Circularly Polarized Quasimonochromatic High Harmonic Generation

Quasimonochromatic (QM) coherent extreme ultraviolet (EUV) pulses have emerged as crucial tools for spectroscopic and imaging applications. Such pulses could be obtained by selectively enhancing a specific harmonic order in high-harmonic generation (HHG). However, it is still challenging to directly generate circularly polarized QM-EUV via HHG, which is essential in ultrafast circular dichroism spectroscopies and circular-dichroism-integrated imaging techniques. Here, we experimentally demonstrate the generation of circularly polarized QM-EUV radiation by driving nonaligned CO_{2} molecules with bichromatic counter-rotating circularly polarized fields. An unreported selective enhancement of a single harmonic order, distinct from the previously observed phenomena in HHG from gases, is observed. The enhanced harmonic is over 50 times stronger than the adjacent harmonics and at least 25 times stronger than the other high harmonics. More importantly, its efficiency is higher than the counterpart with the monochromatic linearly polarized driving laser. This enhancement originates from the resonance due to the overlap between the excited state and photon-dressed autoionizing state. The polarization of the singly enhanced harmonic is experimentally characterized with an EUV polarizer, indicating an ellipticity up to 0.92.