Green upconversion lasing and thermo-optical bistability in ZBNA microspheres

We demonstrate upconversion lasing and fluorescence from active microspheres fabricated from a novel fluorozirconate, Er³⁺-doped glass, ZBNA, when pumped around 978 nm through a tapered optical fibre. An ultralow, green lasing threshold of ∼3 μW for 550 nm emissions is measured. This is one order of magnitude lower than that previously obtained for ZBLAN microspheres. Optical bistability effects observed within the microspheres indicate that this material is suitable for low-frequency, all-optical switching. The bistable mechanism is discussed and attributed to shifts of the resonances due to thermal expansion of the sphere, where the heat is generated by phonon transitions excited after optical pumping around 978 nm. We also report multiple bistability loops within the microspheres. In a separate experiment, the latching behaviour of the microspheres is illustrated.