This thesis investigates the effect and exploitation of photonic crystals (PhCs) for light extraction in light-emitting diodes (LEDs). LEDs have come some way to meeting the requirements for several applications, but are limited in their suitability, particularly for étendue-limited applications requiring high directivity. Light extraction from LEDs presents a challenge, due to total internal reflection at the surface of the LED chip and current commercial light extraction techniques offer limited control over the direction of emission.PhC LEDs can operate in either a weak regime, where the PhC acts principally as a diffraction grating, or in strong regime where the PhC introduces a significant periodic (Bloch) component to the optical modes supported in the device. Experimental LEDs incorporating a buried photonic quasi-crystal are investigated and their weak-regime behaviour described by a simple model, with a 2D part concerning the PhC tiling and a 1D part considering the slab waveguiding properties of the device when the PhC is considered as an effective medium. A transfer and scattering matrix model for arbitrary slab waveguides is developed, including a model for emission from an embedded source layer. Experimental devices are shown to operate in the strong regime, possibly for the first time, comprised of large-area periodic arrays of nanorods with embedded light emitters suitable for large area fabrication and electrical contacting. Through confinement of light by index guiding or Fresnel reflection to the periodic layers, Bloch modes can be formed. Exploitation of both the strong and weak regimes in device designs for highly directional LEDs are discussed, with a comparison made between the two approaches. The strong PhC operating regime enabled by this work may facilitate significant performance improvements for practical highly directional LEDs over alternative methods.
|Date of Award||1 Jun 2016|
|Supervisor||Duncan Allsopp (Supervisor) & Philip Shields (Supervisor)|