摘要
集成光学和光学成像是信息光学的两大重要领域,但是它们的发展都受到衍射极限的限制。表面等离激元(SPPs)是一种存在于金属与电介质表面的电磁波,它的强度随距离界面的增大而呈指数减小。SPPs之所以能够突破衍射极限是因为其波矢大于其光波在自由空间的波矢,被认为是下一代纳米集成光路的有效信息载体之一。本文的第一部分工作就是设计了各种基于SPPs的新型纳米光子集成器件,并利用数值模拟的方法验证了这些器件的光学性能。另一方面,金属特异介质是人造的周期性结构,其周期远小于响应波长。其表现出自然界现有材料不具备的令人激动人心的光波传输效应,比如说负折射。它已经在超分辨成像、光学隐身、慢光等方面取得了巨大的成功。本文的另外一项工作就是利用金属特异介质实现远场超分辨成像。本文的具体工作分为以下几个方面：
1. SPPs纳米弯曲波导和聚焦
对于相同宽度的狭缝,SPPs在两种金属狭缝中传播的等效折射率存在差异,设计了一种由两种金属构成的异质狭缝波导。由于SPPs倾向于向高等效折射率金属狭缝中传播,所以在波导中能量主要限制在高等效折射率的金属狭缝中。通过选择合适的结构参数,SPPs的光斑大小可限制在半波长以下。进一步用这种结构构建了金属异质结构纳米弯曲波导,数字模拟证实在弯曲半径等于0时,SPPs的透过率超过了90%,它是一种很好的SPPs波导连接器。由其构建的纳米T型分束器和M-Z干涉仪都取得了较好的传输效果。另一方面,基于同样的原理,设计了楔形金属异质结构用以实现了SPPs纳米聚焦,SPPs主要限制在具有较高等效折射率的金属表面传播,最后在其尖端实现聚焦,增强因子超过了1000倍。另外基于此结构还设计了阵列探针实现了同时对多个位置纳米聚焦。该结构可以在高密度存储、近场光学显微镜、光刻、生物化学传感等方面找到应用。
2.可见光频段SPPs“彩虹”捕获和释放
对于金属-电介质-空气模型,其支持的SPPs等效折射率随电介质高度的增大而逐渐增大,设计了在金属上覆盖渐变高度的电介质光栅结构。计算表明不同光栅高度对应的色散关系不同,随光栅高度的增加,色散曲线的上边带波长会红移。而对应在边带附近,会发生局域效应,光波的群速度会降低。对于特定波长(可见光)激发的SPPs,其群速度会沿着传播方向逐渐减小,最后SPPs局域在结构中特定的位置。因此对于可见光不同波长的SPPs最后会局域在金属表面的不同位置,也就是发生了所谓的“彩虹捕获”效应。在635nm波长,SPPs的光子寿命达到0.36ps。上述渐变高度的电介质光栅结构在实验上制备较为困难,另外还设计了一种在实验上较为容易制备的结构-在金属表面覆盖等高度渐变周期的电介质光栅结构。对不同的光栅周期,对应的色散曲线上边带波长随周期的增大而增大。类似地,该结构也可实现SPPs“彩虹捕获”。此外,通过在金属膜的下表面覆盖等周期的电介质光栅,动态调节光栅的折射率,可将捕获的SPPs逐一释放。该结构可在纳米光缓冲器、分光器、滤波器、数据处理和量子光子存储器等方面上找到应用。
3.远场超分辨成像
在光学波段,金属的介电常数往往是小于0的,而电介质的介电常数大于0,由金属电介质的组合可实现双曲线型色散曲线。那么金属电介质多层膜结构可支持消逝波的传播,并可将消逝波逐渐转化为传播波。本文设计了一种V型金属-电介质多层膜结构实现分辨两个距离小于半波长的线光源,通过结构放大以后,结构外表面两光源像的距离大于半波长,然后通过传统的光学成像系统直接处理可得到放大的像。在V型夹角为900时,分辨极限可达21nm。另外还设计了一种金字塔型金属-电介质多层膜结构,分辨空间八个距离小于半波长的物点(八个点构成一个梯棱台的八个顶点)。通过放大,在结构外表面八个像点两两之间的距离都大于半波长。合理地选择结构的几何参数,可实现分辨其它各种三维结构的八个点光源。
Integrated optics and optical imaging are two important realms in information optics, however, their developments are hampered by the diffraction limit. Surface plasmon polaritons (SPPs) are surface electromagnetic waves propagating along the interface between metals and dielectrics, and their intensity decreases exponentially with the distance away from metal surface. SPP can overcome the diffraction limit because its wave vector is much larger than that of light in air, thus it has been regarded as one of the most effective carriers in next-generation nanophotonic integrated circuits. The first part of this work in this dissertation is to design various SPP based novel nanophotonic integrated devices, such as metal heterostructured bending waveguide, ridged metal heterostructure for nanofocusing, broadband slow SPP systems etc, the optical properties of which have been confirmed by numerical simulation methods. On the other hand, metamaterials are artifical periodical structures with "lattice constants" that are much smaller than the response wavelength. They exhibit many dramtic effects on the light propagation that don't exsit in natural materials, such as negative refraction, and they have been tremendously successful in achieving subdiffraction imaging, optical invisibility, and slow light etc. The second part of this work in this dissertation is to adopt metamaterials to achieve far-field subdiffraction imaging. The work of this dissertation is divided in to the following parts:
1. SPPs bending waveguide and nanofousing
With regard to metal gap waveguides (MGWs) of same width, the effective refractive index for different metal films is different, we design a hetrowaveguide consisting of two kinds of MGWs to guide SPPs. As SPPs tends to propagate with higher effective refractive index, thus the most light energy in the waveguide is confined in the MGW with higher effective refractive index. By properly choosing the geometric parameters, SPPs spot can be confined to very small size below half of the wavelength. We further employ this metal heterowaveguide to construct a SPP bending waveguide, and numerically demonstrate that SPPs transmission exceeds 90% as the bending radius is equal to zero, implying it may serve as an excellent SPP waveguide connector. The metal heterowaveguide based T-shaped spliter and M-Z interferometer are demonstrated to have good transmission in a nanoscale domain. Based on the same principle, we propose a ridged metal heterostructure for nanofocusing. SPPs mainly propagate in the metal surface with higher effective refractive index, and finally are focused at its tip. The enhancement factor exceeds 1000. In addition, based on this structure we further design array probes to achieve multiple nanofocusing for different spatial positions simultaneously. The proposed structure may find potential applications in high-density optical data storage, near-field optical microscopy, optical nanolithography, and bio-and chemosensing etc.
2. SPPs rainbow trapping and releasingn at visible frequencies
Considering metal-dielectric-air model, the effective refractive index of SPPs increases with dielectric thickness, we design a metal film covered by dielectric gratings of graded thickness. Calculated result shows that the dispersion relation is different for different grating thickness, and the wavelength near the upper bandegde of the dispersive curve red shifts with the grating thickness. At the bandedge of dispersive curve, localization effect occurs and light group velocity reduces. For SPPs of a certain visible wavelength, the group velocity is gradually reduced along the propagation direction and finally SPPs are localized at a specific postion in the structure. Thus SPPs of different excitation wavelengths in the visible domain will be localized at different spatial positions along the metal surface, or in other words, trapped rainbow appears. SPP lifetime reaches about 0.36 ps at 635 nm wavelength. Because of the difficulty in fabricating such dielectric gratings of graded thickness in acutual experiment, we propose a more realizable metals film covered by chirped dielectric gratings with same thickness but graded lattice constant. For different lattice constant, the corresponding wavelength near the upper bandegde of dispersive curve is increased with the lattice constant. Similarily, such a structure can aslo achieve SPPs rainbow trapping. In addition, by using another uniform dielectric grating attached to the bottom of the metal film and real-time tuning the refractive index of the grating, the trapped SPPs can be released in sequence. Such a structure may find potential applications in nanoscale buffers, spectrometers, filters, data processors, and quantum optical memories etc.
3. Far-field subdiffraction imaging
Since metal tends to have negative electric permittivity and dielectric have positive one in the optical frequency range, the combination of which may result in hyperbolic dispersive dispersion. Therefore, metal-dielectric multilayers can support the propagation of evanescent waves, and gradually convert evanescent waves into propagation waves. We propose a kind of V-shaped metal-dielectric multilayers to resolve two linear sources below half of the wavelength. Through the structure's magnification, the imaing distance on the output surfaces can be much larger than half of the wavelength, thus the magnified images can be directly processed by conventional optical imaging systems. In the case of 90~0 wedge angle, the resolution limit of the system is down to 21 nm. Besides, we design a kind of pyramid-shaped metal-dielectric multilayers for resolving eight point sources (eight points construct a trapezoidal bevel) with subdiffraction separations in three-dimensional domain. Through magnification, the imaging distances between the nearest-neighbor point sources on the output surfaces of the structure are all larger than half of the wavelength. By properly changing the geometrical parameters, we are able to resolve eight point sources with different hexahedron structures.
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