Introduction:
Recently, the merging of plasmonics and metamaterials areas has led to the achievement of usual optical functionalities in the optical frequencies. The coupling among metallic particles leads to hybridized plasmonic systems, which are very important for tailoring the optical response of the system. In this project, we have utilized the coupling between a super-radiant (radiative mode) and sub-radiant (dark mode) plarmonic particles to realize a plasmonic system with similar features to that of the electromagnetically induced transparency (EIT).   The underlying principle of this EIT plasmonic system is the analogy of the dark mode to a meta-stable level in an atomic system, and the analogy of bright mode to an energy level with dipole-allowed transition.  The coupling between bright mode and dark mode leads to a destructive interference between two different excitation pathways, and a narrow transparency window with slow group velocity. This new approach is then used to construct the first EIT-like optical metamaterial consisting of coupled nanoscale plasmonic resonators. The plasmonic metamaterial has well defined EIT-like effective properties and open up the possibilities to construct devices of various scales and shapes, while maintaining consistent optical properties that do not depend on the dimensions of the device. They can be designed for applications either in free space optics or integrated optical circuit, and in particular for slow light and enhanced nonlinear effects

Plasmonic EIT metamaterial design and simulation:
Figure (a) shows a specific design of nano-plasmonic “molecule” for the realization of the EIT-like system. A simple metal strip may function as an optical dipole antenna, and thus could serve as the radiative or “bright atom” in the EIT-like plasmonic system. The dark “atom” consists of two parallel metal strips with a small separation.  This configuration has symmetric and anti-symmetric modes, whose resonances are separated in the frequency domain. The anti-symmetric mode has counter propagating currents on the two strips, therefore, there is no direct electrical dipole coupling with the radiation wave and it can be considered as a dark mode with significantly longer dephasing time. The field at the red arrow is the sum of the incident light and the dipole response of the radiative “atom”; as a result, it is linearly related to the dipole polarizability. Thus, by investigating the field response we can gain insight into the effective susceptibility of the system. Fig. (b) shows the dependence of the coupling between the two “atoms” on their separation.  At the resonance frequency, destructive interference is clearly observed (a dip in the imaginary part of electric field). To visualize this destructive interference between the two pathways, we compare the 2D distribution of electric field at 428.4 THz for the radiative antenna uncoupled [Fig. (c), left] and coupled with the dark “atom” [Fig. (c), right]. Without coupling to the dark “atom”, the radiative antenna is strongly excited by the incident plane wave with high electric field forming at its end facets. By placing the dark “atom” 40 nm from the radiative one, the electromagnetic field is coupled back and forth between the radiative and dark “atoms”, leading to a destructive interference and a suppressed state in the radiative “atom” with much weaker electric field at its ends. This suggests that the scattering of the system is strongly suppressed at this frequency, and there exists a transmission peak with highly dispersive phase profile, i.e. a slow group velocity.
      

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