Production of intense isolated attosecond pulses with circular polarization by using counter-propagating relativistic lasers

We demonstrate theoretically and numerically that intense isolated circularly polarized (CP) attosecond pulses can be generated from ultrathin foil targets irradiated by two relativistic lasers from opposite sides, where their polarizations are orthogonal to each other. With a proper matching condition, the compressed oscillating plasma mirrors on both sides of the foil are pushed inside by laser radiation pressures, eventually merging together to form a dense electron nanobunch under the effect of orthogonal laser fields. This nanobunch reaches both high density and high energy in only half a laser cycle and smears out in others, resulting in coherent synchrotron emission of a single attosecond pulse with circular polarization. Two-dimensional particle-in-cell simulations show that an intense isolated CP attosecond XUV pulse with an intensity of 1.2 × 10¹⁹ W cm⁻² and a duration of ∼75 as can be obtained by two lasers with the same intensity of 2.1 × 10²⁰ W cm⁻². (paper)