Photonic quasi-crystal LEDs: design, modelling, and optimisation

In this paper we investigate improvement in performance attainable by etching Photonic Crystals and Photonic Quasi-Crystals into the top emitting surface of LEDs. We describe the physical mechanisms of extraction enhancement through ordered surface patterning and investigate benefits in terms of total extraction enhancement, beam directionality, and far field beam quality. Factors such as lattice geometry, etch depth, and epitaxy thickness are investigated. We show that a great variety of far field beam profiles of benefit in applications such as projection TV light engines and direct flat panel display illumination can be obtained simply by adjusting geometric design parameters. Our results show that PCs can provide significant improvement in extraction enhancement for applications requiring non Lambertian beam shapes when etched into standard "production line" epitaxy wafers in comparison to "state of the art" surface roughened thin-GaN LED devices. We investigate PC beam steering effects in these devices confirming that PCs do in fact re-direct light from trapped modes confined within the epi-structure to radiating modes. We also show that by tailoring the thickness of the epi-structure to complement the properties of the photonic crystal, extraction enhancement can be improved by a factor of 9 for some applications

Isotropic photonic band gap and anisotropic structures in transmission spectra of two-dimensional 5-fold and 8-fold symmetric quasiperiodic photonic crystals

We measured and calculated transmission spectra of two-dimensional quasiperiodic photonic crystals (PCs) based on a 5-fold (Penrose) or 8-fold (octagonal) symmetric quasiperiodic pattern. The photonic crystal consisted of dielectric cylindrical rods in air placed normal to the basal plane on vertices of tiles composing the quasiperiodic pattern. An isotropic photonic band gap (PBG) appeared in the TM mode, where electric fields were parallel to the rods, even when the real part of a dielectric constant of the rod was as small as 2.4. An isotropic PBG-like dip was seen in tiny Penrose and octagonal PCs with only 6 and 9 rods, respectively. These results indicate that local multiple light scattering within the tiny PC plays an important role in the PBG formation. Besides the isotropic PBG, we found dips depending on the incident angle of the light. This is the first report of anisotropic structures clearly observed in transmission spectra of quasiperiodic PCs. Based on rod-number and rod-arrangement dependence, it is thought that the shapes and positions of the anisotropic dips are determined by global multiple light scattering covering the whole system. In contrast to the isotropic PBG due to local light scattering, we could not find any PBGs due to global light scattering even though we studied transmission spectra of a huge Penrose PC with 466 rods.Comment: One tex file for manuscript and 12 PNG files for figures consisting of Fig.1a-d, 2,3, ...

Computational design and microfabrication of photonic quasicrystals

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Spectral evolution of femtosecond pulses in nonlinear waveguides: measuring continuum generation and group velocity with NSOM

We describe spectral measurements using NSOM of the nonlinear propagation of femtosecond pulses in waveguides. Nonlinearity produces self phase modulation, which broadens the spectrum of the propagating light as it travels along the guide. This process is the basis for supercontinuum sources that span octaves in frequency, which are important for metrology[1] and carrier-envelope phase measurement[2]. Our experiments uniquely measure the spectral evolution of this continuum as the light propagates along the waveguide by sampling the evanescent field with an uncoated NSOM tip, allowing measurement of the spectrum as it develops over long (mm) length scales, and also allowing the study of the submicron evolution of the spectra both along and across the guide. The evolution in one example is shown in figure 1(a). We will compare the measured continuum generation with our nonlinear propagation modeling, and discuss how the observed spectra differ from predictions of simple models

Realisation of ultra-low loss photonic crystal slab waveguide devices

In this paper we demonstrate low loss transmission both above and below the primary band-gap for a photonic crystal (PC) super-prism device consisting of 600 lattice periods. By modifying the refractive index of the holes, we reduce overall insertion loss to just 4.5 dB across the entire visible spectrum. We show that the remaining loss is predominantly due to impedance mismatch at the boundaries between patterned and unpatterned slab waveguide regions. Experimental loss measurements compare well with finite difference time domain simulations

Tuning localized plasmon cavities for optimized surface-enhanced Raman scattering

Mesostructured metallic substrates composed of square pyramidal pits are shown to confine localized plasmons. Plasmon frequency tuning is demonstrated using white light reflection spectroscopy with a wide range of structure dimensions from 400 to 3000 nm. Using a simple plasmon cavity model, we demonstrate how the individual pit morphology and not their periodicity controls the resonance frequencies. By measuring the surface-enhanced Raman scattering (SERS) signals from monolayers of benzenethiol on the same range of mesostructures, we extract a quantitative connection between absorption, field enhancement, and SERS signals. The match between theory and experiment enables effective design of plasmon devices tailored for particular applications, such as optimizing SERS substrates. © 2007 The American Physical Society.This work was supported by EPSRC NanoPhotonics Portfolio EP/C511786/1.Peer Reviewe

Complete and absolute photonic bandgaps in highly symmetric photonic quasicrystals embedded in low refractive index materials

It is firmly established that periodic lattice structures can support photonic bandgaps (PBG). However, complete and absolute photonic bandgaps (CAPBG) have only been achieved in high dielectric constant mediums such as GaAs (ε=13.6). An artificial quasiperiodic photonic crystal based on the random square-triangle tiling system was designed and fabricated. The photonic quasicrystal possesses 12-fold symmetry and was analysed using a finite difference time domain (FDTD) approach. High orders of symmetry in photonic quasicrystals have been shown to provide isotropic bandgaps across all the directions of propagation of light. As an outcome of these properties, this new class of photonic quasicrystal has been shown, for the first time, to possess a secondary non-directional CAPBG for a relatively low index material, silicon nitride (ε=4.08). These materials are compatible with integrated optical technologies. This allows the fabrication of efficient integrated optical PBG devices such as WDM filters and multiplexers to become a real possibility