Coupling between engineered plasmonic resonators and material absorption lines plays an important role in the fundamental behavior of biochemical sensors, surface enhanced spectroscopy techniques and active plasmonic devices. Often, the design of and interpretation of these coupled mode systems draws inspiration and intuition from analogies to atomic and molecular physics systems. They have, for example, been shown to mimic quantum interference effects, such as electromagnetically induced transparency and Fano resonances. Recent work on the subject has involved engineering features such as the coupling strength to the absorption line via field enhancement or the frequency mismatch between the two resonances.
Here we focus on a number of significant effects that instead are directly determined by the composition of the loss mechanisms that characterize the damping of the plasmonic resonance. These are radiative damping and intrinsic loss due to e.g. material absorption. The ratio between the two loss rates defines one of three regimes of operation for the plasmonic resonator.[2,3] We show, both theoretically and experimentally, that depending on the regime of operation, coupling between the plasmonic resonance and absorption band can manifest itself as either an electromagnetically induced transparency-like effect or alternatively strongly enhanced absorption. Notably, the overall magnitude of the effect is also a function of the ratio between the different loss rates.
These results imply a number of important consequences. For example, because the composition of the loss mechanisms associated with a plasmonic mode can vary strongly depending on its resonant frequency, very different behavior can result for surface enhanced absorption measurement performed in e.g. the visible versus the infrared. Alternatively, because the radiative damping rate of a plasmonic resonance can be tuned by tailoring the particle geometry, one can engineer whether coupling to a material absorption resonance will lead to either increased absorption or the opening of a narrow transparency window. Finally, we show that tuning the ratio of the radiative to intrinsic loss can dramatically alter the magnitude of the response. This provides another mechanism, in addition to field enhancement, with which to optimize the response of plasmonic sensors used for surface enhanced spectroscopy applications. Overall, these results and their theoretical description provide important insights into the nature of signals observed in a range of surface enhanced spectroscopy measurements and plasmonic devices that leverage coupling to material absorption bands as well as new opportunities for their design.