Introducing a slow-light medium into an optical microresonator extends the lifetime of a photon circulating in the device by several orders of magnitude.

In the whispering gallery beneath the dome of St Paul’s Cathedral in London, whispers can be heard at any point in the gallery because of the efficiency with which sound waves travel along its curved wall. Similarly, in a whispering-gallery-mode resonator, light waves propagate efficiently along the device’s periphery. This and other properties make these structures outperform conventional optical resonators in several applications, such as microlasers. Now Yannick Dumeige from the University of Rennes 1, France, and colleagues at the French National Center for Scientific Research (CNRS), show that introducing a slow-light medium into a whispering-gallery-mode microresonator extends a photon’s lifetime in the resonator by several orders of magnitude. This feature of a resonator defines the length of time a photon can circulate in the device, and thus be stored, before being absorbed or scattered.

The traditional way of enhancing a photon’s lifetime in an optical microresonator is to fabricate the resonator with smoother walls or better mirrors and to use optical materials that are less lossy. Dumeige and co-workers take a different tack. They incorporate erbium ions into a glass microsphere, which forms a whispering-gallery-mode resonator, to generate a phenomenon called coherent population oscillations in the ions. This effect slows down light circulating in the resonator, increasing a photon’s optical path. As a result, the lifetime of the photon is extended from 210 ps to a record-breaking 2.5 ms—25 times larger than the previous record of 100 $𝜇$s for optical whispering-gallery-mode resonators. The authors’ approach could be used to make microarrays of solid-state optical memories that can store photons for long periods of time.

This research is published in Physical Review Letters.

–Ana Lopes

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