Pushing Photons for More Efficient LEDs

UC Santa Barbara researchers continue to push the boundaries of LED design a little further with a new method that could pave the way toward more efficient and versatile LED display and lighting tech­nology. Jonathan Schuller and colla­borators developed a new approach, which could allow a wide variety of LED devices from virtual reality headsets to auto­motive lighting to become more sophis­ticated and sleeker at the same time. “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,” said Schuller. This improved performance, he explained, is achieved without the external packaging components that are often used to mani­pulate the light emitted by LEDs.

Illustration of the GaN nanopillar system with InGaN multiple QWs grown on a GaN base layer on a double-side polished sapphire substrate. (Source: P. P. Iyer et al., NPG / UCSB)

Light in LEDs is generated in the semi­conductor material when excited, nega­tively charged electrons traveling along the semi­conductor’s crystal lattice meet positively-charged holes and transition to a lower state of energy, releasing a photon along the way. Over the course of their measurements, the researchers found that a significant amount of these photons were being generated but were not making it out of the LED. “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,” Schuller said. “And so we realized that you could design around that normally trapped light using traditional meta­surface concepts.”

The design they settled upon consists of an array of 1.45-micrometer long gallium nitride nanorods on a sapphire substrate, in which quantum wells of indium gallium nitride were embedded, to confine electrons and holes and thus emit light. In addition to allowing more light to leave the semi­conductor structure, the process polarizes the light, which Prasad Iyer said, “is critical for a lot of applications.” The idea for the project came to Iyer a couple of years ago as he was completing his doctorate in Schuller’s lab, where the research is focused on photonics tech­nology and optical phenomena at subwave­length scales. Metasurfaces were the focus of his research. “A meta­surface is essentially a subwave­length array of antennas,” said Iyer, who previously was researching how to steer laser beams with meta­surfaces. He understood that typical meta­surfaces rely on the highly direc­tional properties of the incoming laser beam to produce a highly directed outgoing beam.

LEDs, on the other hand, emit spon­taneous light, as opposed to the laser’s stimulated, coherent light. “Spon­taneous emission samples all the possible ways the photon is allowed to go,” Schuller explained, so the light appears as a spray of photons traveling in all possible directions. The question was could they, through careful nanoscale design and fabrication of the semi­conductor surface, herd the generated photons in a desired direction? “People have done patter­ning of LEDs previously,” Iyer said, but those efforts in­variably split the into multiple directions, with low effi­ciency. “Nobody had engineered a way to control the emission of light from an LED into a single direction.” (Source: UCSB)

Reference: P. P. Iyer et al.: Unidirectional luminescence from InGaN/GaN quantum-well metasurfaces, Nat. Phot., online 1 June 2020; DOI: 10.1038/s41566-020-0641-x

Link: Solid State Lighting and Energy Electronics Center, University of California Santa Barbara, Santa Barbara, USA

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