Single-crystalline hexagonal ZnO microtube optical resonators

Abstract

High quality ZnO microtubes with hexagonal cross sections, fabricated via an oxidation-sublimation process, are studied as novel optical resonators. Whispering gallery modes, Fabry-P erot modes, and an additional set of modes with different polarizations are directly observed in the visible spectral range at room temperature by using the spatially resolved spectroscopic technique. The diameter and wall thickness of the microtube are crucial for light modulation. The experimental results are explained and fitted with a plane wave interference model and Cauchy dispersion formula for refractive indices.

A novel ultra-thin-walled ZnO microtube cavity supporting multiple optical modes for bluish-violet photoluminescence, low-threshold ultraviolet lasing and microfluidic photodegradation

“…17 The WGMs are generally supported by the hexagonal geometry of ZnO corresponding to the crystallographic plane of (0001), providing strong optical confinement and feedback by total internal reflection (TIR) as a high-Q cavity. 12,15,[18][19][20][21] The excitonexciton (ex-ex) collision has been proposed as a substitute for electron-hole plasma as a means of lowering the threshold of lasing in ZnO microcavities. 12,[21][22][23][24] The lasing threshold can be further reduced by introducing metals and graphene surface plasmons because of the high confinement of the optical fields and strong photonexciton interactions.…”

Fabrication of ZnO Nanostructures

“…Up to now, WGM could be observed in ZnO structures with triangular (m = 3) [97], hexagonal (m = 6) [8] or dodecagonal (m = 12) [98] cross section whereby the hexagonal structure is the most common one as measured by the number of publications. Note that even microtubes are able to exhibit WGM signatures [99]. All modes can unambiguously be identiϐied by their mode energies for a given cavity cross section width d (in our case the longest possible distance between two cross section corner points) and the ZnO refractive index n. Approximately, these mode energies E can be calculated via a simple, geometrical plane wave picture However, as Eq.…”

Zinc Oxide Nanophotonics

“…There are several investigations about the enhancement of visible emission from ZnO by WGMs [76,81]. The graph in Figure 5F shows luminescence spectra of the visible emission band enhanced due to a preferred constructive interference of light.…”