Metasurface Design Approach Achieves Unidirectional Luminescence in LEDs

Using a metasurface-based design, researchers at the University of Santa Barbara, California (UCSB) developed a way to extract more photons from LEDs and direct the light emitted from LEDs with greater precision. Their approach could be used to create more efficient and versatile LED display and lighting technologies.

LEDs emit spontaneous light, in contrast to the coherent light emitted by a laser. Spontaneous emission samples all the possible ways the photon is allowed to go, so the light appears as a spray of photons traveling in all possible directions. The UCSB team wanted to find out whether the photons generated in an LED could be herded in a desired direction through nanoscale design and fabrication of the LED’s semiconductor surface.

“We realized that if you looked at the angular distribution of the emitted photon before patterning, it tended to peak at a certain direction that would normally be trapped within the LED structure,” professor Jonathan Schuller said. “And so we realized that you could design around that normally trapped light using traditional metasurface concepts.”

The researchers designed an array of 1.45-μm-long gallium nitride (GaN) nanorods on a sapphire substrate, in which quantum wells of indium gallium nitride (InGaN) were embedded. The quantum-well structures generated narrow, unidirectional transmission and emission lobes at arbitrary engineered angles. The team demonstrated sevenfold and hundredfold enhancements of total and air-coupled external quantum efficiencies, respectively. In addition to allowing more light to be emitted, the process polarized the light.

“What we showed is a new kind of photonic architecture that not only allows you to extract more photons, but also to direct them where you want,” Schuller said. This improved performance, he said, was achieved without the bulky external packaging components that are often used to manipulate the light emitted by LEDs.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-0641-x).