In a result that could reshape how scientists think about light-driven electronics, researchers at Institute of Science Tokyo have shown that the internal architecture of a crystal—not its surface—can single-handedly determine the direction of an electric current generated by circularly polarized light. The study, published in Nano Letters, demonstrates that a phenomenon known as the circular photogalvanic effect, or CPGE, arises purely from the bulk structure of a two-dimensional organic–inorganic hybrid perovskite. By carefully choosing how light strikes the crystal, the team was able to strip away confusing surface signals and reveal an unambiguous intrinsic response, opening a clearer path toward spin-based technologies that could one day replace conventional charge-based electronics.
Most of the electronic devices in use today store, process, and transmit information using the charge of the electron. But electrons carry a second property that has long fascinated physicists: spin, an intrinsic form of angular momentum. Harnessing spin alongside charge is the goal of spintronics, a field that promises devices that are smaller, faster, and far more energy-efficient than their purely charge-based counterparts. The challenge lies in generating and controlling spin-polarized currents without bulky magnets or cryogenic conditions, which is where optical phenomena such as CPGE enter the picture.
The circular photogalvanic effect is a striking example of how light can do more than simply liberate charge in a material. When circularly polarized light—light whose electric field rotates in either a left- or right-handed spiral—falls on certain noncentrosymmetric materials, it can drive a net photocurrent whose direction flips depending on the handedness of the light. Because this effect is closely tied to spin-polarized electronic states, it offers a purely optical route to generating spin currents in materials that are not magnetized, making it especially attractive for studying and exploiting spin–orbit interactions, the quantum coupling between an electron’s motion and its spin.
Yet a fundamental question has clouded the field: when researchers measure a helicity-dependent photocurrent, is it truly coming from the bulk polarity of the crystal, or is it an artifact of surfaces and interfaces? In conventional CPGE measurements, signals from crystal surfaces often mix with bulk contributions, making it nearly impossible to determine the microscopic origin of the observed current. Professor Kouji Taniguchi of Science Tokyo, who led the new study, noted that CPGE is closely related to spin-polarized electronic states and has attracted attention as a route for generating spin-polarized photocurrents by light, but that this mixing of surface and bulk signals has made the microscopic origin difficult to pin down.
To resolve the ambiguity, the Science Tokyo team—including graduate student Ichi Naruse and Assistant Professor Po-Jung Huang of the Department of Chemistry—turned to two-dimensional organic–inorganic hybrid perovskites, abbreviated 2D-OIHPs. These layered crystals are built from alternating sheets of lead iodide and organic 1-(p-tolyl)ethylammonium cations. The design is deliberate: lead atoms endow the crystal with strong spin–orbit interaction, while the molecular cations carry permanent electric dipoles that confer a spontaneous macroscopic polarity on the whole crystal. In other words, the material combines the ingredients needed for strong CPGE with a tunable internal polarity set entirely by molecular arrangement.
The crucial experimental innovation was a symmetry-selective measurement geometry. The researchers illuminated the crystal with circularly polarized light at normal incidence—striking the surface at 90 degrees—and detected a photocurrent perpendicular to the crystal’s polarization axis that reversed direction when the light’s handedness was switched. When they rotated the electrodes by 90 degrees to measure along the polarization direction, the helicity-dependent current vanished entirely, exactly as the symmetry of the bulk crystal predicts. This directional asymmetry is the fingerprint of a bulk effect: surface contributions, which have different polarizations and spin orientations, are suppressed under normal incidence while the bulk response survives.
The importance of geometry became even clearer in control experiments. When the researchers tilted the light to a 45-degree incidence angle, helicity-dependent currents appeared in both the x- and y-axes, indicating that oblique illumination reintroduces contributions that obscure the intrinsic bulk signal. Normal incidence, the team concluded, is essential for cleanly isolating the bulk CPGE. The finding gives experimentalists a practical recipe: by simply choosing how light enters the crystal, they can separate bulk and surface photoresponses in atomically layered hybrid materials, a capability that should accelerate the characterization of spin-dependent properties across this growing material family.
To prove that bulk polarity truly dictates the direction of the photocurrent, the researchers went a step further and synthesized chiral-polar crystals containing either right-handed or left-handed versions of the organic molecule—mirror-image enantiomers. The chirality of the molecules determines the direction of the crystal’s overall polarization, so the two enantiomeric crystals exhibit opposite polarization directions. Remarkably, the sign of the CPGE photocurrent reversed between the two crystals, providing direct evidence that the molecular arrangement within the bulk sets the polarity of the crystal and, with it, the direction of the light-generated current. Flipping the handedness of the building blocks flips the response of the entire device.
The implications extend well beyond a single material system. By establishing that molecular design controls bulk polarity and thereby the direction of spin-polarized photocurrents, the study offers a rational strategy for engineering light-driven charge and spin responses from the bottom up. Taniguchi and his colleagues suggest that the findings could contribute to the development of helicity-sensitive photodetectors, spin-photonic devices, and next-generation opto-spintronic materials based on 2D hybrid perovskites. A photodetector that distinguishes left- from right-circularly polarized light, or a spin current source that requires nothing more than a laser and a cleverly designed crystal, are no longer distant concepts but plausible engineering targets.
The work, made available online on September 1, 2026, and published in Volume 26, Issue 36 of Nano Letters on September 16, 2026, also carries a broader lesson for materials science: symmetry is not merely a descriptive label but an operational tool. By aligning measurement geometry with the symmetry of the bulk crystal, researchers can silence unwanted surface signals and read out the intrinsic response of a material with unprecedented clarity. As hybrid perovskites continue to mature as a platform for optoelectronics, the ability to dial in polarity through molecular arrangement—and to verify it through symmetry-selective excitation—may prove to be one of the field’s most valuable design principles, bringing spin-based information technologies a step closer to practical reality.
Subject of Research: Bulk circular photogalvanic effect in two-dimensional organic–inorganic hybrid perovskites
Article Title: Molecular arrangement controls crystal polarity and reverses photocurrent direction
Article References: Molecular arrangement controls crystal polarity and reverses photocurrent direction. (n.d.). Original publication
Image Credits: AI Generated
DOI: Not provided
Keywords: circular photogalvanic effect, hybrid perovskites, spintronics, crystal polarity, circularly polarized light, spin–orbit interaction, chiral crystals, photocurrent, 2D materials, opto-spintronics, molecular design, Nano Letters
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Bethany Barker. (October 3, 2026). Crystal Polarity Steers Light-Driven Currents in Hybrid Perovskites. Scienmag. https://scienmag.com/crystal-polarity-steers-light-driven-currents-in-hybrid-perovskites/
Bethany Barker. “Crystal Polarity Steers Light-Driven Currents in Hybrid Perovskites.” Scienmag, 3 October 2026, https://scienmag.com/crystal-polarity-steers-light-driven-currents-in-hybrid-perovskites/. Accessed 3 October 2026.
Bethany Barker. “Crystal Polarity Steers Light-Driven Currents in Hybrid Perovskites.” Scienmag. October 3, 2026. https://scienmag.com/crystal-polarity-steers-light-driven-currents-in-hybrid-perovskites/
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Tags: 2D materialsadvances in non-mchiral crystalscircular photogalvanic effectcircular photogalvanic effect in two-dimensional hybrid materialsCircularly polarized lightcrystal architecture control of optoelectronic responsescrystal polaritycrystal polarity and internal structure influence electronic propertiesharnessing optical phenomena for spin-based device developmenthybrid perovskitesinfluence of crystal symmetry on photocurrent directionintrinsic bulk effects versus surface signals in optoelectronicslight-driven currents in hybrid perovskitesmolecular designNano Lettersopto-spintronicsorganic-inorganic hybrid perovskites for light-driven electronicsphotocurrentspin–orbit interactionspintronicsspintronics and spin-polarized current generation
