Tunable Chiral Terahertz Radiation With Colossal Circular Nonlinear Optical Response From Two‐Dimensional Perovskites

Chirality is a fundamental geometric property endowing materials with distinct optical behaviors, pivotal in the frontier of nonlinear optics and photonics. Chiral two‐dimensional (2D) hybrid organic‐inorganic perovskites (HOIPs) integrate intrinsically broken inversion symmetry with molecular chirality, offering exceptional potential for tailoring nonlinear optical responses. Here we report the generation of tunable chiral terahertz (THz) radiation from the prototypical chiral 2D HOIPs single crystals (R/S‐(BrBA) 2 PbBr 4 ), exhibiting giant and broadband circular nonlinear optical response with asymmetry factor that approaches the theoretical upper limit of 1, corresponding to near‐complete discrimination between opposite circular polarizations. By directly resolving the helicity‐dependent components of the emitted THz fields, we show that the helicity‐dependent response originates at the emission stage, rather than arising from propagation or post‐emission polarization filtering. Beyond polarization modulation, oscillatory features associated with chiral phonon modes were observed in the emitted THz signals, suggesting a coupling between THz chirality and collective lattice dynamics. These results establish the THz domain as a fertile regime for chiral light–matter interaction and open new pathways for ultrafast chiral spectroscopy, polarization control, and coherent manipulation of collective excitations in quantum and functional materials.