Circularly Polarized Lasing and Ultrafast Optical Helicity Switching in Chiral Perovskite Microcavities

Robust helicity that can be generated intrinsically and controlled on ultrafast timescales would open attractive opportunities for terahertz photonic switching and chirality‐sensitive quantum devices. Yet such control remains challenging because the intrinsic chiral perturbation of molecular emitters is usually too small to establish a robust helicity bias that can break symmetry and sustain the ultrafast emergence and switching of a helicity‐selected optical state. In this study, we address this challenge by engineering chiral R/S quasi‐2D perovskite microcavities (R/S‐2DPMs) and establishing a hierarchical framework for chirality amplification in this platform. The resulting R/S 2DPMs support single‐mode lasing with quality factors on the order of 10 3 . Polarization‐resolved measurements show pronounced dissymmetry amplification from molecule dissymmetry of g mol ∼ 10 −4 to spontaneous emission dissymmetry of g sp ∼ 10 −2 and then through stimulated emission dissymmetry of g laser ∼ 0.78. Time‐resolved spectroscopy further reveals opposite ultrafast helicity switching on 1 ∼ 3 ps timescales in R/S‐enantiomeric microcavities. These results establish chiral quasi‐2D perovskite microcavities as a compact platform that converts weak microscopic chirality into robust circularly polarized lasing and ultrafast helicity switching, opening new opportunities for chiral photonics and helicity‐programmable coherent light sources.