A team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences in collaboration with Peking University in Beijing has demonstrated a new way to control the momentum of broadband light. In a widely-used optical component known as a whispering gallery micro cavity (WGM).
Integrated photonic circuits, which rely on light rather than electrons to move information. Promise to revolutionize communications, sensing and data processing. But controlling and moving light poses serious challenges. One major hurdle 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.
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The whispering arch in Grand Central Station in New York City.
“The broadband optical chaos in micro cavity is creating a universal tool to access many optical states,” said Linbo Shao, a graduate student in the lab of Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering, at SEAS. “Previously, researchers need multiple special optical elements to couple light in and out WGMs at different wavelengths, but by this work we can couple all color lights with a single optical coupler.”
A WGM a type of optical micro resonator used in a wide variety of applications. From long-range transmission in optical fibers to quantum computing. Further, WGMs named after 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 occurs in the Echo Wall in the Temple of Heaven in China and in the whispering arch in Grand Central Station in New York City.
Although, 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. However, it is difficult to couple the optical fields from wave guides to the optical fields in whispering galleries in photonic circuits because the waves are traveling at different speeds.
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 micro resonator, the researchers were able to create and harness so-called chaotic channels. In which the angular momentum of light not conserved and can change over time. By alternating the shape of the resonator, the momentum is tuned. The resonator designed to match momentum between wave guides and WGMs. Importantly, the coupling is broadband and occurs between optical states that would otherwise not couple.
However, the research provides new applications for micro cavity optics and photonics in optical quantum processing, optical storage and more.
“The work illustrates a fundamentally different approach to probe this important class of micro resonators. 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 Cal Tech, who was not involved in this research.
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Next, the team will explore the physics of optical chaos in other optical platforms and materials. Including photonic crystals and diamonds.
[source: Harvard John A. Paulson School of Engineering and Applied Sciences]