Researchers at Institute of Science Tokyo, led by Professor Kouji Taniguchi and Assistant Professor Po-Jung Huang with graduate student Ichi Naruse, have devised a way to generate spin-polarized photocurrents from the bulk of a noncentrosymmetric crystal while cleanly separating that signal from surface effects - a long-standing obstacle in using the circular photogalvanic effect (CPGE) for opto-spintronic devices.
CPGE generates a helicity-dependent photocurrent - one whose direction depends on whether the illuminating light is left- or right-circularly polarized - in materials with strong spin–orbit coupling and broken inversion symmetry. Because electron spin and momentum become locked together in such materials, CPGE offers a purely optical way to generate and probe spin-polarized currents without an external magnetic field, making it attractive for opto-spintronic devices and helicity-sensitive photodetectors. The catch: conventional measurements excite only a thin near-surface region of the material, where bulk and surface electronic states coexist and their contributions to the measured photocurrent are difficult to disentangle.
The team worked with 2D organic–inorganic hybrid perovskite (2D-OIHP) crystals, where heavy lead atoms supply strong spin–orbit interaction and dipolar organic cation layers impose a bulk, macroscopic polarity. By illuminating these crystals with circularly polarized light at normal incidence, rather than the angled geometries more commonly used, the researchers found they could suppress the surface-state contribution via the measurement's symmetry, leaving only the bulk spin–momentum-locked response. The resulting photocurrent appeared perpendicular to the crystal's polarization axis and reversed with the light's handedness, but vanished when measured parallel to that axis, confirming the signal's bulk origin, since tilting the incidence angle to 45° let surface contributions reappear in both directions.
The researchers further showed that this bulk spin-polarized photocurrent could be switched in direction simply by changing which molecular enantiomer was built into the crystal: left- and right-handed versions of the same organic cation induce opposite crystal polarization, which in turn reversed the sign of the measured CPGE signal. That gives a chemical, rather than external-field, handle for controlling spin-dependent photoresponse.
"CPGE is closely related to spin-polarized electronic states and has attracted attention as a route for generating spin-polarized photocurrents by light," said Taniguchi, adding that the approach "can contribute to the development of helicity-sensitive photodetectors, spin-photonic devices and next-generation opto-spintronic materials based on 2D hybrid perovskites."