Evolution of Whispering Gallery Modes in Li-Doped ZnO Hexagonal Micro- and Nanostructures

Rocío Ariza,Belén Sotillo,Paloma Fernández,Fernando Pavón,Ana Urbieta

doi:10.3390/app10238602

Abstract

The formation of optical cavities in Li-doped ZnO nanostructures was investigated. By means of the vapor–solid method, long micro- and nanostructures with a hexagonal cross-section were grown. These morphologies were favorable for Fabry-Pérot and whispering gallery modes to appear. A variety of structures with different sections was studied using µ-photoluminescence in both the transverse electric (TE) and transverse magnetic (TM) polarizations, showing well-pronounced optical resonant modes. The results showed a dominance of whispering gallery modes that were in good agreement with the calculated refractive index. The quality factor (Q) and finesse (F) were estimated, which demonstrated the quality of Li:ZnO structures as optical cavities.

Highlights

ZnO is a well-known material with applications in very different fields from photonics to cosmetics.A wide bandgap, relatively high refractive index, large exciton binding energy, and non-ohmic behavior are behind many of these applications
The evolution of the optical resonant modes established in the hexagonal cross-sections of ZnO:Li structures was investigated
The incorporation of Li in the growth process improved the quality of the optical cavities and produced structure morphologies that were suitable to confine the light in the cross-section

Summary

Introduction

ZnO is a well-known material with applications in very different fields from photonics to cosmetics.A wide bandgap, relatively high refractive index, large exciton binding energy, and non-ohmic behavior are behind many of these applications. The lasing capabilities at room temperature have received a lot of attention in the last few years, with special attention being paid to hexagonal structures, rods, plates, or nails at the micro- and nanoscale [2,3]. Dielectric whispering gallery resonators are the focus of this application. They are microstructures with disk, cylindrical, or spherical shape, in which the light circulates around the lateral walls, causing multiple internal reflections. This effect usually leads to high Q factors, and low lasing threshold powers. The wurtzite crystal structure offers unique properties for creating microcavities for this purpose [2,3,4,5,6,7]

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Results

Conclusion