Researchers have discovered that stacks of two-dimensional materials naturally form microscopic cavities that trap light and electrons, altering quantum behavior without the need for mirrors. This finding, observed using a novel terahertz spectroscope, could enable new ways to control exotic quantum states. The study was published in Nature Physics.
Two-dimensional materials, prized for effects like superconductivity and exotic magnetism, have long puzzled scientists seeking to understand and manipulate their quantum properties. A team led by James McIver, assistant professor of physics at Columbia University, revealed a previously unseen mechanism in these materials during experiments originating at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany.
The research is part of the Max Planck-New York Center on Nonequilibrium Quantum Phenomena, involving collaborations with Columbia, the Flatiron Institute, and Cornell University. To probe the materials, the team developed a compact terahertz spectroscope that shrinks light wavelengths from about 1 millimeter to 3 micrometers, allowing direct observation of electron movements in thin samples thinner than a human hair.
Initial tests on graphene uncovered unexpected standing waves formed by hybrid light-matter quasiparticles known as plasmon polaritons. "Light can couple to electrons to form hybrid light-matter quasiparticles. These quasiparticles move as waves and, under certain conditions, they can become confined, much like the standing wave on a guitar string," explained Hope Bretscher, a postdoctoral fellow at MPSD and co-first author.
The key insight: the materials' edges naturally act as mirrors, creating cavities that confine light and electrons. In multilayer devices, these cavities—spaced tens of nanometers apart—allow plasmons to interact strongly. "We've found that the material's own edges already act as mirrors," said Gunda Kipp, a PhD student at MPSD and first author.
The team, including Marios Michael, developed an analytical theory using geometric parameters to predict quasiparticle frequencies and light-matter coupling. This tool could help design materials for specific quantum phases by varying factors like carrier density or temperature. "We've uncovered a hidden layer of control in quantum materials and opened a path to shaping light-matter interactions," said McIver.
Described as serendipitous, the discovery paves the way for broader applications in quantum technologies. The spectroscope is now being used to explore other 2D materials in Hamburg and New York.