Metamaterials manipulate light on a microchip

Controlling light on a microchip is no easy feat, but new theoretical designs for miniaturized optical devices made of metamaterials could make it a little easier.

A unified theory from researchers at Penn State University combines metamaterials and transformation optics. Transformation optics (TO) uses refraction in a rigorously mathematical way by applying the 150-year-old Maxwell equations describing the propagation of light onto metamaterials – artificial constructs with custom-designed refractive indexes. Metamaterials have been used in cloaking devices and perfect lenses, but those are just the tip of the optical iceberg.

Transformation optics devices that perform diverse, simple functions can be integrated to build complex photonic systems for optical communications, imaging, computing and sensing.

“This field [transformation optics] is in its early stages, so there are many contributions to be made,” said Douglas Werner, professor of electrical engineering. “Our big contribution is in figuring out how to develop TO designs with the simplest material parameters without impacting performance, and linking the devices together to form an on-chip integrated photonic system.”

Controlling light on a microchip could, in the short term, improve optical communications and allow sensing of any substance that interacts with electromagnetic waves. In the medium term, optical integrated circuits – the equivalent of the integrated electronic circuits found in cellphones and computers – for infrared imaging systems are feasible. High-speed all-optical computing is possible down the road, but this path requires some twists on well-known equations and the construction of structures smaller than the wavelength of light.

Among the Penn State designs are light collimators, waveguide couplers, TO splitters, waveguide crossings and TO benders, which turn the light around corners without loss. Each of these devices is only 5-10 µm in size, and many could fit on a centimeter-sized chip.

“In order to get the best design for a targeted application, thousands of simulations may have to be performed using powerful optimization techniques developed in our group,” Werner said. Doctoral candidate Jeremiah Turpin wrote algorithms for the simulation tools, and postdoctoral researcher Qi Wu developed the designs to be simulated.

TO devices that perform diverse, simple functions can be integrated together to build complex photonic systems for optical communications, imaging, computing and sensing, the researchers said. The current non-TO method is to design each device using different techniques and materials that may not be compatible on a single platform.

The Penn State method, on the other hand, employs graded index metamaterial structures, such as patterned air holes or rods, on a silicon-on-insulator platform that can be easily integrated into on-chip photonic systems, providing broad bandwidth and low losses.

All of the designs, described in Light: Science & Applications (doi: 10.1038/lsa.2012.38), can be realistically built with current fab processes, Werner said.

“It’s like a CAD tool,” he said, citing the computer-aided design tools used in manufacturing. “We’ve developed customized transformation optics simulation and optimization tools for designing optical devices. Beyond that, TO is flexible enough that it opens up the possibility of creating all sorts of new devices that don’t currently exist.”