Plasmonics is one of the hottest topics, on which a vast amount of research is being conducted by physicists, chemists, biologists and engineers. Plasmons are collective oscillations of free electrons in both metals and semiconductors. Nanoparticles of sizes 20nm-150nm have plasmon polarization resonances for optical illuminations of range 390nm-700nm. Plasmons localize the incident optical wavelengths (of μm size) to ~10nm. Therefore, when radiation is converted to plasmons —electric field trapped as polarization— the intensity of the electric field increases 105 times compared to the incident radiation.
Emergence of such a localization results some genuine features in nanomaterials. Field concentration provides enhanced light-matter interaction. When a quantum object (e.g. molecules, quantum dots) is attached to nanomaterials, polarization field induced by plasmons interacts strongly with the quantum emitter. This enables one to read a single quantum memory efficiently.
The enhancement of the light-matter interaction by several orders of magnitude makes plasmonics important for optoelectronic devices. Photosensors, on which the defense technology is based and that we mostly use in our daily life, can be improved and miniaturized using plasmonic effects. Plasmonic sensors can detect much weaker signals compared to conventional photonic sensors. Field enhancement can also be utilized in thin film solar cells as interaction (light-trapping) centers for increasing the conversion efficiency.
Localization also provides higher resolution —on using the same excitation energy— in plasmonic imaging techniques. The metabolism of a single cell can be studied. Plasmonic nanoparticles (NPs) are used in cancer and viral therapies. Metal nanoparticles (MNPs) decorated with proper antibodies attach to the target cell and the localized strong field burns the cancer (infected) cell out.
The reason for the processor speeds of the computers being stuck at the GHz regime is insufficient information transfer capacity of the electronic circuits. Fiber optical cables can transfer information 1000 times faster compared to electronics due to their bandwidth. However, fiber optic cables are large in dimensions compared to electronic circuits. Plasmonics provide a solution also to this problem. Properly designed nanowires can carry both electronic and plasmonic information in nano-dimensions [Ozbay, Science, 311, 189 (2006)].
Field localization also allows nonlinear optical effects to show up. Second (or higher) harmonic generation (2ω, 3ω … signals), sum frequency generation (ω3=ω1+ω2) can be observed at lower excitation intensities. Nonlinear effects also lead the emergence of the quantum entanglement. An interesting feature of plasmon, demonstrated at several experiments, is that; entanglement is preserved for much longer times compared to the lifetime of plasmon itself. This makes plasmonic circuits also a proper candidate for quantum computers.
Here, in Institute of Nuclear Sciences, we conduct the leading research on nonlinear plasmonics both theoretically and experimentally (with national and international collaborations). In our recent articles, we show that nonlinear frequency conversion processes [e.g. second harmonic generation (SHG)] can be enhanced 1000 times by coupling the plasmonic nanoparticle with a quantum emitter (e.g. molecule, quantum dot). In a recent experiment of ours, we observe second harmonic (2ω) signal from a cluster of metal nanoparticles decorated with fluorescent molecules, using only a continuous wave (CW) laser excitation. Normally, such experiments are conducted using ultrashort lasers, of peak power at least 1000 times greater compared to CW lasers. In our simulations, we demonstrate that a 1000 enhancement factor for the nonlinearity (due to the path interference effect of Fano resonances introduced by the molecule) compensates the lower CW laser intensity.
We also conduct research on the entanglement properties of such nonlinear systems.
In addition, our group is the first to demonstrate the extension of the plasmonic excitation lifetime, when plasmons are coupled to high-quality quantum objects. Such lifetime extensions lead to the process of spaser —surface plasmon amplification by stimulated emission of radiation. Our treatment also explain the narrowing in the spectrum of spaser emission [Noginov et al., Nature 460, 1110, (2009) ] which emerges due to lifetime extension.