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Optomechanical transducer links sound, light and RF

NIST researchers build GaAs-based piezo-optomechanical circuit

Researchers working at the National Institute of Standards and Technology (NIST) in America have developed a GaAs-based 'piezo-optomechanical' circuit that converts signals among optical, acoustic and radio waves. The teams thinks systems based on this kind of design could move and store information in next-generation computers.

The work was published in Nature Photonics and presented at the March 2016 meeting of the American Physical Society in Baltimore. 

While Moore's Law has proven remarkably resilient, engineers will soon begin to encounter fundamental limits. As transistors shrink, heat and other factors will begin to have magnified effects in circuits, researchers are considering designs in which electronic components interface with other physical systems that carry information such as light and sound.

Interfacing these different types of physical systems could circumvent some of the problems of components that rely on just one type of information carrier if researchers can develop efficient ways of converting signals from one type to another (transduction). 

Light, for example, is able to carry a lot of information and typically doesn't interact with its environment very strongly, so it doesn't heat up components like electricity does. But light is difficult to store for long periods, and it can't interact directly with some components of a circuit. On the other hand, acoustic wave devices are already used in wireless communications technology, where sound is easier to store for long periods in compact structures since it moves much more slowly.

NIST researchers and their collaborators have built a piezoelectric optomechanical circuit at the heart of which are optomechanical cavities supporting co-localised 1550nm photons and 2.4GHz phonons which are combined with photonic and phononic waveguides. They say that working in GaAs makes it easy to manipulate the localised mechanical mode either with an RF field through the piezoelectric effect (which produces acoustic waves that are routed and coupled to the optomechanical cavity by phononic-crystal waveguides), or optically. 

Each optomechanical cavity consists of an array of air holes in a tiny GaAs beam. The holes act like mirrors for light (photons). At the same time, the nanoscale beam confines phonons(mechanical vibrations), at a GHz frequency. The photons and phonons exchange energy so that vibrations of the beam influence the buildup of photons inside the cavity, while the buildup of photons inside the cavity influences the size of the mechanical vibrations. The strength of this mutual interaction, or coupling, is one of the largest reported for an optomechanical system.

One of the researchers' main innovations came from joining these cavities with acoustic waveguides. By channeling phonons into the optomechanical device, the group was able to manipulate the motion of the nanoscale beam directly. Because of the energy exchange, the phonons could change the properties of the light trapped in the device.

To generate the sound waves, which were at GHz frequencies, they used piezoelectric materials, which deform when an electric field is applied to them and vice versa. By using a an interdigitated transducer (IDT), which enhances this piezoelectric effect, the group was able to establish a link between radio frequency electromagnetic waves and the acoustic waves. The strong optomechanical links enable them to optically detect this confined coherent acoustic energy down to the level of a fraction of a phonon. 

They also observed controllable interference effects in sound waves by pitting electrically and optically generated phonons against each other. According to one of the paper's co-authors, Kartik Srinivasan, the device might allow detailed studies of these interactions and the development of phononic circuitry that can be modified with photons.

"Future information processing systems may need to incorporate other information carriers, such as photons and phonons, in order to carry out different tasks in an optimal way," says Srinivasan, a physicist at NIST's Center for Nanoscale Science and Technology. "This work presents one platform for transducing information between such different carriers."

'Coherent coupling between radio frequency, optical, and acoustic waves in piezo-optomechanical circuits' by K. Balram et al;  Nature Photonics. March 28, 2016. 

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