Transmission through metallic array slits curved with perpendicular waveguides

The transmission of normally incident plane wave through an array of subwavelength metallic slits curved with perpendicular waveguides has been explored. Two-dimensional electromagnetic fields inside and near the metal film are simulated by the finite-difference time-domain (FDTD) method. The perpendicular waveguides in the middle of slits affect the modes of slits in two different manners: commonly, the extended length of the waveguides leads to the increase of the resonant wavelength; shifting the waveguides location along the vertical slits, red or blue shift of resonant wavelengths can be achieved, however to some confirmed length of the perpendicular waveguides, there is the only one resonant wavelength. In addition, a new explanation model is proposed in which the resonance is ascribed to three reasons: the Fabry-Perot cavity theory, the surface current flow, and the surface charges. All the calculated results are well explained by our proposed model.     

金属亚波长狭缝中凹槽对其透过特性的影响

利用时域有限差分方法,研究了单个二维金属亚波长狭缝中空气凹槽的结构、位置等参数变化对其透过特性的影响. 整个缝槽结构可以类比为LC振荡环路,凹槽结构变化时,不同程度地改变振荡电路的电感和电容,从而使共振透射峰出现移动. 凹槽位置变化时,电路的驱动源改变,从而改变缝槽结构中的电磁场分布,使得共振透射峰出现分裂. 利用LC共振原理分析了凹槽各个参数变化时共振透射峰的变化情况. 关键词： 表面等离子体 透射率 LC振荡')" href="#">LC振荡     

Role of propagating slit mode in enhanced transmission through slit arrays in a metallic films

The mechanisms responsible for enhanced transmission of electromagnetic wave through an array of subwavelength slits in a metallic film are analyzed. Theoretical model of the enhanced transmission which takes into account the penetration of electromagnetic field into real metal is developed. Semi-analytical model based on Fabry–Perot formula is considered. Comparison of theoretical model, semi-analytical model and results of numerical simulation of Maxwell equations in time-dependent form (FDTD method) for silver with various geometric parameters is presented. The roles of surface plasmons and plasmon localized along slits are studied.     

Squeezing millimeter waves into microns

Microstructured metallic devices will play a vital role in the continuing search to manipulate the passage of electromagnetic radiation relevant to optical, microwave, and communication technologies. Here, we investigate the electromagnetic response of a completely novel and ultrathin (< wavelength) structure within which is buried a metal-clad waveguiding layer ("core") of subwavelength width. By removing metal from the core cladding to form a periodic array of slits, radiation is coupled into a standing wave within the layer and the structure resonantly absorbs or transmits radiation of wavelength more than 100 times its thickness. Additionally, such structures display the truly remarkable capability of compressing half of the standing-wave wavelength into a fraction of the expected distance.     

Waveguide resonance of subwavelength metallic slits

The transmission characteristics of a metallic film with subwavelength periodic slits are investigated by using the two-dimensional finite-difference time-domain method (2D-FDTD). Two models are constructed to show the dependance of the transmission spectrum on the slit structure. A sandwiched structure is used to exhibit the contribution of the metallic wall inside slits to the extraordinary high transmission. And a filled slit structure is employed to reflect the relation between the average refractive index inside the slits and the transmission spectrum of the structure. The transmission characteristics of two structures can be explained well with the waveguide resonance theory.     

Controllable electromagnetic transmission based on metallic nanoslit with a microcavity

In this paper, we explore the electromagnetic transmission through a vertical metallic nanoslit with a horizontal microcavity. The introduction of microcavity gives rise to the changes of slit transmission spectrum in the number and amplitude of transmission peak and dip, owing to the electromagnetic coupling between slit and microcavity. The transmission through the slit with microcavity strongly depends on whether the isolated slit is in resonance or not, and the microcavity resonance is an additional factor that influences the transmission. The transmission spectrum can be adjusted by changing the opening position of slit, and the slit depth, the width and depth of microcavity. Our results are analysed and discussed from the view of Fano resonance. These results may be of value in tuning electromagnetic wave in subwavelength optics and designing plasmonic devices based on composite metallic nanoslit structures.     

Role of surface plasmon polaritons in anomalous transmission of an electromagnetic wave through two arrays with subwavelength slits

The anomalously large transmission of an electromagnetic wave through structures consisting of two periodic arrays of subwavelength slits in films has been investigated. The conditions ensuring zero transmittance of this system have been determined. The role of surface plasmon polaritons in transmission anomalies has been analyzed. An analysis of the systems consisting of three arrays of slits has revealed specific features of the transmittance that are independent of the number of slits. It has been demonstrated that, at a wavelength corresponding to the excitation of a surface plasmon polariton in the gap between two periodic arrays of subwavelength slits, the transmittance is zero (i.e., transmission is suppressed). The investigation has been carried out using numerical simulations with the Fourier modal method.     

Influence of slit width on the electromagnetic transmission of a periodic metallic grating

The light transmittance of a periodic metallic grating with varied slit widths has been investigated. The transmission peaks move to the shorter wavelength direction with an increase in the width of slits while keeping the other parameters unchanged. It was demonstrated that the slit width affects the spectral transmittance of the metal grating significantly. It was also found that the effective refractive index and cavity modes in slits are responsible for this phenomenon. Cavity modes play an important role in extraordinary transmission of the sub-wavelength aperture grating. When a complete resonant mode forms in the slits, a high transmission will appear. A wider slit results in a smaller efficient refractive index and thus affects the cavity mode in the slits. These two elements cause the transmission peaks to move to the shorter wavelength direction with widening of slits. The results obtained here may provide a useful guide to design metallic slit grating devices.     

Transmission of light through metallic Z-shaped slit array with inner metallic bar

The transmission of a normally incident wave through a Z-shaped channel metallic slit array with metallic bar inside has been investigated by using finite-difference time-domain method. It is obtained that transmission spectra are nearly the same of the slit array with straight channel as that with Z-shaped channel in the condition the material of the slit array the same as that of the inner bar. If the Au bar is replaced by Al of the Au slit array, both resonance modes red shift obviously, especially for the structure with bent channel. Along with the width of the inner bar increasing, the localized waveguide resonance mode red shifts regularly with a tiny decrease of the peak value of all the kinds of composed structure introduced here, and the surface plasmon resonance mode red shifts regularly accompanied by peak value changing uniformly for the structures with only one type of metal. However, the surface plasmon resonance behaves different obviously, its center peak first moves to a larger wavelength fast, then red shifts slowly, for the Au-film Al-bar structure, but that moves in a very small wavelength range for the Al-film Au-bar one. The results obtained here are helpful to design subwavelength optical devices.     

Light passage through a film with subwavelength slits

The effect of anomalously high transmission of an electromagnetic wave through a periodic array of subwavelength slits in a metal film was studied. Results of numerical simulation were compared with theoretical and semi-analytical calculations of anomalous transmission through a silver film. Transmission through a perfectly conductive metal film with subwavelength slits was considered; anomalous transmission through it was detected.     

Extraordinary THz transmission through subwavelength semiconductor slits under antiparallel external magnetic fields

We theoretically study the resonant transmission of electromagnetic waves at the THz frequencies through subwavelength semiconductor slits under external static magnetic fields. The dispersion relations of the surface plasmon polaritons (SPPs) inside a subwavelength slit are analytically derived. It is found that the SPPs propagating along one direction and its reverse are symmetric when parallel external magnetic fields are applied, but are asymmetric when antiparallel external magnetic fields are applied. The transmission properties of periodic subwavelength semiconductor slit arrays with the antiparallel magnetic fields in each unit cell are investigated by the mode expansion technique. The two significant transmission characteristics are observed: (i) The resonant peaks are redshifted with increasing external magnetic fields; (ii) The transmissions in the two opposite directions through the slit arrays are asymmetric. The origin of the transmission asymmetry is reasonably explained by the magnetic-field induced asymmetric SPP propagation losses.     

Optical transmission through subwavelength slit apertures in metallic films

Transmission of polarized light through subwavelength slit apertures is studied in visible and near infrared range wavelengths. We examine the roles played by the slit apertures, such as length, depth, period and number of slits. The experiment results, including dispersion curves, demonstrate among other things that the surface plasmon polariton and Fabry-Perot waveguide modes collectively dictate the transmission properties of subwavelength slit arrays and that as they approach each other, not only large gaps are formed but also mode interconversion occurs. These findings are discussed and compared to theoretical predictions.     

Microwave transmission of a compound metal grating

An array of subwavelength slits in a metallic substrate supports a series of Fabry-Perot-like resonances, where each harmonic results in a transmission peak. Addition of extra slits per period yields a compound grating with a structure factor associated with the basis. In this study each repeat period is comprised of a central slit flanked by a pair of narrower slits. It supports three resonances for every Fabry-Perot-like solution. New and useful insight into this phenomenon is gained by describing each of the modes in terms of the band structure of diffractively coupled surface waves.     

Mie谐振耦合的亚波长金属孔宽带高透射传输

表面等离激元共振激发的亚波长金属孔透射较Bethe理论有大幅度的提高,然而,由于共振对频率的敏感性以及金属在光频的高损耗特性,表面等离激元共振难以实现亚波长金属孔的宽带高透射传输.本文采用放置在金属孔两边的硅纳米颗粒的Mie谐振耦合取代表面等离激元共振,实现亚波长金属孔的宽带高透射传输.全波仿真结果表明,采用Mie谐振耦合的亚波长金属孔(r/λ=0.1)光传输,透射系数超过90%的带宽达到65 nm,与表面等离激元共振诱导的透射增强相比,峰值增高了1.5倍,3 dB带宽拓宽了17倍.根据耦合模理论,建立了Mie谐振耦合亚波长金属孔透射的等效电路模型,并在临界耦合状态下反演出电路模型中的元件参数值.进一步研究发现,仅通过改变等效电路模型中的耦合系数,就可全面揭示Mie谐振耦合亚波长金属孔透射的传输规律,并得到与全波电磁仿真完全一致的结果,从而找到光与放置硅纳米颗粒的亚波长金属孔相互作用的数学表达,也给予人们在光学领域按照电路设计方法构建相应功能模块的启示.     

Controlling the phase delay of light transmitted through double-layer metallic subwavelength slit arrays

We demonstrate that the phase of light transmitted through double-layer subwavelength metallic slit arrays can be controlled through lateral shift of the two layers. Our samples consist of two aluminum layers, each of which contains an array of subwavelength slits. The two layers are placed in sufficient proximity to allow coupling of the evanescent fields at resonance. By changing the lateral shift between the layers from zero to half the period, the phase of the transmitted electromagnetic field is increased by pi, while the transmitted intensity remains high. Such a controllable phase delay could open new capabilities for nanophotonic devices that cannot be achieved with single-layer structures.     

Plasmon-assisted two-slit transmission: Young's experiment revisited

We present an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam. This modulation is attributed to an interference phenomenon at each of the slits, instead of at the detector. The interference arises as a consequence of the excitation of surface plasmons propagating from one slit to the other.     

Analytical model of the enhanced light transmission through subwavelength metal slits: Green’s function formalism versus Rayleigh’s expansion

We present an analytical model of the resonantly enhanced transmission of light through a subwavelength nm-size slit in a thick metal film. The simple formulae for the transmitted electromagnetic fields and the transmission coefficient are derived by using the narrow-slit approximation and the Green’s function formalism for the solution of Maxwell’s equations. The resonance wavelengths are in agreement with the semi-analytical model [Y. Takakura, Phys. Rev. Lett. 86, 5601 (2001)], which solves the wave equations by using the Rayleigh field expansion. Our formulae, however, show great resonant enhancement of a transmitted wave, while the Rayleigh expansion model predicts attenuation. The difference is attributed to the near-field subwavelength diffraction, which is not considered by the Rayleigh-like expansion models. PACS 42.25.Bs; 42.65.Fx; 42.79.Ag; 42.79.Dj     

微型腔单一亚波长金属狭缝的光波透射

 利用时域有限差分法模拟了厚金属层上单一亚波长狭缝内壁刻有一微型腔结构光波传输,结果发现在特定波长发生异常的增强透射。结合透射峰值对应波长近场分布,利用麦克斯韦方程组和波导理论对增强透射进行解释,认为光波在微型腔和狭缝中形成共振腔模起了重要作用:在狭缝内形成纵模,可以形成透射峰;在腔内形成不同的腔模也可以形成透射模。     

Inward electromagnetic wave coupling in hybrid subwavelength structures illuminated with secondary imaging

Subwavelength structures hybridized with slits and grooves in metallic films exhibit extraordinary optical properties for manipulation of light. A Green method is developed to investigate the wave coupling physics in the hybrid subwavelength structures, while source-image arrangement in bounded spaces is employed to obtain the Green's functions for the hybrid units. We find that secondary imaging is induced at a horizontal boundary to provide an inward wave propagation mechanism to couple the wave scattered by groove back into slit. Sophisticated self-consistent framework derived from our method that involves the wave couplings is then developed to study the physics origin of the extraordinary optical properties. The case of a slit in a metal film with patterned grooves on the output side for beaming and that of a two-layer metal film with indented double slit for beyond-limit focusing are analyzed. Our Green method, which automatically includes the Fourier high-order modes, is concise in formulation and thus yields accurate solutions. Explicit analytical models are rigorously developed to address wave coupling physics problems in subwavelength structure.     

Whispering gallery mode propagation in photonic crystals in front of subwavelength slit arrays: Interplay with extraordinary transmission

We numerically study the propagation of light in 2D photonic crystals (PC) made of infinite cylinders of high refractive index, via the excitation of their morphological dependent resonances, in the presence of metallic arrays of subwavelength slits, either corrugated or not, that may produce extraordinary transmission. In this way, we confirm and illustrate previous theoretical findings on PCs and show new effects when combined with slits. Among them, we show that the whispering gallery mode excitation in the PC couples dominates the transmission of the slit arrays alone. Appropriate design of the system parameters and the illumination conditions, selects the transport and confinement of energy.