Upconversion emission in rare earth doped materials under near infrared excitation using silica microspheres as focusing lenses

The upconversion emission properties of Er3+-Yb3+ and Tm3+-Yb3+ codoped glass and glass ceramic samples covered with silica microspheres (3.8 μm diameter) were analyzed. A 950 nm laser beam focused by the microspheres produces a set of photonic nanojets on the sample surface. Due to upconversion processes, the optically active ions located in each microsphere focus emit light in the visible region. The emission from each sample was measured, yielding an upconversion intensity in the focal area higher than the emission from the bare substrate. To estimate the effective size of the emission area under a single microsphere, a deconvolution of the measured focal spot with the Point Spread Function of the experimental setup was performed. The results obtained by Finite-Difference Time-Domain simulations are in good agreement with the experimental values.