Researchers at Japan's Institute of Science Tokyo have demonstrated that the circular photogalvanic effect (CPGE) — a light-driven current whose direction depends on the handedness of circularly polarized light — can originate entirely within the bulk of a crystal, with no surface contribution. The result, published in Nano Letters (DOI: 10.1021/acs.nanolett.6c02601), gives materials designers a concrete handle for controlling spin-polarized photocurrents in layered semiconductors.

The finding matters for spintronics, a field that aims to exploit electron spin rather than charge to build smaller and more energy-efficient devices. CPGE is attractive because it generates spin-polarized photocurrents optically, in materials that carry no magnetization at all. Until now, however, researchers could not cleanly answer a basic engineering question: does the measured photocurrent come from the crystal's bulk or from its surface?

"In conventional CPGE measurements, signals originating from crystal surfaces and interfaces are often mixed with bulk contributions, making it difficult to determine the microscopic origin of the observed photocurrent," said Kouji Taniguchi, professor in the Department of Chemistry at Science Tokyo, who led the study with graduate student Ichi Naruse and assistant professor Po-Jung Huang.

Two layers, two jobs

The team worked with two-dimensional organic–inorganic hybrid perovskites (2D-OIHPs) built from alternating lead iodide layers and layers of 1-(p-tolyl)ethylammonium cations. Each component plays a distinct role. The lead atoms supply strong spin–orbit coupling, which ties electron motion to spin. The organic cations carry permanent electric dipoles that give the crystal a spontaneous macroscopic polarity — the asymmetry the CPGE requires.

To separate bulk from surface responses, the researchers illuminated the crystals with circularly polarized light at normal incidence — 90° to the crystal surface. At that angle, the symmetry of the measurement geometry suppresses surface contributions, while bulk states can still generate a CPGE signal.

The method produced a clear, directional result. Along the x-axis, perpendicular to the crystal's polarization, the team detected a photocurrent that reversed direction whenever they flipped the light's handedness. They then rotated the electrodes 90° to measure along the y-axis, parallel to the polarization. There, they detected no helicity-dependent current at all — exactly what the bulk crystal's symmetry predicts.

The contrast was decisive. At a 45° incidence angle, where the geometry no longer suppresses surface contributions, helicity-dependent currents appeared along both the x- and y-axes. The y-axis signal, in other words, shows up only when surface effects are allowed. Normal incidence is therefore essential for isolating the intrinsic bulk CPGE.

Chirality as a control knob

The researchers then ran a second, chemical test of the bulk origin. They synthesized two chiral-polar crystals using enantiomers — right-handed and left-handed versions of the same organic molecule. The two enantiomeric crystals exhibit opposite polarization directions, and the sign of the CPGE photocurrent reversed between them. Molecular handedness, fixed inside the bulk of the crystal, determines which way the current flows.

That is the commercially interesting result: the direction of a spin-polarized photocurrent can be set by molecular design rather than by device geometry or external fields. The approach also gives researchers a general method for distinguishing bulk from surface photoresponses in atomically layered hybrid materials, which should help characterize their spin-dependent properties.

"The findings of this study can contribute to the development of helicity-sensitive photodetectors, spin-photonic devices and next-generation opto-spintronic materials based on 2D hybrid perovskites," Taniguchi said.

The work remains at the level of fundamental materials physics — the paper reports a mechanism demonstration, not a device or a manufacturing process. But perovskite thin films already have an established fabrication base in photovoltaics, and the ability to program photocurrent direction through enantiomer selection points toward optical spin-current sources that need no magnetic field. Whether that translates into practical opto-spintronic components will depend on how well chiral 2D perovskites survive integration with conventional semiconductor process flows.