Time-resolved cathodoluminescence scanning electron microscope

This proposal aims to bring to the UK an amazing microscope which will provide new and powerful capability in understanding the properties of light emitting materials and devices. These materials are key to many technologies, not only technologies that utilise the light emission from materials directly (such as energy efficient light bulbs based on light emitting diodes) but also a range of other devices which utilise the same family of materials such as solar cells and electronic devices for power conversion. Some of these technologies are in current use, but their efficiency and performance can be enhanced by achieving a better understanding of the relevant materials. Other target technologies are further from the market, but may represent the building blocks of our future security and prosperity. For example, the new microscope will provide information about light sources which emit one and only one fundamental particle of light (photon) on demand. Such "quantum light sources" are a potential building block for quantum computers and for quantum cryptography schemes which represent the ultimate in secure data transfer. How will the new microscope allow us to advance the development of all these technologies? It is based on a scanning electron microscope, which utilises an electron beam incident on a sample surface to achieve resolutions almost three orders of magnitude better than can be achieved using a standard light microscope. It thus accesses the nanometre scale, which is vital to addressing modern day electronic devices. Standard electron microscopy accesses the topography of a surface, but the incoming electron beam also excites some of the electrons within the material under examination into states with a higher energy. When these electrons relax back down to their usual low energy state, light may be given out, and the colour and intensity of that light is incredibly informative about the properties of the material under examination. This light emission can be mapped on a scale of ~10 nanometres so that nanoscale structures ranging from defects to deliberately engineered quantum objects can be addressed. This technique is known as cathodoluminescence, and has been in use for many years. The new capability of our proposed system is that it will map not only the colour and intensity of the light emission, but also allow us to measure the timescales on which an electron relaxes back down to its low energy state. We use the phrase "in the blink of an eye" to describe something that happens extraordinarily quickly. A real eye blink takes at least 100 milliseconds, whereas the relevant timescales for the electron to return to its low energy state could be almost 10 billion times quicker than this! The new microscope will be able to measure processes occurring on this time scale, by addressing how long after an electron pulse excites the material a photon is emitted. It will even be able to distinguish between photons with different wavelengths (or colours) being emitted on different time scales. Crucially, coupling this time-resolved capability with the ability to vary the temperature, we will be able to infer not only the time scales on which electrons relax to low energy sites emitting a photon, but also the time scales by which electrons reduce their energy by other, non-light-emitting routes. These non-light-emitting processes are what limit the efficiency of light emitting diodes, for example. Overall, across a broad range of materials, we will build up an understanding of how electrons interact with nanoscale structure to define a material's electrical and optical properties and hence what factors limit or improve the performance of devices. The proposed system will be the most advanced in the world, and will give UK researchers working on these hugely important photonic and electronic technologies a global advantage in developing new materials, devices and ultimately products.