Manipulation of near field polarization by far field excitation

Metal nanoparticle assemblies of designed structure are investigated as substrates for polarization manipulation in the near field region. Gold nanoparticles are known for their optical response due to the excitation of surface plasmons. Surface plasmons in coupled particles can strongly modulate light either in the far or near field region. The most common near field application of coupled particles is as field enhancing substrates for amplifying signals of molecules, for example, Raman signals, IR signals or fluorescence signals. However, the capabilities of metal nanoparticle assemblies can be extended beyond field amplification. Groups of particles can function as small antennas which convert far field excitation into localized fields with specific polarization. Through simulations we demonstrate that the near field polarization can be partially controlled through suitable design of nanoparticle configuration. The benefit of this configuration is that no probe excitation or other localized excitation is needed. The far field signal is converted into specific spots with designed polarization, which is not necessarily the same as excitation. Polarization is manipulated through the coupling of different surface plasmon modes. This polarization modulation extends down to the few nanometer scale and may provide us more control of interaction of light with nano-scale emitters or molecules.