In a step toward reaching the ultimate light-extraction efficiency for high-power, high-brightness LEDs, researchers at Philips Lumileds (San Jose, CA) and Philips Research (Eindhoven, The Netherlands) have created a blue-emitting LED that uses a photonic-crystal structure to reach an estimated unencapsulated light-extraction efficiency of 73%.1
The fact that this efficiency was reached for an unencapsulated LED is important for two reasons. First the organic materials used for LED encapsulants degrade under high intensities, and are thus unsuitable for the highest-power LEDs.
Second, the use of an encapsulant decreases the brightness of an LED; this happens because the encapsulant, which has a higher refractive index than air, is in direct contact with the LED emitter. This means that light rays escape from the emitter into the encapsulant at a higher angle than if there were an air gap, increasing the "optical size" of the LED. (For a typical encapsulated LED, the brightness is decreased by about a factor of two.)
The gallium nitride (GaN)-based LED is very thin (700 nm) and has a hexagonal photonic-crystal pattern on its surface with a lattice constant of 455 nm and a depth of 250 nm. The new version outdoes other photonic-crystal-based LEDs at least in part because it is so thin that few optical modes are allowed by the LED, allowing the researchers to optimize for the few modes that are produced.
The LED consists of multiple quantum wells grown on sapphire, emits light at a peak wavelength of 450 nm, and is 200 microns by 200 microns in size. The reflector under the emitter is low-loss silver. To figure out where the remaining (non-extracted) light went, the researchers did a simulation and found that about half the light was absorbed in the quantum wells and the other half by the reflector.
1. Jonathan J. Wierer et al., Nature Photonics, Vol. 3, p. 163, March 2009.