Ultralow threshold green lasing and optical bistability in ZBNA (ZrF ₄-BaF₂-NaF-AlF₃) microspheres

Upconversion lasing and fluorescence from active microspheres fabricated from a novel fluorozirconate, Er³⁺doped glass, ZBNA (ZrF₄- BaF₂-NaF-AlF₃), when pumped at 978 nm via a tapered optical fiber is demonstrated. An ultralow, green lasing threshold of ∼3 μW for 550 nm emissions is measured. This is one order of magnitude lower than that previously reported for ZBLAN (ZrF₄-BaF₂- LaF₃-AlF₃-NaF) microspheres. Optical bistability effects in ZBNA microspheres are reported and the bistable mechanism is discussed and attributed to shifts of the whispering gallery modes due to thermal expansion of the sphere, where heating is achieved by optical pumping around 978 nm. The effect of the bistability on the upconversion lasing is examined and we report multiple bistability loops within the microspheres.