Plasmon-enhanced light emission from hybrid nanowires: towards electrically driven nanowire lasers

Semiconductor nanowires are important quantum systems that can emit light at a broad range of wavelengths efficiently, where quantum effects resulting from their small size enhance the emission considerably. One issue that makes working with these tiny devices very challenging is the ability to inject current into them effectively and generate light by an electric current. We intend to explore a new means of growing these nanowires using tiny droplets of liquid helium that enables us to control the structure of these nanowires with exquisite precision. We can build these systems from the bottom up, i.e., by the addition of atoms/molecules one by one to helium droplets, allowing us to produce nanowires that are extremely difficult to make by any other means. These helium droplets are collections of helium atoms which may range in size from as few as several dozen helium atoms all the way to in excess of 100 billion atoms. The droplets possess some remarkable physical properties owing to the very low temperature, 0.37 K, which makes each droplet a superfluid. Any atoms or molecules added to a helium droplet cool rapidly to this temperature because of the exceptionally high thermal conductivity of superfluid helium and the fact that excess energy can be removed rapidly from the droplet by evaporative loss of the weakly bound helium atoms.

When many atoms and/or molecules are added to the droplets they can aggregate and form objects that have nanoscale dimensions. For example, very recently it has been shown that metal nanoparticles composed of a few hundred to several million atoms can be made via this route, and nanowires can be grown by adding atoms/molecules to one-dimensional quantised vortices present in large helium droplets. Furthermore, these nano-objects can be removed from the droplets by collision with a solid surface, delivering a soft-landing. These preliminary studies, several of which originated from our team, are highly significant because they pave the way for the use of helium droplets as a tool in synthetic nanoscience. The great promise offered by helium droplets is the almost unlimited combination of materials that can be added, which will aggregate into nanoparticles or nanowires with a high degree of control.

With the helium droplet technology, we can produce nanowires which have a thin filament of metal at their core, clad with a range of semiconductors, or vice versa, which are very difficult to make with other synthetic methods due to the non-wetting between metals and semiconductors. These nanowires will take advantage of surface plasmons to enhance the optical emission greatly, and smooth metallic coatings during growth would enable us to contact the nanowires efficiently; hence they are ideal to develop very small lasers driven by electric currents, and ultimately can be used to construct nano-sized electro-optical devices. In this regard, the nanowires that we will explore will have significant advantages: they are standalone and moveable, allowing them to be manipulated, transported and integrated into nanophotonic circuits.

We have a range of state-of-the-art lasers that we can use to investigate the optical properties of these systems where the light is collected by high-resolution microscope systems that can operate with samples cooled to temperatures as low as 4 K. We will also use e-beam and electron microscopy techniques to write contacts onto these nanowires in controlled patterns so that we can excite individual nanowires both electrically and optically, and collect and analyse their emission. Applications we envisage range from sensors, quantum light sources and photovoltaic devices to nanolasers where we can control the emission wavelength over a large range. The project aims to deliver radical advances in both fundamental nanoscience and applied nanotechnology.