When arranged in just the right ways, two-dimensional materials can display unusual and valuable quantum effects such as superconductivity and exotic types of magnetism. Understanding why these effects arise, and how to control them, remains one of the biggest challenges for physicists and engineers. A new study published in Nature Physics has uncovered a previously unseen property that may explain how these mysterious quantum phases form and evolve.
Using a novel terahertz (THz) spectroscopy method, researchers found that thin stacks of 2D materials — commonly used in laboratories worldwide — can naturally create what are called cavities. These tiny spaces confine both light and electrons into even smaller regions, significantly altering their interactions and behavior.
“We’ve uncovered a hidden layer of control in quantum materials and opened a path to shaping light-matter interactions in ways that could help us both understand exotic phases of matter and ultimately harness them for future quantum technologies,” said James McIver, assistant professor of physics at Columbia and lead author of the paper.
The work traces its origins to Hamburg, where McIver led a research group at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD). The institute is part of the Max Planck-New York Center on Nonequilibrium Quantum Phenomena, a collaboration among MPSD, Columbia, the Flatiron Institute, and Cornell University. Researchers at the Center study how stable physical systems respond when pushed away from equilibrium.
McIver’s team explores these questions through light. “2D materials, with their fascinating macroscopic properties, often behave like black boxes. By shining light on them, we can literally shed light on the hidden behavior of their electrons, revealing details that would otherwise remain unseen,” said Gunda Kipp, a PhD student at MPSD and first author of the paper. One obstacle, however, is that the wavelengths of light needed to probe 2D materials are far larger than the materials themselves, which are thinner than a human hair.
To overcome this scale mismatch, the researchers developed a chip-sized spectroscope that compresses THz light — the range where many quantum effects occur — from about 1 millimeter down to just 3 micrometers. This compact design made it possible to directly observe how electrons move within 2D materials. They first tested their approach using graphene, a well-known form of carbon, to measure its optical conductivity.
What they found was unexpected: distinct standing waves.
“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 that produces a distinct note,” explained MPSD postdoctoral fellow and co-first-author Hope Bretscher.
In a guitar, the string’s fixed ends define where the wave can form. Pressing a finger on the string shortens the wave, changing the pitch of the note. In optics, a similar process occurs when two mirrors trap light between them, creating a standing wave inside what scientists call a cavity. When a material is placed inside that cavity, the trapped light can repeatedly interact with it, altering its electronic properties.
However, the researchers discovered that mirrors might not even be necessary.
“We found that the material’s own edges already act as mirrors,” said Kipp. With their THz spectroscope, they observed that excited streams of electrons reflect off the edges to form a type of hybrid light-matter quasiparticle called a plasmon polariton.
The McIver lab studied a device made up of multiple layers, each of which can act as a cavity separated by a few tens of nanometers. The plasmons that form in each layer can, in turn, interact — often strongly. “It’s like connecting two guitar strings; once linked, the note changes,” said Bretscher. “In our case, it changes drastically.”
The next step was to understand what determines the frequencies of these quasiparticles and how tightly light and matter couple together. “With co-author and MPSD postdoctoral fellow Marios Michael, we developed an analytical theory that only needed a handful of geometric sample parameters to match the observations of our experiments,” said Kipp. “With just a click of a button, our theory can extract the properties of a material and will help us design and tailor future samples to obtain specific properties. For example, by tracking resonances as functions of carrier density, temperature, or magnetic field, we may uncover the mechanisms driving different quantum phases.”
While this study focused on plasmons, the new chip-scale THz spectroscope could detect other types of quasiparticles oscillating in many different 2D materials. The team is already testing new samples in both Hamburg and New York.
“This whole project was a bit of a serendipitous discovery. We didn’t expect to see these cavity effects, but we’re excited to use them to manipulate phenomena in quantum materials going forward,” said Bretscher. “And now that we have a technique to see them, we’re intrigued to learn how they might be affecting other materials and phases.”
