Competition between extinction and enhancement in surface-enhanced raman spectroscopy

Conjugated metallic nanoparticles are a promising means to achieve ultrasensitive and multiplexed sensing in intact three-dimensional samples, especially for biological applications, via surface-enhanced Raman scattering (SERS). We show that enhancement and extinction are linked and compete in a collection of metallic nanoparticles. Counterintuitively, the Raman signal vanishes when nanoparticles are excited at their plasmon resonance, while increasing nanoparticle concentrations at off-resonance excitation sometimes leads to decreased signal. We develop an effective medium theory that explains both phenomena. Optimal choices of excitation wavelength, individual particle enhancement factor, and concentrations are indicated. The same processes that give rise to enhancement also lead to increased extinction of both the illumination and the Raman-scattered light. Nanoparticles attenuate the incident field (blue) and at the same time provide local enhancement for SERS. Likewise, the radiation of the Raman-scattered field (green) is enhanced by the nearby sphere but extinguished by the rest of the spheres in the suspension upon propagation.