Fig. 4: Off-axis spectroscopy.

a Schematic illustration of off-axis VUV fluorescence spectroscopy with both reflection and transmission, where the inset is a scanning electron microscope (SEM) image of a typical sample with microcrystalline particles. b, c NaYF4 and NaYF4:2%Tm emission bands, respectively, under 177 and 980 nm excitation. d NaYF4:2%Tm,20%Yb emission bands under 980 nm excitation (Ref. 31)