A plasmonic nanoantenna probed by a plane-polarized optical field in a medium with no gain materials can show zero absorption or even amplification, while exhibiting maximal polarizability. This occurs through coupling to an adjacent nanoantenna in a specially designed metamolecule, which is pumped by an orthogonal optical field with /2  phase shift. The introduced scheme is a classical counterpart of an effect known in quantum optics as enhancement of the index of refraction (EIR). In contrary to electromagnetically induced transparency (EIT), where the medium is rendered highly dispersive at the point of zero susceptibility and minimum absorption, in the EIR the system exhibits large susceptibility and low dispersion at the point of zero or negative absorption. The plasmonic analogue of the EIR allows for coherent control over the polarizability and absorption of plasmonic nanoantennas, offering a novel 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. It may also open up the way for lossless or amplifying propagation of optical waves in zero-index to high refractive index plasmonic metamaterials. Optical response of surface plasmons are mostly governed by metal and ambient medium parameters, geometry of structures and also by plasmon hybridization, which can result in novel resonance line-shapes, enabling the plasmonic systems to mimic some quantum optical effects such as Fano interference and EIT [1-5]. The functionality of the plasmonic nanostructures is significantly improved through active plasmonics and specifically by all-optical

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Classical analogues of the well-known effect of electromagnetically induced transparency (EIT) in quantum optics have been the subject of considerable research in recent years from microwave to optical frequencies, because of their potential applications in slow light devices, studying nonlinear effects in low-loss nanostructures, and development of low-loss metamaterials. A large variety of plasmonic structures has been proposed for producing classical EIT-like effects in different spectral ranges. The current approach for producing plasmon-induced transparency is usually based on precise design of plasmonic "molecules," which can provide specific interacting dark and bright plasmonic modes with Fano-type resonance couplings. In this paper, we show that classical interactions of coupled plasmonic and excitonic spherical nanoparticles (NPs) can result in much more effective transparency and slow light effects in metamaterials composed of such coupled NPs. To reveal more details of the wave-particle and particle-particle interactions, the electric field distribution and field lines of Poynting vector inside and around the NPs are calculated using the finite element method. Finally, using extended Maxwell Garnett theory, we study the coupled-NP-induced transparency and slow light effects in a metamaterial comprising random mixture of silver and copper chloride (CuCl) NPs, and more effectively in a metamaterial consisting of random distribution of coated NPs with CuCl cores and aluminum shells in the UV region.

In quantum optical Enhancement of Index of Refraction (EIR), coherence and quantum interference render the atomic systems to exhibit orders of magnitude higher susceptibilities with vanishing or even negative absorption at their resonances. Here we show the plasmonic analogue of the quantum optical EIR effect in an optical system and further implement this in a linear all-optical switching mechanism. We realize plasmon-induced EIR using a particular plasmonic metasurface consisting of a square array of L-shaped meta-molecules. In contrast to the conventional methods, this approach provides a scheme to modulate the amplitude of incident signals by coherent control of absorption without implementing gain materials or nonlinear processes. Therefore, light is controlled by applying ultra-low intensity at the extreme levels of spatiotemporal localization. In the pursuit of potential applications of linear all-optical switching devices, this scheme may introduce an effective tool for improving the modulation strength of optical modulators and switches through the amplification of input signals at ultra-low power.

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A novel ultrafast all-optical switching mechanism is demonstrated theoretically and experimentally based on a plasmonic analogue of the effect of Enhancement of Index of Refraction(EIR) in quantum optics. In the quantum optical EIR the atomic systems are rendered by coherence and quantum interference to exhibit orders of magnitude higher index of refraction with vanishing or even negative absorption near their resonances. Similarly, in the plasmon induced EIR a probe signal can experience positive, zero or negative extinction while strongly interacting with a metallic nanorod in a metamolecule which is coherently excited by a control beam. The same mechanism is observed in the collective response of a square array of such metamolecules in the form of a metasurface to modulate the amplitude of a signal by coherent control of absorption from positive to negative values without implementing gain materials or nonlinear processes. This novel approach can be used for challenging the control of light by light at the extreme levels of space, time and intensity by applying ultra-short pulses interacting with ultrafast surface plasmons or extremely low intensity pulses at the level of single photon to a nanoscale single plasmonic metamolecule. The scheme also introduces an effective tool for improving the modulation strength of optical modulators and switches through amplification of the input signal.

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