Nanoscale LED Bypasses Efficiency Droop to Take on Laser Qualities

Using microscopic LEDs in a laboratory setting, a team of researchers including scientists from the National Institute of Standards and Technology (NIST) has increased the brightness of laser light, as well as the ability to manufacture it in a controlled environment. The newly conceived and demonstrated design may hold value for scientists overcoming limitations involving the light source’s efficiency, enhancing brightness 100× to 1000× over conventional, submicron-size LEDs. The architecture could support work spanning both large-scale and miniaturized applications.

While the materials the team used in its design do not vary from those used in conventional LEDs, there is a significant difference in physical shape. Whereas conventional LEDs shine light from a component that features a flat, planar body, the new light source consists of long, thin strands of zinc oxide. The team refers to the strands as fins. A fin is only about 5 µm in length, stretching approximately a tenth of the thickness of an average human hair.

An array of fins resembles a microscopic comb that extends to areas as large as 1 cm, and successfully receives more electrical current than existing LEDs. Though modern LEDs do increase their brightness as the currents of corresponding electrical feeds increase, brightness eventually dissipates.

The phenomenon is known as an “efficiency droop” — and it stands in the way of LEDs supporting applications in communications technology and medicine.

A comb-like array of fin LEDs, some of which are glowing (bright spots at tips). Courtesy of NIST.

“We saw an opportunity in fins, as I thought their elongated shape and large side facets might be able to receive more electrical current,” said NIST’s Babak Nikoobakht, who conceived the design, according to NIST. “At first we just wanted to measure how much the new design could take. We started increasing the current and figured we’d drive it until it burned out, but it just kept getting brighter.”

NIST said the team’s cursory goal for the research was to create a microscopic LED for use in very small applications. Ultimately, the novel design shone prominently in wavelengths bordering violet and ultraviolet ranges.

The fin LED pixel design includes the glowing zinc oxide fin (purple), isolating dielectric material (green), and metal contact (yellow atop green). Courtesy of NIST.

A typical LED of less than 1 sq µm shines with about 22 nW of power, Nikoobakht said. The new LED can produce up to 20 µW, suggesting it can overcome an efficiency droop.

Because the LEDs’ comparatively broad emission eventually narrowed to two wavelengths of intense violet, the LED qualified as a tiny laser in the team’s experiments. Such a bright and compact device could empower chip-scale applications, such as those used for chemical sensing, high-definition displays, and hand-held communication.

“Converting an LED into a laser takes a large effort,” Nikoobakht said. “It usually requires coupling an LED to a resonance cavity that lets the light bounce around to make a laser. It appears that the fin design can do the whole job on its own, without needing to add another cavity.”

Participating scientists are from the IBM Thomas J. Watson Research Center, Rensselaer Polytechnic Institute, and the University of Maryland. The U.S. Army Cooperative Research Agreement partially supported the research.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.aba4346).