Strong Light-Matter Interaction in ZnO Nanowires

Abstract

Zinc oxide (ZnO) has been investigated for UV/blue emitters and optoelectronics because of its wide band gap energy (3.37 eV).1-3 In addition, several interesting properties are observed in ZnO. First, the large exciton binding energy and oscillator strength result in the primary excitation of the excitons at room temperature.4,5 Second, light is efficiently guided for the photonic confinement in nanowires.6-8 Another property is the quantum mechanical lightmatter interaction between a travelling photon and exciton, [9][10][11] which is much more intense compared to other semiconductors. Moreover, the light-matter interaction is enhanced in nanowires due to the confined mode volume and the enhanced oscillator strength.6-11 Thus, a strong exciton-photon coupling leads to the exciton-polariton in the nanowires, where the optical properties are not only determined by the electronic states but also by the geometries and dimensions.Among the various nanostructures of ZnO, the most extensively investigated are one-dimensional nanowires due to their natural cavity mode and optical gain.1-5 In addition, the active guiding of light takes place in nanowires because the vacuum wavelength of the guided light is longer than the wire diameter. However, the light-matter interaction has not been well understood in the nanowaveguide of ZnO. In this Note, we study the light-matter interaction in ZnO nanowires. The abnormal spectral spacing of Fabry-Pérot-type modes was observed in the isolated single nanowire, which indicated the strong exciton-photon coupling in a nanocavity. The lasing modes became blue-shifted with increase in the excitation intensity, which implied the weakening of the exciton states and thus the reduction of the light-matter interaction. At the high excitation intensity, additional lasing modes were observed in the low energy regime, which was attributed to the formation of electron-hole plasma state and band gap renormalization. The lasing in the electron-hole plasma state was also explained by the light-matter interaction in nanowires.The typical photoluminescence spectrum of ZnO nanowires is presented in the inset of Figure 1(a), which was obtained at the excitation intensity of 10 μJ/cm 2 . The UV peak at 3.25 eV indicated the exciton state emission.1-3 The nearly absent visible emission (2-3 eV) suggested that the defect states were minimized in the nanowires.4,5 The emission spectra of the single nanowire were also obtained as a function of the excitation intensity to investigate the photonic confinement effect. The sharp peak (mode a) was observed at the excitation intensity of 50 μJ/cm 2 (Figure 1(b)), where the bandwidth of the peak was reduced by ~20 times compared to the normal emission band. This narrow band indicated that the lasing threshold was 50 μJ/cm 2 in this nanowire.12,13 With increase in the excitation intensity above the lasing threshold, new sharp peaks (modes b and c) appeared in the lower energy regime, which were attributed ). The lasing modes in the renormalized band gap (mode b a...