High-brightness polarized light-emitting diodes

Elison Matioli,Stuart Brinkley,Claude Weisbuch,Kathryn M Kelchner,Steven Denbaars,Yan-Ling Hu,James Speck,Shuji Nakamura

doi:10.1038/lsa.2012.22

Abstract

Light-emitting diodes are becoming the alternative for future general lighting applications, with huge energy savings compared to conventional light sources owing to their high efficiency and reliability. Polarized light sources would largely enhance the efficiency in a number of applications, such as in liquid-crystal displays, and also greatly improve contrast in general illumination due to the reduction in indirect glare. Here, we demonstrate light-emitting diodes presenting high-brightness polarized light emission by combining the polarization-preserving and directional extraction properties of embedded photonic-crystals applied to non-polar gallium nitride. A directional enhancement of up to 1.8-fold was observed in the total polarized light emission together with a high polarization degree of 88.7% at 465 nm. We discuss the mechanisms of polarized light emission in non-polar gallium nitride and the photonic-crystal design rules to further increase the light-emitting diode brightness. This work could open the way to polarized white-light emitters through their association with polarization-preserving down-converting phosphors. Researchers have designed a light-emitting diode (LED) that produces bright directional polarized blue light. The device, developed at the University of California at Santa Barbara in the USA by Elison Matioli and collaborators, is based on a variant of the semiconductor gallium nitride, grown on a specifically crystal direction that yields emission of polarized light. The researchers improved light extraction from the device by drilling aligned arrays of holes at precisely defined intervals into the substrate. This photonic crystal structure selectively enhances the emission of polarized light for particular emission angles by up to a factor of 1.8. High-brightness LEDs emitting polarized light are of interest for flat-screen displays, and also for household lighting because they minimize the glare from light reflections.

Highlights

Due to a continuously improved performance, light-emitting diodes (LEDs) are the major contender for future general lighting sources,[1] and play an important role in a growing number of other applications—from backlight for high-efficiency televisions and mobile phone displays, to car lights and headlights—replacing the classical white sources owing to their high efficiency, brightness, reliability and low operation cost
Polarized light sources would largely improve the efficiency of most of these applications: from general illumination, with an improved contrast due to reduced glare,[2] which minimizes eye discomfort and eye strain,[3] to highefficiency displays which operate through the spatial modulation of polarized light[4]
The polarization ratio as defined in equation (1) is sensitive to the presence of peaks from the extracted modes by the PhCs at 06. These results show that the embedded PhCs enhance the total polarized light emission maintaining the high intrinsic polarization ratio of m-plane GaN LEDs

Summary

INTRODUCTION

Due to a continuously improved performance, light-emitting diodes (LEDs) are the major contender for future general lighting sources,[1] and play an important role in a growing number of other applications—from backlight for high-efficiency televisions and mobile phone displays, to car lights and headlights—replacing the classical white sources owing to their high efficiency, brightness, reliability and low operation cost. Common light sources are usually unpolarized, since the electric field of the light emitted has no preferred orientation. This is the case for most of the nitride-based LEDs commercialized nowadays. The diffraction of light by optical gratings (or photonic crystals (PhCs)) offers one possible polarization-preserving light-extraction mechanism, in addition to the high light-extraction efficiency demonstrated in GaN LEDs.[16,17]. The coherent nature of the diffraction by PhCs retains the original polarization of light and offers directional light extraction, resulting in high-brightness polarized LEDs. We develop a theoretical model of light emission in m-plane GaN and of its interaction with PhCs, and discuss the important parameters to design and enhance the directional polarized light emission. The association of such high-brightness polarized blue LEDs with polarization-preserving down-converting phosphors,[18] such as oriented dye systems[19] or quantum dashes,[8] could open the way to polarized white-light emitters

MATERIALS AND METHODS

CONCLUSIONS