Contactless technique exposes contrasting light responses in mirror-image materials

A new contactless technique is giving scientists an unusually direct look at how mirror-image materials respond to light—and the results suggest that these materials may act as microscopic filters for electron spin. Researchers from the Institute of Chemistry and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem have developed a method that measures light-induced charge separation in chiral two-dimensional perovskites without first turning the material into a complete electronic device. Their findings, published in Small, could make it faster to identify materials for next-generation sensors, spintronic components and optoelectronic technologies.

The materials studied by the team exist in two structural forms known as enantiomers. Like a left hand and a right hand, the R and S forms contain the same chemical components but are arranged as mirror images that cannot be perfectly superimposed. Under ordinary, unpolarized light, the two materials can appear nearly identical. Circularly polarized light, however, has an electric field that rotates either clockwise or counterclockwise as the light travels. That twist gives the material an opportunity to distinguish between the two directions, revealing its underlying chiral structure.

The researchers, led by Dr. Joanna Dehnel and Dr. Igal Levine, wanted to determine whether this structural handedness could influence not only how much light the material absorbs, but also what happens to the electrons generated after absorption. In conventional experiments, scientists typically attach metal electrodes to a sample so they can measure electrical currents or voltages. Yet contacts can introduce defects, alter charge movement or create signals that are difficult to separate from the material’s intrinsic behavior. The device itself may therefore become part of the experiment.

To avoid that problem, the team introduced circularly polarized time-resolved surface photovoltage, or CP-TRSPV. The approach uses pulses of circularly polarized light to create electron–hole pairs in the material and then monitors the resulting surface photovoltage without requiring a top electrical contact. A surface photovoltage develops when photoexcited charges separate, producing an imbalance of electrical potential near the surface. By recording how that signal changes over time, researchers can follow charge separation and recombination from nanoseconds through milliseconds.

This broad time window is crucial because the processes involved occur on very different scales. The initial interaction between light and the electronic structure takes place extremely rapidly. Electrons and holes can be created and separated almost immediately, while trapping, release and recombination processes continue for much longer. CP-TRSPV allows these stages to be distinguished without the complications introduced by a fabricated device, offering what the researchers describe as a direct way to listen to the material’s electrical response before it is wired into a circuit.

When the team illuminated the R and S forms with opposite helicities of circularly polarized light, the mirror-image samples responded in opposite ways. The R-form produced a stronger response under right-circularly polarized light, whereas the S-form responded more strongly under left-circularly polarized light. A racemic sample, containing a mixture of the two mirror-image forms and therefore lacking an overall chiral preference, showed no significant difference between the two light helicities. This control result strongly connected the effect to the handedness of the crystal rather than to an accidental difference in sample composition or measurement conditions.

The most striking observation was the scale of the electrical asymmetry. The researchers measured a photovoltage anisotropy factor, known as gSPV, reaching approximately −0.7 for the R material and 0.17 for the S material. The photovoltage difference was roughly 1,000 times larger than the corresponding difference in optical absorption. In other words, the mirror-image materials were not merely absorbing slightly different amounts of light. They were converting that light into separated electrical charges with a far greater handedness-dependent contrast.

The findings are consistent with chiral-induced spin selectivity, or CISS, a phenomenon in which a chiral structure preferentially transmits or separates electrons according to their spin orientation. Spin is a quantum property of electrons that can be thought of, in simplified terms, as an intrinsic form of angular momentum. In a chiral crystal, the combination of molecular arrangement, electronic motion and spin–orbit interactions may favor one spin direction over another as charges move through the material. The result is a structure that behaves somewhat like a microscopic spin filter, potentially separating charges more efficiently when the light’s helicity matches the material’s handedness.

The researchers emphasize that the long-lived photovoltage does not mean that electron spins remain coherent for milliseconds. Instead, the initial spin-selective process occurs rapidly and creates unequal populations of separated charges. Those populations can persist as charges become trapped, released or gradually recombine, leaving behind a measurable electrical signal long after the original spin-dependent event has ended. This distinction is important because it links an ultrafast quantum process to a macroscopic voltage that can be tracked with laboratory instruments.

The new method could change how researchers screen chiral materials. Rather than fabricating a complete device for every new chemical composition, scientists could first use CP-TRSPV to test whether a material produces a strong helicity-dependent response, how quickly charges separate and how long the resulting signal survives. Chiral two-dimensional perovskites are especially attractive because their layered structures can combine strong light absorption with tunable electronic properties, while their handedness may enable control of charge and spin without an external magnetic field. Possible future applications include detectors for circularly polarized light, spin-based electronics and optoelectronic systems that distinguish between different light helicities. By turning a difficult-to-observe quantum effect into a time-resolved electrical signal, the Hebrew University team has provided a potentially high-throughput route toward discovering materials whose hidden microscopic asymmetries could power future technologies.

News Publication Date: 8-Aug-2026

Web References: https://doi.org/10.1002/smll.74908

References: Small, DOI: 10.1002/smll.74908

Keywords

Chiral perovskites, circularly polarized light, surface photovoltage, CP-TRSPV, chiral-induced spin selectivity, CISS, electron spin, spintronics, optoelectronics, nanotechnology, materials science, photovoltage anisotropy