Multiwavelength UV Upconversion in Lanthanides Assisted by Photonic Crystals

Upconversion luminescence consists of the absorption of low-energy photons followed by the emission of a higher-energy photon. It has mainly been studied in lanthanides to upconvert monochromatic near-infrared excitation. However, upconverting broad excitations in near-infrared and visible to ultraviolet is crucial for applications such as solar-to-fuel conversion or environmental remediation. Moreover, upconversion luminescence is limited by the low absorption cross-sections of lanthanides. In this work, we engineered the Bloch modes of a photonic crystal to assist a multiwavelength upconversion mechanism and demonstrated a 28-fold enhancement of ultraviolet upconversion luminescence in Yb3+-Tm3+-doped thin films. The selected materials are free from absorption losses, and the proposed structure exhibits resonances that control an excited-state transition of Tm3+ and the ultraviolet light extraction. Each of these two contributions was quantified, and the measured photonic band structures were well reproduced by electromagnetic simulations.