EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Optical cavities have been achieved in different materials and morphologies. However, ZnO stands out due to its optical properties [1]. Nanostructures have attracted much attention because of their necessity in miniaturized devices. ZnO shows unique properties for this purpose. The wurtzite crystal structure leads to long structures formations, which, along with its relative high refractive index, causes multiple internal reflections. In this work, the vapour-solid method was used to grow micro- and nanostructures of Li doped ZnO. The obtained morphologies are elongated structures with hexagonal cross-section being suitable to confine the light. This morphology assists the Fabry-Pérot (FP) and the whispering gallery modes (WGM) to appear. Nevertheless, the optical cavity behaviour depends on the shape and diameter of the cross-section.

The evolution of the resonant modes has been studied by means of µ-photoluminescence spectra and maps of two selected structures with different shape and diameter. In order to determine the main resonant mode occurring in the cavity, both polarizations were measured: Transverse-Electric (TE) and Transverse-Magnetic (TM). WGM are the dominant mode in both structures while residual FP shows a very low contribution. The calculated refractive index for the structures is in good agreement with the reported values in the literature. In turn, the quality factor and the finesse confirm the quality of Li doped ZnO in regard to undoped ZnO. These results show the potential of Li doped ZnO for applications like sensors [2] and optical filters [3].

[1]Gargas, D.J, et al. Whispering gallery mode lasing from zinc oxide hexagonal nanodisks. ACS Nano 2010, 4, 3270–3276. [2]Khanum, R, et al. Defect engineered ZnO whispering gallery modes via doping with alkali metal ions for label-free optical sensors. J. Appl. Phys. 2019, 125. [3]Absalan, H. A Four-Channel Optical Demultiplexer Using Photonic Crystal-Based Resonant Cavities. J. Opt. Commun. 2018, 39, 369–373.