Direct Observation of Whispering Gallery Mode Polaritons and their Dispersion in a ZnO Tapered Microcavity

We report direct observation of the strong exciton-photon coupling in a ZnO tapered whispering gallery (WG) microcavity at room temperature. By scanning excitations along the tapered arm of the ZnO tetrapod using a micro-photoluminescence spectrometer with different polarizations, we observed a transition from the pure WG optical modes in the weak interaction regime to the excitonic polariton in the strong coupling regime. The experimental observations are well described by using the plane wave model including the excitonic polariton dispersion relation. This provides a direct mapping of the polariton dispersion, and thus a comprehensive picture for coupling of different excitons with differently polarized WG modes.

Figure 1

(a) SEM image of the ZnO tetrapod. (b) Magnified SEM image of the resonator shows perfect surface quality. (c) The relation of the measured radius $R$ and scan position $x$ is fitted by $R=367–4.7\times x(\mathrm{nm})$.

Figure 2

(a) WGM-free (black curve) spectrum and the WGM spectra obtained at a certain position of the tapered arm with unpolarized (green dashed curve), TE (blue dotted curve) and TM polarized (red dashed-dotted curve) detection, respectively. (b) Spatially resolved PL mapping along the tapered arm with the excitation laser polarization $\mathit{E}\parallel \mathbf{c}$ axis and detection unpolarized.

Figure 3

PL mapping along the ZnO tapered arm with the excitation polarization $\mathit{E}\parallel \mathbf{c}$-axis and detection polarizations of (a) TE and (b) TM. (c) and (d) are the corresponding waterfall plots of the spectra. The colored lines in (a) and (b) are the theoretical fittings. The PL intensity scale of (a) and (b) is the same as that of Fig. 2b. (e) and (f) the complete theoretical computation dispersion for TE and TM, respectively. the energy of $A$-, $B$- and $C$-exciton were shown with dotted lines, the lower (red), upper polariton branch (blue), pure WGM (black) are shown.