Anomalous Quenching of Second-Harmonic Generation by Symmetry-Breaking Spin Order in CrOCl

Abstract

Second-harmonic generation (SHG) is a sensitive optical probe of symmetry breaking because the allowed nonlinear susceptibility components are directly constrained by crystal and magnetic symmetry. In magnetic materials, the onset of spin order commonly activates additional SHG channels or redistributes intensity among polarization geometries, often producing an enhanced nonlinear response. Here we demonstrate the opposite phenomenology in the van der Waals antiferromagnet CrOCl: SHG is strongly quenched across the incommensurate and commensurate magnetic transitions. The suppression occurs broadly across polarization angles, in both co- and cross-polarized detection channels, and in all fitted magnetic-dipole tensor elements. By ruling out structural distortion, classical optical effects, and optical-resonance shifts as dominant origins, we assign the anomalous quench to a bulk magnetic-dipole SHG process whose radiative matrix element is reduced by reconstruction of the Cr d-orbital wave function assisted by spin–orbit coupling. This mechanism can be understood as magnetic-order-induced rotation of orbital pseudospin within the even-parity d manifold, revealing a novel route to nonlinear optical control that is distinct from conventional symmetry activation.