarization switching timescales that are four orders of magnitude faster than the current state of the art and, at x-ray wavelengths, approaching the atomic unit of time of approximately 24 attoseconds. Such polarization control has significant potential in the study of ultrafast atomic and molecular processes. The output alternates between either orthogonal linear or circularly polarized light without the need for any polarizing optical elements. This facilitates operation at the high brightness x-ray wavelengths associated with FELs. As the method uses an afterburner configuration it would be relatively easy to install at existing FEL facilities, greatly expanding their research capability.