Researchers have experimentally steered electrons in bilayer graphene using circularly polarized light rather than a traditional magnetic field. First reported in Science by an international team including scientists from ICFO, Columbia University, NIMS Japan, and Nanyang Technological University, this breakthrough harnesses the quantum wave-like nature of electrons to generate bent electronic flows and transverse Hall responses without applying external magnetism.
Altering the trajectory of charge carriers in conductive materials required external magnetic forces, generating standard Hall effects. By shifting the control vector from macroscopic magnets to microscopic light polarization, physicists have opened a pathway for manipulating electronic properties in two-dimensional materials.
Engineering Electron Trajectories With Light
When an electrical voltage is sustained across a conducting material, charge carriers typically move along the electric field in straight paths akin to a rolling ball. Applying a magnetic field bends these trajectories, inducing transverse signals known as Hall responses. In a study published in Science, researchers demonstrated that circularly polarized infrared light can achieve this bending effect in bilayer graphene without any magnetic field present.
The collaborative investigation originated from a 2016 discussion between Justin Song from Nanyang Technological University and Frank Koppens, an ICREA Professor at ICFO. As Justin Song explains, “Electrons are not just particles, but can have a quantum wave-like nature.” In quantum materials such as bilayer graphene, that wave pattern exhibits a complex winding designated as quantum geometry. Koppens and Song hypothesized that this quantum geometry could be harnessed to steer electron flow using light.
Turning theory into hardware proved challenging. Jianbo Yin, a researcher in Koppens’ team and first author of the study, noted that the initial device architecture was complex. The team had to fabricate numerous experimental iterations, eventually collaborating with Cheng Tan and James Hone at Columbia University to refine device quality, alongside material contributions from Kenji Watanabe and Takashi Taniguchi of NIMS Japan.
Valley Selectivity and Berry Curvature Control
Bilayer graphene features two distinct pockets of electron valleys, designated as K and K’. When researchers apply a perpendicular electric field, the quantum geometrical properties of the electrons in these two valleys normally cause them to bend in opposite directions, resulting in the cancellation of their individual Hall effects.
To overcome this cancellation, the research team applied circular polarized infrared light onto the bilayer graphene device. This optical illumination selectively excited one specific valley population of electrons, generating a distinct photovoltage perpendicular to the standard electron flow.
“We now engineered the device and setup in such a way that current only flows with light illumination,” Koppens highlights. “With this, we were able to avoid the background noise that hampers measurements and achieve a sensitivity in the detection several orders of magnitude better than any other 2D material.”
Conventional photodetectors often require large voltage biases that trigger unwanted dark currents flowing even in the absence of light. By utilizing out-of-plane electric fields as a tuning knob, the researchers found they could control the bending angle of the electrons—quantified via Hall conductivity. Adjusting this voltage knob tunes the Berry curvature, a characteristic of quantum geometry, leading to giant Hall conductivity.
Toward Advanced Infrared Detection
This optical manipulation of charge carriers bridges fundamental quantum mechanics with practical optoelectronic engineering. By eliminating dark currents and leveraging valley-selective excitation, the discovery sets the stage for infrared detection and imaging applications.
As Koppens concludes, such discovery could have major implications in applications for infrared and teledetection architectures, proving that light itself can act as a steering mechanism for quantum materials.
