Ordered Arrays of ZnO Nanowires: Piezoelectric properties and stimulated emission from single wires

The current interest in ZnO nanostructures is based on its possible applications in UV optoelectronics. In particular, ZnO nanowires (NWs) appear to be promising candidates for the realization of dense arrays of nanosized lasers.
ZnO NW arrays were grown by a modified vapor liquid solid process which results in the Au catalyst remaining at the base of the nanowires. The fabrication process is template assisted involving a gold nanodisk array. This allows a site-specific growth of the ZnO NWs in an ordered hexagonal arrangement. The typical separation of the wires is around 500 nm, the NWs have a narrow size distribution centered at 300 nm and a typical height of 1.5 mm. The effective piezoelectric coefficient of individual ZnO NWs will be shown for various sizes and length. High-excitation phenomena, in particular excitonic nonlinearities and lasing will be reported. Under low excitation the NWs show bulk-like PL spectra. However, there is still a one-dimensional confinement of exciton-polaritons resulting in the absence of the P-band emission resulting from polariton-polariton scattering in thin wires. Stimulated emission is observed in single NWs up to 150 K. Upon raising the excitation fluency a spectrally and temporally narrow emission peak shows up while the P-band emission is quenched. We will show that free carriers are involved in the laser emission. We observe, that the lasing is influenced by details of the wire geometry. Calculations show the increasing leakage of the guided modes into the vacuum when decreasing the wire diameter. There is also a significant leakage of the resonator modes into the substrate. This loss can be reduced significantly by introducing thin layers of gold at the resonator facets as done here.