A new design for light-emitting diodes (LEDs) may hold the key to overcoming a long-standing limitation in the light sources’ efficiency. The concept, demonstrated with microscopic LEDs in the lab, achieves an increase in brightness of 100 to 1,000 times over conventional tiny, submicron-sized LED designs as well as the ability to create laser light — all characteristics that could make it valuable in a range of large-scale and miniaturized applications.
LEDs have existed for decades but the development of bright LEDs ushered in a new era of lighting; however, even modern LEDs have a limitation: feeding an LED more electricity makes it shine more brightly but soon the brightness drops off, making the LED highly inefficient. Called “efficiency droop,” the issue stands in the way of LEDs being used in a number of promising applications, from communications technology to killing viruses.
While the new LED design overcomes efficiency droop, the researchers did not initially set out to solve this problem. Their main goal was to create a microscopic LED for use in very small applications such as a lab-on-a-chip. The team experimented with a new design for the part of the LED that shines. Unlike the flat, planar design used in conventional LEDs, the researchers built a light source out of long, thin, zinc oxide strands they refer to as fins. (Each fin is only about 5 micrometers in length, stretching about a tenth of the way across an average human hair’s breadth.) Their fin array looks like a tiny comb that can extend to areas as large as 1 centimeter or more.
Their design shone brilliantly in wavelengths straddling the border between violet and ultraviolet, generating about 100 to 1,000 times as much power as typical tiny LEDs. A typical LED of less than a square micrometer in area shines with about 22 nanowatts of power; the new one can produce up to 20 microwatts, suggesting that the design can overcome efficiency droop in LEDs for making brighter light sources.
The team made another discovery as they increased the current. While the LED shone in a range of wavelengths at first, its comparatively broad emission eventually narrowed to two wavelengths of intense violet color. The tiny LED had become a tiny laser. Converting an LED into a laser usually requires coupling the LED to a resonance cavity that lets the light bounce around to make a laser; however, it appears that the fin design can do the job on its own, without needing to add another cavity. A tiny laser would be critical for chip-scale applications not only for chemical sensing but also in next-generation handheld communications products, high-definition displays, and disinfection.