What is it about?
In our earlier work, we have shown that the response decays within a few tens of nanometers. However, several literature reports presented oscillations of the response (the resonance wavelength) of NP films for layers hundreds of nanometers thick, far beyond what would be theoretically plausible for a localized plasmon response. We replicated these findings with thick layers of polyelectrolytes or vacuum-deposited silica, and collaborated with Takumi Sannomiya, a theorist at the Tokyo Institute of Technology, to solve the riddle. We found that the response is a convolution of a short-range plasmonic response, with a long-range interference effect - the air/silica interface at the top, and the gold NP/silica interface at the bottom are both partially reflective, and form a Fabry-Pérot interferometer, or etalon. The thickness of the dielectric layer controls the distribution of light energy in the different modes (absorption, transmission and reflection), thus modulating the observed spectrum. Though the bottom surface is composed of individual NPs, which do not couple in the near-field (their plasmonic responses for thin film are independent of one another, rather than extended plasmon modes), they do couple in the far-field, forming a partially reflective mirror. This mirror differs from simple metallic films in its spectral response, and in imparting a phase-shift on the reflected light that is different from that of a full mirror.
The following have contributed to this page: Dr Ofer Kedem