Physics and applications of semiconductor nanowire photonic devices

Semiconductor nanowires offer an attractive technological route for the development of nanoscale photonic devices. Given certain dimensional constraints, nanowires can support guided modes, and their end facets can function as effective mirrors, which supply optical feedback. Furthermore, with an appropriate pump mechanism, nanowires of direct bandgap materials can exhibit gain. Thus, nanowires naturally embody all the necessary elements of a laser cavity, which render them ideal sources for efficient coupling into nanophotonic elements as well as local excitation of biological specimens. The challenges they present are not of a chemical nature: they can be synthesized inexpensively, in large quantities and with very high quality. Rather, the key difficulties lie in their manipulation and assembly into useful devices and circuits. This thesis introduces new methods for the fabrication of nanowires into such devices and offers physical insights into their operation. In the first part, the focus is on a device geometry consisting of a nanowire sandwiched between a highly-doped silicon substrate, which functions as a common bottom contact, and a top metal electrode. We find that the nature of the nanowire/substrate and nanowire/metal interfaces completely determines the operation of the device. In fact, nanowire electroluminescence is only possible when a thin insulating layer is included between the nanowire and the substrate. A fascinating consequence of this is that we can obtain electroluminescence (both from the nanowire and the substrate) when the nanowire and the substrate are of the same conductivity type. In the second part, we study the behavior and limitations of zinc oxide nanowire lasers by means of optical excitation. Our findings show that nanowires present a fundamental trade-off: while nanowires with narrow diameters (compared to the wavelength of light in the material) satisfy the need for compactness, they present large losses that prevent lasing. We also introduce a "head on" detection geometry with which we measure the far field profile of a nanowire laser and thus identify the oscillating modes.