Graphene Quantum Dots Exhibit Anomalous Inverse Faraday Effect with Orders of Magnitude Weaker OAM Transfer

The interaction between light and two-dimensional materials presents exciting opportunities for controlling fundamental phenomena, and researchers are now exploring how light can induce magnetism in these systems. Zi-Yang Xu, Wei E. I. Sha from Zhejiang University, and Hang Xie from Chongqing University, report a surprising discovery concerning graphene quantum dots, revealing an anomalous inverse Faraday effect when illuminated with twisted light, known as optical vortices. Their work demonstrates that these vortices generate light-induced magnetic moments within the graphene, and crucially, they observe reversed magnetic moments in certain areas, leading to currents that rotate in the opposite direction to the light’s twist. This unexpected behaviour, explained through detailed analysis of the light’s phase and the material’s properties, establishes a new understanding of how light’s angular momentum can be converted into magnetism in carefully designed two-dimensional materials, potentially opening avenues for novel optical control and spintronic devices.

Chiral photon interactions with two-dimensional materials enable unprecedented control of quantum phenomena. This research reports anomalous inverse Faraday effects (IFE) in graphene quantum dots (GQDs) under linearly polarized optical vortex illumination. The investigation explores how these GQDs respond to light possessing a helical phase, specifically optical vortices, and demonstrates unusual IFE behaviour. This effect arises from the interplay between the chiral nature of the optical vortex and the unique electronic structure of the GQDs, leading to a circular polarization conversion efficiency that deviates from conventional expectations. The findings offer potential for novel optoelectronic devices and chiral light manipulation techniques.

Angular Momentum Transfer in Graphene Quantum Dots

Scientists developed a framework to investigate light-matter interactions in graphene dots (GQDs), revealing an unexpected phenomenon related to the transfer of orbital angular momentum (OAM). Researchers employed this framework to analyze the behavior of GQDs when illuminated with vortex beams, focusing on the generation of photocurrents and the transfer of angular momentum. To probe these effects, the team selected a hexagonal GQD, comprising 54 atoms, and computationally irradiated it with a Laguerre-Gaussian 01 (LG01) vortex beam. The absorption spectrum was calculated, revealing a low-frequency peak at 1.

847 eV (672nm) corresponding to lower-energy transitions and degenerate excited states crucial for OAM transfer. The team then computed time-dependent wavefunctions under resonant LG01 irradiation, directly yielding photoinduced currents categorized as type-II and type-III. Type-II currents oscillate in synchrony with the in-plane electric field, while type-III currents form quasi-steady vortices around the GQD center, signifying the acquisition of z-directional OAM and manifesting as an anomalous inverse Faraday effect. To quantify this effect, scientists systematically displaced the GQD within the beam field and computed the resulting orbital magnetic moments (OMMs). Results for LG01, LG02, and LG13 modes demonstrated radial and azimuthal sign reversals in the OMM distributions, with oscillation periods matching the LG nodal index. This research establishes a new understanding of OAM-to-magnetization conversion in engineered two-dimensional systems, revealing a previously unobserved phenomenon of periodic magnetic moment reversal.

Graphene Dots Convert Light to Magnetization

Scientists have demonstrated a new method for converting light’s orbital angular momentum (OAM) into magnetization within graphene dots (GQDs), revealing a previously unobserved phenomenon. The team employed a time-dependent perturbation framework to probe vortex-driven responses, specifically photocurrents and angular momentum transfer within the GQDs. Experiments focused on a hexagonal GQD, comprising 54 atoms, irradiated with a LG01 vortex beam, and revealed a low-frequency peak in the absorption spectrum at 1. 847 electron volts (672 nanometers). Analysis of the photoinduced currents showed two distinct types: type-II currents oscillating synchronously with the in-plane electric field, and type-III currents forming quasi-steady vortices around the GQD center, signifying electrons acquiring z-directional OAM and manifesting as IFE.

Further investigation quantified the generated orbital magnetic moment (OMM) by systematically displacing the GQD within the beam field. Results for LG01, LG02, and LG13 modes showed radial and azimuthal sign reversals in the OMM distributions, with oscillation periods matching the LG nodal index. The team observed an anomalous IFE, where the GQD acquired negative OAM when irradiated with vortex beams carrying positive OAM. This paradox was resolved through the discovery of a mechanism for negative OAM transfer, confirming the self-consistency of the findings. Detailed analysis of the light-induced torque on electrons revealed that the observed phase differences arise from the vortex beam’s azimuthal phase structure, inducing position-dependent phase shifts and rotational dynamics. The team demonstrated that the spatial variation in the electric field undergoes anticlockwise rotation, matching the type-III current vortices. These findings provide a quantitative interpretation of the observed phenomena, bridging the gap between electromagnetic theory and the observed charge polarization.

Graphene Dots Exhibit Inverse Faraday Effect

Researchers have demonstrated a new understanding of how light interacts with nanoscale materials, specifically graphene dots, revealing an unexpected phenomenon in the conversion of angular momentum to magnetism. The team discovered that linearly polarized vortex illumination induces magnetic moments within the graphene dots, but these moments exhibit a counterintuitive bipolar distribution, reversing polarity at off-axis positions. This anomalous inverse Faraday effect arises from the complex interplay between the transferred orbital angular momentum and the material’s electronic structure, generating rotational patterns not predicted by conventional models. Through a combination of theoretical modeling and spatially resolved measurements, the scientists rigorously validated this behavior, establishing a framework for understanding vortex-driven photoresponses in two-dimensional materials. Their analysis, employing time-dependent perturbation theory and eigenmode decomposition, revealed that the observed magnetic moment reversal stems from phase differences within the vortex electric field, effectively challenging the expected direction of angular momentum transfer.