A team of engineers has achieved a significant breakthrough in technology by developing a surface acoustic wave phonon laser, capable of producing vibrations akin to mini-earthquakes on a microchip scale. This innovation has the potential to enhance the performance of smartphones and other wireless devices, making them smaller, faster, and more energy-efficient. The research was led by Matt Eichenfield, an incoming faculty member at the University of Colorado Boulder, alongside scientists from the University of Arizona and Sandia National Laboratories. Their findings were published on January 14, 2024, in the journal Nature.
Understanding Surface Acoustic Waves
The core of this new technology lies in surface acoustic waves (SAWs), which behave similarly to sound waves, but travel exclusively along a material’s surface rather than through air or deep inside the material. Large earthquakes generate powerful SAWs that can cause significant damage, while on a much smaller scale, these waves are integral to modern technology. According to Eichenfield, “SAW devices are critical to many of the world’s most important technologies,” noting their presence in smartphones, key fobs, garage door openers, GPS receivers, and radar systems.
Inside a smartphone, SAWs serve as precise filters that convert radio signals from cell towers into tiny mechanical vibrations. This process enables chips to differentiate useful signals from background noise, enhancing communication efficiency. The newly developed phonon laser introduces an innovative method for generating these surface waves, using a device that produces controlled vibrations similar to earthquake waves, but at a micro level.
A New Era for Signal Processing
The phonon laser operates differently from traditional lasers. Instead of emitting light, it generates vibrations, functioning as an analog to conventional diode lasers. These diode lasers create light by bouncing it between mirrors on a semiconductor chip, amplifying the beam through interaction with energized atoms. Eichenfield emphasized the goal of developing a similar device for SAWs, stating, “We wanted to make an analog of that kind of laser but for SAWs.”
The researchers constructed a bar-shaped device about half a millimeter long, composed of layered materials. The base is silicon, a standard in computer chip manufacturing. Above this layer is lithium niobate, a piezoelectric material that generates oscillating electric fields when vibrated. The top layer consists of an extremely thin sheet of indium gallium arsenide, which accelerates electrons to high speeds even under minimal electric fields. Together, these components allow vibrations on the lithium niobate surface to interact with fast-moving electrons in the indium gallium arsenide.
The operation of the device resembles a wave pool, where electric current through the indium gallium arsenide generates surface waves in the lithium niobate layer. These waves reflect back and forth, similar to light bouncing between mirrors in a laser. Each pass strengthens the wave, while backward movement diminishes its power. Eichenfield noted, “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.”
The researchers successfully generated surface acoustic waves vibrating at approximately 1 gigahertz, and they believe the design could achieve frequencies in the tens or even hundreds of gigahertz. Traditional SAW devices typically reach a maximum of around 4 gigahertz, making this new system significantly faster.
Eichenfield expressed optimism about the implications of this advancement, stating, “This phonon laser was the last domino standing that we needed to knock down. Now we can literally make every component that you need for a radio on one chip using the same kind of technology.” The phonon laser promises to simplify the signal processing tasks currently handled by multiple chips in smartphones, potentially transforming the landscape of wireless technology.
