'Earthquake on a chip' uses 'phonon' lasers to make mobile devices more efficient

Engineers have created a device that produces tiny, earthquake-like vibrations on the surface of a chip. They say it could one day be harnessed for signal processing inside everyday electronics, potentially paving the way to smaller, faster and more efficient wireless devices.

In a new study published Jan. 14 in the journal Nature, the scientists described their device as a surface acoustic wave (SAW) phonon laser that generates very small, rapid vibrations.

"Think of it almost like the waves from an earthquake, only on the surface of a small chip," lead study author Alexander Wendt, a graduate student at the University of Arizona Wyant College of Optical Sciences, said in a statement.

In nature, SAWs are produced on a massive scale when tectonic plates slide against each other and cause earthquakes.

SAWs are also used as filters in smartphones to help clean up wireless signals. A phone's radio receives radio waves from a cell tower and then converts them into tiny mechanical vibrations, making it easier for chips to remove unwanted noise.

Multiple chips convert radio waves into SAWs and back again every time you send a text, make a call or access the internet.

SAWs in modern technology

Although they're conceptually similar to seismic surface waves released by earthquakes, SAWs are far too small to be measured on any scale like the moment magnitude scale, which is used to estimate the energy released by movement in Earth's crust.

SAW devices are essential to many of the world's most important technologies, senior study author Matt Eichenfield, a professor of quantum engineering at the University of Colorado Boulder, said in the statement. This includes cell phones, key fobs, garage door openers, most GPS receivers, and radar systems.

The scientists said a completely solid-state, single chip that generates coherent SAWs at very high frequencies, without needing an external radio-frequency source, has never been achieved before.

Traditional SAW components typically require two separate chips plus a power source. The team's design aimed to deliver similar functionality using a single chip — potentially enabling much higher frequencies to be powered by a typical smartphonebattery.

The researchers built the device by stacking ultrathin layers of different chip materials into a tiny "bar" about 0.02 inches (0.5 millimeters) long.

This included a silicon base; a thin layer of lithium niobate, a type of piezoelectric crystal that converts electrical signals into mechanical vibrations; and a final layer of indium gallium arsenide, a semiconductor material that can accelerate electrons to extremely high speeds when exposed to an electric field.

The system works by repeatedly amplifying vibrations as they bounce back and forth inside the structure, similar to how light intensifies in a diode laser between two mirrors. Surface vibrations in the lithium niobate interact with electrons in the indium gallium arsenide, boosting the energy of the waves as they move forward.

"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 in the statement.

The team generated surface waves at around 1 gigahertz — equal to billions of vibrations per second — and believes the design could be pushed into the tens or hundreds of gigahertz. That's well beyond the capabilities of typical SAW devices, which often top out around 4 GHz, the researchers said.

The long-term goal is to simplify how phones handle wireless signals — namely, by designing a single chip that can convert radio waves into SAWs and back again, using surface waves for much of the signal processing. Doing so could potentially enable future wireless devices to filter and route signals on smaller chips, using less power.

"This phonon laser was the last domino standing that we needed to knock down," Wendt added. "Now we can literally make every component that you need for a radio on one chip using the same kind of technology."