Whispering Gallery Microcavities Control Momentum of Light

A method to control the momentum of broadband light using whispering gallery microcavities* (WGMs) could enable easier application of integrated photonic circuits. Optical devices typically require the coupling of light between different components. However, conservation of momentum usually limits the bandwidth of the coupling. It becomes challenging to couple the optical fields because the waves from the two fields are traveling at different speeds.

Coupling the optical fields from waveguides to the optical fields in whispering galleries in photonic circuits is like trying to transfer a package between a bike and a car on a highway. Without the chaos, coupling photons to an optical mode is inefficient (left). With the chaos, the photons could be efficiently delivered to the optical mode (right) . Courtesy of Yin Feng and Xuejun Huang and Harvard John A. Paulson School of Engineering and Applied Sciences.

To resolve this difference in momentum, researchers from Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with researchers from Peking University, developed chaotic channels in which the angular momentum of light is not conserved and can change over time.

The team showed that by slightly deforming the shape of the micro-resonator, the momentum of light could be tuned and restrictions on momentum could be relaxed.

Researchers further found that the resonator could be designed to match momentum between the waveguide and the WGM, thus to produce coupling that was broadband and that occurred between optical states that would otherwise not couple. The chaotic scattering of the light within the deformed structure could transform optical modes of different angular momenta within a few picoseconds.

The team demonstrated efficient coupling from the visible to the NIR bands, between a nanowaveguide and whispering gallery modes, with quality factors exceeding 10 million. The change in broadband was shown to enhance the device conversion efficiency of the third-harmonic generation by greater than three orders of magnitude over conventional evanescent-wave coupling.

This research could lead to novel applications for microcavity optics and photonics in optical quantum processing and optical storage. Researchers believe that the observed broadband and fast momentum transformation could be used to enable applications such as multicolor lasers, broadband memories and multiwavelength optical networks.

SEAS researcher Linbo Shao said that the broadband optical chaos in the microcavity created one universal tool for accessing a range of optical states. “Previously, researchers needed multiple special optical elements to couple light in and out of WGMs at different wavelengths, but through this work we can couple all colors of light with a single optical coupler,” he said.

*Named for the whispering galleries of St. Paul’s Cathedral in London, WGMs are used as microresonators in a variety of applications, from long-range transmission in optical fibers to quantum computing.

The research was published in Science (doi: 10.1126/science.aao0763).