Tiny, artificial earthquake-like waves generated by lasers may be the key to smaller, faster, and more efficient computer chips, say a group of American engineers.
At the core of the research is a new surface acoustic wave phonon laser developed by researchers spread between the University of Colorado at Boulder, the University of Arizona, and Sandia National Laboratories. The team revealed their design in a recent paper published in the journal Nature.
Surface Acoustic Waves
The laser operates on surface acoustic waves (SAWs), which behave somewhat like sound waves but travel only along the top layer of a material rather than penetrating deeper. At large scales, earthquakes are SAWs moving across the planet’s surface, leaving destruction in their wake. However, at smaller scales, these waves underpin some of our most essential wireless technologies.
“SAWs devices are critical to many of the world’s most important technologies,” said Matt Eichenfield, senior author of the new study and Gustafson Endowed Chair in Quantum Engineering at CU Boulder. “They’re in all modern cell phones, key fobs, garage door openers, most GPS receivers, many radar systems, and more.”
SAWs are not used directly in cellular communications but play an important role in maintaining signal fidelity. When your phone receives a wireless signal, that signal can easily become corrupted by noise and other signals. To filter the signal, it is converted into a SAW inside the phone, allowing chips to remove undesired noise. Then those SAWs are converted back into their original radio waves.
Laser Device
The team used this phenomenon in a new way to generate a phonon laser. This new type of laser operates similarly to a typical such device, except that it produces Earthquake-like vibration.
“Think of it almost like the waves from an earthquake, only on the surface of a small chip,” lead author said Alexander Wendt, a graduate student at the University of Arizona.
While SAW devices predate their work, the team has made major refinements to the technology. Their SAW devices require only a single chip, compared to the double-chip devices common today. Additionally, their laser produces higher-frequency SAWs that operate solely on battery power.
Redesigning the Laser
Traditional diode lasers bounce light between two tiny mirrors along the surface of a semiconductor chip. As light interacts with the excited atoms in the semiconductor, it gains energy, producing a more intense beam.
“Diode lasers are the cornerstone of most optical technologies because they can be operated with just a battery or simple voltage source, rather than needing more light to create the laser like a lot of previous kinds of lasers,” Eichenfield said. “We wanted to make an analog of that kind of laser but for SAWs.”
Their design is bar-shaped and only 0.5 mm long, consisting of a stack of materials including silicon, lithium niobate, and indium gallium arsenide. Silicon is a familiar material known for its use in computer chips, while lithium niobate has the notable property of producing oscillating electric fields when it vibrates. Curiously, the converse is also true: when electric fields are present in lithium niobate, it begins to vibrate. Indium gallium arsenide accelerates electrons to tremendous speeds when it encounters a weak electric field, which can be driven by vibrations in lithium niobate.
Vibrating Laser
Like how light bounces between mirrors in a traditional laser, the SAW laser operates by having an electric current in the indium gallium arsenide produce waves in the lithium niobate, which hit a reflector and bounce back. As the process repeats in a loop, the waves gain more energy moving forward than they lose moving backward, causing strength to accumulate.
“It loses almost 99% of its power when it’s moving backward, so we designed it to get a substantial amount of gain moving forward to beat that,” Wendt said.
Eventually, the wave grows large enough to leak out of one side of the device. The SAW grew to 1 gigahertz during the experiments, rippling at billions of times per second. However, the team believes that tens or hundreds of gigahertz may eventually be possible after continued work on the device. That would far eclipse the 4 gigahertz that most SAW devices are capable of.
Such an advancement would enable smaller cellular devices that require less power while offering even greater performance. Currently, phones use multiple chips for radio-to-SAW conversion, but the team believes that their work would enable that processing to be streamlined to a single chip per device.
“This phonon laser was the last domino standing that we needed to knock down,” Eichenfield concluded. “Now we can literally make every component that you need for a radio on one chip using the same kind of technology.”
The paper, “An Electrically Injected Solid-State Surface Acoustic Wave Phonon Laser,” appeared in Nature on January 14, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.