UNIVERSITY PARK, Pa. — Xingjie Ni, assistant professor of electrical engineering at Penn State, has received a National Science Foundation (NSF) Faculty Early Career Development Program (CAREER) Award to develop new architecture for smaller scale, on-chip optical devices that also have greater light controllability. If successful, this research could lead to improved wearable displays, medical devices and other compact, lightweight optical equipment.

In order to create the smaller, more accurate optical devices, Ni proposes a hybrid architecture that combines two existing technologies: metasurfaces and integrated photonics. 

Metasurfaces are ultrathin nanostructures that can be applied to an object’s surface to control the behavior of light when it hits that surface. Typically, when light hits the surface of an object, it scatters. When the human eye perceives this scattered light, the brain is able to interpret the shape of the object, its distance from the viewer and other visual information. However, Ni previously developed a technology exploiting the metasurface that can be applied to the object to eliminate the detection of this information by erasing the object information carried by the scattered light. Because of this, the eye and brain will not perceive the object at all, earning this metasurface-based technology the nickname of “invisibility cloak.” The same metasurface concept enables the creation of ultrathin flat lenses or other optical components.

While this ability to manipulate light in specific and controlled ways is a core component to optical advances, one limitation is that it requires free space, that is, space between the light source and the lens, or between the lens and the object. For certain applications, such as optical computing, the device needs to be small enough to fit on a chip without any free-space light propagation. 

“There is already an existing technology called integrated photonics,” Ni said. “Just as electrons are routed through electrical wires in an electronic circuit, light is routed through waveguides on a photonic integrated chip. It incorporates many light-controlling components into a single chip, enabling miniature optical circuits similar to integrated electronic chips. While the photonic integrated circuit offers great advantages in terms of size, reliability, scalability and power consumption, it does not have the capability to manipulate light in the small scale.”

With this NSF CAREER Award, Ni will create a new hybrid architecture that combines integrated photonics and metasurfaces to take advantage of both the ability to put light waves on a chip and the ability to control the light precisely. 

“We can make lenses [with metasurfaces] on top of photonic integrated waveguides, and we can focus light, steer light or even project complicated images from such a chip,” Ni said.

Combining the two technologies has many advantages for practical applications. One of these applications is related to near-eye displays.

“When you think about current virtual-reality glasses like Microsoft’s HoloLens or Facebook’s Oculus, one big problem is the size of the devices,” Ni said. “They have to use big headsets containing all of the bulky optics and display components. With our technology, essentially we will need only a thin glass chip with photonic integrated waveguides and metasurfaces. These waveguides will guide light in front of the eyes, and the metasurface on top of it will project the image directly to the retina.”

Another application of this research is in optogenetics, a biological technique in which light is used to control neurons in the brain. In this scenario, a tiny photonic chip could guide light using integrated waveguides and then channel the light out at a very specific position to control the neuron activity. 

“The current optogenetic technology does not provide much of the spatial resolution when light is delivered into the brain,” Ni said. “With our photonic integrated metasurface chip, we can precisely control light to point it at a targeted position or focus it at a targeted depth. The precision can be at a single-neuron level."

Ni also explained that because there are multiple lenses on a single waveguide, different light signals can be sent out and collected back simultaneously. The collected optical signals can give the researchers real-time information about what is happening inside the brain. 

In addition to the research, Ni is looking forward to implementing the outreach and education portion of his NSF CAREER Award by hosting a local exhibition for K-12 students.

“Most often, when people think of optics, they think of big optical components like a lens or a prism, but we are interested in letting people know that optics is fascinating at nanoscale,” he said. “We would like to give teachers and students information on what modern nano-optics can do, which is very different from traditional optics.” 

NSF CAREER Awards offer the foundation’s “most prestigious awards in support of early-career faculty who have the potential to serve as academic role models in research and education and to lead advances in the mission of their department or organization,” according to the foundation’s website.