Local Symmetry Breaking Drives Picosecond Spin Domain Formation in Polycrystalline Semiconducting Films

Photoinduced spin-charge interconversion in semiconductors with spin-orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. A central question is whether the resulting spin-associated charge currents are robust to structural disorder, which is inherent to polycrystalline semiconductors that are desirable for device applications. Using femtosecond circular polarization-resolved pump-probe microscopy on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin-polarized positive and, unexpectedly, negative spin domains on the micron scale formed through lateral currents. Further, the polarization of these domains and lateral transport direction is switched upon switching the polarization of the pump helicity. Micron scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spin domain formation is driven by the presence of strong local inversion symmetry breaking via inter-grain structural disorder. We propose that this leads to spatially varying Rashba-like spin textures that drive spin-momentum locked currents, leading to local spin accumulation. Our results establish ultrafast spin domain formation in polycrystalline semiconductors as a new optically addressable platform for nanoscale spin-device physics.