Optical cavity modes of a single crystalline zinc oxide microsphere

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.

Whispering gallery modes in intrinsic TiO2 microspheres coupling to the defect‐related photoluminescence after visible excitation

“…The sharp resonances shown in Fig. on top of the broad luminescence background are caused by the coupling of the luminescence to WGMs, as observed in a number of other systems .…”

“…Similar to the situation in ZnO , visible light can be used in photocatalytic processes involving TiO 2 . This work shall show that a recently established coupling method for ZnO microspheres – similar to free space coupling – enables the excitation of WGMs in intrinsic TiO 2 microspheres without doping elements. Here, we shall discuss the coupling of defect induced photoluminescence (PL), which can be excited in the visible, to WGMs in a TiO 2 microsphere resonator.…”

Zinc Oxide Nanophotonics

“…The different types of WGMs differ by the length of their closed optical pathway, the angle of incidence impinging light wave fronts on the cross section boundary, and the number of reflections at boundary that give rise to phase shifts of the light waves for each total internal reflection. Spherical shape of ZnO optical cavity has been reported [73], however, most of ZnO nanostructures in which WGMs have been observed so far exhibit hexagonal cross sections that reflect the hexagonal wurtzite crystal structure. The WGMs emission have been investigated in hexagonal ZnO micro and nanodisks [68,74,75], nanonails [76][77][78], and wires [79,80].…”

“…Accurate values for the WGMs peak positions can be determined at the intersections of a series of solid lines for different mode number N (from 202 to 212). Interestingly, Moriangthem et al fabricated ZnO microsphere that exhibited WGMs evolution in both entire visible range as well as near UV region with narrow resonance peaks [73]. Using size dependence observation and theoretical comparison with experiments, comprehensive investigations were performed to confirm that the emission was from WGMs.…”

Random lasing of ZnO thin films grown by pulsed-laser deposition

“…Various types of cavities have been envisaged: individual cavities formed inside micro-or nano-single structures such as microdisks [11,16,17], microspheres [18,19], nanorods, nanowires and nanobelts [20][21][22]. In thin films, cavities are not so well identified and one should explain the effect by random lasing: multi-scattering on the edge of crystallite interfaces or grain boundaries is known to form closed-loop path [23] which could be considered as random ring cavities for light [4,13].…”