Pushing light to new limits

Marie Freebodymariefreebody@physics.org

Australian researchers are rewriting the rules on how light behaves when confined in ever-smaller optical fibers. Everything has its limits, and light-carrying optical fibers are no exception. Until now, it was thought that, as the size of the optical fiber shrinks, light becomes more and more confined until it reaches a point beyond which it cannot be squeezed any smaller, and it rapidly begins to diverge. This ultimate point was thought to occur when the strand of fiber is just a few hundred nanometers in diameter.

At the University of Adelaide, next-generation nanoscale optical fibers are being developed with a high-index contrast between core and cladding, inhomogeneous cross sections and subwavelength dimensions. The triangular-shaped glass core region, which measures just 450 nm, is represented in both the larger panel (A) and the inset (B), surrounded by three large holes. In panel (A), the large gray area surrounding the core is glass; with a total diameter of 150 µm, it makes up the rest of the fiber. Images courtesy of Wen Qi Zhang, Heike Ebendorff-Heidepriem and Tanya Monro.

Now, Shahraam Afshar and colleagues Wen Qi Zhang, Heike Ebendorff-Heidepriem and Tanya Monro at the University of Adelaide have discovered that they can push beyond that limit by almost a factor of two. They can do this thanks to a breakthrough in the theoretical understanding of how light behaves at the nanoscale, and thanks to the use of a new generation of nanoscale optical fibers being developed at the university.

“Rapid progress in fabrication of optical planar waveguides and microstructured fibers within cladding with high refractive indices has provided access to an emerging class of optical waveguides,” Afshar explained. “Unlike standard waveguides, these have a high-index contrast between core and cladding, inhomogeneous cross sections and subwavelength dimensions.”

Conventional wisdom dictates that pulse propagation in optical waveguides is described by the nonlinear Schrödinger equation. However, when it comes to the novel waveguides developed by Afshar’s team, all bets are off, and the equation no longer describes the behavior of light accurately.

“We have discovered that the ultimate limit of squeezing light into an area is smaller than what is expected according to standard theories,” Afshar said. “This means that we can access much higher nonlinearity in optical waveguides, which is important both in terms of fundamental studies of guided nonlinear optics and their applications.”

This discovery is expected to lead to more efficient tools for optical data processing in telecommunications networks and optical computing, as well as new light sources. But, according to Afshar, any application relying on guided nonlinear optics can benefit from the findings.

The next step for the Australian group is to explore induced nonlinear polarization and supercontinuum generation.