Multi-wavelength UV Upconversion in Lanthanides assisted by Photonic Crystals
Upconversion luminescence consists of the absorption of low-energies photons followed by the emission of a higher energy photon. The process has mainly been studied in lanthanides to upconvert monochromatic near-infrared excitation to near-infrared or visible light, and has been exploited only to a limited extent to upconvert broad excitations to ultra-violet. In addition, upconverting near-infrared and visible light to ultra-violet is crucial for applications such as solar-to-fuel conversion or environmental remediation. However, upconversion luminescence is limited by the low absorption cross-sections of lanthanides. In this work, we engineered Bloch modes in a photonic crystal to assist a multi-wavelength upconversion mechanism and demonstrated a 28-fold enhancement of ultra-violet upconversion luminescence of Yb3+-Tm3+ doped thin films. Materials were selected and optimized to design nanostructures without parasitic absorption losses. The geometric parameters of the photonic crystals were scanned to match a slow-light resonance with an excited-state transition of Tm3+ and thus enhance incident visible light absorption. Ultra-violet light extraction was also enhanced by photonic crystal Bloch modes. Each of these two contributions were quantified and the measured photonic band structures were well reproduced by electromagnetic simulations.