Deep UV random lasing from NaGdF₄:Yb³⁺,Tm³⁺ upconversion nanocrystals in amorphous borosilicate glass

The realization of lanthanide-doped upconversion nanocrystals embedded in a robust and transparent solid medium is highly desired to achieve deep UV (<300 nm) lasing. Here, we fabricate NaGdF₄:Yb³⁺, Tm³⁺ upconversion nanocrystals inside amorphous borosilicate glass to support ∼290 nm random lasing emission under 980 nm excitation. We found that with careful control of the growth process, which is the key to achieve high-crystallinity nanocrystals, the nanocrystals can suppress defect-related quenching and enhance ¹I₆ → ³H₆(⁶I_J → ⁸S₇/₂) transition of Tm³+ (Gd³⁺) ions under five-photon absorption excitation so that high optical gain (>45 cm⁻¹) at ∼290 nm can be obtained.