Exciton Polaritons Confined in a ZnO Nanowire Cavity

Semiconductor nanowires of high purity and crystallinity hold promise as building blocks for miniaturized optoelectrical devices. Using scanning-excitation single-wire emission spectroscopy, with either a laser or an electron beam as a spatially resolved excitation source, we observe standing-wave exciton polaritons in ZnO nanowires at room temperature. The Rabi splitting between the polariton branches is more than 100 meV. The dispersion curve of the modes in the nanowire is substantially modified due to light-matter interaction. This finding forms a key aspect in understanding subwavelength guiding in these nanowires.

Figure 1

(a) Average photoluminescence spectrum of a ZnO nanowire for excitation with sub-band-gap light (1.72 eV), showing green and UV photoluminescence as well as second harmonic generation. (b)–(e) Spatially resolved excitation images taken at the emission energies indicated in panel (a) by the colored bars. (f) TEM image of the investigated wire. The wire had a length of $5.19\mu \mathrm{m}$ and a diameter of 273 nm. (g) Excitation line traces along the length of the wire ($c$ axis) at emission energies indicated by the colored bars in panel (a). The profiles are scaled with the indicated factors for clarity. (h) Calculated excitation profiles for the energies shown by the color bars in (a).

Figure 2

(a) Average spectrum (black line) and enhancement spectrum (blue line) of a wire excited with $\hslash \omega =1.72\mathrm{eV}$, which shows, near the exciton resonances, a peak splitting in the enhancement spectrum of 164 meV. (b) Average spectrum (black line) and enhancement spectrum (blue line) of a wire excited with $\hslash \omega =1.72\mathrm{eV}$. The enhancement spectrum shows a peak splitting of 102 meV near the exciton resonances and modulations due to Fabry-Pérot interference in the transparent region (2.7–3.2 eV). (c) Exciton-polariton dispersion (red lines) calculated with Eq. (1) from the resonances of the A $(3.309\mathrm{eV})$, B $(3.315\mathrm{eV})$, and C $(3.355\mathrm{eV})$ excitons (dotted horizontal lines) and a photon mode confined in the nanowire cavity (dotted curve). For clarity, we do not show the strongly damped modes in the anticrossing energy-gap. The crosses indicate the energetic peak positions obtained from the Fabry-Pérot maxima of the enhancement spectrum shown in panel (b). These maxima are equidistantly spaced at $m_z{}^{*}(\pi /L)$ in $k$ space ($L=\mathrm{\text{the length of the wire}}=5.19\mu \mathrm{m}$) and give the slope of the exciton-polariton dispersion between 2.7 and 3.2 eV. (d) Calculated enhancement resulting from the dispersion curve shown in (c). (e) Excitation power dependence of the peaks in the enhancement spectrum.