Light travels at different speeds and in different phases in different components of an integrated circuit and for light to couple between optical components, it needs to be moving at the same momentum.
A team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences, in collaboration with Peking University in Beijing, has been able to demonstrate an innovative new way to control the momentum of broadband light in a widely-used optical component known as a whispering gallery microcavity (WGM).
“The broadband optical chaos in a microcavity creates 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 needed multiple special optical elements to couple light in and out WGMs at different wavelengths, but with this work we can couple all colour lights with a single optical coupler.”
A WGM is a type of optical microresonator used in a wide variety of applications, from long-range transmission in optical fibres to quantum computing.
WGMs get their name from the whispering gallery in 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.
Optical whispering galleries work much the same way in that light waves trapped in a highly-confined, circular space orbit around the inside of the cavity - the cavity is able to trap and then carry the wave.
It is, however, difficult to couple the optical fields from waveguides to the optical fields in whispering galleries in photonic circuits because the waves are traveling at different speeds.
To address the problem of different momentum the research team turned to ‘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. Crucially, the coupling is broadband and occurs between optical states that would otherwise not couple.
The research provides new applications for microcavity optics and photonics in optical quantum processing, optical storage and more.
The team will explore the physics of optical chaos in other optical platforms and materials, including photonic crystals and diamonds.
The paper, whose co-authors also include researchers from Washington University in Saint Louis, the California Institute of Technology, and the University of Magdeburg, has been published in Science.