The concept that your earbuds could replicate everything your smartphone does, but better, may not be far off, thanks to a new category of synthetic materials potentially sparking the next revolution of wireless technologies and the work of researchers.
A recently published paper in Nature Materials by researchers from the University of Arizona Wyant College of Optical Sciences and Sandia National Laboratories detailed their success in combining specialised semiconductor and piezoelectric materials not traditionally used together and subsequently achieving substantial nonlinear interactions among phonons.
Previous innovations, including phonon amplifiers using these materials, have paved the way for potentially smaller, more efficient, and more powerful wireless devices such as smartphones and data transmitters.
Because these materials could facilitate devices to be smaller, require less signal strength and use less power, their applications in the IoT space are significant; from reducing consumption in battery-powered sensors to enhancing data transmission rates in smart cities dependent on reliable connectivity.
The material at the heart of it: phononics
At the heart of these advancements rests phononics, akin to photonics but harnessing phonons; particles that transmit mechanical vibrations within materials, analogous to sound but at frequencies beyond human hearing. Phononics and photonics exploit similar physical principles, offering novel pathways for technological progress. The key difference is that while photonics manipulates light (photons), phononics achieves similar feats using phonons.
Senior author of the study, Matt Eichenfield, who holds positions at both UArizona and Sandia National Laboratories, said: “Most people would probably be surprised to hear that there are something like 30 filters inside their cell phone whose sole job it is to transform radio waves into sound waves and back.”
As a component of front-end processors, these piezoelectric filters, fabricated on specialised microchips, play a crucial role in converting both sound and electronic waves repeatedly during data transmission in smartphones, according to Eichenfield.
He explained that because these can’t be made from the same materials, like silicon used in other essential chips of the front-end processor, the physical dimensions of the device are larger than optimal. Additionally, the process of alternating between radio waves and sound waves incurs losses that accumulate and diminish performance over time.
“Normally, phonons behave in a completely linear fashion, meaning they don’t interact with each other,” Eichenfield added. “It’s a bit like shining one laser pointer beam through another; they just go through each other.”
Nonlinear phononics refers to the interaction of phonons within special materials, as detailed by Eichenfield. In their paper, the researchers showcased “giant phononic nonlinearities.” The synthetic materials developed by the team enable phonons to interact far more intensely than in conventional materials.
“In the laser pointer analogy,” Eichenfield explained, this would be like changing the frequency of the photons in the first laser pointer when you turn on the second,” he said. “As a result, you’d see the beam from the first one changing colour.”
The researchers demonstrated with these new phononic materials that one phonon beam can indeed alter the frequency of another. They also illustrated that phonons can now be manipulated in ways previously achievable only with transistor-based electronics.
Integrating acoustic components
The team has been working to integrate all necessary components for radio frequency signal processors onto a single chip using acoustic wave technologies instead of transistor-based electronics, in a manner compatible with standard microprocessor manufacturing.
Their latest paper validates the feasibility of this approach. Previously, the team successfully developed acoustic components including amplifiers and switches. With the introduction of acoustic mixers in their latest work, they have completed the final piece of the puzzle.
“Now, you can point to every component in a diagram of a radiofrequency front-end processor and say, ‘Yeah, I can make all of these on one chip with acoustic waves,'” Eichenfield said. “We’re ready to move on to making the whole shebang in the acoustic domain.”
According to Eichenfield, consolidating all components necessary for creating a radio frequency front end on a single chip could potentially reduce the size of devices like cell phones and other wireless communication gadgets by up to a factor of 100.
The team demonstrated its proof of concept by integrating highly specialised materials into microelectronic-scale devices capable of transmitting acoustic waves. Specifically, they combined a silicon wafer with a thin layer of lithium niobate – a synthetic material widely used in piezoelectric devices and cell phones – and introduced an ultra-thin layer (less than 100 atoms thick) of a semiconductor containing indium gallium arsenide.
“When we combined these materials in just the right way, we were able to experimentally access a new regime of phononic nonlinearity,” said Lisa Hackett, Engineer at Sandia and the lead author on the paper. “This means we have a path forward to inventing high-performance tech for sending and receiving radio waves that’s smaller than has ever been possible.”
In this setup, acoustic waves travelling through the system exhibit nonlinear behaviours as they pass through the materials. This phenomenon can be harnessed to alter frequencies and encode information. Nonlinear effects, a mainstay in photonics for transforming invisible laser light into visible laser pointers, have faced technological and material limitations in phononics. For instance, while lithium niobate ranks among the most nonlinear phononic materials known, its practical application is hampered by the inherent weakness of these nonlinearities in isolation.
Through the addition of indium gallium arsenide semiconductor, Eichenfield’s team established an environment where acoustic waves traversing the material influence the distribution of electrical charges within the indium gallium arsenide semiconductor film. This interaction causes the acoustic waves to mix in controlled manners, opening up the potential for diverse applications.
“The effective nonlinearity you can generate with these materials is hundreds or even thousands of times larger than was possible before, which is crazy,” Eichenfield concluded. “If you could do the same for nonlinear optics, you would revolutionise the field.”
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