There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices, including lasersand nonlinear optical frequency converters. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet-blue light-emitting diodes, lasers and photodetectors. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films. Here we report the observation of ultraviolet-blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry-Perot modes (Q approximately 10(3)) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV microJ x cm(-2)) supports the idea that the electron-hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet-blue coherent light sources.

Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.

Quantum wire lasers are generally fabricated through complex overgrowth processes with molecular beam epitaxy. The material systems of such overgrown quantum wires have been limited to Al-Ga-As-P, which leads to emission largely in the visible region. We describe a simple, one-step chemical vapor deposition process for making quantum wire lasers based on the Al-Ga-N system. A novel quantum-wire-in-opticalfiber (Qwof) nanostructure was obtained as a result of spontaneous Al-Ga-N phase separation at the nanometer scale in one dimension. The simultaneous excitonic and photonic confinement within these coaxial Qwof nanostructures leads to the first GaN-based quantum wire UV lasers with a relatively low threshold.Quantum confinement of charge carriers in more than one dimension in quantum wires and quantum dots has been predicted to yield improved performance of semiconductor lasers, relative to conventional quantum well devices. 1,2 The carrier confinement is expected to lead to reduced threshold currents and narrower spectral line widths. The potential advantages of the quantum wire lasers could make them ideal for a variety of applications that require coherent light sources with low power consumption and high-speed digital modulation capability. A major challenge, however, has been the development of the fabrication technology for preparing quantum wire heterostructures that are compatible with laser applications. Almost all quantum wire lasers reported thus far have been made through molecular beam epitaxy, microfabrication, and lithographical techniques on the GaAs/InP system for visible light emission. 1,3,4 Significant technical hurdles exist for the direct fabrication of GaN-based quantum wire lasers, despite their obvious potential in short-wavelength photonic devices and hightemperature/high-power optoelectronics. 5-8 Herein, we report the first realization of self-organized, monolithically singlecrystalline GaN/Al x Ga 1-x N (x ) 0.75) core-sheath onedimensional (1D) nanostructures. Quantum wires of GaN (refractive index of 2.54) with diameters as small as 5 nm are cladded by Al 0.75 Ga 0.25 N (refractive index of 2.25) sheaths with uniform thicknesses in the range of 50-100 nm, forming a novel quantum-wire-in-optical-fiber (Qwof) structure. As a manifestation of the quantum confinement, a blue shift of the photoluminescence (PL) has been observed. Moreover, the simultaneous excitonic and photonic confinement within these coaxial Qwof nanostructures leads to the first GaN-based quantum wire UV lasers with relatively low thresholds.Recently, UV lasing has been demonstrated by our groups for the ZnO and GaN nanowire systems. [9][10][11] In these earlier studies, the entire faceted nanowire (generally with a diameter of >100 nm) serves both as a gain medium and as a FabryPerot optical cavity. This nanowire nanolaser configuration places a lower limit on the lasing nanowire diameter, approximately λ/2n (where n is the refractive index), below which the nanowire is no longer capable of sustaining even a leaky ...

No abstract