Imagine a guitar string strummed more than a billion times a second. No human can hear the sound, but this ultra-high-frequency hum may one day power the world’s most sensitive transducers or help to unlock the secrets of the quantum realm.
Described in a new study published in Photonics Research and selected for inclusion in Spotlight on Optics, researchers at the University of Florida and Carnegie Mellon University (CMU) have built a chip-sized device that uses forces exerted by photons to “strum” a single-crystal 4H silicon carbide (SiC) microdisk, inducing it to vibrate at frequencies that can be detected with the right techniques and equipment.
The research may open new doors to integrated sensing technologies that could operate from deep-sea labs to distant space probes.
Lead author of the study, Electrical & Computer Engineering Ph.D. student Yuncong Liu and his co-authors built many iterations and sizes of these complex microdisks in an effort to explore how these tiny devices operated in various configurations.
They were intrigued by the observation that the smaller the diameter of the disk, the higher the frequency produced when it interacted with a nearby passing laser beam “waveguided” by a nanoscale light rail.
How did they hear these impossible sounds?
Using light.
Carefully guided through the device by the on-chip waveguides, light moves through the chip and interacts with the disk. Researchers were able to detect and measure vibrations by monitoring the changes in the input and output of the laser beam.
Picture a miniature bell ringing at a pitch so high as to be practically singing in silence. The researchers are able to use light to “listen” to its tiny natural vibrations—caused purely by heat and motion—without intentionally disturbing the system. In tests, the chip achieved a vibration clarity (or “quality factor”) rarely seen at room temperature.
For the final devices, the team leveraged advanced nanofabrication techniques, including high-resolution electron-beam lithography and dry etching processes similar to those in the advanced semiconductor chip manufacturing industry. They ultimately crafted a microdisk resonator just 4 microns in radius—about 10 to 20 times smaller than a human hair’s diameter, the smallest reported in 4H-SiC.
Turn up the light
The researchers noticed a peculiar byproduct of the interactions between the disks and lasers—as the power of the laser increased, the disks were induced to “sing” in a self-sustaining way, something that had never before been observed.
As Feng explained, photons from the laser were actually leaking into the microdisk, forming a sort of racetrack, allowing the photons to zoom around the track, exerting force on the walls of the device and creating the resonance.
“As we increased the power, the microdisk device didn’t just vibrate—it sang,” explained Liu. “The laser began feeding energy into the motion, amplifying it into a self-sustained rhythm.”
The collaboration
This project was born from a convergence of expertise. Qing Li, Ph.D., associate professor at Carnegie Mellon University’s ECE department, focused his group’s work on the photonic properties of 4H-SiC for years, but its mechanical potential remained an unexplored frontier.
“Meantime, my group has been studying SiC electromechanical and optomechanical resonators and switches over the past 15 years, by exploiting the excellent mechanical and thermal properties of SiC crystals, especially 3C-SiC thin films,” said Feng, the Wally Rhines Professor in Quantum Engineering in ECE at UF.
Feng elaborated, “The two groups naturally leveraged our complementary experiences. Yuncong shuttled between UF and CMU, and his excellent collaborative efforts led to this exciting new milestone in SiC optomechanics.”
The future
This work could pave the way for ultra-compact high-precision clocks, communication systems that rely on light instead of electricity and transducers sensitive enough to detect vanishingly small masses and forces of single molecules. It opens new doors to integrated sensing technologies that could operate from deep-sea labs to distant space probes.
“These results are very encouraging for optomechanics in wide-bandgap materials,” said Feng. “This platform can bridge today’s photonic chip technology with emerging quantum devices to enable ultra-compact and powerful hybrid systems for efficient and coherent information processing.”
In addition to NSF, this work was supported by the Defense Advanced Research Projects Agency.
Other collaborators include Wenhan Sun, Ph.D. student at CMU, and Hamed Abiri, Ph.D. student at Georgia Institute of Technology. A link to the CMU ECE News on this work is here.