We investigate the luminescent spectrum of thin films of dye molecules embedded in a polymer layer and on top of an array of aluminum nanoparticles. The emission couples to quasi-guided modes that are efficiently coupled out to free space by scattering with the particle array. This outcoupling provides a significant enhancement of the emission in defined directions. The mode density changes and the spectrum is modified by varying the thickness of the layer. In consequence, the luminous efficacy of the layer of dye can be controlled by the layer thickness and the characteristics of the particle array. This system can be regarded as a luminescent metamaterial with designed properties that can be exploited in The invention of the efficient blue Light Emitting Diode (LED) 1 has led to the Solid State Lighting (SSL) revolution that will replace inefficient incandescent and fluorescent lamps by much more efficient light sources. Blue light is necessary for the generation of broadband white light. In a white LED this generation is usually realized through a luminescent or phosphorescent material, which is called the phosphor.2 Blue light from efficient InGaN quantum wells is absorbed by the phosphor, which emits light of a longer wavelength. White light is generated by mixing the emission of the phosphor with the fraction of non-absorbed blue light. Research on light generation in white LEDs has mainly focused on the improvement of the quantum efficiency of the phosphor by modifying the material composition and on the change of the emission spectrum in order to optimally match it to the photopic sensitivity curve of the eye.
3In this article we demonstrate a radically different approach to modify the emission intensity and spectrum of phosphors for SSL. This approach relies on coupling the emission to quasi-guided modes in the phosphor layers. These modes are very efficiently outcoupled to free space radiation by scattering with periodic arrays of metallic nanoparticles. Metallic nanoparticles support localized surface plasmon polaritons (LSPPs), which are the coherent oscillations of the free charges in the metal driven by the electromagnetic field. 4 LSPPs are resonant phenomena leading to the increase of the polarizability of the particle and, consequently, to the scattering efficiency. 5 In order to use resonant plasmonic nanoparticles in real applications, we need to have large arrays or ensembles of these nanostructures. Therefore, we investigate the modification of the emission from phosphor layers on top of periodic arrays of metallic nanoparticles. These and similar arrays have received recently a significant attention because of their capacity of supporting collective plasmonic resonances. These collective resonances are the result of the enhanced radiative coupling of LSPPs through scattering and diffraction by the particle array. Two different kinds of collective resonances in periodic arrays of plasmonic particles have been described in the literature. 6 The first kind arises in waveguides with the particl...