Nanophotonics, Plasmonics & Metamterials
Metallic nanostructures allow us to break the diffraction limit of light providing much stronger interactions of light and matter than nature usually allows. Our research includes nanoscale plasmon lasers, sensitive biodetectors, quantum plasmonics and artificial nonlinear optical materials and forms a large part of the Centre for Plasmonics and Metamaterials.
Academic Staff: Prof. Stefan Maier
The field of plasmonics uses collective electronic resonances in metal nanostructures to compress light fields into tiny volumes, often much less than an optical wavelength. Such structures can be designed to concentrate light to enable extremely sensitive chemical detection, down to just a handful of biomolecules, via a dramatic amplification of their optical absorption characteristics. Similar approaches are being pursued at the other end of the spectrum, where we use designer “spoof plasmon” surfaces to confine THz radiation close to the surface where it can be strongly absorbed. A well-known example of devices relying on localised plasmon resonances are glucose-sensors for diabetics.
SERS, surface enhanced Raman Spectroscopy, exploits the same plasmonic field enhancement effect to achieve enhancement of vibrational coupling of molecules to light by many orders of magnitude. Imperial researchers led by Prof Cohen developed the use of coupled plasmons between colloidal gold nanostructures to move towards a method capable of detecting disease-specific enzymes with unprecedented levels of sensitivity.
Integrated Plasmonic Semiconductor Devices
Ever since the 1990s, researchers have endeavoured to make lasers ever smaller. While it is logical that lasers should be compact if we are to integrate them in optical circuitry similar to integrated electronics, two important physical motivations arise: firstly, smaller lasers have reduced laser thresholds leading to lower power consumption and higher efficiency; secondly, small lasers can be incredibly fast. There are numerous reasons why these features are important, but the ability to control and manipulate light matter interactions is at the heart of contemporary solid state and optical physics. In the past few years, we have been exploring the speed of these metallic nano-lasers based on Zinc Oxide. Indeed, these lasers are as fast as expected, but by being so small as they are, they can be switched on and off in less than a pico-second – in the so-called ultrafast regime! Other work involves transferring our nano-laser technology to more conventional III-V semiconductor materials.
Silicon photonics is a promising platform for short haul data communications. Plasmonics concepts can have a major impact on this technology, both from the perspectives of miniaturisation and functional capabilities. We have been developing plasmonic components to focus light to the nano-metre scale where light can be extremely intense. Usually, microscope lenses can only focus light down to the diffraction limit, which is about half of the wavelength of light being focussed – about 750 nm at the telecommunications wavelength of 1500 nm. However, with metal nanostructures, light’s focus is only limited by how small one can fabricate something – about 25 nm in this case. We have also shown how the focussed fields can be used to achieve extremely strong nonlinear frequency mixing. This is an exciting breakthrough as it lifts many of the limitations of conventional nonlinear optics. We are currently investigating a number of key concepts relevant to data communications with silicon.
We also make quantum metamaterial “superlenses”, using quantum theory to design structures where light beams propagate without spreading by diffraction. We look at them with a new s-SNOM microscope, that maps IR spectra at a resolution of 100th of a wavelength, and we also use it to measure chemical distributions within single cells for the first time.
Our work in nonlinear plasmonics main involves harmonic generation, whereby we double or triple the frequency of laser light using metallic nano-particles to create the conversion process. In particular, there is a long history of studying second harmonic generation in metallic antennas as intrinsically, these materials should not exhibit a second order response at all. Recently, we have investigated the nonlinear response of individual antennas, to understand better how to build a more effective non-linear metamaterial. Previous work in the literature has stuck to the rule that antennas must break centro-symmetry to be effective – however, our work shows a counter example – a non-centro symmetry antenna that out-performs a centrosymmetric one! Our work thus reveals the importance of a microscopic interpretation when making nonlinear metamaterials. Using this information, we have designed the most effective non-linear antennas for second harmonic generation reported to date. This work may also lead to exciting capabilities in quantum optics, by exploiting the artificial nonlinearity we are engineer.