A team of engineers has made major strides in generating the tiniest earthquakes imaginable.
The team's device, known as a surface acoustic wave phonon laser, could one day help scientists make more sophisticated versions of chips in cellphones and other wireless devices—potentially making those tools smaller, faster and more efficient.
The study was conducted by Matt Eichenfield, an incoming faculty member at the University of Colorado Boulder, and scientists from the University of Arizona and Sandia National Laboratories. The researchers published their findings Jan. 14 in the journal Nature.
The new technology utilizes a phenomenon known as surface acoustic waves, or SAWs. SAWs act a little like soundwaves, but, as their name suggests, they travel only on the top layer of a material.
Earthquakes, for example, generate large SAWs that ripple over the planet's surface, shaking buildings and causing damage in the process.
Much, much smaller SAWs, meanwhile, are an important part of modern life.
"SAWs devices are critical to the many of the world's most important technologies," said 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."
In a smartphone, SAWs already act as little filters. Radios inside your phone receive radio waves coming from a cell tower. They then convert those signals into tiny vibrations, which allows chips to easily remove unwanted signals and noise. Then, the same device turns those vibrations back into radio waves.
In the current study, Eichenfield and his team developed a new way of making SAWs using a "phonon laser." It works like a run-of-the-mill laser pointer, except that it generates vibrations.
"Think of it almost like the waves from an earthquake, only on the surface of a small chip," said Alexander Wendt, a graduate student at the University of Arizona and lead author of the new study.
Most SAWs devices today require two different chips and a power source to generate these waves. The team's device, in contrast, works using just a single chip and can potentially produce SAWs at much higher frequencies paired only with a battery.
A new kind of laser
To understand how the team's new SAW device works, it helps to think about a traditional laser.
Most lasers around today, known as "diode lasers," work by bouncing a beam of light between two microscopic mirrors on the surface of a semiconductor chip. As that light bounces back and forth, it bangs into atoms in the semiconductor material that have an electric field running through them from a battery or other power source. In the process, those atoms eject even more light, and the beam becomes more powerful.
"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."
To do that, the team developed a device that's shaped like a bar and measures about half a millimeter from end to end.
The device is a stack of materials: In its finished form, it's made from a wafer of silicon, the same material in most computer chips. On top of that is a thin layer of a material called lithium niobate. Lithium niobate is a "piezoelectric" material, which means that when it vibrates, it also produces oscillating electric fields. Equivalently, when oscillating electric fields are present, they create vibrations.
Last, the device includes an even thinner layer of indium gallium arsenide—an unusual material that, when hit with a weak electric field, can accelerate electrons to incredibly fast speeds.
Altogether, the team's stack allows vibrations on the surface of the lithium niobate to directly interact with electrons in the indium gallium arsenide.
Doing the wave
The device works a bit like a wave pool.
When the researchers pump their device with an electric current in the indium gallium arsenide, waves will form in the thin layer of lithium niobate. Those waves slosh forward, hit a reflector, then slosh back—similar to light bouncing between two mirrors in a laser. Every time those waves move forward, they get stronger. Every time they move backward, they get a little weaker.
"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.
After several bounces, the wave becomes very large. The device lets a little of that wave leak out one side, which is equivalent to how laser light builds up and leaks out from between its mirrors.
The group was able to generate SAWs that rippled at a rate of about 1 gigahertz, or billions of times per second. But the researchers also think they can easily increase that to frequencies in the many tens of gigahertz or even hundreds of gigahertz.
That's much higher frequency than traditional SAW devices which tend to top out at about 4 gigahertz.
Eichenfield says the new device could lead to smaller, higher performance, and lower power wireless devices like cell phones.
In a smartphone, for example, numerous different chips convert radio waves into SAWs and back again multiple times every time you send a text, make a call, or access the internet.
His team wants to streamline that process, designing single chips that can do all that processing using SAWs alone.
"This phonon laser was the last domino standing that we needed to knock down," Eichenfield said. "Now we can literally make every component that you need for a radio on one chip using the same kind of technology."
