Lasing characteristics of optically-pumped single ZnO micro/nanocrystal

Abstract

Zinc oxide (ZnO) has a wide band-gap energy of 3.37 eV and a large exciton binding energy of 60 meV which is considerably larger than the thermal energy at room temperature (26 meV), and therefore, efficient exciton emission in ultraviolet (UV) region can be expected. Especially, ZnO micro/nanocrystals are quite attractive as building blocks for efficient UV opto-electronic devices. We have been investigating micro-cavity UV lasing from variously-shaped ZnO micro/nanocrystals, and micro-cavity lasing from ZnO nanowire and nanosheet have been confirmed, so far. Recently, we could fabricate ZnO micro/nanosphere crystals by a simple laser ablation method of ZnO sintered target in the air. In this study, we report UV micro-cavity lasing from an optically-pumped single ZnO micro/nanosphere crystal, for the first time. The spherical-micro-cavity lasing characteristics were investigated and discussed by comparisons with theoretical considerations in terms of quality factor and mode spacing of its lasing spectra with modal structures. From those considerations, it was found that the lasing mechanisms within a ZnO sphere crystal was attributed to whispering-gallery- mode (WGM) cavity lasing, and a ZnO sphere crystal had a good light confinement property due to the internal total reflections. Since the fabrication method is very simple and productive without any time-consuming crystal-growth process, ZnO micro/nanosphere crystals can be promising building blocks for UV opto-electronic devices such as a UV laser diode. In addition, since a ZnO micro/nanosphere can operate as an active WGM refractometric sensor for small molecules in UV region, high sensitivity enhanced by high quality factor, refractive index, and wavelength dispersion can be expected.