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.
Rinnert et al. (Thu,) studied this question.