Effects of photonic structures on upconversion

Photonic crystals modify the local density of photon states. These variations influence the emission properties of a dipole embedded within the photonic crystal. Furthermore, field enhancement can be observed within photonic crystals. In this paper, we investigate how these effects influence upconversion processes in β-NaYF4:Er. For this purpose we use finite-difference time-domain (FDTD) simulations of a grating-waveguide-structure in combination with a rate equation model of the upconversion processes in β-NaYF4:Er. The grating parameters are optimized to achieve large field enhancements within the structure for the combination of sand p-polarized light. Furthermore, the variation of the spontaneous emission rates for dipole emitters within the structure is simulated. The varied transition rates, as well as the field enhancement, serve as input parameters for the rate equation model for upconversion. Using this approach, the influence of the structure on the upconversion quantum yield is calculated. For a simulated initial irradiance of 1000 W/m2, we find enhancement factors of up to four for the field enhancement in the upconverter region and up to a factor of three for the upconversion quantum yield. In consequence, the incorporation of upconverting material in photonic structures in very promising to increase upconversion efficiencies.