Christopher Wiesmann

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.

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.

Efficiency of InGaN LEDs incorporating Surface Plasmon Polaritons

We estimated the influence of surface plasmon polaritons on the internal efficiency of LEDs by 3D FDTD calculations. It turns out, that SPP LEDs outperform standard LEDs if non-radiative losses are high.