Professor Yu and Team Receive Air Force MURI Grant

May 14 2014 | By Holly Evarts | Image: Nanfang Yu

Nanfang Yu, assistant professor of applied physics in the Department of Applied Physics and Applied Mathematics, is part of a team of researchers from Columbia, Harvard, Purdue, Stanford, and UPenn who have won a $6.5 million five-year grant from the Air Force Office of Scientific Research (AFOSR) Multidisciplinary University Research Initiative (MURI) program. Their project, "Active Metasurfaces for Advanced Wavefront Engineering and Waveguiding," is targeted at developing “flat” optical devices based on “metasurfaces”—ultra-thin optical components—to control light propagation in free space and in optical waveguides.

Schematic of a beam-steering metasurface used for optical radar applications.
Schematic of a beam-steering metasurface used for optical radar applications. The metasurface is made of a 2D array of optical scatterers that introduce different amounts of delay to different portions of the incident optical wavefront, which leads to a reflected light beam pointing to desirable directions. The reflection direction is controlled electrically by tuning the phase response of the optical scatterers.

“This is critical research that will study the interaction between light and low-dimensional designer structures and will address the challenge of miniaturizing optical components and devices,” says Yu.

The researchers hope to get rid of current bulky optical elements such as compound lenses and complex optical instrumentations and replace them with either highly integrated chips that run light as signals or with planar optical components that can realize sophisticated control of light when a light beam traverses through them. The team’s findings could lead to flat microscopes, flat beam-steering devices for optical radar, and integrated photonic circuits that process quantum information.

Metasurfaces are made of two-dimensional (2D) arrays of designer scatterers, such as optical antennas, which are miniature version of radio antennas and have nanometer-scale dimensions. The key feature of metasurfaces is that the optical scatterers are all different optically; that is, the scattered light from them can have different amplitude, phase, or polarization, so that metasurfaces can introduce a spatially varying optical response that can control light in extremely flexible ways. As a result, metasurfaces make it possible to realize functionalities that conventionally require 3D optical components or devices with a much larger footprint, such as, for example, focusing or steering light beams, or switching optical signals on integrated photonic chips.

“The interface between two materials is usually thought of as merely a passive boundary,” Yu explains. “The essence of metasurfaces, however, is to make an interface useful and functional via clever designs. Such designer optical interfaces can mold optical wavefronts of light propagating in free space into arbitrary shapes and can control surface waves propagating along the interfaces. We are seeing new physics and novel device functionalities introduced by metasurfaces that are distinctly different from those observed in three-dimensional materials.”

An optical radar uses light instead of microwave or radio wave used in conventional radar to probe objects far away from the observer. Usually carried by an airplane to scan the terrestrial surface, optical radar technology has the benefits of higher resolution and the ability to access the chemical information, if the frequency of the light is tuned to match with the absorption bands of the chemical to be detected. 

But, says Yu, the major challenge with optical radar technology is that the laser beam must be scanned very fast in a two dimensional way over a solid angle. The current method uses a steering mirror to mechanically bend the light beam, but it is slow and prone to mechanical problems. In addition, the system is bulky and needs to be housed in a big radar cone, cumbersome for the airplane carrying the optical radar. 

One of the aims of the MURI project is to demonstrate a flat optical radar made of a 2D-array of phased optical elements that can bend the propagation of light as the beam traverses the flat optical radar. The bending direction is controlled by electronics integrated with the optical elements so the flat optical radar will not have any movable compartments and can be easily integrated directly onto the surface of an airplane.

And it’s not just radar that Yu is thinking about. Flat devices are lightweight and can be made flexible. “So they are more portable and can conform to non-conventional surfaces, like the human body,” he says. “Common optical instruments, from microscopes to cameras to telescopes, could be made flat and could even be wearable. There are amazing things we could develop with this new technology!”