Research On The Surface Plasmon Enhanced Efficiency Of LED By Coupling With A Metallic Grating

Surface plasmon photonics play an important role in the nano-optics, and with the development of theory and mirco-processing technology, the research of metal nanostructure has form a new academic direction. In recent years, the research of plasma optical and plasmon-based devices has aroused extensive attention. The charge density oscillation on metal/dielectric interface is inspired by electromagnetic waves, and the mixed waves of the density oscillation and electromagnetic waves are called surface plasmon, which is bound on the surface, with features such as enhanced near-field, short wavelength, and limited surface characteristics. It may break the diffraction limit by taking advantage of local features of surface plasmon at sub-wavelength, achieving the optical coupling between micro and nano-interface. It will be widely applied in the new light source, optical storage, the new chip and nano-optical imaging.One of the restrictions of GaN-based LED' application in lighting is its low external quantum efficiency. Since the great refractive index between GaN and air, the most of emitted light reflects at the interface, forming trapped modes. The light is absorbed within the LED, causing large loss of energy, which is harmful to the LED. Recently, surface plasmon has been used to enhance the light efficiency of LED and satisfied results were obtained. When the momentum conservation between photons and surface plasmon is realized, the emitted photons can exist as form of surface plasmon, and if the surface plasmons can be converted to radiative photons, the overall external quantum efficiency will be enhanced. When a metal structure is placed nearby the emitting layer, the SPPs generated on the metal/dielectric surface can increase the emission efficiency. The energy of emitter is first transferred into SPPs modes via the emitter's near field and then into radiation modes. With approaching to the SP frequency, the density of SPPs modes become dramatically large, and this can increase the energy transfer from the emitter to SPPs, enhancing the spontaneous emission rate. The content of this article is divided into the following sections:1. We discussed the influence of optical band electromagnetic waves on the movement of free electrons in the metal. Then several dispersion modes of metal were introduced, among which the Drude modes with its modified form were emphasized. On this base, the formation and characteristics of surface plasmon were analyzed. Then several important parameters and excitation mechanism were introduced. In addition, metal nano-rods example showed the existence and nature of localized surface plasmon.2. Several numerical calculation methods were introduced:Method of Moments (MoM), Beam Propagation Method (BPM), Finite Element Method (FEM) and Finite-Difference Frequency-Domain (FDTD), among which FDTD was emphasized.3. The radiation of a single dipole was simulated by using three-dimensional FDTD method. In the simulation, a single dipole was substitute for the electron hole pair in LED. Firstly three different metal films were compared, and properties of silver, gold and copper showed that silver film leaded to a peak in the blue light band, indicating that silver was more suitable for the manufacture of blue LED. Secondly surface plamon on the silver film was analyzed. The comparison between silver film and grating showed that grating can lead to much more enhancement of radiation. Thirdly different pitches of the grating were scanned, and measurable pitch was selected. The results show that surface plasmon coupling played a crucial role in the radiation enhancement process. The effects of enhancement extremely depended on the orientation of dipole and the duty cycle of the grating, and tens of folds radiation enhancement was obtained at some wavelengths. Lastly the enhancement effect of dipole radiation caused by different silver grating pitches and dipole orientation were discussed. Because the LSP was sensitive to the metal nano-structure, when the duty cycle of grating was changed, far-field electromagnetic enhancement was obtained at some wavelengths. When the orientation of diople was perpendicular to Ag/GaN interface, much more radiation of dipole and far-field electromagnetic enhancement were obtained.The results were significant to increase the efficiency of lighting devices by using metallic grating.