Refraction enhancement in plasmonics by coherent control of plasmon resonances

Optical materials exhibit significant losses over the resonance frequency of their constituent atoms and so they are practically implemented at frequencies far from resonances. Electromagnetically induced transparency (EIT) provides a method for effective suppression of optical loss in a narrow window over the resonance, where the medium exhibits significant dispersion but at the expense of zero susceptibility.

The classical or plasmonic analogs of the EIT effect are introduced and widely used in the context of electromagnetic or optical metamaterials (MMs). In another interesting phenomenon in quantum optics known as enhancement of index of refraction (EIR), the optical loss of the medium can be zero or even negative at the region of maximal susceptibility and negligible dispersion.

This condition is interesting for applications where a strong electromagnetic response of the medium with negligible loss is required, such as zero- or negative-index metamaterials (MMs). Here we introduce a plasmonic analog of the EIR which allows for coherent control over the polarizability and absorption of plasmonic nanoantennas.

It can open up the way for loss-compensated propagation of optical waves in zero-index to high-refractive-index plasmonic MMs. The scheme also offers an approach to all-optical switching and coherent control of transmission, diffraction, and polarization conversion properties of plasmonic nanostructures, as well as propagation properties of surface plasmon polaritons on metasurfaces.