Harnessing chaos for broadband coupling

OCT . 23 2017
Peking University, Oct. 23, 2017: Integrated photonic circuits, which rely on light rather than electrons to carry and transfer information, promise to revolutionize communications, sensing and data processing. But controlling and moving light poses serious challenges. One major hurtle is that light travels at different speeds and in different phases in different components of an integrated circuit. For light to couple between optical components, it needs to be moving at the same momentum.

Now, a research group at Peking University, collaborated with Washington University in Saint Louis, Harvard University, California Institute of Technology, and University of Magdeburg, has demonstrated a new way to control the momentum of broadband light in a widely-used optical component known as a whispering gallery microcavity (WGM).

A WGM is a type of optical microresonator used in a wide variety of applications, from long-range transmission in optical fibers to quantum computing. WGMs are named for the whispering galleries of St. Paul’s Cathedral in London, where an acoustic wave (a whisper) circulates inside a cavity (the dome) from a speaker on one side to a listener on the other. The similar phenomena occur in the Echo Wall in the Temple of Heaven in China and in the whispering arch in Grand Central Station in New York City.

Optical whispering galleries work much the same way. Light waves trapped in a highly-confined, circular space — smaller than a strand of hair — orbit around the inside of the cavity. Like the whispering wall, the cavity traps and carries the wave.  “WGMs are exceptional for various fundamental and applied studies that benefit from strongly enhanced light-matter interactions,” said 
Xiao Yun-Feng , professor in physics at Peking University and the corresponding author of the study.

However, it is difficult to couple the optical fields from waveguides to whispering galleries in photonic circuits because the waves are traveling at different speeds.

The guy riding a bike is delivering photons to a fast running car. It illustrates the coupling process between a straight optical waveguide and an optical cavity. (Left) Without the chaos, coupling photons to an optical mode is inefficient. (Right) With the chaos, the photons could be efficiently delivered to the optical mode.
(Artwork by 
Feng Yin and Huang Xuejun )

Think of a WGM as a highway roundabout and optical fields as UPS trucks. Now, imagine trying to transfer a package between two trucks while both are moving at different speeds. Nearly impossible, right?

In order to solve for this difference of momentum — without breaking Newton’s law of the conservation of momentum — the research team created a little chaos. By deforming the shape of the optical microresonator, the researchers were able to create and harness so-called chaotic channels, in which the angular momentum of light is not conserved and can change over time. By alternating the shape of the resonator, the momentum can be tuned; the resonator can be designed to match momentum between waveguides and WGMs. Importantly, the coupling is broadband and occurs between optical states that would otherwise not couple.

The broadband optical chaos in microcavity is creating a universal tool to access many optical states. Previously, researchers need multiple special optical elements to couple light in and out WGMs at different wavelengths, but by this work they can couple all color lights with a single optical coupler.

“This research provides a new platform for microcavity optics and photonics, and could promote nonlinear optics, optical quantum processing and optical storages,” said Gong 
Qihuang, Cheungkong Professor in Physics at Peking University, a co-author of this paper, also the president of Chinese Optical Society, and the vice president of International Commission for Optics.

Next, the team will explore the physics of optical chaos in other optical platforms and materials, including photonic crystals and diamonds.

 “The work illustrates a fundamentally different approach to probe this important class of microresonators while also revealing beautiful physics relating to the subject of optical chaos,” said Kerry Vahala, the Ted and Ginger Jenkins Professor of Information Science and Technology and Professor of Applied Physics at Caltech.

More information: Xuefeng Jiang, Linbo Shao, Shu-Xin Zhang, Xu Yi, Jan Wiersig, Li Wang, Qihuang Gong, Marko Loncar, Lan Yang, and Yun-Feng Xiao, Chaos-assisted broadband momentum transformation in optical microresonators, Science, DOI: 10.1126/science.aao0763

Edited by: Zhang Jiang
Source: School of Physics