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University Of Minnesota Engineers Bend Light With GHz Acoustic Waves

University of Minnesota engineering researchers have developed a chip on which microwave frequency acoustic waves and light waves are generated and confined together so that the sound can efficiently control the light. The research was published in Nature Communications.

Light-sound interactions have long been exploited in acousto-optic devices based on bulk crystalline materials. Conventionally, these devices operate in MHz frequency range where the acoustic wavelength is much longer than the optical wavelength and a long interaction length is required to attain significant coupling.

With nanoscale transducers, acoustic waves with sub-optical wavelengths can now be excited to induce strong acousto-optic coupling in nanophotonic devices. In this case the group demonstrated microwave frequency surface acoustic wave transducers co-integrated with nanophotonic resonators on piezoelectric AlN substrates.The figure above  illustrates a sound wave passing across an integrated optical waveguide, overlaid with a colour map of the light field in it.

"At this high frequency, the wavelength of the sound is even shorter than the wavelength of light. This is achieved for the first time on a chip," said Semere Tadesse, a graduate student in the University of Minnesota's School of Physics and Astronomy and the first author of the paper. "In this unprecedented regime, sound can interact with light most efficiently to achieve high-speed modulation."

The chip is made with a silicon base coated with a layer of AlN that conducts an electric change. Applying alternating electrical signal to the material causes the material to deform periodically and generate sound waves that grow on its surface. The researchers used nanofabrication technology to make arrays of electrodes with a width of 100 nanometers to excite the sound waves at a frequency higher than 10GHz.

"Our breakthrough is to integrate optical circuits in the same layer of material with acoustic devices in order to attain extreme strong interaction between light and sound waves," said Mo Li, assistant professor in the Department of Electrical and Computer Engineering and the lead researcher of the study.

In addition to applications in communications, researchers are investigating the interaction between single photons (the fundamental quantum unit of light) and single phonons (the fundamental quantum unit of sound). The researcher plan to use sound waves as the information carriers for quantum computing.

'Sub-optical wavelength acoustic wave modulation of integrated photonic resonators at microwave frequencies' by Semere Ayalew Tadesse & Mo Li, appears in Nature Communications 5, Article number: 5402 doi:10.1038/ncomms6402

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