An estimated unencapsulated light-extraction efficiency of 73% has been reached by a photonic-crystal structured blue-emitting LED created by researchers of Philips Lumileds (San Jose, CA) and Philips Research(Eindhoven, The Netherlands).

There are two reasons why this unencapsulated efficiency is so important a step toward the ultimate light-extraction efficiency for high-power, high-brightness LEDs. Firstly, the organic materials are not suitable for the highest-power LEDs for its degrading under high intensities. And secondly, the higher refractive index of the encapsulant than air decreases the brightness of the LED by about a factor of two.

The 700-nm gallium nitride-based LED has a hexagonal photonic-crystal pattern on its surface with a lattice constant of 455 nm and a depth of 250 nm. With such thinness, few optical modes are allowed by this new version and thus can be optimized by researchers.

The LED emits light at a peak wavelength of 450 nm, and is 200 microns by 200 microns in size with low-loss silver as reflector under the emitter. The remaining (non-extracted) light was half absorbed in the quantum wells and the other half by the reflector, found the researchers in a simulation experiment.