Krister Bergenek

Directional light extraction from thin-film resonant cavity light-emitting diodes with a photonic crystal

We report directional light extraction from AlGaInP thin-film resonant cavity light emitting diodes (RCLEDs) with shallow photonic crystals (PhCs). Diffraction of guided modes into the light extraction cone enhances the light extraction by a factor of 2.6 compared to unstructured RCLEDs, where the farfields still show higher directionality than Lambertian emitters. The external quantum efficiency is 15.5% to air and 26% with encapsulation, respectively. The PhC-RCLEDs are also more stable to a temperature induced wavelength shift than unstructured RCLEDs.

Enhanced light extraction efficiency from AlGaInP thin-film light-emitting diodes with photonic crystals

We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInP light-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index neff is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant G=1.5k0, which is 47% better than an unstructured reference device.

Strong High Order Diffraction of Guided Modes in Micro-Cavity Light-Emitting Diodes With Hexagonal Photonic Crystals

Photonic crystals (PhCs) have now been firmly established as an efficient means for light extraction from light emitting diodes (LEDs). We analyze the diffraction properties from thin GaN micro-cavity LEDs with hexagonal lattices that feature three guided TE modes only. In contrast to common design rules, we find that high order diffraction contributes significantly to the light extraction and increases the directionality of the emitted light. The implementation of the PhC leads to an enhancement in light extraction by a factor of up to 1.8 and the directionality of the light is greatly improved with a radiant intensity enhancement factor of 4.3, which can only be explained by the higher order diffraction that has been hitherto neglected. Furthermore, we show that higher order diffraction contributes significantly to the high azimuthal extraction uniformity we observe, suggesting that the use of quasi-crystal lattices is not necessary. We use a model including mode absorption where each in-plane angle of the guided modes is treated separately in order to explain the experimental results.

Beam-shaping properties of InGaN thin-film micro-cavity light-emitting diodes with photonic crystals

Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone according to Bragg´s law. The extraction angle of a single mode is determined by the phase match between the guided mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs) with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice type and lattice constant of the PhC. Two- six- and twelve-fold symmetry was observed in the azimuthal plane from 1D lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by 80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations. Keywords: light-emitting diodes, photonic crystal, cavity, InGaN

Theoretical Investigation of the Radiation Pattern From LEDs Incorporating Shallow Photonic Crystals

A theoretical approach based on coupled-mode theory is presented in order to determine the radiation pattern of LEDs incorporating a shallow photonic crystal. From this, a fundamental limit for the directionality of the diffraction of a single guided mode is given. Additionally, the Fabry-Perot resonances are shown to have significant impact on the directionality of diffracted light. For a realistic green-emitting InGaN LED in thin-film configuration the optimum reciprocal lattice vector is derived in terms of absolute diffracted intensity and directionality within a limited acceptance angle. The latter can be as high as 1.8 times the directionality of a Lambertian emitter. Furthermore, the spontaneous emission distribution between guided modes heavily influences the diffracted intensity.

Luminescent ceramics for LED conversion

Many LED-based applications would benefit from more efficient and/or high lumen output devices that enable usage in both white and single color illumination schemes. In the present article we briefly review the materials research history leading to optical ceramic converters and discuss their typical characteristics. Recently demonstrated high performance values in terms of efficacy and external quantum efficiency in orange (amber) spectral region are described.

Optimization of optical systems for LED spot lights concerning the color uniformity

Spotlighting is one illumination field where the application of light emitting diodes (LED) creates many advantages. Commonly, the system for spot lights consists of a LED light engine and collimating secondary optics. Through angular or spatial separated emitted light from the source and imaging optical elements, a non uniform far field appears with colored rings, dots or patterns. Many feasible combinations result in very different spatial color distributions. Several combinations of three multi-chip light sources and secondary optical elements like reflectors and TIR lenses with additional facets or scattering elements were analyzed mainly regarding the color uniformity. They are assessed by the merit function Usl which was derived from human factor experiments and describes the color uniformity based on the visual perception of humans. Furthermore, the optical systems are compared concerning efficiency, peak candela and aspect ratio. Both types of optics differ in the relation between the color uniformity level and other properties. A plain reflector with a slightly color mixing light source performs adequate. The results for the TIR lenses indicate that they need additional elements for good color mixing or blended light source. The most convenient system depends on the requirements of the application.