A Microscopic Roundabout for Light

Artist impression of the light circulator. The yellow beam enters at the upper left port and is forced to leave the resonator at the lower left port. The red beam enters at that port but cannot follow the reverse path of the yellow beam as it is forced to propagate to the lower right exit. (Source: H.-J. Boluijt, AMOLF)

Circu­lators are important components in communi­cation tech­nology. Their unique way of routing light usually requires centimeter-sized magnets, which are difficult to minia­turize for use on optical chips. Researchers at AMOLF and the Univer­sity of Texas have circum­vented this problem with a vibra­ting glass ring that interacts with light. They thus created a micro­scale circu­lator that direc­tionally routes light on an optical chip without using magnets.

Circu­lators allow to transmit infor­mation without loss among more than two nodes in a network, which is why they are already widely used in optical networks. Circu­lators have several entrance and exit ports between which they route light in a special way: light entering a parti­cular port is forced to exit in a second port, but light entering that second port exits in a third port and so on. “Light propa­gation is symmetric in nature, which means if light can propa­gate from A to B, the reverse path is equally possible. We need a trick to break the symmetry”, says AMOLF group leader Ewold Verhagen. “Usually this trick is using centi­meter-sized magnets to impart directionality and break the symmetric nature of light propa­gation. Such systems are difficult to minia­turize for use on photonic chips.”

Verhagen and his colleagues create circu­lating behavior using a micro­scale glass ring reso­nator with a different trick. They let light in the ring interact with mechanical vibra­tions of the same structure. The researchers used this principle in earlier work to demon­strate one-way optical trans­mission. “By shining light of a control laser in the ring, light of a different color can excite vibra­tions through a force known as radiation pressure, but only if it propa­gates in the same direction as the control light wave”, Verhagen explains. “Since light propa­gates dif­ferently through a vibrating structure than through a structure that is standing still, the optical force breaks symmetry in the same way as a magnetic field would.”

Turning the one-way street for light they demon­strated before, into a useful optical round­about was not as straight­forward as it may seem, as postdoc John Mathew points out: “The challenge is to dictate the parti­cular exit to which light can be routed, such that it always takes the next port.” The researchers found the solution in optical inter­ference. Careful control of the optical paths in the structure ensures that light from each input construc­tively interferes in exactly the right output. “We demon­strated this circu­lation in experi­ments, and showed that it can be actively tuned. The frequency and power of the control laser allow the circu­lation to be turned on and off and change handed­ness”, says Mathew.

The new round­about for light is actually the first magnet-free, on-chip optical circu­lator. Although the research is funda­mental in nature, it has many possible appli­cations. Verhagen: “Devices like this could form building blocks for chips that use light instead of electrons to carry infor­mation, as well as for future quantum computers and communi­cation networks. The fact that the circu­lator can be actively controlled provides additional functionality as the optical circuits can be reconfigured at will.” (Source: AMOLF)

Reference: F. Ruesink et al.: Optical circulation in a multimode optomechanical resonator, Nat. Commun. 9, 1798 (2018); DOI: 10.1038/s41467-018-04202-y

Link: Center for Nanophotonics (E. Verhagen), AMOLF, Amsterdam, The Netherlands

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