【作者】 汤冬华；

【导师】 陈历学；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2009， 博士

【摘要】 随着理论和技术水平的发展,亚波长光子结构得到了广泛而深入的研究,为集成全光系统的实现奠定了基础。但随着光子结构的缩小,光子能量的高效率输入和输出变的非常困难,并逐渐成为制约光子技术发展的一个瓶颈。本论文利用理论分析与数值模拟相结合的方法,研究光子晶体亚波长波导中能量的输入和输出特性,探索高效率的能量输入方法,以及能量的定向辐射特性,期望给目前光子学的发展提供有价值的结果。论文提出利用级联光子晶体结构实现光子晶体波导能量的定向辐射,通过分析光子晶体波导结构的传输和色散特性,以及级联周期结构的自准直传输特性,得到了频率匹配的级联结构,实现了光子晶体波导能量的高效率定向辐射输出。数值模拟结果显示输出效率约80%,输出光束的发散角小于6?。系统研究了级联结构的参数,包括级联层的厚度,级联层宽度,耦合强度,以及波导辐射场的发散角,对定向辐射光束特性的影响。研究了不同弯曲级联结构对输出光束的影响,结果表明正向弯曲导致透射光束在近距离发生强汇聚,远场发散角变大;而负方向弯曲则使光束的近场汇聚减弱,从而使得远场的方向性以及能量传输效率增强。此外我们还研究了级联结构实现定向辐射的带宽特性,发现工作带宽随结构的变化而变化,以我们所用具体结构而言,在自准直频率附近2.4%的带宽范围内,都可以实现高效率的定向辐射输出。论文提出并研究了光子晶体波导能量的多方向定向辐射原理以及实现方法。通过分析波导能量定向辐射的机理,指出表面结构所对应的等效辐射源分布,决定了辐射光束的方向;并且提出基于离散表面调制,以及多波导分支两种方法改变透射表面结构特性,实现了多方向的定向辐射。利用相干点源干涉模型推导了输出能量随方向的变化规律,并且利用数值模拟方法加以验证。结果表明这两种结构均能实现高效率的Nb(Nb = 1,2,···)个定向传播光束,并且所得到的输出光束个数Nb,发散角?θ等参数与理论分析结果相吻合。利用该结构,研究了光子晶体波导与多介质波导耦合的特性,得到了高效率的1 ? N能量分束与耦合器。研究了表面等效辐射源相对位置关系对辐射光束的特性的影响,得到了光束特性的调节方法。论文还研究了辐射源非对称(包括振幅,位相,以及位置坐标)情况下辐射光束的特性,发现了与对称结构中辐射光束的不同特性。利用非对称结构,能够实现发散角更小的光束,同时具有结构相对简单,体积更小,损耗更小的优点。由于结构的非对称特性,不同频率准直透射光束的传播方向不同,所以利用非对称结构,不仅能够实现透射光束的准直辐射,而且还能实现不同频率信号的空间分离。论文提出了基于负折射率介质球面透镜实现宽光束到亚波长光子晶体波导的高效率能量输入方法。利用数值模拟方法,以及负折射率介质的SRR/Rod模型,实现了宽度为16λ0的高斯光束到宽度为0.5λ0的光子晶体波导之间的耦合,耦合效率达到93%,而该结构的厚度只有4～6λ0,其中λ0为光波在真空中的波长。利用负折射率介质的Snell定律,研究了负折射率介质球面透镜的汇聚特性,推导了其焦距公式,及其随入射角度的变化关系,同时分析比较了与目前广泛研究的平板型负折射透镜的区别。系统分析了入射光束的频率,入射光束传播方向与光轴的夹角,以及光束的横向(相对于负折射率透镜光轴)偏移对入射效率的影响,得出了该耦合结构的特性。同时对比研究了普通正折射率透镜作为能量输入耦合结构时的特点,发现本文所提出的结构具有体积更小(工作距离只有正折射率情况下的1/3),效率更高(大于90%)的优点。论文还研究了基于三角点阵光子晶体等效负折射效应而实现的光子晶体透镜耦合结构,分析了其等频面的形态,以及有效负折射率的变化特性。设计了具体的负折射率光子晶体透镜结构,研究了其耦合特性变化规律,并比较分析了与SRR/Rod模型耦合的不同。更多还原

【Abstract】 With the developments of theory and technology of nano-structures, sub-wavelengthphotonic structures are widely investigated, and a solid foundation for all-optic systemhas been founded. However, with the decreasing of volumes of photonic elements, theinput and out coupling become more and more difficult, which is becoming a bottle-neckof modern nano photonics. In this thesis, using theoretical analysis, and full-vector nu-merical simulation methods, the input and output coupling properties of photonic crystalwaveguides are investigated. We aim to explore high efficient input and directional outputmethods, and provide valuable results to the modern nano-photonics.A highly efficient directional output mechanism based on cascaded photonic crystal(PC) waveguide and self-collimated PC structures is proposed and investigated both the-oretically and numerically using the finite-difference time-domain (FDTD) method. Weanalyze the transmission properties of the PC waveguide, and those of the self-collimationbeams in the cascading PC. Then a frequency matching method is used to find properstructures for directional emission, and highly directional beaming is obtained numeri-cally. Results show that the output efficiency is about 80%, and divergence angle lessthan 6?. We systematically investigate the effects of the structural parameters on thecoupling efficiency, which include the thickness, width of the cascading layers, and alsothe coupling intensity between the WG and the cascading layers. We then introduce anarbitrary curving the cascading layers, and investigate the effect of curvature radius onthe directional beaming. We find that a positive curvature makes the beaming distanceshorter, and highly focused at near field region. However, a minus curvature enlargesthe directional distance. The bandwidth of the directional emission is also investigated,and results show that, for the specific structure investigated in this thesis, within a rela-tive bandwidth of 2.4% near the self-collimation frequency, all the frequencies can realizedirectional emission.The mechanism of multiple directional emission of PC waveguide is investigated.By analyzing the directional emission process of PC waveguide, we find that the distribu-tion of the effective sources on the surface determines the emission profile of the beam.An interference model of multiple coherent point source is developed to describe this phe- nomena, which is verified by numerical results. Two structures of discretely modulatedexit surface, and multiple waveguide branch are investigated in detail to realize multipledirectional emission. Results shows that, using these two structures, one can obtain Nb(Nb = 1, 2,···) directional beams, also Nb, and the divergence angle are predicted ex-actly using the interference model. When coupled with traditional dielectric waveguides,a high efficiency 1 ? Nb power splitter is numerically realized. The emission propertiesof asymmetric structures are also investigated. Results show that the divergence angleis smaller, and the structure is more simple, and also the loss is less than the symmetri-cal ones. Due to the asymmetric property, the signals with different frequencies are alsofiltered in space when directionally emitted from the waveguide.A highly efficient input mechanism based on spherical lens made of negative re-fractive index (NRI) material is proposed and investigated theoretically and numericallyusing FDTD method. Using the SRR (split ring resonator) and rod model of NRI mate-rial, high efficiency input coupling between a wide beam (with a waist width of 16λ0) anda sub-wavelength PC waveguide (with a width of 0.5λ0) is realized (λ0 is the wavelengthin vacuum). The coupling efficiency is more than 93%, and the thickness of the coupleris only (4～6)λ0. Using the Snell’s law of NRI material, we derive a focusing lengthformula, and investigate the changes with incident angle. We find the focusing propertiesof the lens to wide parallel beams, which is difference with the ?at NRI lens. We sys-tematically investigate the effects of incident frequency, de?ection angle, and transverseshift of the beam on the coupling efficiency, and also make a comparison with those of atraditional lens made of positive refraction. Our results show that the advantages of NRIlens coupler lies in the smaller size, and higher efficiency. When the NRI lens coupler ismade of photonic crystal, but not SRR/Rod structure, we investigate the profiles of equalfrequency surface, and effective negative refraction of an designed triangle lattice PCstructure. A NRI lens made of PC is proposed, and the coupling properties is investigatedand compared with those of SRR/Rod model.更多还原