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Materials scientists have been studying crystals - large and small - for many years. However, very tiny crystals - crystals only a few atoms across - exhibit some really surprising properties, which we are only just starting to understand. In terms of their optical properties, these very small crystals, which we call quantum dots, exhibit behaviour more similar to that of an individual atom, than that of a large crystal. This surprising observation - which is a consequence of the confinement (or trapping) of charge carriers within a very small region - is more than just a weird academic curiosity. Scientists hope to exploit quantum dots to allow improved performance in light sources such as laser diodes, and to develop completely new light sources which might be used in novel computers or in secure communication. For light sources emitting in the red or infra-red, researchers are already starting to realise some of these goals using a material called indium gallium arsenide. However, for light emission in the blue - which is particularly relevant to applications such as high density data storage and satellite-based communications networks - quantum dots made from different materials are required. For light emission in the blue spectral region, quantum dots made from indium gallium nitride (or InGaN) could be used. Quite apart from their convenient wavelength of emission, InGaN quantum dots might be rather flexible, since their emission can be adjusted by applying an external electric field. Also, by surrounding the InGaN quantum dots with an optimal matrix material, it may be possible to force them to exhibit their peculiar properties at room temperature, whereas quantum dots emitting in the red usually have to be cooled down to temperatures more than 200 degrees below freezing before they work properly. Unfortunately, InGaN quantum dots also have disadvantages. They are usually formed on top of layers of another semiconductor - gallium nitride. Gallium nitride is quite difficult to make, and contains many mistakes, or defects, in the crystal. The defects may become electrically charged, and the presence of this charge alters the properties of the quantum dot. Since the electrical charge on the defect varies with time, so does the behaviour of the quantum dot - leading to problems with the operation of a quantum dot device. In order to try to understand the properties of the InGaN quantum dots more thoroughly, and to improve the properties of quantum dot devices, we have decided to incorporate the quantum dots into optical cavities. An optical cavity is a structure within which light may be confined. By trapping the light emitted by the quantum dot within a small volume, we can force the quantum dot and the light to interact strongly, and this can lead to more efficient emission from the quantum dot. By understanding the interactions between the light and the quantum dot, we can also use the cavity as a tool to probe the details of the quantum dot's behaviour and its interactions with any defects in its immediate surroundings. We hope to use the cavities to tailor the quantum dots' properties so that they are easier to exploit in future applications. However, making the cavities is very challenging, particularly since we have to find routes to do this which do not damage the quantum dot. Since this is a very complex problem, we have set up an international collaboration in order to attack it more effectively. Two British research groups with expertise in InGaN quantum dots will collaborate with an American research group which has world-leading capability in cavity fabrication. Together, we hope to be able to develop quantum dot - cavity systems which allow very strong interactions between the quantum dot and the cavity. In the future such systems will be used not only as a probe to study the quantum dot properties but as a major building block of novel light sources.
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