Dynamical chiral high-harmonic generation via subcycle symmetry engineering

The polarization degree of freedom of lightwaves governs light-matter coupling mechanisms and interaction strengths, serving as a pivotal observable for probing quantum processes. In strong-field-driven systems, symmetry-constrained selection rules strictly dictate the efficiency and polarization states of nonlinear optical harmonics. Recent observations of interband Berry phases in the nonadiabatic dynamics of Bloch states reveal the feasibility of manipulating quantum-state evolution via the quantum geometry of the Bloch electrons. This quantum phase-controlled approach enables circumvention of conventional selection rules to extend the harmonic polarization control into the subcycle regime, yet experimental validation remains unexplored. Here, we present an all-optical scheme that achieves full chiral and elliptic control of high-harmonic generation via subcycle coherent waveform manipulation with phase-locked two-color driving fields. This approach exploits symmetry-breaking-induced deviations from conventional selection rules in both the light field and crystalline solids. By engineering the spectral phase between orthogonal harmonic components, which inherently encodes both the dipole phase and the interband Berry phase, we achieve deterministic polarization control of even-order harmonics. Crucially, polarization manipulation is mediated by the driving field’s symmetry parameters, enabling full Poincaré sphere control when combined with crystal rotation. This all-optical control scheme will advance high-harmonic spectroscopy as a chiral quantum light source, and providing broadband coherent emission control across extended spectra.