Mirror-image crystals reverse the direction of light-driven currents

The circular photogalvanic effect (CPGE), a phenomenon that generates helicity-dependent photocurrents in noncentrosymmetric materials, can originate purely from a crystal’s internal structure without a contribution from the surface, a study from Science Tokyo reveals.

Researchers demonstrated this effect in 2D organic–inorganic hybrid perovskites using circularly polarized light at normal incidence, which helped distinguish the bulk response from surface contributions. The finding establishes a strategy for regulating spin-polarized photocurrents and advancing opto-spintronic technologies.

The findings are published in Nano Letters.

Light-driven currents for spintronics

Most electronic devices we use today rely on electron charge to store, process and transmit information. However, scientists are exploring another electron property, known as spin, to develop new ways to control electronic signals. These technologies, known as spintronics, could make electronic devices smaller and more energy-efficient.

The circular photogalvanic effect (CPGE) has the potential to advance spintronics and provides a purely optical means to generate spin-polarized photocurrents in nonmagnetized materials.

When circularly polarized light falls on certain materials, it generates a photocurrent whose direction depends on the handedness (such as left- or right-handedness) of the light. This effect is particularly useful for generating and studying spin-dependent photocurrents in materials with strong spin–orbit interactions, where electron motion is closely linked to spin.

In certain materials, circularly polarized light can generate a photocurrent whose direction changes depending on whether the light is right- or left-handed.

However, when designing such materials, an important question remains: Does the photocurrent originate from the polarity of the bulk of the material or from its surface?

Tracing photocurrents to the crystal bulk

To address this question, researchers from Institute of Science Tokyo (Science Tokyo) in Japan demonstrate that CPGE originated from the bulk of two-dimensional organic–inorganic hybrid perovskite (2D-OIHP) crystals.

The team was led by professor Kouji Taniguchi and included graduate student Ichi Naruse and assistant professor Po‑Jung Huang of the Department of Chemistry at Science Tokyo.

“CPGE is closely related to spin-polarized electronic states and has attracted attention as a route for generating spin-polarized photocurrents by light. However, 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,” explains Taniguchi.

The researchers demonstrated the effect in 2D-OIHP crystals made of alternating lead iodide layers and 1-(p-tolyl)ethylammonium cation layers. The lead atoms give the crystal strong spin–orbit interaction, while the molecular cations have permanent electric dipoles that give the crystal a spontaneous macroscopic polarity.

Normal incidence separates bulk and surface

To determine the origin of CPGE, the researchers exposed 2D-OIHPs to circularly polarized light incident at 90° to the crystal surface. They detected a photocurrent perpendicular to the crystal’s polarization that reversed direction when they switched the light’s polarity or handedness. The photocurrent measurement was done along the x-axis in this setting.

The researchers then rotated the electrodes by 90° (aligned with the y-axis) and measured the photocurrent parallel to the polarization direction.

The researchers did not detect a helicity-dependent photocurrent in this direction, as predicted by the symmetry of the bulk crystal, supporting an intrinsic bulk origin of CPGE. However, at a 45° light incidence angle, the researchers detected helicity-dependent currents in both directions (x- and y-axes), indicating that normal incidence (at 90°) is important for detecting the intrinsic bulk CPGE.

The bulk and surface states have different polarizations and spin orientations. Under normal incidence of light, the symmetry of the measurement geometry suppresses the surface contributions, while the bulk states can still produce a CPGE signal.

Molecular handedness controls photocurrent direction

The researchers further verified the bulk origin of CPGE by synthesizing chiral-polar crystals containing either right- or left-handed organic molecules (enantiomers). The crystal chirality determines the direction of polarization, with the two enantiomeric crystals exhibiting opposite polarization directions. The sign of the CPGE photocurrent reversed between the two crystals, clearly indicating that bulk polarity determines the direction of the generated photocurrent.

By revealing the bulk origin of CPGE and showing that molecular design can control the resulting photocurrent direction, this study provides a potential strategy for harnessing light-driven charge and spin responses. The approach also offers a way to distinguish bulk and surface photoresponses in atomically layered hybrid materials, helping researchers better understand and design 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,” says Taniguchi.

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Sadie Harley

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Robert Egan

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