All-dielectric bowtie waveguide with deep subwavelength mode confinement

Plasmonic waveguides and conventional dielectric waveguides have favorable characteristics in photonic integrated circuits. Typically, plasmonic waveguides can provide subwavelength mode confinement, as shown by their small mode area, whereas conventional dielectric waveguides guide light with low loss, as shown by their long propagation length. However, the simultaneous achievement of subwavelength mode confinement and low-loss propagation remains limited. In this paper, we propose a novel design of an alldielectric bowtie waveguide, which simultaneously exhibits both subwavelength mode confinement and theoretically lossless propagation. Contrary to traditional dielectric waveguides, where the guidance of light is based on total internal reflection, the principle of the all-dielectric bowtie waveguide is based on the combined use of the conservation of the normal component of the electric displacement and the tangential component of the electric field, such that it can achieve a mode area comparable to its plasmonic counterparts. The mode distribution in the all-dielectric bowtie waveguide can be precisely controlled by manipulating the geometric design. Our work shows that it is possible to achieve extreme light confinement by using dielectric instead of lossy metals.     

THz波段介质涂敷空芯金属圆波导的传输特性

针对THz波段介质涂敷空芯金属圆波导传输特性的精确分析问题,基于波导中场方程及边界条件建立关于传播常数的特征方程,并且采用改进的Muller法求解特征方程得到涂敷圆波导主模HE11模的传播常数。计算中对THz波段的非理想导体电导率采用经典弛豫效应模型。仿真结果表明:内径为1.8 mm的银波导,当聚苯乙烯涂敷层厚度为17 m时,HE11模在1.5~3.0 THz的衰减常数在1 dB/m以下,且具有较好的色散特性;内径为2.2 mm的银波导,在2.5 THz时其衰减常数随涂敷层厚度的增加先增大后减小,且存在最佳介质涂敷层厚度,可实现THz波低损耗传输。     

A long-range hybrid THz plasmonic waveguide with low attenuation loss

Numerical solutions are obtained for the proposed novel hybrid terahertz plasmonic waveguide structure, namely the silicon metal silicon (SMS) waveguide. It is shown that the SMS waveguide can overcome the diffraction limit while still maintaining a sizeable propagation length. The geometric dependence of the mode characteristics of this structure is analyzed in detail, showing strong confinement and low loss with propagation lengths exceeding 14 mm at normalized mode areas of 1.72 × 10⁻². By using the FEM method (Comsol), the guiding properties of the hybrid terahertz surface plasmon polariton (HTSPP) waveguide are numerically analyzed at the THz frequency, and a combination of double-structured comparisons of the best features of the terahertz plasmonic waveguide is made. Depending on the height used and how the mode confinement is measured, various modal designs, such as double microwire structures, are developed. The structures indicate that we verified the possibility of low attenuation loss of hybrid THz plasmonics propagation. The effective mode area A_eff, energy distribution, and propagation length L_p versus height for waveguides with Si microwire and SiO₂ are shown. The numerical calculation results reveal a potential for use in applications such as optical force in trapping and transporting biomolecules, and in high-density integrated circuits.     

Enhancing Bremsstrahlung production from ultraintense laser-solid interactions with front surface structures

A new type of a coaxial multi-layer plasmonic waveguide is proposed. The mode propagation properties are analyzed at the communication working wavelength. Theoretical investigations reveal that the enhanced optical confinement can be achieved in the two low-index dielectric media layers. The mode size can be sub- or deep sub-wavelength scale. The mode propagation loss can be well compensated by replacing the high-index dielectric media with gain material to achieve longer propagation length with better mode confinement. The comparisons of the mode properties between the proposed waveguide and waveguides studied in the published literatures are also considered. These investigations potentially lay the groundwork for the further applications of nanowire type multilayer hybrid structures. This structure could also enable various applications such asnanophotonic waveguides, high-quality nanolasers, and optical trapping and biosensors.     

High-index and low loss polyethylene terephatate optical waveguides

In this paper, the fabrication and characterization of a high-index low loss polyethylene terephatate (PET) optical waveguide is presented. Dip-coating technique has been used to fabricate the optical waveguide using PET. Various waveguide properties, such as mode effective refractive index, optical loss, dispersion and waveguide thickness as a function of temperature of waveguides and substrates have been reported. Optical losses of as low as 0.7 dB cm^–1 waveguides can be obtained if the formation temperature can be controlled between 30 and 35°C. In addition, we suggested that it is a suitable material for optoelectronic waveguide devices.     

K-Na离子交换单模平面波导的制备及其表征

通过K-Na离子交换技术制备了多模玻璃平面波导.采用棱镜耦合技术测量了波导的有效折射率,用IWKB方法拟合得到K-Na离子交换波导的折射率分布符合高斯分布,由色散曲线得到单模波导的制备条件(即扩散深度范围),从而得出单模波导的离子交换时间范围,制备出单模波导,并通过求解WKB色散方程得出单模波导的表面折射率.用普通数码相机,通过对离子交换平面玻璃光波导传输线进行数字成像,根据传输线上的光强分布拟合出光强传输衰减曲线,计算出单模波导的传输损耗大约为0.4 dB/cm.     

紫外表面等离激元在基于氧化锌纳米线的半导体-绝缘介质-金属结构中的输运特性研究

研究了紫外表面等离激元在半导体纳米线-绝缘介质-金属构成的波导结构中的输运问题,借助有限元方法,对这种波导所支持导模的电磁能分布、有效折射率、传播长度和有效模场面积随电磁参数和几何结构参数的依赖关系进行了分析.计算结果表明:以氧化锌纳米线作为增益介质,绝缘材料选择折射率小的空气,金属选择铝能够实现对输出光场的亚波长约束,有效模场面积达到λ2/100,同时保持低的传输损耗和高场强限制能力;有望用作纳米光源,使得相关的生物探测器件和医疗诊断设备实现更高的灵敏度和更小的体积.     

Terahertz demonstrations of effectively two-dimensional photonic bandgap structures

We demonstrate effectively two-dimensional (2D) terahertz (THz) photonic bandgap (PBG) structures for transverse electromagnetic (TEM) mode propagation within metal parallel plate waveguides (PPWG). The 2D-PBG structures consisting of square arrays of dielectric cylinders were characterized by THz time-domain spectroscopy (THz-TDS). THz photonic bandgaps were observed, as determined by the 160 mum lattice constant, the 65 mum diameter, and the dielectric constant of the cylinders. The experimental measurements were fitted with excellent agreement to 2D theory, confirming that for TEM mode propagation, effectively 2D propagation experiments can be achieved within the bounded space of the PPWG.     

磁光平面波导的单向传播特性

表面磁等离子体(surfacemagnetoplasmons,SMPs)是一种在电介质和偏置磁场作用下磁光材料界面处传播的近场电磁波.其独特的非互易传播特性引起了大量科研工作的关注,但在具体的波导结构设计上仍存在很多问题.本文研究了一种银-硅-磁光材料的3层平面波导结构,SMPs在磁光材料和硅的界面处传播,发现在特定的频率范围内,SMPs的基模及高阶模式均具有正向或反向的单向传播特性.分别计算了旋磁与旋电材料平面波导的色散方程,研究了硅层厚度与外加磁场对能带结构及SMPs单向传播区域的影响,发现无论是旋磁或旋电材料的结构,硅层厚度的增加使高阶模式使高阶模式出现在更低的频率位置,使单向传输带宽变小甚至消失,外加磁场的变大使磁光材料的能带结构频率增大的同时带隙中也引入了高阶模式.计算了2种磁光材料平面波导的正向和反向的单向传播带宽宽度,发现旋磁材料YIG的单向SMPs模式出现在GHz波段,最大单向带宽可达到2.45 GHz;旋电材料InSb的单向SMPs模式出现在THz波段,最大单向带宽达到3.9 THz.     

Fabrication and characterization of polyethylene terephatate optical waveguides

In this paper, the fabrication and characterization of polyethylene terephatate (PET) thin film integrated optical waveguide is presented. Dip-coating technique has been used to fabricate the optical waveguide using PET. Various waveguide properties, such as mode effective refractive index, optical loss, dispersion and waveguide thickness as a function of temperature of waveguides have been reported. The optical loss are found to be <1 dB cm^–1.     

基于耦合介质纳米线的深亚波长局域波导

提出了一种基于耦合介质纳米线的深亚波长局域波导,通过两根紧邻的高折射率介质纳米线的耦合,该波导可以将光场有效束缚在纳米线之间的低折射率纳米缝隙中. 计算模拟的结果表明,该波导的有效模场面积达到Λ²₀/200,比单根纳米线波导小一个数量级,这种深亚波长的模场束缚能力可以与表面等离激元混合波导相比拟. 计算模拟的结果还表明,纳米线可能带有的低折射率氧化膜、低折射率衬底的存在、以及纳米线间尺寸存在的一定差异对于该波导结构的实际应用都不会产生很大 关键词： 介质波导 亚波长局域 表面等离激元波导 纳米线     

利用3D打印技术制备太赫兹器件

高性能的太赫兹器件在控制太赫兹波方面起到重要的作用,因此寻求一种简单有效的太赫兹器件加工方案非常必要。本文以太赫兹波导和太赫兹滤波器为例,分别选用Kagome型光子晶体结构的波导和一维光子晶体结构的滤波器,运用商用的3D打印机加工样品,并采用透射式太赫兹时域光谱系统对样品的参数进行测量。实验结果表明:加工的波导在0.2~1.0 THz范围内传输损耗平均值约为0.02 cm~(-1),最小值可达到0.002 cm~(-1),且可运用机械拼接的方式将多个波导进行简单的连接从而获得更长的波导而不引起严重的损耗;滤波器的透射谱在0.1~0.5 THz之间有两个明显高损耗带;这两组实验结果均与理论预计非常接近。本文运用太赫兹波导和滤波器的实例证实了3D打印技术加工太赫兹器件的可行性,将会成为获取性能可控、价格低廉的太赫兹器件的有效途径。     

Propagation characteristics of the silica and silicon subwavelength-diameter hollow wire waveguides

The basic propagation properties of the silica and silicon subwavelength-diameter hollow wire waveguides have been investigated by comparison. It shows that the silica and silicon subwavelength-diameter hollow wire waveguides have some interesting properties, such as enhanced evanescent field in the cladding, enhanced intensity in the hollow core, and large waveguide dispersion. For the different confinement ability, the enhanced field in the hollow core and cladding of the silica subwavelength-diameter hollow wire is much stronger than that of the silicon one for the same size.     

Dispersion properties of hollow-core photonic bandgap fibers based on a square lattice cladding

In this paper the dispersion properties and confinement loss of hollow-core photonic bandgap fibers (PBGFs) based on a square lattice (SL) with rounded square air-holes was investigated for the first time, by using a full-vector finite element method (FEM). The waveguide group velocity dispersion (GVD) curves with different core diameter D, air hole size d, rounded diameter d_c and hole pitch Λ are presented. The influence of the number of cladding rings on dispersion and confinement loss were also calculated. It was found that as Λ or d increases, the width of PBG becomes wider, and that D and the number of cladding rings have a smaller influence on waveguide GVD. The ratio between bandgap width and central wavelength in our simulation is about 38.1%, which is larger than that of hollow-core PBGFs with triangular lattice (TL) (∼25%). By simulation, the desired zero dispersion wavelength or desired dispersion slope could be obtained by properly choosing the value of d_c or Λ. Compared to TL PBGF, at least nine cladding rings is needed to achieve the confinement loss less than 0.1 dB/m for future application.     

一种低损耗的对称双楔形太赫兹混合表面等离子体波导

本文研究了一种适用于太赫兹频段的对称双楔形混合等离子体波导.采用有限元法对其混合模式特征进行数值模拟,阐述了波导的传播长度、归一化有效模场面积和品质因子等特性随波导几何参数的变化规律.结果表明,对称双楔形混合等离子体波导在太赫兹频段可获得良好的模场约束能力和低损耗特性,在有效模场面积达到λ2/10280时,传播长度达到51 × 103 μm.波导参数最优时的平行对称楔形波导在忽略波导间串扰时,波导间距小于16 μm时的耦合长度约为8958 μm.通过对比发现,相比于先前的对称微楔形波导结构和对称蝴蝶结波导结构,对称双楔形混合等离子体波导在传输特性和耦合特性方面都具有更大的优势,将在光学力捕获、生物分子传输以及高密度集成电路设计等方面具有潜在的应用前景.     

Simplified universal dispersion curves for channel waveguides

Simplified universal dispersion curves for graded-index diffused channel waveguides have been obtained with the new normalized WKB equation using the effective index method. It is shown that, similarly to planar guides, the propagation constant of all the guided modes of a particular channel can be obtained by two parameters, called the normalized mode orders. The dispersion relations between the propagation constant and the normalized mode order of the channel have been obtained for a number of profile shapes and waveguide dimensions.     

Analytical solution of the scalar wave equation for slightly deformed optical Bragg waveguide

Using a simple analytical method, we study the electromagnetic-wave propagation in an optical Bragg waveguide which is slightly deformed at one side. We show the results of computations in the form of dispersion curves and provide a comparison with standard circular Bragg waveguides and step-index circular waveguides. We show that the correspondence between the cutoff V values for the standard step-index fiber and the new Bragg waveguide, which is slightly flattened on one side, is quite close for the lowest mode under the weak guidance condition.     

Dispersion modified slot optical waveguides

Advent of slot waveguide structures had opened a new era where light can be confined in low index slot guarded by high index slabs. Already in use SOI slot waveguides (contrast ratio is 2.42) have two distinct properties over the conventional waveguides, i.e. high E-field amplitude, optical power, optical intensity in low index materials, and strong E-field confinement localized to nanometer-size low index regions. We hereby propose a low refractive index contrast ratio slot waveguide structure (ratio is 1.18) comprising of commercially available glass material. Novelty lies in showing high E-field amplitude, optical power, optical intensity, and strong E-field confinement in low index slot regions despite of lowest ever reported contrast ratio. A systematic numerical study on the higher order dispersion characteristics of the widely studied SOI-based slot structure and of our proposed low refractive index contrast slot structure is carried out. It has been demonstrated that low refractive index contrast ratio slot optical waveguide GVD properties are quite different than SOI slot optical waveguide. The less normal dispersion existing in this kind of waveguide could have an impact on their applications in various nonlinear or linear applications.     

Anomalous propagation loss in photonic crystal waveguides

Propagation loss can occur in photonic crystal waveguides without complete optical confinement. We employ a highly efficient transfer-matrix method which allows for accurate and reliable extraction of the propagation loss even at an extremely low level. The results for a two-dimensional photonic crystal waveguide shows that the loss exponentially decays via the waveguide wall thickness. An anomalous phenomenon is found where the loss for guided modes near the upper band gap edge can be several orders of magnitude smaller than that for modes in the middle of the band gap. This anomaly can be well explained by the localization degree of guided modes at different frequency domains.     

混合表面等离子体激元的纳米激光器设计

为了实现纳米激光器的性能优化,设计了一种基于纳米线、半圆形氟化镁、三角形空气槽和金属脊结构的纳米激光器模型。模型中耦合在低折射率电介质层中的SPP模式和纳米线波导可以在低折射率间隙下像电容器那样存储光能,从而使低折射率的空气槽场强明显增大。应用有限元法在COMSOL Multiphysics软件下,分析了该纳米激光器模型的电场分布、模式特性、品质因数和增益阈值随着设计结构几何参数变化的规律,通过各部分折线图的综合分析来得出模型性能的数据。分析表明：该模型的光场约束能力较强且传播损耗较低,其中归一化面积最小可达到0.004 8,有效传输损耗最小可达到0.002。波导模场区域和限制因素表明,该激光器模型可以实现输出光场的亚波长约束。该模型基本实现了低增益阈值、低传输损耗和高品质因数的要求。