Symmetry-breaking control of high-harmonic generation in solids: theory, mechanisms, and applications

High-harmonic generation (HHG) in solids, a frontier of extreme nonlinear optics, has recently witnessed remarkable progress through symmetry-breaking manipulation. Here we provide a systematic review of how lifting inversion, time-reversal, and rotational symmetries enables precise control over the efficiency, spectral structure, polarization state, and spatial coherence of solid-state HHG. We outline the core theoretical frameworks, including semiconductor Bloch equations and time-dependent density functional theory, and elucidate the underlying microscopic mechanisms, such as transition dipole phases, Berry curvature, quantum path interference, and Bloch-wave phase matching. Symmetry-breaking routes are categorized into three classes: in two-dimensional materials, symmetry is tuned by layer number, strain, doping, and stacking configuration; in topological materials, breaking time-reversal or inversion symmetry activates Berry connection and curvature effects, revealing the intrinsic link between chiral anomaly and topologically enhanced nonlinear responses; in heterostructures and interfaces, van der Waals stacking and twisted moiré superlattices reconfigure symmetry. These advances not only deepen the understanding of strong-field light–matter interaction but also lay the foundation for attosecond light sources, all-optical probing of topological properties, and on-chip nonlinear photonic devices. We conclude by discussing key challenges, including conversion efficiency, phase matching, and device integration, and we offer perspectives on future directions such as multiscale cooperative control, intelligent material design, and dynamically programmable devices.