A team of researchers at the Indian Institute of Science Education and Research (IISER),Pune is trying to understand the interaction of light with nanostructures and molecules at micro and nano scales. Their research is based on a rapidly emerging sub-discipline of nanophotonics known as plasmonics. We are primarily interested in propagation,manipulation and localisation of light at the nanoscale. We achieve this by utilising the properties of surface plasmons,that are the collective oscillations of the free electrons in a metal, said Dr G V Pavan Kumar,Photonics and Spectroscopy Laboratory,Division of Physics and Chemistry,IISER. Depending on the geometry of the nanostructures,optical waves can couple with these electron oscillations in the form of propagating surface waves or localised excitations. Such sub-wavelength surface plasmons can be harnessed for technologically important applications,specifically in optical nano-circuits and nano-optical sensing. The research team includes PhD students Arindam Dasgupta and Partha Pratim Patra,and MS students Danveer Singh and Rohit Chikkaraddy. Together,the team is trying to find answers to some key questions: How does light interact with nanoparticles and nanowires smaller than its wavelength? How can we build an efficient optical antenna with nanomaterials? How does one enhance light scattering by molecules and nanomaterials? What happens to the structure of illuminated bio-molecules near metallic surfaces? How can a single molecule or nanoparticle be detected optically? The project began in May 2010 and might go on for another five years. Single molecules behave very differently as compared to an ensemble of molecules. This has been observed in various scenarios including in living cells,where single biomolecules can play a critical role in regulating the function of the cell. Single molecules on the surface of various materials,too,have relevance in catalysing many chemical reactions. We employ a technique called Surface Enhanced Raman Scattering (SERS),which is a combination of plasmonics and Raman scattering. The idea behind this method is to adsorb single molecules on metallic nanostructures,and scatter laser light off them. The scattered radiation spectra (in this case Raman scattered light) will have signatures of the single molecule adsorbed by the nanostructures, explained Dr Kumar. This phenomenon can be understood as follows: when light is incident on the metallic nanostructures,they act as optical antennae by enhancing the local optical intensity at the vicinity of the molecule,further leading to enhanced interaction between the incident light and the molecule. This enhanced interaction is due to the plasmon field created by the nanostructure and is good enough to detect single molecule signatures. Such optical techniques are vital to design and develop nano-optical biosensors,where the size of the detector should be as small as possible. One of the primary goals of nanophotonics is to design and develop nano-optical elements that can be utilised to propagate and localise light at the nanometer scale. Surface plasmons of metals can serve this purpose. One can squeeze optical signals into minute nanowires by using light to produce electron density waves called plasmons. These may be utilised to create nano-circuits of light,similar to electronic circuits. Plasmonic circuits can be used to build computer chips to create fast interconnects to transfer large amounts of data across a chip. Plasmonics can also improve the resolution of optical microscopes,the efficiency of light-emitting diodes,and the sensitivity of chemical and biological sensors. Recent research by other groups has shown that plasmonic materials could alter the electromagnetic field around a nano-object to the extent that it could be made invisible. This has relevance in the development of optical cloaks.
