介质覆层下金属周期结构变化对TE波异常透射特性的影响

根据金属光栅结构变化对TE波异常透射特性影响的研究需要,建立了相应的模型。应用时域有限差分(FDTD)方法分别计算了单缝结构、多缝结构、不同宽度和不同周期等结构下的透射分布特征。研究发现添加凹槽会对金属表面能量的传递起阻碍作用。透射频域宽度随薄膜宽度增加而增加。随着狭缝宽度的增加,透射率分布曲线包络线趋于平坦,主透射峰短波长侧透射曲线分布基本不变,而主透射峰及其长波长侧的透射曲线分布变宽。这说明宽度的变化影响了表面共振模式,从而影响透射的分布。单个狭缝的透射与多周期结构相比,透射率曲线几乎重合,表明狭缝对表面模式没有调制作用,各狭缝的透射相对独立。     

基于塔姆激元-表面等离极化激元混合模式的单缝加凹槽纳米结构的增强透射

金属单缝纳米结构因为结构简单、易于集成,常用在基于表面等离极化激元(surface plasmon polaritons,SPPs)的纳米结构中构建光源.但是,金属亚波长单缝结构一直存在透射率低的问题,如何提高其透射率一直是研究的重点.为了更好地提高金属亚波长单缝的透射率,本文对之前文献提出的分布式布拉格反射镜(distributed bragg reflector,DBR)和金属银薄膜纳米缝结构进行改进,在金属银薄膜两侧设计凹槽.当TM偏振光由DBR侧入射至DBR-银纳米缝结构时,DBR-银膜界面上和银膜入射侧凹槽一起激发的塔姆激元(Tamm plasmon polaritons,TPPs)和SPPs,以及纳米缝和银膜出射侧凹槽对的SPPs同时激发,利用凹槽激发的SPPs和银膜表面处的TPPs-SPPs混合模式的干涉相长耦合作用,通过塔姆激元的局域场增强效应和两侧凹槽与单纳米缝的干涉相长耦合作用进一步提高了表面等离极化激元模式的激发效率,再加上纳米缝中的类法布里-珀罗腔共振效应,使纳米缝的透射率得到增强.本文采用有限元方法研究了DBR-银纳米缝结构上单纳米缝加凹槽的透射特性.经过一系列参数优化,使DBR-银纳米缝凹槽结构的最大透射率增加到0.22,相对于TiO_2-银纳米缝结构的透射率(0.01)提高了22倍,比文献[23]得到的最大透射率0.166有所提高.研究结果在纳米光源设计、光子集成电路和光学信号传输等相关领域具有一定的应用价值.     

基于SPPs-CDEW混合模式的亚波长单缝多凹槽结构全光二极管

全光二极管是集成光子回路上最基本的光子器件,如何有效增强全光二极管的单向透射性,提高消光比一直是学者们研究的重点.当前,利用表面等离极化激元(surface plasmon polaritons,SPPs)和复合衍射衰逝波(composite diffracted evanescent wave,CDEW)的亚波长金属微纳结构构建全光二极管器件还鲜有研究.因此,开发出一种可调制的全光二极管,对未来制备复杂的光子回路具有重要意义.本文提出了一种基于SPPs-CDEW混合模式设计全光二极管的方法和结构,该结构结合纳米缝中的类法布里-珀罗共振效应,利用结构参数对SPPs进行调控,实现了光束单向透过的功能.首先,利用理论推导和有限元算法分析了单缝-对称双凹槽纳米结构的透射增强现象,提出了透射增强和削弱的物理机理.其次,计算出规约化透射率随单狭缝和凹槽对之间距离变化的远场透射谱,给出的理论和数值计算结果符合得很好.最后,通过此透射谱精确确定凹槽的位置和数量,得出上表面对称分布五对增强透射凹槽、下表面六对抑制透射凹槽的最优全光二极管结构,有效增强了全光二极管的单向透射性,提高了消光比,最大消光比可以达到38.3 dB,即正向透射率是反向透射率的6761倍,比已有文献提高了14.6 dB,并在850 nm左右有70 nm宽的工作波长带宽(20 dB).本文提出的光二极管结构简单,宽带宽工作,易于集成,耦合效率高,研究结果对光学信号传输、集成光回路、超分辨率光刻等相关领域具有潜在的应用价值.     

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.     

Enhanced optical transmission through a subwavelength 1D aperture

We present finite difference time domain simulations of the enhancement of the optical transmission through a single subwavelength slit in a thick metal film caused by resonances of the electromagnetic fields in and near the slit. These can be enhanced by including a periodic modulation on the in-coupling surface, and the dependence of the transmission on the grating structural parameters is analysed in detail.     

Polarization-Independent Directional Beaming of Light by a Subwavelength Metal Slit

We present the directional beaming effect of light at the terahertz frequency by using a subwavelength slit in the metal film. The metal is dressed with anisotropic dielectric so that both the transverse electric （TE） and transverse magnetic （TM） polarized waves can be well guided on the metal surface and reach the phase matching. By using a periodical array of dielectric ridges and grooves around the slit, the guided waves can be scattered out of the slit and interfere with the transmitted light directly through the slit. The results performed by finite-difference at time-domain computations indicate that the directional beaming of light can be obtained simultaneously for both the TE and TM polarized waves after optimizing the geometric parameters. The structure may find great applications in polarization-independent optical devices such as couplers, connectors, beam collimator, and etc.     

Extraordinary Transmission through Metallic Grating with Subwavelength Slits for S-Polarization Illumination

Based on the rigorous coupled-wave analysis algorithm, we have systematically analysed the effect of the geometrical parameters of a dielectric film coated metallic grating with subwavelength slits on extraordinary optical transmission for s-polarization illumination. Results show that the dielectric film which sustains a waveguide electromagnetic mode on the top of the metallic lamellar grating can strongly enhance the transmittance, the positions of the transmission peaks are mainly determined by the period of the metallic grating, the thickness and refractive index of the dielectric film. This structure shows potential applications in excellent polarizers or polarization-isotropic devices at infrared spectral range by appropriately choosing the geometrical parameters.     

Multiple paths to enhance optical transmission through a single subwavelength slit

In this Letter, we explore transmission properties of a single subwavelength slit flanked by a finite array of grooves made on a thick metallic film. We identify three main mechanisms that can enhance optical transmission: groove cavity mode excitation (controlled by the depth of the grooves), in-phase groove reemission (controlled by the period of the groove array), and slit waveguide mode (controlled by the thickness of the metal film). By tuning these geometrical parameters, enhancements of transmission of light by up to 2 orders of magnitude can be achieved when all three mechanisms coincide. Experimental verification of these findings is also shown for structured silver films fabricated by focused-ion-beam milling.     

磁光光子晶体结构的法拉第效应增强

在超薄薄膜的基础上,基于时域有限差分法原理,利用FDTD Solutions仿真软件分别研究了基于两种多层膜结构和一种金属光栅结构的磁光光子晶体法拉第旋光效应。研究表明,多层膜结构的法拉第旋光效应增强原理为入射光在薄膜中心层的透射谱谐振,而金属光栅周期结构的法拉第效应增强是通过金属光栅激发表面等离子体实现的;在三种结构中,金属光栅周期结构具有更广的法拉第偏转角增强域。进一步通过参数优化,实现对金属光栅周期结构工作波长的可调节性研究,为薄膜型磁光器件设计提供了理论依据。     

Influence of V-shaped plasmonic nanostructures on beam propagation

The influence of V-shaped plasmonic nanostructures caused by focused ion beam milling on transmission properties are addressed in this paper. In order to evaluate the influence, a finite-difference and time-domain numerical method is employed for simulation of the plasmonic structures flanked with the corrugations with equal groove depth at the exit side of a metal film which is coated on a quartz substrate. Our simulation results have shown that a V-shaped subwavelength central slit/aperture is helpful for beam shaping. It plays a positive and dominant role in transmissivity. However, the V-shaped grooves cause a red shift of the peak wavelength and a broadening of the cut-off wavelength. This leads to the beam diverging in the region of the far field. In the case of the existence of both the V-shaped central slit and the V-shaped grooves, the influence of the grooves is less compared to that of the central slit, and can be ignored theoretically. PACS 02.60.Cb; 02.70.Bf; 07.79.Fc; 42.25.Bs     

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.     

侧面泵浦高功率双包层光纤激光器的表面弧形金属槽结构的制作和分析

侧面耦合高功率泵浦光技术是获得高功率稀土光纤激光器的关键技术之一。介绍了一种用于侧面泵浦高功率双包层光纤激光器的表面弧形金属槽结构的制作工艺,并且针对已经实验制作出来的表面连续弧形金属凹槽的侧面耦合结构进行了严格的矢量电磁场理论计算,将120°弧形凹槽等效近似为多层等高不等宽的矩形槽并运用透射矩阵算法对这种近似后的多层矩形结构进行优化设计。这种结构可以用于大功率激光二极管阵列的侧面泵浦中,尤其可用于条形半导体激光器侧面泵浦双包层掺杂光纤,以制作各种大功率稀土光纤激光器。     

Transmission and directionality control of light emission from a nanoslit in metallic film flanked by periodic gratings

We investigate the transmission and directional properties of light emission from a nanoslit film flanked by periodic gratings, formed on a silver film, embedded in a high index dielectric medium. Using FDTD calculations it is demonstrated that the transmission has strong dependence on dimension of the dielectric film. The directional property of the emitted beam is controlled by tapering height of output grating elements and this effect is explained by standard theory of antenna array. We propose and examine the potential of such slit grating structure as a Plasmonic antenna in axial injection of light from single mode fiber to photonic crystal waveguide.     

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.     

Giant optical transmission of a subwavelength slit optimized using the magnetic field phase

In this Letter, we show that the energy equivalent to that incident on a 4.7 microm wide strip can be squeezed through a 50 nm wide slit in a metal film surrounded by grooves. This corresponds to a transmission efficiency of 9400%, which can be even further enhanced by increasing the number of grooves. We use the phase of the magnetic field to explain that the ideal slit-to-groove distance is just over half the plasmon wavelength. In addition, we also optimize the groove depth and width. Such optimized transmission enhancement is very important for near-field devices.     

Enhanced optical reflection and multiple-peak transmission from a subwavelength slit with surface corrugations

We have observed a new phenomenon of the enhanced optical reflection and transmission from a subwavelength metallic slit with surface corrugations. The reflected energy is focused by the periodic structure and our experiment shows that it is more effective than a planar metal surface in real situations. The transmission spectrum performs the feature of a multipeak narrow-band transmission with a wavelength spacing of 1.4 nm resulting from the FP cavity resonance in the substrate film. Such optical response properties of our sample will be very useful for applications in many fields including wavelength-division-multiplexing (WDM) optical communications systems.     

Numerical Investigation of Surface Plasmons Associated Subwavelength Optical Single-Pass Effect

Surface plasmons （SPs） associated optical single-pass effect has been investigated in novel subwavelength metallic structures, including single slit and grating structures. With influence of SPs, these metallic structures can enhance transmission in incident direction and suppress it in the opposite direction, exhibiting a single-pass effect. The finite difference time domain method is employed to study the influences of structure parameters on far-field transmission, near-field electric field distribution and extinction ratio of the single-pass transmission effect. A maximal extinction ratio of 47.83dB is achieved in the grating structure.     

二维光栅与周期性缝隙阵列组合薄膜结构的杂散光抑制

传统方格型二维光栅与周期性缝隙阵列的组合薄膜结构具有雷达和光学波段双带通的电磁特性. 但由于其杂散光能量集中分布而严重制约它在高精度探测以及成像领域中的应用. 本文提出了一种全新的组合薄膜结构, 即由满足一定约束条件的圆孔型二维光栅和十字缝隙阵列构成. 基于Fraunhofer衍射理论建立组合薄膜结构标量衍射模型, 通过对比两种组合薄膜结构的衍射光分布, 理论分析与实验测试均表明: 圆孔型光栅与十字缝隙阵列组合薄膜结构不仅能够提高其光学透过率, 而且还使其杂散光分布均匀, 降低了其杂散光总比率, 从而有效抑制杂散光, 进一步增强了二维光栅与周期性缝隙阵列组合薄膜结构在实际光学系统中的可靠性. 关键词： 二维光栅 组合薄膜结构 衍射光强 杂散光     

Complex Modulation in Plasmon-Assisted Transmission Spectra of a Two-Slit Structure

We study the plasmon-assisted transmission of two kinds of slit structures in a 290-nm-thick silver film on a glass substrate. For the two-slit structure, the total transmission intensity spectra displays a complex modulation, which is attributed to different propagation constants of the surface plasmon polaritons （SPPs） on the silver-air and silver-glass interfaces. Replacing one of the two slits by a shallow corrugation results in a simple sinusoidal intensity modulation because of the only effective SPP excitation and propagation on the silver-air interface. These confirm the role of different SPP propagations and interference in the transmission properties of metal nanoslits.     

双层金属纳米光栅的TE偏振光异常透射特性

根据在亚波长金属光栅表面添加电介质会引起TE偏振光的透射异常性, 应用严格耦合波理论和时域有限差分方法, 研究了双层金属纳米光栅在TE偏振光入射时产生的异常透射现象. 利用等效折射率方法建立了双层金属光栅的等效模型, 得到了TE偏振光透射率与聚合物的折射率、厚度以及金属层厚度的变化关系. 确认了结构中聚合物是透射异常出现的必要条件, TE偏振光以波导电磁模式在其中传播, 并认为类Fabry-Perot腔谐振是透射峰值产生的主要原因.