Luminescent Metamaterials for Solid State Lighting

Abstract

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...

Light-emitting metasurfaces

“…The angular range of emission at a given wavelength can be as narrow as 1-2° ( Figure 14B) while extending over bandwidths of 5-10 nm at a given emission angle [76,77]. When spectrally integrated, this directional enhancement [77] can reach values of about 14, while integration over both spectral and angular range [177] may give an overall enhancement of about 4.5. The spectral and angular band in which directivity occurs is controlled through lattice periodicity, while the magnitude of the enhancement and the bandwidth can to some degree be tuned by the scatterer polarizability.…”

“…More recently, studies also extend to 2D active materials [170,171] and metal halide perovskites [172]. While original studies on light-emitting metasurfaces were largely motivated by the notion that the high Purcell factors of plasmon scatterers promote emission while high local field enhancement would accelerate pumping [173][174][175][176], in fact the common denominator of successful devices is that diffractive resonances enhance pump incoupling and emission outcoupling [57,77,177]. The main achievements of this strategy are detailed in the next section.…”

Modified emission of extended light emitting layers by selective coupling to collective lattice resonances

“…In order to quantify the impact of the change in the spectral content of the emission, we determine the red luminous efficacy (η) in the direction defined by the solid angle as the ratio between the luminous flux and the spectral power density. One must note that, unlike for the spectral luminous efficacy where the integration is performed over the entire visible range, the red luminous efficacy is integrated over a narrower range of frequencies within the red part of the spectrum [39]. The luminous efficacy defines how well a source produces visible light as perceived by the human eye, and can be expressed as…”

Theory of Radiative Lifetime of an Activator Ion due to Surface Plasmons

“…We derive this parameter using a semi-classical approach, and show its dependence on the dielectric constants of the system and on the distance from the metallic layer. Recently there has been considerable interest on utilizing surface plasmons in metallic thin films to enhance internal quantum efficiency of luminescent materials [1][2][3][4][5][6][7][8][9][10][11][12] and light emitting diodes (LED). …”

“…The s-polarized waves will not yield any surface polarization waves, and will not be considered further. The conditions of continuity of the electric and magnetic fields require that at the interfaces, z = a and z = −a E 1x = E 2x [12] ε 1 E 1z = ε 2 E 2z [13] H 1y = H 2y [14] The subscripts, 1 and 2 will denote solutions corresponding to medium 1 and medium 2 respectively. Using Eqs.…”