New Type of Optical Soliton Waves

These optical microcavities are where solitons are created. The solitary waves circle around the tiny disks at the speed of light (Source: Qi-Fan Yang / Caltech)

These optical microcavities are where solitons are created. The solitary waves circle around the tiny disks at the speed of light (Source: Qi-Fan Yang / Caltech)

Applied scientists led by Caltech’s Kerry Vahala have disco­vered a new type of optical soliton wave that travels in the wake of other soliton waves, hitching a ride on and feeding off of the energy of the other wave. Solitons are localized waves that act like particles: as they travel across space, they hold their shape and form rather than dispersing as other waves do. They were first dis­covered in 1834 when Scottish engineer John Scott Russell noted an unusual wave that formed after the sudden stop of a barge in the Union Canal that runs between Falkirk and Edinburgh. Russell tracked the resulting wave for one or two miles, and noted that it preserved its shape as it traveled, until he ulti­mately lost sight of it.

He dubbed his discovery a “wave of trans­lation.” By the end of the century, the pheno­menon had been described mathema­tically, ulti­mately giving birth to the concept of the soliton wave. Under normal conditions, waves tend to dissi­pate as they travel through space. Toss a stone into a pond, and the ripples will slowly die down as they spread out away from the point of impact. Solitons, on the other hand, do not.

In addition to water waves, solitons can occur as light waves. Vahala’s team studies light solitons by having them recirculate inde­finitely in micro­meter-scale circular circuits called optical micro­cavities. Solitons have applications in the creation of highly accurate optical clocks, and can be used in micro­wave oscil­lators that are used for navi­gation and radar systems, among other things. But despite decades of study, a soliton has never been observed behaving in a dependent — almost parasitic — manner.

“This new soliton rides along with another soliton — essentially, in the other soliton’s wake. It also syphons energy off of the other soliton so that it is self-sustaining. It can eventually grow larger than its host,” says Vahala, Ted and Ginger Jenkins Professor of Information Science and Techno­logy and Applied Physics and executive officer for applied physics and materials science in the Division of Engi­neering and Applied Science.

Vahala likens these newly discovered solitons to pilot fish, carni­vorous tropical fish that swim next to a shark so they can pick up scraps from the shark’s meals. And by swimming in the shark’s wake, the pilot fish reduce the drag of water on their own body, so they can travel with less effort. Vahala is describing the new type of soliton, dubbed the “Stokes soliton.” The new soliton was first observed by Caltech graduate students Qi-Fan Yang and Xu Yi. Because of the soliton’s ability to closely match the position and shape of the original soliton, Yang’s and Yi’s initial reaction to the wave was to suspect that labo­ratory instru­mentation was malfunctioning. “We confirmed that the signal was not an artifact of the instrumen­tation by observing the signal on two spectro­meters. We then knew it was real and had to figure out why a new soliton would spon­taneously appear like this,” Yang says.

The micro­cavities that Vahala and his team use include a laser input that provides the solitons with energy. This energy cannot be directly absorbed by the Stokes soliton. Instead, the energy is consumed by the “shark” soliton. But then, Vahala and his team found, the energy is pulled away by the pilot fish soliton, which grows in size while the other soliton shrinks. “Once we under­stood the envi­ronment required to sustain the new soliton, it actually became possible to design the micro­cavities to guarantee their formation and even their pro­perties like wave­length,” Yi says. (Source: Caltech)

Reference: Qi-Fan Yang et al.: Stokes solitons in optical microcavities, Nat. Phys., online 5 September 2016; DOI: 10.1038/nphys3875

Link: T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California, USA

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