The best of both worlds

A wealth of photonic devices including precision sensors and quantum light sources depend on strong light-matter interactions.

Two leading platforms for enhancing (usually weak) light-matter interactions are photonic crystal waveguides and whispering gallery resonators.

In photonic crystal waveguides light is scattered back and forth by a periodic wavelength-scale modulation of the dielectric constant. This multiple scattering reduces the speed at which light pulses propagate, resulting in slow light that has more time to interact with matter. While photonic crystals can be used to slow or even stop light, the enhanced interaction strength is accompanied by increased sensitivity to scattering losses due to imperfections and roughness in the periodic modulation, which limits their performance in practical applications.

On the other hand, whispering gallery resonators guide light along the boundary large (many wavelengths in size) dielectric particles. These boundaries can be exceptionally smooth, resulting in ultralow scattering losses, allowing light to circulate millions of times before being lost from the cavity. However, one is limited to cavity sizes somewhat larger than the operating wavelength and cannot confine light to the nanoscale volumes achievable using photonic crystals.

Published yesterday in Nature Photonics, a collaboration between researchers at NIST and the University of Massachusetts report a novel hybrid whispering gallery microring resonator. The outer edge of the microring is smooth, leading to whispering gallery modes with high quality factors. The inner edge of the microring is periodically modulated, forming a one-dimensional photonic crystal that reduces the group velocity of the whispering gallery modes by a factor of 10. Moreover, defects in the inner edge photonic crystal can be used to confine the whispering gallery modes to a small part of the ring, enhancing their interaction with localized emitters such as quantum dots.

The exceptional lifetimes of whispering gallery modes combined with the ability to tailor modal properties using periodic modulations of the inner edge makes this a highly promising platform for a variety of nonlinear optics applications.