Light-emitting diodes (LEDs) have been changing the way we see the world since the 1960s. Their usage in everyday life is pervasive and continues to increase thanks to the cutting-edge research being done in the field of optics.

To highlight breakthroughs in LEDs, the editors of Energy Express, a bi-monthly supplement to Optics Express, the open-access journal of the Optical Society (OSA), has published a special Focus Issue on Optics in LEDs for Lighting. The issue is organized and edited by Guest Editors Jae-Hyun Ryou and Russell Dupuis of the Georgia Institute of Technology.

"The papers in this Focus Issue represent the outcome of state-of-the-art research and development by recognized experts in the field of LEDs, said Ryou. "These latest advances are truly exceptional and will prove to be invaluable to advancements in lighting technology."

Summary
LEDs continue to prove themselves as the future in lighting, with applications in everything from vehicle headlights to stadium displays to video cameras. In addition to their current commercial applications, LEDs have opened up an era of solid-state lighting (SSL) with capabilities of emitting photons of either primary colors or white light.

With their continuous improvements in luminous efficiency compared with conventional light sources, LEDs will lead to significant energy savings when used as a ubiquitous light source for general lighting applications.

The papers in this Focus Issue feature state-of-the-art research and development that address the technical challenges and possible solutions for visible LEDs to be widely used in SSL, while also focusing on the major challenges associated with improving luminous efficiency and simultaneously delivering superb color quality at a reasonable cost.

Key Findings and Select Papers
The following papers are some of the highlights of the Energy Express Focus Issue on Optics in LEDs for Lighting. All are included in Volume 20, issue S4 and can be accessed online here.

+ Typical III-N-based visible LED structures are grown on sapphire substrates; however, a possible way to lower the capital cost of LED-based SSL technologies is to fabricate the devices on silicon substrates. Kei May Lau, et al. report blue-emitting LEDs on silicon substrates to lower the manufacturing cost of visible LEDs.

The paper addresses many important technical issues associated with LEDs on silicon substrates, such as strain management and crack-formation in the epitaxial structure, thermal management of the chips, and external quantum efficiency of the devices including light extraction. pp. A956

+ It is believed that current and future SSL is based on LEDs and phosphors. Jeff Tsao and Jonathan Wierer, et al. challenge this common belief that the narrow spectral linewidth and the high capital cost of lasers makes them unsuited for general illumination purposes.

They discuss the use of lasers for higher power and efficiency at high current densities for SSL and experimentally demonstrate that four-color (RYGB) laser white illuminant is virtually indistinguishable from high quality state-of-the-art white reference illuminants.

This result suggests that lasers can also be a serious contender for solid-state lighting in some applications. pp. A982

+ In order to improve extraction efficiency, hence external quantum efficiency, of LED devices, photons generated in the active region should escape out of the naturally formed slab waveguide structure formed by the LEDs' epitaxial layers.

A paper by Seong-Ju Park, et al. demonstrates that tungsten metal can be used not only as a mask for epitaxial lateral overgrowth but also for the formation of an air void underneath it to improve both internal quantum efficiency and extraction efficiency of the LEDs.

Whereas several similar approaches have been demonstrated, this study is unique in the formation of an air void as an optical scatterer without resorting to a complicated etching process. pp. A943

+ For the improvement of internal quantum efficiency, C. C. Yang and Yean-Woei Kiang, et al. investigate surface plasmon coupling with radiating dipoles (electron-hole pairs) experimentally and theoretically.

The team demonstrates improvement in the efficiency droop, a term commonly used by the LED community to refer to the reduction in emission efficiency with increasing injection current, as well as in internal quantum efficiency.

They also numerically study the effects of coupling based on a coupling model between a radiating dipole and the localized surface plasmon induced by Ag nanoparticles. pp. A914

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