Researchers at the City College of New York have confirmed something that sounds like a quantum physics fever dream: in materials just a few atoms thick, light, electric charge, and magnetism stop acting like awkward strangers at a party and start behaving like close friends. Physicist Vinod M. Menon's Laboratory for Nano and Micro Photonics (LaNMP) has been charting this fast-growing area of quantum science, and they're not just doing it for the academic thrill. They believe these unusual interactions could eventually power advanced optoelectronic devices and quantum technologies that manipulate light, charge, and electron spin together - because why settle for manipulating just one thing at a time?
In a review published in Nature Materials, titled "Excitons in van der Waals magnetic materials," the team examines recent progress involving layered magnetic semiconductors. These materials allow light-generated excitations called excitons to interact with magnetic order and with magnetic waves known as magnons. An exciton, for those not fluent in quantum, forms when incoming light energizes an electron, causing it to move and leave behind a positively charged "hole." The electron and hole remain linked, forming an electrically neutral particle that can still interact strongly with light. Magnons, on the other hand, are collective waves that travel through the organized magnetic structure of a material - think of them as the ocean waves of the magnetic world.
Scientists have spent years trying to unite the optical properties of exciton-rich semiconductors with magnetism. Earlier strategies included adding magnetic atoms to semiconductors or stacking atomically thin semiconductors on top of magnetic materials - essentially trying to force a friendship. Van der Waals magnetic semiconductors provide a more direct approach: within these crystals, excitons and magnetic moments can emerge from the same electronic orbitals. This shared origin allows light and magnetism to influence one another inside the material itself. "In these materials, light and magnetism no longer operate as separate channels," said Pratap Chandra Adak, a postdoctoral researcher in Menon's group and lead author of the review. "An exciton is not just a passive light-driven excitation sitting on top of the magnetism. It can sense the spin order and magnons, and under the right conditions, even help control the magnetic state itself."
The review examines several important material platforms, including chromium triiodide, nickel phosphorus trisulfide, and chromium sulfur bromide. Research on these two-dimensional magnets has revealed several ways that excitons and magnetic behavior can affect each other. Excitons can significantly strengthen magneto-optical effects, allowing scientists to identify magnetic states by observing changes in the polarization of light. Magnetic order can also alter the energy of excitons and influence where they are confined within a material. Interactions between excitons and magnons can connect optical signals with magnetic activity occurring at gigahertz frequencies. The researchers also discuss exciton polaritons, hybrid particles that combine properties of light and matter and can transport optical information through a material - because nature apparently decided that photons and electrons weren't enough.
"Over the past few years, this field has moved from detecting magnetism in atomically thin crystals to actively exploring how magnetic order can control light-matter interactions," said Menon, professor of physics and senior author of the review. "The goal of this article is to bring those developments into a coherent framework and identify where the field can go next." The researchers identify several potential applications that would depend on precise control of light and magnetism at extremely small scales. These include magneto-photonic memory and data readout, all-optical logic, adjustable light-emitting devices, magneto-optic lasers, and polaritonic technologies. Another promising application involves quantum transducers, which convert signals between microwave and optical frequencies - a capability that could become important for connecting components in future quantum networks.
Despite the rapid progress, much of this field remains unexplored. Many possible materials have not yet been studied in detail, and scientists still need better theoretical models that can predict how excitons, electron spins, lattice vibrations, and photons behave when they interact at the same time - because multitasking is hard, even for quantum particles. Future research could investigate moiré magnetic excitons, the optical control of spin textures, magneto-photonic devices, magnetic exciton polariton condensation, and the conversion of microwave signals into optical signals for quantum communication. Other co-authors include Florian Dirnberger of the Technical University of Munich; Swagata Acharya of the National Laboratory of the Rockies; Akashdeep Kamra of Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau; and Xiaodong Xu of the University of Washington. The work at CCNY was supported by DARPA and the Gordon and Betty Moore Foundation.