Plasmonics and quantum theory
Controlling the interaction between light and matter is fundamental to science and technology – from probing entanglement in quantum physics to harnessing the spectacular information carrying capacity of optical fibres. Nanoscale fabrication techniques, such as electron-beam lithography, allow us to create new materials with increasing sophistication and freedom of design, however controlling light at the nanoscale remains a challenge.
Traditionally light can only be controlled on length scales down to a little below the wavelength of light, a few hundred nanometres, this accounts for the usual resolution limit of optical microscopes and telescopes. Without a means to control light on length scales down to a few nm, both nanoscience and nanotechnology will fail to deliver their potential. However, a new paradigm called plasmonics has emerged, an approach based on localised surface plasmon resonances of metal nanostructures, that allows to control light below the wavelength limit and down to the required nanometre length scales.
Typically in optics we can consider the response of materials such as metals, dielectrics etc. to be macroscopic, i.e. to ignore the fact that they are structured at the nanoscale, comprising atoms/molecules. Plasmonics pushes us into a new real because the degree of confinement of optical fields that plasmonics affords means that the atomistic nature of matter can not be ignored.
To date, most of the research into surface plasmon resonances has been limited to frequencies near metallic plasma frequencies (i.e. in the visible spectral region). Semiconductors, such as graphene, with plasma frequencies in the THz and IR spectral ranges, offer the potential for confining surface plasmon resonances at lower frequencies. Furthermore, semiconductors offer a unique and hugely beneficial advantage over metals: since the surface charge density can be modified by, for example chemical doping, plasma frequencies and SPP properties can be tailored. An extension of this is the exciting possibility of all-optical plasmon control, i.e. ‘photo-doping’ a semiconductor with visible frequency light, so that plasma frequencies may be tuned by a visible frequency light source, allowing active materials that can be switched on very fast (picosecond) timescales, something that is essential for high-bandwidth and/or time resolved applications.
Understanding the optical response of materials at the nanoscale requires to go beyond the usual macroscopic (classical) approach to electrodynamics. To obtain a fuller picture quantum optics and quantum information science techniques can be used that include the full quantum nature of light-matter interactions. We have recently established a theoretical component in these areas that complements our plasmonics experiments and explores new directions towards realisable quantum technologies. Future activities will include exploring spatial dispersion in material responses to electromagnetic stimuli and investigating the role of quantum effects in thermodynamics at the nanoscale.
This theme is led by Professor Bill Barnes, Professor Euan Hendry, Dr Janet Anders, Dr Tom Philbin and Dr Simon Horsley, supported by Professor Roy Sambles, Professor Rob Hicken and Professor Alastair Hibbins.