Smaller and faster: how will smartphones change with new technologies?

The team’s device uses a phenomenon known as surface acoustic waves (SAW). SAWs act in a similar way to sound waves, but, as their name suggests, they propagate only through the upper layer of the material. Earthquakes, for example, generate large seismic waves that propagate across the planet’s surface, shaking buildings and causing damage in the process. Meanwhile, much, much smaller SAWs are an important part of modern life.

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“SAW devices are fundamental to many of the world’s most important technologies,” explains Matt Eichenfield, incoming professor at the University of Colorado at Boulder, lead author of the new study and holder of the Gustafson Chair in Quantum Engineering at the University of Colorado at Boulder. “They are present in all modern cell phones, key fobs, garage door openers, most GPS receivers, numerous radar systems and more.”

In a smartphone, electromagnetic waves (SAW) already act as small filters. The radios inside the phone receive radio waves from a cell tower. They then convert those signals into small vibrations, allowing the chips to easily eliminate unwanted signals and noise. The same device then converts those vibrations back into radio waves.

In the current study, Eichenfield and his team developed a new way to make shock waves (SAW) using a phonon laser. It works like a regular laser pointer, with the difference that it generates vibrations. “Think of it almost like waves from an earthquake, only on the surface of a small chip,” adds Alexander Wendt, a graduate student at the University of Arizona and lead author of the new study.

Most current SAW devices require two different chips and a power supply to generate these waves. The team’s device, on the other hand, runs on a single chip and can produce SAW at much higher frequencies with just one battery.

To understand how the team’s new SAW device works, it’s helpful to think of a traditional laser. Most current lasers, known as “diode lasers,” work by bouncing a beam of light between two microscopic mirrors on the surface of a semiconductor chip. As the light bounces, it collides with the atoms of the semiconductor material, which are subjected to an electric field from a battery or other energy source. In the process, those atoms emit even more light and the beam becomes more powerful.

“Diode lasers are the cornerstone of most optical technologies because they can be powered by a simple battery or voltage source, rather than needing more light to create the laser, like many previous types of lasers,” Eichenfield said. “We wanted to create an analogue of that type of laser, but for seismic waves.”

To do this, the team developed a device that is shaped like a rod and measures approximately half a millimeter from end to end. The device is a stack of materials: In its final form, it is made of a silicon wafer, the same material found in most computer chips. On top of it is a thin layer of a material called lithium niobate. Lithium niobate is a “piezoelectric” material, meaning that when it vibrates, it also produces oscillating electric fields. Likewise, when oscillating electric fields exist, they create vibrations.

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Finally, the device includes an even thinner layer of indium gallium arsenide, an unusual material that, when exposed to a weak electric field, can accelerate electrons to incredibly fast speeds.

Together, the team’s stack allows vibrations on the surface of the lithium niobate to interact directly with the electrons in the indium gallium arsenide.

When the researchers pump their device with an electric current into the indium gallium arsenide, ripples form in the thin layer of lithium niobate. Those waves travel forward, hit a reflector, and then move back, similar to light bouncing between two mirrors in a laser. Each time those waves move forward, they become stronger. Every time they move back, they get a little weaker

After several bounces, the wave becomes very large. The device lets a small amount of that wave escape from one side, which is equivalent to how laser light accumulates and filters between its mirrors. The group managed to generate SAW waves that undulated at a speed of about 1 gigahertz, or billions of times per second. However, researchers also believe that they can easily increase this frequency to several tens or even hundreds of gigahertz.

This is a much higher frequency than traditional SAW devices, which tend to max out at about 4 gigahertz.

Eichenfield reports that the new device could lead to smaller, higher-performance, lower-power wireless devices like cell phones. In a smartphone, for example, numerous different chips convert radio waves to SAW and vice versa multiple times every time you send a text message, make a call, or access the Internet. His team wants to streamline that process, designing individual chips that can do all that processing using only SAW.

“This phonon laser was the last domino we had to knock over,” says Eichenfield. “We can now literally make all the components needed for a radio on a single chip using the same technology.”