Further Enhancement of Directional Light Extraction Efficiency of LED in Infrared Region using Triangular Nanophotonic Crystals of Different Composite Materials
TL;DR: In this article, the mathematical formulations for the coupling of light modes are discussed while analysis is done using FDTD method for improving the light extraction efficiency in LED using triangular nano-patterned layers of different composite materials.
Abstract: Improvement of ‘Light Extraction Efficiency’ in LED using triangular nano-patterned layers of different composite materials has been studied. The mathematical formulations for the coupling of light modes are discussed while analysis is done using FDTD method.
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TL;DR: The results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.
Abstract: Since 1993, InGaN light-emitting diodes (LEDs) have been improved and commercialized, but these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their light-emission efficiencies have been limited. Here we describe a method to enhance this efficiency through the energy transfer between quantum wells (QWs) and surface plasmons (SPs). SPs can increase the density of states and the spontaneous emission rate in the semiconductor, and lead to the enhancement of light emission by SP–QW coupling. Large enhancements of the internal quantum efficiencies (etaint) were measured when silver or aluminium layers were deposited 10 nm above an InGaN light-emitting layer, whereas no such enhancements were obtained from gold-coated samples. Our results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.
1,349 citations
TL;DR: It is shown how one can thereby compute semianalytical reflection and transmission through crystal tapers of almost any length, using only a single pair of modes in the unit cells of uniform gratings, which becomes more accurate as the taper becomes more gradual, with no significant increase in the computation time or memory.
Abstract: We prove that an adiabatic theorem generally holds for slow tapers in photonic crystals and other strongly grated waveguides with arbitrary index modulation, exactly as in conventional waveguides. This provides a guaranteed pathway to efficient and broad-bandwidth couplers with, e.g., uniform waveguides. We show that adiabatic transmission can only occur, however, if the operating mode is propagating (nonevanescent) and guided at every point in the taper. Moreover, we demonstrate how straightforward taper designs in photonic crystals can violate these conditions, but that adiabaticity is restored by simple design principles involving only the independent band structures of the intermediate gratings. For these and other analyses, we develop a generalization of the standard coupled-mode theory to handle arbitrary nonuniform gratings via an instantaneous Bloch-mode basis, yielding a continuous set of differential equations for the basis coefficients. We show how one can thereby compute semianalytical reflection and transmission through crystal tapers of almost any length, using only a single pair of modes in the unit cells of uniform gratings. Unlike other numerical methods, our technique becomes more accurate as the taper becomes more gradual, with no significant increase in the computation time or memory. We also include numerical examples comparing to a well-established scattering-matrix method in two dimensions.
272 citations
TL;DR: In this article, epitaxial structures which modify the distribution of guided modes were introduced to enhance the potential for efficient light extraction by photonic crystal assisted gallium nitride light-emitting diodes.
Abstract: We relate the currently limited efficiency of photonic crystal (PhC)-assisted gallium nitride light-emitting diodes (LEDs) to the existence of unextracted guided modes. To remedy this, we introduce epitaxial structures which modify the distribution of guided modes. LEDs are fabricated according to this concept, and the tailored band structure is determined experimentally. We investigate theoretically the consequences of this improvement, which significantly enhances the potential for efficient light extraction by PhCs.
203 citations
TL;DR: In this paper, a nitride-based light emitting diode with a mesa sidewall roughening process that increases light output power was reported. But, the performance of the PEC treated GaN-Ga/sub 2/O/sub 3/air layers was not evaluated.
Abstract: In this letter, we will report on a nitride-based light emitting diode with a mesa sidewall roughening process that increases light output power. The fabricated GaN-based light-emitting diode (LED) wafers were first treated through a photoelectrochemical (PEC) process. The Ga/sub 2/O/sub 3/ layers then formed around the GaN : Si n-type mesa sidewalls and the bottoms mesa etching regions. Selective wet oxidation occurred at the mesa sidewall between the p- and the n-type GaN interface. The light output power of the PEC treated LED was seen to increase by about 82% which was caused by a reduced index reflectance of GaN-Ga/sub 2/O/sub 3/-air layers, by a rough Ga/sub 2/O/sub 3/ surface, by a microroughening of the GaN sidewall surface, and by a selective oxidation step profile of the mesa sidewall that increases the light-extraction efficiency from the mesa sidewall direction. Consequently, this wet PEC treated process is suitable for high powered nitride-based LEDs lighting applications.
87 citations
TL;DR: In this paper, the authors investigated the transmittance characteristics of sinusoidal nanopatterning grating structure for greatly improving the light extraction efficiency of light-emitting diodes.
Abstract: To accurately utilize the sinusoidal nanopatterning grating structure for greatly improving the light extraction efficiency of light-emitting diodes, the transmittance characteristics of these nanostructures as a function of the normalized period and the normalized depth are investigated quantitatively. It is found that when the normalized period of nanostructure is less than 0.46; in other words, only zeroth order transmission light is propagating, the light extraction efficiency can be enhanced immensely due to the higher transmittance of surface structure. When the period scale of nanostructure is more than the emitted wavelength, the transmittances decrease as the structure height increase. However, as the period dimension is less than the emitted wavelength, the transmittance of surface profile is increased as the depth increases. Besides, in order to easily analyze and effectively design the transmittance characteristics of these nanopatterning structures integrated in LEDs, we propose that the developed scalar method and the effective medium theory can be used accurately. The accuracy of both uncomplicated methods is quantitatively evaluated by the comparison of diffraction efficiencies predicted by the scalar theory and effective medium method, to exact results calculated by the rigorous coupled wave analysis method.
72 citations