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Highly crystalline and smooth 1% Ho3+ doped ZnO microspheres with diameters ranging from 0.5 to 15 μm were synthesized using laser ablation technique. Near band edge whispering gallery mode (WGM) resonances from Ho:ZnO microspheres with a single oscillation route in the UV range are observed with 355 nm excitation. Apart from the significantly enhanced visible WGM resonances associated with intrinsic oxygen vacancy defects by Ho3+ doping, the visible range WGM resonances associated with the Ho3+ 5F5 → 5I8 emission at 629–690 nm are also observed with a 488 nm excitation. The WGMs of Ho3+ f-f emission possess lower threshold and high Q-factor values.

Toward achieving an efficient micro-optical temperature sensor in a wide visible spectral range, Sm3+:ZnO micro-spheres are fabricated by the laser ablation technique. Visible range whispering gallery mode resonances are observed and are found to be lasing above low optical threshold with a quality factor of the order of 103. A temperature-induced change in whispering gallery modes is observed with a sensitivity of 0.04 nm/°C from the micro-sphere of diameter 10.5 μm. Using these results, cost-effective and bio-compatible miniature temperature sensors that operate in the visible spectral range could be devised.

In this paper, the whispering gallery mode lasing from Sm 3+ doped ZnO micro-spheres operating in the visible wavelength range 550-750 nm is studied. Highly smooth and crystalline Sm 3+ :ZnO micro-spheres of various diameters were synthesized by simple laser ablation of sintered target in air. The dopant concentration has been restricted within the solid solubility limit of 1 mol%. The Sm 3+ ion substitution in ZnO matrix results in the formation of inter-band levels by the characteristic 4f level of Sm 3+ in the forbidden gap of ZnO. Thus, when the micro-spheres are excited with 488 nm, four major visible emissions associated with Sm 3+ f − f transitions namely, transitions from 4 G 5/2 to its low-lying multiplets of 6 H J (J = 5/2, 7/2, 9/2 and 11/2), are obtained. These emissions are coupled to the WGMs of the Sm 3+ :ZnO micro-cavity resulting in sharp resonances in the visible region. Interestingly, the resonances are found to be lasing above lower pumping powers. The Q-factor obtained is of the order of 10 3 for spheres of size ranging 1.8-10.2 µm. The best Q-factor obtained is 3744 for 561.61 nm peak (FWHM 0.15 nm) from 8 µm sphere. These results would find a place in visible range lasing and sensor applications.