Shining Light on Quantum Materials: Interplay of Symmetry, Quantum Geometry, and Nonlinear Photocurrents

ABSTRACT Nonlinear photocurrents offer a versatile microscopic and broadband spectroscopic tool for quantum materials: optical excitation from terahertz to optical frequencies can generate a dc current that is read out electrically without engineered junctions or external bias fields. Canonically, second‐order dc photocurrents are differentiated into shift, injection, and ballistic currents, associated with real‐space wave‐packet displacement during photoexcitation, asymmetric excitation of Bloch states with different group velocities, and scattering‐controlled transport of injected carriers. A modern viewpoint recasts these mechanisms in terms of quantum geometry, linking intrinsic nonlinear response to Berry connections, Berry curvature, and quantum metric, while retaining the role of relaxation and disorder in steady‐state experiments. This framework applies across a broad range of systems, including polar and magnetic quantum materials, van der Waals heterostructures, topological phases, and flat‐band systems. Looking forward, many‐body effects and light‐wave‐driven symmetry control may extend nonlinear photocurrents from probes of quantum matter toward tools for steering nonequilibrium quantum states.