Chiral High-Harmonic Generation via Subcycle Symmetry Engineering.

We demonstrate complete, all-optical control over the chirality and ellipticity of high-harmonic generation by manipulating dynamical symmetry with a phase-locked two-color field. This control arises from subcycle symmetry breaking governed by the quantum geometry of laser field-driven electronic states, which modifies the polarization selection rules. Specifically, by engineering the spectral phase between orthogonal harmonic components encoding both the transition dipole phase and interband Berry phase, we realize deterministic polarization control of even-order harmonics. When applied to representative materials such as Bi_{2}Se_{3}, BiTeI, and GaSe, this scheme reveals polarization responses that vary qualitatively with material-specific electronic structure, underscoring the profound influence of quantum geometry. The polarization manipulation is dictated by the driving field parameters, allowing full coverage of the Poincaré sphere when combined with crystal rotation.