Investigation Of The Extraordinary Light Transmission Through The Subwavelength Metallic Structures By FDTD Numerical Simulations

Since the first discovery of extraordinary optical transmission (EOT) throughsubwavelength hole arrays on the metal films, which was reported by Ebbesen et al.on Nature in1998, the goal of understanding the physical issues about thelight-matter interactions in the scale of subwavelength and nano-meter has attractedenormous interest around the world, and sprung on the intense research activities andutilizing these phenomena, whose rapid development makes it become a researchhotspot in the current optical field. Although the underlying mechanisms of thesepeculiar phenomena are still in a controversy, numerous studies have confirmed thatsurface plasmon polaritons (SPP), being a coherent collective oscillation of freeelectrons on the metal surface excited by the incident electromagnetic waves, shouldplay significant roles during the optical transmission processes. Actually, how tocomprehensively understand and clearly reveal the relevant physical essences hasalready been an open question in this research field. Therefore, it is very necessary tofurther carry out theoretical studies to explore some new phenomena and new effects,and try to discover their new principles, which will be of benefit to the futuredevelopment of the new nano photonic devices and their optimal designing work.In this thesis, we employ the finite-difference time-domain (FDTD) numericalsimulation method and the self-compiled calculation program with C computerlanguage to investigate physical properties of the optical transmission through a silversubwavelength slit structure illuminated by the infrared incident waves. And then weintensively elucidate the relationships of these phenomena with the structuralparameters. Finally their physical origins are discussed. The main contents and resultsof this thesis are given as follows:1. The physical processes of EOT from a subwavelength metallic slit illuminated byan incident transverse-magnetic wave and the involved electromagnetic fielddistributions in different cross-sections of the slit are investigated thoroughly. The simulation results reveal that the transmission efficiency seems to periodicallyoscillate with multiple peaks when the slit depth is varied, but it tends to give no morethan one peak profiled tendency for the change of the slit width. The ratio of theoutput energy dispensing between the radiative and surface wave components isidentified to closely depend on the slit width. On the other hand, with increasing theslit width, the field coupling intensity in the slit is found to reduce with accompany ofthe degrading uniform distribution of the spatial mode.2. We investigate the upgrade transmission properties of a spatially quantized singlesubwavelength metallic slit. We find for the first time that the optical transmittancewill be further increased about ten times or suppressed effectively to zero, when thesmooth slit is discretized into several unit components by inside placing transversegrooves with suitable geometrical parameters (such as the width, depth, and therefractive index of the filling). The theoretical analysis suggest that the formation offundamental plasmon resonances inside the slit units and their multiple scattering andmutual coupling processes play a dominant role for the observations. This kind of slitquantization method can be used to improve the propagation distance of SPP signalsin nano-photonic devices.3. We simulate the electromagnetic field distributions of the transmission lightthrough a single subwavelength metallic slit when the output space is set as differentpermittivities, and found for the first time that the near-field beam focusing andcollimating effect especially in the cases of high permittivity and relatively large slitwidth. The available transverse full-width and half-magnitude (FWHM) value of thefocal spot can be compressed into the subwavelength scales. Moreover, our detailedinvestigations demonstrate that the characteristic parameters of the beam focusingspot (for example, the focal length and the focus depth) tends to increase with the slitwidth, output permittivity, and incident wavelength. This new finding will beexpected to have potential applications in nanolithography, sensing probe, andsuper-resolution microscopy. 4. In order to understand the physical origins of the near-field beam focusing of thesingle metallic subwavelength slit with the high permittivity of output space, wepropose a interference model of the quasi-cylindrical waves scattered by the metallicslit, with accompany of the secondary light sources radiation processes. According tothe theory, the quasi-cylindrical waves are firstly scattered from the slit exit corners,and then they have a spatial interference to form the periodic intensity fringes, whichcan be acted as the secondary light sources to thereafter reemit electromagnetic waveswith the phase coherence. And their subsequent spatial interference in the near-fieldregion finally leads to the beam focusing of the transmission light. The theoreticalpredictions agree well with the simulation results. We believe that this theoryprovides a new sight into the physical interaction of the subwavelength metallicstructures and incident light waves.