SERS

	Plasmonic nanoparticles and Surface Enhanced Raman Scattering (SERS).

The Raman effect is the small shift in the wavelength of light as it scatters off a molecule. This occurs when the photon excites one of the molecule vibrational modes leading to a small loss in photon energy. This energy loss, equal to the energy of the excited vibrational mode, results in a wavelength shift of the photon. The Raman spectrum gives information about the molecule vibrations and can be used to uniquely identify the molecule. This is important in chemical sensing for environmental monitoring.

The problem is the Raman effect is very weak. However, it was discovered in the 1970's that the Raman effect increased enormously (6 orders of magnitude or more) when the molecules were on a rough gold or silver surface. This is known as the Surface Enhanced Raman effect. It is now known that the effect is due to the excitation of surface plasmons on the rough metal film and this leads to stronger coupling of the light to the molecule.

In my group we applied our theory of the coupling of plasmonic particles to model mathematically the Raman enhancement phenomenon [1]. Our theory shows that the metal particle supporting LSPs acts as an antenna. It is much better at collecting the incident light than the molecule. On resonance it produces strong electric fields that excite the Raman modes of the molecule. Moreover, the Raman-scattered light is picked up again by the metal particle and efficiently re-radiated. One can think of this as an impedance matching problem. The optical antenna matches the incoming radiation to the molecule and vice versa.

By including coupling between plasmonic particles in the Raman enhancement effect there are many ways of modifying the interactions - watch this space!

The Raman enhancement effect is due to the strong electric fields excited by surface plasmons. This is very strong at the point between two gold spheres. This shows the distortion of the LSP charge which becomes confined to the region between the spheres [1].

The metal nanoparticle as like an optical antenna, efficiently coupling the incident light to the molecule and then collecting the Raman scattered light from the molecule and re-radiating it.

[1] T. J. Davis, D. E. Gomez, K. C. Vernon: "Evanescent coupling between a Raman-active molecule and surface plasmons in ensembles of metallic nanoparticles" Physical Review B 82, 205434 (2010)