Method gets LEDs to act more like lasers

A new method could pave the way toward more efficient and versatile LED display and lighting technology.

The new approach 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,” says Jonathan Schuller, a professor of electrical and computer engineering at the University of California, Santa Barbara.

This improved performance, he says, can be achieved without the external packaging components that scientists often use to manipulate the light LEDs emit.

Light in LEDs is generated in the semiconductor material when excited. Negatively charged electrons travel along the semiconductor’s crystal lattice, meet positively-charged holes (an absence of electrons), 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 says. “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 (GaN) nanorods on a sapphire substrate. Quantum wells of indium gallium nitride (InGaN) are embedded in the nanorods to confine electrons and holes and thus emit light.

In addition to allowing more light to leave the semiconductor structure, the design polarizes the light, which co-lead author Prasad Iyer says, “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. He was focused on metasurfaces—engineered surfaces with nanoscale features that interact with light.

“A metasurface is essentially a subwavelength array of antennas,” says Iyer, who was previously 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 says, 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 says, 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.”

The research appears in Nature Photonics.

Source: UC Santa Barbara

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