When light interacts with a noble metal nanoparticle, the free electron gas in the metal is externally driven to perform oscillations at its plasma eigenfrequency (ω0). This resonant excitation of electrons (localized surface plasmon resonance) generates in close vicinity of the metal surface a high-frequency electromagnetic vibration, i.e. an enhanced optical field on the nanoparticle.

Let us assume the increased local electric field amplitude on the metal surface is Eloc(ω0), where ω0 is the plasmon resonance frequency. If a molecule is located inside the local field, the generated Raman scattering can be elastically scattered off the metal surface. In other words, both the incident light (E0) and the Raman scattering are enhanced via elastic scattering off the metal nanoparticle. The intensity of Raman scattering from the molecule is proportional to the square of the amplitude at ω0. And this scattering radiation is again amplified to an intensity that is proportional to the square of the amplitude at ω0 ± ωR. Overall, the intensity of the enhanced Raman scattering (ISERS) is enhanced two times by the metal substrate: 
ISERS ~ |Eloc(ω0)|2 x |Eloc(ω0 ± ωR)|2
For ω0 » ωR, the SERS intensity can be approximated by the fourth power of the local electric field amplitude on the metal surface:
ISERS ~ |Eloc(ω0)|4
This equation is called the |E|4 approximation for the SERS enhancement (which is a good approximation only for the near infrared excitation): a moderate enhancement of E results in a huge SERS signal enhancement (|Eloc|4).
   Our group is working in the field of plasmonic nanoparticles and SERS. The design and synthesis of bifunctional nanoparticles for in situ monitoring of chemical reactions at the catalytic interfaces is the main focus of our research. Characterization of heterogeneous catalysis using conventional methods is challenging because of the many different components that are difficult to be separated and purified from the mixture, especially the components appearing only at the catalytic interfaces. In SERS, beside the high sensitivity, only the components close to the metal are highlighted and different molecules there exhibit ... [Read more...]