Enhanced Upconversion Luminescence by Two-Dimensional Photonic Crystal Structure

Upconversion nanoparticles (UCNPs) convert near-infrared excitation into visible emission with efficiencies far greater than those of two-photon absorption or second harmonic generation, enabling upconversion with low intensity, incoherent light. For widespread applications, however, further enhancement of upconversion efficiency is desired. Photonic crystal (PhC) structure embedded with UCNPs provides a new way to engineer the photonic environment and enhance upconversion luminescence. We incorporate silica-coated UCNPs into a two-dimensional (2D) thin film PhC structure, which exhibits enhanced local electric field at the near-infrared (NIR) excitation wavelength of UCNPs. Thanks to the nonlinearity of the upconversion process, the local field enhancement is amplified and results in a significantly enhanced luminescence intensity. We observed approximately 130- and 350-fold enhancements for green and red luminescence, respectively, and present a detailed analysis of the enhancement mechanism. Unlike the plasmonic nanostructure, which tends to cause severe luminescence quenching, the purely dielectric photonic crystal structure generally shows little quenching and provides a good alternative for many applications.