Polarization Control in High Harmonic Generation Using Molecular Structures in Nonaligned Molecules

The symmetry of the target system plays a decisive role in the polarization of high harmonic generation (HHG). Molecules breaking the isotropic symmetry can be utilized to manipulate HHG polarization, but it has long been believed that prealignment is necessary to manifest the microscopic molecular structural effect within the macroscopic ensemble. In this work, we show that the molecular structural effect can be exploited in nonaligned molecular ensembles with appropriate 2-dimensional driving fields, despite the ensembles exhibiting isotropic macroscopic symmetry. The feasibility of this scheme is comprehensively elaborated with a multiscale theory from the perspective of symmetry breaking and is experimentally validated employing bichromatic counterrotating circularly polarized driving fields as an example. By varying the intensity ratio of the bichromatic components, substantially chiral high harmonics are generated from nonaligned molecules associated with the highest HHG efficiency, where, by contrast, the spectral chirality is nearly zero from the reference atom. Remarkably, we observe a simultaneous enhancement of both the chirality and yield of the harmonics from CO 2 , overcoming a commonly observed trade-off of the HHG efficiency for higher spectral chirality. Our findings hold the potential for a straightforward and robust pathway toward attosecond light sources with high brightness and large ellipticity.