Highly circularly polarized monochromatic high harmonic pulses via a reflective phase retarder at 27.9 eV

A circularly polarized (CP) single-order high harmonic pulse light source, operating at a photon energy of 27.9 eV, is developed. The production of CP harmonic pulses with a degree of polarization exceeding 99% is achieved by utilizing a high-throughput phase retarder composed of SiC mirrors. Notably, our phase retarder exhibits minimal deviation from quarter-wave phase retardation, with a precision of less than ±λ/50 across a 3.1 eV bandwidth. This wide bandwidth enables effective polarization conversion of attosecond pulses preserved. By implementing the phase retarder, we converted the linear polarization to circular with an ellipticity of 0.93.