We show that more than increasing the polarized light emission, PhCs offer a tool to control and design the angular emission pattern of LEDs. We demonstrate highly polarized emission with a degree of polarization of 88.7% at 465 nm and enhancement up to 1.8-fold in directional polarized emission. 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 coherent nature of the diffraction by PhCs not only retains the original polarization of light but also offers directional light extraction, resulting in high-brightness polarized LEDs. In this work, we embed periodically distributed air rods (embedded PhCs) within the LED structure to form a highly diffractive optical medium that offers high light-extraction efficiency. 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. The common light-extraction improving schemes applied to LEDs rely on randomizing the light reflected off the interfaces (roughened interfaces 14 or patterned substrates 15) which increases the light output of m-plane LEDs but also randomizes the original intrinsic polarization of the emitted light. High polarization ratios (defined in equation (1)) have been reported in m-plane GaN LEDs, 10, 11, 12, 13 however, such LEDs present poor light-extraction efficiency due to total internal reflection at the flat GaN/air interface. 6, 7 Today, the availability of high-quality free-standing substrates along non-polar and semipolar crystal planes of GaN allowed the demonstration of high-effciency optoelectronic devices. Non-polar m-plane GaN LEDs were first developed and more intensively investigated due to the possible reduction of polarization-induced electric fields in the QWs, which for c-plane GaN LEDs degrade their radiative recombination rate as a result of quantum confined stark effects. A strongly linearly polarized source, however, is obtained in m-plane GaN LEDs where the asymmetric in-plane biaxial stress on the quantum wells (QWs) orients the light emitting dipoles preferentially along the in-plane a direction. This is also the case for most of the nitride-based LEDs commercialized nowadays. However, common light sources are usually unpolarized, since the electric field of the light emitted has no preferred orientation. 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 also minimizes eye discomfort and ultimately eye strain, 3 to high-efficiency displays which operate through the spatial modulation of polarized light 4 (for completeness, we also note that polarized light and other forms of artificial light could be harmful for the life of animals and other species relying on natural light cycles to live 5). Similar content being viewed by othersĭue to a continuously improved performance, light-emitting diodes (LEDs) are not only the major contender for future general lighting sources, 1 but also 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.

This work could open the way to polarized white-light emitters through their association with polarization-preserving down-converting phosphors.

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. 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. 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. 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. The oscillations in one rope are in a vertical plane and are said to be vertically polarized.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.

Unpolarized light intensity equation