All-optical switching via coherent control of plasmon resonances

A novel ultrafast all-optical switching mechanism is demonstrated theoretically and experimentally based on a plasmonic analog of the effect of \textit{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 that 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 the amplification of the input signal.