Defect engineered ZnO whispering gallery modes via doping with alkali metal ions for label-free optical sensors

Abstract

A systematic investigation on the proper utilization of defect levels present in ZnO is very much in demand to avail many applications of photonics in visible and near infrared (NIR) regions. In this paper, we have engineered intrinsic defects of zinc oxide (ZnO) to achieve high-quality intense whispering gallery modes (WGMs) in a single ZnO microsphere optical resonator by doping with alkali metal ions. Here, a single microsphere of undoped and doped ZnO was considered to investigate WGMs by recording luminescence spectra using a microphotoluminescence system under green laser excitation having a central wavelength of 532 nm and a fixed power of 55 mW/cm2. We have found that there is a significant enhancement in the intensity of WGMs in the case of doped ZnO in comparison to undoped ones. Among all the doped ZnO microspheres, 2 mol. % Li-doped ZnO yields the strongest and intense WGMs, which are accompanied by high-quality (Q)-factors. Furthermore, the pump power dependence measurement performed in 2 mol. % Li-doped ZnO reveals the lasing action in the visible optical window by explicitly exploiting the defect levels present in the material. Thus, our proposed defect engineered ZnO microsphere may represent a promising optical microresonator for developing highly sensitive WGMs based optical sensors.