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Abstract
A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 µm are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors.
Highlights
Zinc oxide (ZnO), a wide bandgap (~3.37eV) semiconductor material, has been studied in both forms of thin films and micro/nano structures in the past decades for their potential applications in various fields because of its tunable optical and electronic properties [1,2,3,4,5]
We present the synthesis of ZnO microspheres with smooth surface and study the optical resonances of whispering gallery modes (WGMs) from single crystalline ZnO microsphere at room temperature using micro-PL (μ-PL) measurements
The analysis of elemental compositions for these microspheres is performed with energy-dispersive X-ray (EDX) spectroscopy at an acceleration voltage of 10KV
Summary
Zinc oxide (ZnO), a wide bandgap (~3.37eV) semiconductor material, has been studied in both forms of thin films and micro/nano structures in the past decades for their potential applications in various fields because of its tunable optical and electronic properties [1,2,3,4,5]. ZnO micro/nanostructures with hexagonal cross sections can show distinct mode confinement routes such as those of FabryPerot (FP) modes and whispering gallery modes (WGMs) and can be utilized as compact resonators These structures have drawn considerable interest in applications of optical resonators such as microcavity lasing due to their unique geometry, high crystallinity, and excellent optical properties [20, 21, 25,26,27,28,29]. A smooth surface morphology of ZnO microspheres is crucial since a high quality factor and low lasing threshold of micro/nanocavities are only achievable without large scattering losses from rough surfaces Under such circumstances can whispering gallery modes, which are waves traveling around the circumference of the resonator due to the total internal reflection, exhibit sufficiently sharp resonances in various optical spectra. The observed optical resonances are compared with analytically estimated values and are further verified with a modified eigenvalue formalism implemented with the axially symmetric finite-element method (FEM) [35, 36]
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