Ultra-compact graphene plasmonic filter integrated in a waveguide

Graphene plasmons have become promising candidates for deep-subwavelength nanoscale optical devices due to their strong field confinement and low damping. Among these nanoscale optical devices, band-pass filter for wavelength selection and noise filtering are key devices in an integrated optical circuit. However, plasmonic filters are still oversized because large resonant cavities are needed to perform frequency selection. Here, an ultra-compact filter integrated in a graphene plasmonic waveguide was designed, where a rectangular resonant cavity is inside a graphene nanoribbon waveguide. The properties of the filter were studied using the finite-difference time-domain method and demonstrated using the analytical model. The results demonstrate the band-pass filter has a high quality factor(20.36) and electrically tunable frequency response. The working frequency of the filter could also be tuned by modifying the cavity size. Our work provides a feasible structure for a graphene plasmonic nano-filter for future use in integrated optical circuits.     

基于倏逝场耦合的石墨烯波导光传输相位特性仿真与实验研究

石墨烯材料应用到各种光波导器件中正成为新一代光子器件的重要发展方向之一,目前基于石墨烯的光纤和集成光子器件研究越来越受到国内外的重视. 本文建立了一种由微纳光纤耦合光倏逝场,并在石墨烯薄膜中传输的模型. 通过有限元分析法,研究了光在这种石墨烯波导中传输光场的强度分布和相位特性,并通过实验进行了验证. 结果表明,沿着微纳光纤-石墨烯光波导传播的倏逝场的强度分布和相位均受石墨烯材料作用,石墨烯材料能有效聚集和导行波导中传输的高阶模,在单位传输长度上具有更密集的等相位面. 本文提出了一种利用微纳光纤耦合光倏逝场研究石墨烯相位响应特性的新方法,对基于石墨烯波导的新型调制器、滤波器、激光器和传感器等光子器件的设计和应用具有一定的参考意义. 关键词： 石墨烯平面光波导 倏逝波 光场强度 相位     

Plasmonic split disk resonator based on graphene

In this paper, we designed and simulated an ultra-compact plasmonic filter consisting of a graphene ribbon laterally coupled to a graphene split disk resonator. Introducing a split inside the disk leads to split of the resonance modes symmetrically and asymmetrically. The spacing between the symmetric and asymmetric modes can be controlled through changing length of the split in the disk and the simulation results reveal that all modes identically shift as a result of changing the Fermi level of the disk. finite-difference time-domain method is used to investigate the device performance.     

Ultra-compact directional couplers and Mach-Zehnder interferometers employing surface plasmon polaritons

Optical directional couplers and Mach-Zehnder interferometers employing surface plasmon polaritons are proposed and their characteristics are analyzed numerically using a finite-difference time-domain method. It is shown that these devices can have transverse size smaller than the incident wavelength and can then be regarded as true subwavelength photonic elements. The ultra-compact characteristic of these devices can be utilized as a basis of next generation of integrated photonic circuits. Furthermore, calculations also show that these devices behave differently from their traditional all-dielectric counterparts. For the proposed optical directional coupler, the phase difference between the two output ports will no longer be 90° even when the two ports have the same power outputs.     

Multichannel W3 Y-branch filter in a two dimensional triangular-lattice photonic crystal slab

We demonstrate the design, fabrication and characterization of a highly efficient multichannel W3 Y-branch filter in a two dimensional triangular-lattice photonic crystal slab. The coupling properties between high-order waveguide modes and fundamental resonant modes are investigated. By finely adjusting the size of resonant cavities, four higher-order mode channels with different output wavelengths are experimentally realized, which is in agreement with the theoretical simulations. The results show that this kind of filter may be useful in optical integrated circuits with high coupling and transmission efficiency.     

Uneven splitting-ratio 1×2 multimode interference splitters based on silicon wire waveguides

Two types of 1×2 multi-mode interference(MMI) splitters with splitting ratios of 85:15 and 72:28 are designed.On the basis of a numerical simulation,an optimal length of the MMI section is obtained.Subsequently,the devices are fabricated and tested.The footprints of the rectangular MMI regions are only 3×18.2 and 3×14.3(μm).The minimum excess losses are 1.4 and 1.1 dB.The results of the test on the splitting ratios are consistent with designed values.The devices can be applied in ultra-compact photonic integrated circuits to realize the "tap" function.     

Slow light transmission in a photonic crystal power splitter with parallel outputs

In this paper, we investigate coupling of light to slow modes in a photonic crystal power splitter composed of a Y-junction and two 60° bends. First, a combination of two cascaded bends which is commonly used in integrated photonic crystal circuits is studied in slow light frequency regime. We propose a structure that its transmission spectrum covers the high group-index frequencies near the band edge. Also, by structural modifications, high transmission near to 95% is achieved in slow light bandwidth. Next, we study the complete structure of a photonic crystal power splitter with parallel outputs based on a Y-junction integrated with two 60° bends. Using modified bends and reducing sharpness of Y-junction, the efficiency of splitting increases in both high and low group-index frequency bands. The optimized structure has an average efficiency of 82% in slow mode regime. This structure can be used in photonic crystal based slow light devices, such as Mach-Zehnder interferometers.     

石墨烯-硅基混合光子集成电路

近年来硅基光子学已经慢慢走向成熟,它被认为是未来取代电子集成电路,实现下一代更高性能的光子集成电路的关键技术.这得益于硅基光子器件与现代的互补金属氧化物半导体工艺相兼容,能够实现廉价的大规模集成.然而,由于受硅材料本身的光电特性所限,在硅基平台上实现高性能的有源器件仍然存在着巨大挑战.石墨烯-硅基混合光子集成电路的发展为解决这一问题提供了可行的方案.这得益于石墨烯作为一种兼具高载流子迁移率、高电光系数和宽带吸收等优点的二维光电材料,能够方便地与现有硅基器件相集成,并充分发挥自身的光电性能优势.本文结合我们课题组在该领域研究的一些最新成果,介绍了国际上在石墨烯-硅基混合光子集成电路上的一些重要研究进展,涵盖了光源、光波导、光调制器和光探测器四个重要组成部分.     

Design and fabrication of a compact multimode interference splitter with silicon photonic nanowires

A compact multimode interference （MMI） splitter with silicon photonic nanowires on silicon-on-insulator （SOI） substrate is designed and fabricated. The footprint of the MMI section is only approximately 3×10 （μm）. The simulation results show that the device may have a low excess loss of 0.04 dB. The transmission loss of the silicon photonics wire is measured to be 0.73±0.3 dB/mm. The compact size and low transmission loss allow the device to be used in ultra-compact photonic integrated circuits. The device exhibits a good light splitting function. In a spectral range of 1549-1560 nm, the excess loss is 1.5 dB and the average imbalance between the two channels is 0.51 dB.     

光子晶体线缺陷波导中的折射率相位移调制增强效应

在传统的基于全内反射原理的低折射率比介质波导所构建的相位移调制型光学器件中,调制区域的长度通常在毫米到厘米量级.由于器件横向尺寸保持在微米量级,因此狭长结构成为了传统光波导器件的典型特征,这限制了光学器件集成度的提高,严重制约了集成光路的进一步发展.光子晶体的出现为高密集成光路的发展提供了一条新的途径.本文使用平面波展开方法计算了光子晶体线缺陷波导中的色散曲线.研究发现:在色散曲线下边缘处,材料折射率的一个微小变化可以引起传输常数的较大变化,如果工作频率点选择在带下边缘附近,则可以大幅度减小相位移调制型器件调制区域的长度.本文使用时域有限差分方法进一步验证这种增强效应,计算结果表明,对于0.46%的折射率变化,光子晶体线缺陷波导中的相位调制长度仅为均匀媒质中相位移调制长度的11.7%.通过以进一步研究,这种增强效应有望应用与高密度集成光路.     

Silicon and porous silicon mid-infrared photonic crystals

A 3D silicon micromachining method based on proton beam writing combined with electrochemical anodization of p-type silicon enables fabrication of mid-infrared photonic crystals made of silicon and porous silicon. Here, example structures of silicon 1D and 2D photonic crystals are demonstrated. Progress and problems of fabricating 3D photonic crystals made of silicon are discussed. The strategy of fabricating photonic crystals purely made of porous silicon, and the characterization method of all these mid-infrared structures, are discussed. Due to the flexibility of this fabrication method, photonic devices and integrated photonic circuits may be built on a single chip, for which two 2D silicon photonic crystals with one on top of the other are demonstrated.     

Acoustic analog of monolayer graphene and edge states

Acoustic analog of monolayer graphene has been designed by using silicone rubber spheres of honeycomb lattices embedded in water. The dispersion of the structure has been studied theoretically using the rigorous multiple-scattering method. The energy spectra with the Dirac point have been verified and zigzag edge states have been found in ribbons of the structure, which are analogous to the electronic ones in graphene nanoribbons. The guided modes along the zigzag edge excited by a point source have been numerically demonstrated. The open cavity and “Z” type edge waveguide with 60° corners have also been realized by using such edge states.     

基于石墨烯纳米带的齿形表面等离激元滤波器的研究

提出了一种基于石墨烯纳米带的齿形表面等离激元波导滤波器, 并且用时域有限差分法研究了这种结构. 单个齿形的滤波器可以实现带阻滤波, 其滤波特性可以用基于散射矩阵的解析模型解释. 滤波器的透射谱特性可以通过调节齿的长度、宽度以及石墨烯的化学势来改变. 由于石墨烯的化学势可以用门电路来调节, 这种结构的滤波器可以在器件加工完成后灵活地调节其工作波长. 同时研究了多齿滤波器, 这种结构可以实现宽带滤波, 文中对具有不同齿数、周期的滤波器的透射谱进行了细致的研究. 研究结果对实现基于石墨烯的大规模集成光电子器件提供了重要的理论参考.     

一维光子晶体缺陷模的滤波特性及应用研究

研究了一维光子晶体缺陷膜的滤波特性,重点研究了影响其滤波特性的因素及其影响规律.并对利用一维光子晶体技术开展多通道滤光片设计的可行性进行了研究.结果表明,利用光子晶体的滤波特性,可以设计出传统光学薄膜设计方法难以实现的多通道窄带滤光片,并根据实际需要,设计了一种双通道窄带滤光片.     

二维光子晶体非对称Mach-Zehnder自准直传感器研究

基于自准直效应,通过在二维空气柱型光子晶体非对称Mach-Zehnder干涉仪长臂上设置传感区域,设计了一种自准直传感器.平面波展开法确定了入射光的自准直频率范围,时域有限差分法分析了传感器的灵敏度达到68 nm/RIU,采用单频光入射实现了传感模拟.该传感器完全依赖自准直导光,无需引入任何缺陷波导,大大降低了制作工艺要求,其大小只有十几个微米,能够满足超紧凑、高灵敏度、廉价和无标记的要求.     

光子石墨烯中赝磁场作用下的谷霍尔效应

将石墨烯中赝磁场的产生机理运用于光子石墨烯,通过在光子石墨烯中引入晶格有规律单轴形变的方式,理论分析得到了谷依赖的均匀赝磁场,并通过数值模拟的方法观察到明显的谷霍尔效应.这种谷霍尔效应的显著程度随晶格形变度的增加而加强.在具有一定损耗的电介质材料构成的形变光子石墨烯中仍可观察到明显的谷霍尔效应.随着电介质材料损耗的增加,谷霍尔效应导致的波束转弯效果依然能够保持,只是强度逐渐变弱.类似于自旋电子学中的自旋霍尔效应,这种光子石墨烯中等效赝磁场作用下的谷霍尔效应在未来谷极化器件的设计和应用中具有重要意义.     

近红外波段硅基金属光子晶体平板的亚波长成像特性

利用时域有限差分方法,理论上研究了由正方形金属嵌入背景材料硅中组成的二维正方晶格和三角晶格光子晶体的亚波长成像特性。采用Drude模型描述金属银的色散特性,在近红外波段该模型可以很好地描述金属的实际介电常数。通过结构参数的设计,在上述两种结构中实现了波长在1550nm附近的亚波长成像,并且发现金属对入射光的吸收使得成像位置处的光强稍有降低,但是对于光子晶体亚波长成像的质量并无影响。相对于通常的硅基空气孔型光子晶体亚波长成像器件,该种硅基金属型全固态光子晶体结构更加稳定,因而可以更好地在复杂全光集成回路中加以实际应用。     

Magnetically Tunable Terahertz Switch and Band-Pass Filter

A novel design of a tunable terahertz switch and band-pass filter using liquid-crystal-filled photonic crystal waveguide is demonstrated. The effects of magnetic field on the photonic bandgaps and transmitting properties of a THz wave are investigated by using the plane wave expansion method and finite difference time domain method. The efficient photonic bandgap tuning is predicted such that the two-dimensional liquid-crystal-filled photonie crystal waveguide can serve as a switch and continuously tunable band-pass filter with controlling of the magnetic field.     

Chirality effect in disordered graphene ribbon junctions

We investigate the influence of edge chirality on the electronic transport in clean or disordered graphene ribbon junctions. By using the tight-binding model and the Landauer-Büttiker formalism, the junction conductance is obtained. In the clean sample, the zero-magnetic-field junction conductance is strongly chirality-dependent in both unipolar and bipolar ribbons, whereas the high-magnetic-field conductance is either chirality-independent in the unipolar or chirality-dependent in the bipolar ribbon. Furthermore, we study the disordered sample in the presence of magnetic field and find that the junction conductance is always chirality-insensitive for both unipolar and bipolar ribbons with adequate disorders. In addition, the disorder-induced conductance plateaus can exist in all chiral bipolar ribbons provided the disorder strength is moderate. These results suggest that we can neglect the effect of edge chirality in fabricating electronic devices based on the magnetotransport in a disordered graphene ribbon.     

涂覆双层石墨烯的介质纳米线表面等离子体光波导的模式特性研究

设计了一种由涂覆双层石墨烯的圆形介质纳米线组成的表面等离子体光波导。利用有限元法(FEM),数值分析了纳米线半径、介质夹层厚度、石墨烯化学势以及工作频率对波导所支持的电磁场模式的有效折射率、传播长度、归一化模式面积以及品质因数等模式特性的影响。结果表明,这种新型的混合表面等离子体光波导具有很强的模式束缚能力,归一化模式面积非常小,可以实现极高密度的器件集成,并且传输损耗较低。