Realization of sharp visible WGM lasing from Sm³⁺ :ZnO micro-spheres fabricated by laser ablation technique

In this paper, the whispering gallery mode lasing from Sm³⁺ doped ZnO micro-spheres operating in the visible wavelength range 550-750 nm is studied. Highly smooth and crystalline Sm³⁺ :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 Sm³⁺ ion substitution in ZnO matrix results in the formation of inter-band levels by the characteristic 4f level of Sm³⁺ in the forbidden gap of ZnO. Thus, when the micro-spheres are excited with 488 nm, four major visible emissions associated with Sm³⁺ f - f transitions namely, transitions from 4G5/2 to its low-lying multiplets of 6HJ (J = 5/2, 7/2, 9/2 and 11/2), are obtained. These emissions are coupled to the WGMs of the Sm³⁺ :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 10³ 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.