Coupling And Coherent Effects In Plasmonic Metamaterials

Coupling and coherent effects in metamaterials have emerged in recent years,and the researches in this area have produced a range of interesting effects which have potential applications for optoelectronic devices.In this thesis,the coupling and coherent effects in plasmonic metamaterials have been further investigated,and major emphasis has been given to the plasmonic analogs of electromagnetically induced transparency,mode hybridization,Fano resonances and relevant slow light effects.One of the subjects of this thesis is circular-polarization-dependent mode hybridization and slow light in vertically coupled chiral and achiral plasmonic structures.We have designed an unit cell comprising a z-shaped chiral structure and a pair of strips without chirality,and arranged this unit cell to obtain a square array.Circular-polarization-dependent mode hybridization between the z-shaped structure’s radiant mode and the strip pair’s subradiant mode has been observed,and electromagnetically induced ransparency-like transparency window is obvious for only one circularly polarized state.The underlying mechanisms have been revealed.The effect of metal loss on the mode hybridization and the transparency window have also been studied,and it has been found that transmission coefficient of the transparency window becomes larger as the metal loss decreases.Based on the above effect,circular-polarization-dependent slow light has also been studied in the frequency and the time domain respectively.It has been found that the circularly polarized state that has an obvious transparency window could obtain a distinct slow light effect.The effect of metal loss on the group velocity has been further investigated.It has been found that the group velocities of the two circularly polarized states become smaller as the metal loss decreases.It is verified by the above results that the circular-polarization-dependent slow light has been realized in this structure.It also indicates that tuning and even dynamic tuning of the group velocity could be realized by incorporating gain media to compensate the metal loss.Another subject of this thesis is the trapped mode based on higher-order Fano resonances in a symmetric plasmonic structure.The detuning of the two higher-order eigenmodes participating in the Fano interference is small,and the difference between their quality factors is not significant.A trapped mode is produced by their destructive interference.The current terms of this trapped mode are rigorously anti-symmetric about the mirror along the currents and symmetric about the mirror perpendicular to the currents,so the radiation loss is extremely small.In addition,the spectral lineshape of this trapped mode much resembles that of electromagnetically induced transparency.We have compared the trapped mode by the Fano interference in an isolated unit cell with that in an infinite square array.It has been found that the destructive interference is more effective and the loss of the trapped mode is much smaller in the infinite array.This should be attributed to the reduction of radiation loss by diffraction in this array whose lattice constant is comparable with the wavelength of the trapped mode.Slow light caused by this Fano interference has also been studied in the frequency domain.It has been found that a region of steep normal dispersion emerges around the transparency window,and consequently,large group indexes could be obtained in this region.This indicates that an obvious slow light effect could be realized by this Fano interference.The effect of metal loss on the group velocity has been further investigated in the frequency domain.It has been found that the group velocity becomes smaller as the metal loss decreases.This suggests that tuning and even dynamic tuning of the group velocity could be realized by incorporating gain media to compensate the metal loss.The Fano interference between an eigenmode by collective excitations and an eigenmode by individual excitations in this symmetric structure has been studied.Under the Rayleigh anomaly condition,a surface lattice mode would form through diffractive coupling of individual octupole excitations.This lattice mode interferes with the eigenmode by individual quadrupole excitations.This new type of mode interference results in a trapped mode which possesses both the characteristics of the collective and the localized resonance.In addition,this interference leads to a narrower transparency window.This would compress the bandwidth of the normal dispersion region,and at the same time,enhance the normal dispersion of the refractive index and consequently increase the group index.The model of two-driven coupled oscillators has been modified by setting the dissipation coefficients of the two eigenmodes linearly decreasing in the Fano destructive interference region.This model could reasonably describe the dynamics of this Fano interference,resulting in an expression of extinction ratio,which fits well with the extinction spectra by simulation.Compared with the Fano interference in symmetry-breaking structures,this higher-order Fano resonance in a symmetric structure possesses two signatures.One is that the current terms in this symmetric structure are rigorously anti-symmetric about the mirror along the currents and symmetric about the mirror perpendicular to the currents,so the radiation loss of this trapped mode is extremely small.The other is that the interference between an eigenmode by collective excitations and an eigenmode by individual excitations could take place in this higher-order Fano resonance.In conclusion,the thesis encompasses quite a broad range of aspects including the circular-polarization-dependent mode hybridization and slow light,the trapped mode by higher-order Fano interference in a symmetric structure.These novel phenomena extend the area of coupling and coherent effects in metamaterials,and might find some applications in the optical manipulation devices,small lasers and bio-medical sensing based on surface plasmon resonances.