Upconversion fluorescence imaging of erbium-doped fluoride glass particles by apertureless SNOM

We have imaged fluorescent erbium-doped fluoride glass particles by apertureless scanning near-field optical microscopy. The optical excitation has been performed at lambda = 780 nm whereas fluorescence emission has been collected around lambda = 550 nm. This process, called upconversion by energy transfer, involves two erbium ions and is not linear. Besides an improvement of the lateral resolution, we have observed on some particles that the fluorescence is not homogeneously distributed, but is rather localized in some zones brighter than others. By making tip approach curves, we have also observed that the amount of fluorescence intensity scattered by the tip is increasing when the tip is approaching the sample surface.