Electromagnetic Study of Surface Plasmon and Antireflection Structures

Many aspects of plasmonics are studied theoretically and numerically using the finite-difference time-domain (FDTD) method. An in-house three-dimensional (3-D) FDTD program incorporating the periodic boundary condition and the uniaxial perfectly matched layer is developed with parallel computing. The dispersive materials are realized by means of the auxiliary differential equation method. The transition of plasmonic states bridging from visible frequencies to microwaves is examined. In particular, given the metal permittivity at microwaves, the plasmonic dispersion evolves into the conventional waveguide dispersion. We reveal the coupling effect of plasmonic modes, identify the symmetric and antisymmetric types, and discuss their relations to the conventional TEM (TM0) and TM1 modes. As the symmetric coupled plasmon mode is known to correspond to the TEM (TM0) mode, we find that the antisymmetric coupled surface plasmon mode is essentially related to the TM1 mode. The physical origin of an extraordinary propagation mode in a metallodielectric woodpile structure is investigated. It is proved that this allowed mode results from an equivalent Fabry-Perot cavity inherently presented in the woodpile. Our explanations are confirmed both by simulations and experimental data. We examine the resonance orders of surface plasmons in 2-D MHAs using the finite-difference time-domain method and describe how the electromagnetic intensity is built up at resonance in both space and time. The evanescent-tunneling mode profile in subwavelength cylindrical holes is demonstrated. Moreover, the polarization effect which ultimately decides the direction of the surface plasmon propagation is revealed. We demonstrate that the smoothness of the optical path can be used as a design criterion for omnidirectional antireflection coating profile. This concept is also usedo explain the different performance between gradient-index pro?les such as the Gausssian and Quintic profiles.