Low pump threshold upconversion narrowband emission at 537 nm in a simple system of erbium-doped silica microsphere at the tip of a fiber is reported. The emission comes out radially (not from whispering gallery modes) and is strongly enhanced by a thin metal coating, in particular with platinum. This enhancement seems to be related to the coupling with surface plasmons. The emission wavelength does not correspond to any energy level differences of erbium in silica, and is the same that was reported earlier in a coated fiber.
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