Researchers at the City College of New York are charting a fast-growing area of quantum science centered on materials only a few atoms thick. In these systems, light, electric charge, and magnetism are closely connected rather than behaving independently.
The work comes from physicist Vinod M. Menon’s Laboratory for Nano and Micro Photonics (LaNMP). Researchers believe these unusual interactions could eventually support advanced optoelectronic devices and quantum technologies that manipulate light, charge, and electron spin together.
When Light and Magnetism Interact
In a review published in Nature Materials, titled “Excitons in van der Waals magnetic materials,” the researchers examine 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 forms when incoming light energizes an electron and causes it to move, leaving behind a positively charged “hole.” The electron and hole remain linked, forming an electrically neutral particle that can still interact strongly with light. Magnons are different. They are collective waves that travel through the organized magnetic structure of a material.
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.
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.”
Reading Magnetic States With Light
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.
“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.”
New Possibilities for Quantum Technology
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. These devices convert signals between microwave and optical frequencies, a capability that could become important for connecting components in future quantum networks.
Major Scientific Challenges Remain
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.
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.