Isolated circularly polarized attosecond pulse generated by extreme birefringence effect in magnetized plasma

We propose a method to generate isolated and circularly polarized (CP) attosecond pulses through the interaction of a relativistic linearly polarized laser pulse with magnetized subcritical density plasma. When the strength of the magnetic field is comparable with the normalized laser intensity, the left-handed CP component of the laser can easily transmit through the plasma while the right-handed CP one is almost reflected, which is named the extreme birefringence effect (EBE). EBE serves as a handedness splitter and time-domain gate, reflecting an intense few-cycle CP laser pulse. Along with EBE, a high-density and backward-moving relativistic electron sheet is formed due to the strong electrostatic separation force, further compressing the few-cycle CP laser to a bright and isolated CP attosecond pulse. This work therefore provides a novel approach to generating single attosecond pulses that is directly compatible with existing laser systems operating with multi-cycle pulses and more moderate contrast ratios.