Inside a material, such as an insulator, semiconductor or superconductor, a complex drama unfolds that determines the physical properties. Physicists work to observe these scenes and recreate the script that the actors—electrons, atoms and other particles—play out. It is no surprise that electrons are most frequently the stars in the stories behind electrical properties. But there is an important supporting actor that usually doesn’t get a fair share of the limelight.
This underrecognized actor in the electronic theater is sound, or more specifically the quantum mechanical excitations that carry sound and heat. Scientists treat these quantized vibrations as quantum mechanical particles called phonons(link is external). Similar to how photons are quantum particles of light, phonons are quantum particles of sound and other vibrations in a solid. Phonons are always pushing and pulling on electrons, atoms or molecules and producing new interactions between them.
The role that phonons play in the drama can be tricky for researchers to suss out. And sometimes when physicists identify an interesting story to study, they can’t easily find a material with all the requisite properties or of sufficient chemical purity.
To help overcome the challenges of working directly with phonons in physical materials, Professor Victor Galitski, Joint Quantum Institute (JQI) postdoctoral researcher Colin Rylands and their colleagues have cast photons in the role of phonons in a classic story of phonon-driven physics. In a paper published recently in Physical Review Letters(link is external), the team proposes an experiment to demonstrate photons adequacy as an understudy and describes the setup to make the show work.
“The key idea came from an interdisciplinary collaboration that led to the realization that seemingly unrelated electron-phonon systems and neutral atoms coupled to light may share the exact same mathematical description,” says Galitski. “This new approach promises to deliver a treasure trove of exciting phenomena that can be transplanted from material physics to light-matter cavity systems and vice versa."
Galitski and colleagues propose using a very carefully designed mirrored chamber—like coauthor Benjamin Lev has in his lab at Stanford University—as the stage where photons can take on the role of phonons. This type of chamber, called an optical cavity, is designed to hold light for a long time by bouncing it between the highly-reflective walls.
“We made cavities where if you stick your head in there—of course it's only a centimeter wide—you would see yourself 50,000 times,” says Lev. “Our mirrors are very highly polished and so the reflections don't rapidly decay away and get lost.”
In an optical cavity, the bouncing light can hold a cloud of atoms in a pattern that mimics the lattice of atoms in a solid. But if a cavity is too simple and can only contain a single light pattern—a mode—the lattice is frozen in place. The light has to be able to take on many modes to simulate the natural distortions of phonons in the material.
To create more dynamic stories with phonons, the team suggests using multimode confocal cavities. “Multimode confocal” basically means the chamber is shaped with unforgiving precision so that it can contain many distinct spatial distributions of light.
“If it were just a normal single-mode cavity—two curved mirrors spaced at some arbitrary distance from one another— it would only be a Gaussian shape that could bounce back and forth and would be kind of boring; your face would be really distorted,” says Lev. “But if you stick your face in our cavities, your face wouldn’t look too different—it looks a little different, but not too different. You can support most of the different shapes of the waveform of your face, and that will bounce back and forth.”