【作者】 凌晓辉；

【导师】 范滇元；

【作者基本信息】 湖南大学 ， 计算机应用技术， 2012， 博士

【摘要】 偏振是光的一个重要性质。一直以来,人们都希望能够完全地控制光的偏振态。调控光的偏振在本质上是调控光的电场分量在两个正交方向上的振幅和相位差。传统的控制偏振的方法包括各向异性介质(晶体)波片、手性材料、二色晶体和光栅等。人工微结构材料是一种电磁性质可以人为设计和调控的人工复合材料,由两种以上的物质按一定规则组合而成,具有亚波长的光学尺度,其电磁性质取决于结构组成方式而非构成其结构的物质本身。它往往具有自然物质所不存在的新特性或反常特性或超过常规材料的特性。通过合适地设计其结构单元,可以得到双折射率远大于常规各向异性材料的微结构材料；也可以通过局部地改变结构单元,从而得到想要的光束偏振和相位分布。调控光的偏振还会影响光的另一个基本性质——相干性,因而可以通过控制偏振来调控光的相干性。高的相干性是激光的一个显著特征,这一特征使激光广泛用于工业、军事、医学和科学研究等领域。而对于某些激光应用领域,如激光核聚变、激光显示和激光热处理等,高相干性却是有害的,甚至致命的。因此,这些领域对于非相干激光的需求非常迫切。另一方面,近几年来倍受关注的光自旋霍尔效应实际上也是一种偏振相关的效应。它类比于电子的自旋霍尔效应而来,其中光子的自旋对应于电子的自旋,折射率梯度扮演外场的角色。而光子的两个自旋态对应的即是左、右旋圆偏振。光自旋霍尔效应表现为自旋相关的分裂现象,即一个线偏振光的左、右旋圆偏振分量由于经历不同的几何相位(geometrical phase)而在空间上相互分开。因此,调控光自旋霍尔效应,本质上就是对光的偏振的调控。基于以上认识,本文提出用人工微结构材料来调控光的偏振和自旋霍尔效应,取得了几项创新性成果：(1)提出用不均匀各向异性微结构材料来产生非相干的激光辐照。在横向空间上,我们将大量对偏振具有不同的改变能力的微结构单元随机排列。从光的相干性的观点来看,光束通过这种结构后在远场焦面是非相干的。我们以偏振敏感的L型微结构材料为例,用计算机模拟仿真论证了其在降低散斑对比度和提高激光辐照均匀性等方面的应用潜力。与传统的偏振控制元件相比,基于微结构材料的偏振控制器件的结构尺寸可以控制在波长量级,并且其偏振性质具有很大的可调控空间。这使得它特别有潜力用于微纳光子器件和未来集成光路方面的应用。(2)首先研究了两种多层薄膜微结构中的折射光的自旋霍尔效应。它们分别具有对称和不对称的介质层排列,其菲涅尔系数可以通过调节结构参数而改变。我们发现,光束的自旋-轨道相互作用可随多层结构的光学参数的改变作周期性振荡(来源于法布里-珀罗(Fabry-Perot)共振),因而可以有效地增强、压缩甚至完全抑制自旋相关的横移,从而实现对光自旋霍尔效应的调控。同时,也可以反过来利用光自旋霍尔效应来表征纳米尺度的结构和折射率变化,为研究纳米结构中的物理特性提供一种灵敏的方式。其次,在此基础上,进一步提出用缺陷一维光子晶体的缺陷态来增强光自旋霍尔效应。在斜入射情形下,这种缺陷态具有偏振相关的透射峰。在透射峰附近自旋分裂的横移值可以达到空气-玻璃界面的数十倍,从而极大地增强光自旋霍尔效应。(3)研究了光在单层各向异性超常介质中的反常的自旋霍尔效应。这种反常表现在光束自旋分裂的不对称性。在前人的关于光自旋霍尔效应的研究中,左、右旋圆偏振光的自旋分裂是完全对称的,即它们分居于入射面的两侧(相反的分裂方向),并具有等振幅的横移。而我们发现,由于各向异性超常介质极大的各向异性而导致不对称的几何相,使自旋分裂产生明显的不对称性,即左、右旋圆偏振光可能出现相同方向的横移或是不相等的横移振幅。改变超常介质的结构参数和入射面与光轴的夹角,可以对这种不对称分裂进行调控。(4)提出一种具有特殊几何结构的、不均匀的各向异性微结构材料来调控光的自旋霍尔效应及自旋分裂。它能够改变光束的局部偏振态,并产生自旋相关的、空间变化的几何相,使线偏振光产生自旋分裂。有趣地是,在远场,这种分裂表现为多个独立的左、右旋圆偏振光斑(用Stokes参数的S3分量表示)的交替出现。光斑的个数取决于材料的几何结构,从而使不均匀的各向异性微结构材料有潜力用于调控光束自旋分裂和光子自旋态。实际上,不但材料的几何结构对几何相有贡献,我们还发现,入射的偏振态也会对几何相有贡献。现有的研究都局限在入射光为空间均匀的线偏振光的情形。我们考虑一般性的情况,以轴对称线偏振光为例,考虑空间不均匀的线偏振光(矢量光束的一种)的自旋分裂,均匀线偏振光只是其特殊形式。由于几何相来源于材料和入射偏振两方面的贡献,所以远场的自旋分裂图案也可通过改变入射偏振分布来予以调控。因此,不均匀线偏振光将成为调控光束自旋分裂和光子自旋态的一个新的自由度。更多还原

【Abstract】 Polarization is an important property of light. It is always desirable to have full control of the polarization. In essence, manipulation of the polarization of light can be achieved by controlling the magnitude of its electric field components and their phase difference. Convention methods include anisotropic waveplates, polarizers, chiral media or gratings. Man-made microstructure is a kind of composite medium whose electromagnetic properties are depend on its structure rather than composition. It is composed of two or more kinds of media with its structure unit further less than a wavelength. It always has the properties further exceeded (or not possessed by) the conventional media, or has abnormal properties. One can obtain much larger anisotropy than convention anisotropic media via suitably tailoring the structure geometry of the man-made microstructure. Also, the desirable polarization or phase properties could be acquired by locally designing the structure units.The polarization will also affect another fundamental property of light, that is, coherence, so one can control the coherence of light by steering the polarization. High coherence is a striking property of lasers, which facilitate lasers to be widely applied in those fields requiring high coherent light sources. However, just as a coin has two sides, coherence is harmful to some laser application fields. High coherence usually results in unwanted speckle noise in laser display (or laser projection imaging or laser TV), thereby decreasing the imaging resolution. In laser fusion and laser heat processing, coherence makes the intensity distribution on the focal plane not uniform enough. In some spectroscopy experiments, a high coherence laser is also undesirable. Thus, in these applications, it is urgently needed to eliminate the laser coherence. On the other hand, as a hot topic in the recent years, the spin Hall effect light has attracted much attention. It is an analogy of the spin Hall effect in electronic system in which the spin photon corresponds to the spin electron and the refractive index gradient plays a role of external field. It manifests itself as the spin-dependent splitting of photon spin states. Therefore, in some way, manipulating the spin Hall effect of light is essentially control the polarization.Based on the above knowledge, we propose that man-made microstructures can be employed to manipulate the polarization and the spin Hall effect of light. Some creative work has done as in the follows. (i) We propose an inhomogeneous anisotropic microstructure for producing incoherent laser illumination. The structure can produce locally polarization change for the laser beam. In the far-field focal plane, the superposition beam is incoherent. Without loss of generality, we will exemplify our scheme numerically using a variation version of the typical L-shaped metamaterial, although the approach can be applied to an arbitrary metamaterial geometry offering enough number of free design parameters. The calculating results show that this structure can effectively suppress the speckle contrast and increase irradiation uniformity. The dimension of the structure can be confined to the order of a wavelength, which facilitates the metamaterial-base polarization control device to be potential applied in the future photonic integrated circuit.(ⅱ) We investigate the spin Hall effect of refractive light in two kinds of multi-layer films. The two films have symmetric and non-symmetric structure geometries, respectively, whose Fresnel coefficients can be tunable via changing the optical parameters of the films. We find that the spin-orbital interaction exhibits a sine-like oscillation in the range of negative-zero-positive valves due to the Fabry-perot resonance. Thus, we can significantly enhance or totally suppress the spin-dependent transverse shifts, and then the spin Hall effect of light. Further, we propose a one-dimensional photonic crystal with a defect layer to enhance the spin Hall effect of light. Under the condition of obliquely incidence, the defect modes have polarization-dependent transmission peaks (reflection valleys). Near the peaks, it is possible to acquire large ratio of the Fresnel coefficients, and thereby significantly enhancing the spin Hall effect of light to dozens of times of the ever observed values. At the same time, due to its close dependency on refractive index gradient, it is possible to develop the spin Hall effect as a precise metrology for investigating and describing the subwavelength variation of the structure geometries and refractive indices.(iii) We explore the unusual spin Hall effect of light in an anisotropic metamaterial. which manifests as non-symmetric spin-dependent splitting. In the previous researches, the observed splitting of left and right circular polarization components are all symmetric, that is, they reside on both sides of the incident plane with identical magnitudes. While we find that, due the non-symmetric geometrical phase induced by the strong anisotropy of the metamaterial. the non-symmetric splitting appears. It manifests as the same splitting direction and/or non-identical magnitudes. The asymmetry can be tunable via changing the optical parameters of the medium and the intersection angle between the incident plane and optics axis,(iv) Finally, we propose that the inhomogeneous anisotropic media with specified geometries can be used to manipulate the spin Hall effect of light. This medium can locally control the polarization of light, and apply a spin-dependent and space-variant geometrical phase to beam that passes through it. which cause the separation of the spin components of light. Interestingly, this spin-dependent splitting in the far field exhibits multi-lobe splitting patterns with alternatively left and right circular polarizations, described by the Stokes S3parameters. The lobe number depends upon the structure geometries of the media. So, this medium serves as a potential device to manipulate the spin-dependent splitting and photon spin states. Actually, the geometrical phase is not only associated with the medium properties, but also the incident polarization distribution. For different polarization angle of incident linear polarization, the splitting patterns will rotate. As far as we know, existing researches have concentrated their interests in the case of spatially homogeneous, linearly polarized incident light. We have investigated the case of axisymmetric linearly polarized light. Since the geometrical phase originates from the two kinds of contributions:medium property and polarization distribution, so the far-field splitting pattern can also be controlled by the incident polarization distribution. We believe that the incident polarization distribution servers as a new degree of freedom to manipulate the spin-dependent splitting of beam and photon spin states.更多还原