Chiral Cooling in an Optomechanical System

Microscope image of a silica glass resonator and optical fiber waveguide. Light and sound circulating in this type of resonator are shown to exhibit chiral effects. (Source: U. Illinois)

Energy loss due to scattering from material defects is known to set limits on the perfor­mance of nearly all technologies that we employ for communi­cations, timing, and navi­gation. In optical fiber communi­cation systems, scattering from material defects can reduce data fidelity over long distances thereby reducing achievable bandwidth. Since defect-free materials cannot be obtained, how can we possibly improve on the funda­mental techno­logical limits imposed by disorder?

A research colla­boration between the Univer­sity of Illinois at Urbana-Champaign, the National Institute of Standards and Tech­nology, and the Uni­versity of Maryland has revealed a new technique by which scattering of sound waves from disorder in a material can be suppressed on demand. All of this, can be simply achieved by illu­minating with the appro­priate color of laser light. The result could have a wide-ranging impact on sensors and communi­cation systems.

Gaurav Bahl, an assistant professor of mecha­nical science and engi­neering, and his research team have been studying the inter­action of light with sound in solid state micro-reso­nators. This new result is the culmi­nation of a series of experi­ments pursued by his team over the past several years, and a new scientific question posed in the right place. “Reso­nators can be thought of as echo chambers for sound and light, and can be as simple as micro-spherical balls of glass like those we used in our study,” Bahl explained. “Our research community has long understood that light can be used to create and amplify sound waves in resonators through a variety of optical forces. The resonant echoes help to increase the inter­action time between sound, light, and material disorder, making these subtle effects much easier to observe and control. Since inter­actions within reso­nators are funda­mentally no different from those taking place in any other system, these can be a really compact platform for exploring the under­lying physics.”

The key to suppres­sing scattering from disorder is to induce a mismatch in the propa­gation between the original and scattered directions. To suppress back-scattering of forward-moving sound waves, one must create a large acoustic impedance in the backward direction. This asymmetry for forward and backward propa­gating waves is termed as chira­lity of the medium. Most solid-state systems do not have chiral proper­ties, but these properties can be induced through magnetic fields or through space-time variation of the medium.

“A few years ago, we discovered that chirality can be induced for light using an opto-mecha­nical pheno­menon, in which light couples with propa­gating sound waves and renders the medium trans­parent. Our experi­ments at that time showed that the induced optical trans­parency only allows light to move unidirec­tionally, that is, it creates a preferen­tially low optical impedance in one direction,” Bahl said. “It is then that we met our colla­borator Jacob Taylor, a physicist at NIST, who asked us a simple question. What happens to the sound waves in such a system?”

“Our theo­retical modeling predicted that having a chiral system for sound propa­gation could suppress any back-scat­tering that may have been induced by disorder,” explained Taylor. “This concept arose from work we’ve been doing in the past few years inves­tigating topo­logical protec­tion for light, where chiral propa­gation is a key feature for improving the performance of devices. Initially the plan with Bahl’s team was just to show a difference between the forward and backward propa­gating sound waves, using a cooling effect created by light. But the system surprised us with an even stronger practical effect than expected.”

That simple question launched a new multi-year research effort in a direction that has not been explored previously. Working in close colla­boration, the team discovered that Brillouin light scat­tering, a specific kind of opto­mechanical inter­action, could also induce chirality for sound waves. “We experimentally prepared a chiral opto­mechanical system by circulating a laser field in the clockwise direction in a silica glass resonator. The laser wavelength, or color, was specially arranged to induce optical damping of only clockwise sound waves. This created a large acoustic impedance mismatch between clockwise and counter-clockwise directions of propa­gation,” explained Seunghwi Kim. “Sound waves that were propa­gating the clockwise direction experienced very high losses due to the opto­mechanical cooling effect. Sound waves moving in the counter-clockwise direction could move freely. Surpri­singly, we saw a huge reduction of scat­tering loss for counter-clockwise sound waves, since those waves could no longer scatter into the clockwise direction! In other words, even though disorder was present in the reso­nator, its action was suppressed.”

Disorder and material defects are un­avoidable optical fiber systems, resulting in lower data fidelity, bit errors, and bandwidth limi­tations. The team believes that techno­logies based on this discovery could be leveraged to circum­vent the impact of un­avoidable material defects in such systems. “We’ve seen already that many sensors, such as those found in your phone or in your car, can be limited by intrinsic defects in the materials,” added Taylor. “The approach intro­duced here provides a simple means of circum­venting those challenges, and may even help us approach the limits set by quantum mechanics, rather than our own engi­neering challenges.”

Practical appli­cations of this result may not be too many years off. Reduction of mechanical losses could also directly improve mechanics-based inertial navi­gation sensors that we use today. Examples that we encounter in daily life are accelero­meters and gyro­scopes, without which our mobile phones would be a lot less capable, and our cars and airplanes a lot less safe. (Source: U. Illinois)

Reference: S. Kim et al.: Dynamically induced robust phonon transport and chiral cooling in an optomechanical system, Nat. Commun. 8, 205 (2017); DOI: 10.1038/s41467-017-00247-7

Link: Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, USA


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