【作者】 王琦；

【导师】 辛巍；

【作者基本信息】 东北师范大学 ， 凝聚态物理， 2024， 硕士

【摘要】 手性是指物体无法与其镜像重合的一种几何特性,普遍存在于自然界的各种物质之中,与人类生活息息相关。电磁波与手性物质相互作用时会发生电磁耦合现象,即手性光学特性,其具体表现形式包括非对称传输(Asymmetric Transmission,AT)和圆二色性(Circular Dichroism,CD)等。相关特性自发现以来就受到广泛关注,但由于天然手性材料的光学响应较弱,且通常仅在可见光波段被观测到,因此开发具有优异性能的手性光学器件就显得尤为重要。近些年,人工超表面在电磁波性能调控方面表现出巨大优势,其为上述问题的解决提供了有效方案。由于相关器件结构紧凑、制备过程与半导体加工工艺相兼容等特征,其在片上集成化应用中也展示出发展潜力。然而,人工超表面的性能通常受限于器件的结构单元设计。一旦结构单元被固定后,器件性能将无法改变,故无法满足其在复杂环境下的应用需求。鉴于此,具有动态可调性的人工超表面逐渐引起了大家的关注,人们迫切希望通过对器件性能的精确操控来进一步实现其在众多应用领域中突破。石墨烯具有优异的物理、化学性质,其表面电导率可随化学势的变化得到调控。该过程将伴随石墨烯电磁作用规律的变化,因此是构建动态可调人工超表面的有利候选材料。本论文基于石墨烯的可控特性,提出了两种分别在中红外和太赫兹波段实现非对称传输和圆二色性可调手性超表面。利用有限元数值模拟软件,对上述器件的手性光学特性及其背后的物理机理进行详细分析。其具体研究内容如下:(1)本文提出了一种由三个相互垂直的开口谐振环组成的镂空“L”形单层石墨烯手性超表面。在中红外波段,该超表面分别在8208 nm和8688 nm双波段处具有0.14和0.26的非对称传输值。对上述波长处不同圆偏振光入射下的感应电场和电流分布进行分析,发现具有映体敏感特性的石墨烯表面等离激元的手性选择性激发,为上述非对称传输现象的出现起到了关键作用。通过改变石墨烯化学势的数值大小,该超表面能够在保持非对称传输效率稳定的基础上,实现双响应波段的线性控制。通过计算不同化学势下石墨烯介电常数的实部与虚部相应数值,发现其实部数值的变化直接影响了石墨烯与圆偏振光的作用强度,是产生上述线性变化的物理本质。在此基础上,又设计了双层镂空“L”形石墨烯超表面。通过改变两层超表面结构的相对旋转角度,非对称传输特性实现了进一步提升,最大值为0.38。在目前已报道的石墨烯基超表面中处于先进水平。(2)通过对圆二色性对称性破缺理论进行分析,本文提出了可实现圆二色性超表面的简洁设计思路。设计了一个对称分布的石墨烯条带-矩形金开口环谐振器手性超表面,通过对石墨烯化学势的调控该超表面在8.45 THz和9.75 THz双频段处,分别实现了-0.73和0.65的强圆二色性。通过分析两个工作频率处电场和表面电流的分布,确定石墨烯的非对称表面等离激元的激发是产生圆二色性的物理机制。通过改变石墨烯化学势的大小,可对该超表面圆二色性值进行大范围、连续动态调控,该调控过程伴随着工作频率的蓝移。我们还展示了相关器件在近场成像方面的应用。此外,上述设计方案具有明显的普适性:在金属结构发生改变后,类似的光学圆二色性仍然可以在临近波段范围内被观察到,这也为相关器件的实际应用奠定了基础。更多还原

【Abstract】 Chirality is a geometric property in which an object cannot coincide with its mirror image,and is prevalent in all kinds of matter in nature and relevant to human life.Electromagnetic coupling phenomenon occurs when electromagnetic waves interact with chiral substances,i.e.chiral optical properties,whose specific manifestations include Asymmetric Transmission(AT)and Circular Dichroism(CD).The relevant properties have received much attention since their discovery,but since the optical response of natural chiral materials is weak and is usually observed only in the visible wavelength band,it is particularly important to develop chiral optical devices with excellent properties.In recent years,artificial metesurfaces have shown great advantages in electromagnetic wave performance modulation,and they provide an effective solution to the above problems.Due to the compact structure of the devices and the compatibility of the preparation process with semiconductor processing,they have also demonstrated development potential in on-chip integration applications.However,the performance of artificial metesurfaces is usually limited by the design of the structural unit of the device.Once the structural unit is fixed,the performance of the device cannot be changed,and therefore it cannot meet the needs of its application in complex environments.In view of this,artificial metasurfaces with dynamically tunable properties have gradually attracted attention,and there is an urgent desire to further realise their breakthroughs in numerous applications through precise manipulation of device properties.The chiral optical properties of the above devices and the physical mechanisms behind them are analysed in detail using finite element numerical simulation software.Graphene has excellent physical and chemical properties,and its surface conductivity can be tuned with the change of chemical potential.This process will be accompanied by changes in the electromagnetic interaction law of graphene,making it a favourable candidate for the construction of dynamically tunable artificial metasurfaces.In this thesis,based on the controllable properties of graphene,two chiral metasurfaces are proposed to achieve asymmetric transmission and circular dichroism tunable chiral supersurfaces in the midinfrared and terahertz bands,respectively.Its specific research is described below:(1)In this paper,a skeletonised "L"-shaped monolayer graphene chiral hypersurface consisting of three mutually perpendicular open resonance rings is proposed.In the midinfrared band,the metasurface has asymmetric transmission values of 0.14 and 0.26 at the dual bands of 8208 nm and 8688 nm,respectively.Analysis of the induced electric field and current distributions under the incidence of different circularly polarised light at the above wavelengths reveals that the chiral selective excitation of equipartitioned excitations on the graphene surface with enantiomer-sensitive properties plays a key role in the emergence of the above phenomenon of asymmetric transmission.By varying the numerical magnitude of the graphene chemical potential,the hypersurface is able to achieve linear control of the dual-response band while keeping the asymmetric transmission efficiency stable.By calculating the corresponding values of the real and imaginary parts of the dielectric constant of graphene at different chemical potentials,it is found that the change of its real value directly affects the intensity of graphene’s interaction with circularly polarised light,which is the physical nature that produces the above linear change.On this basis,a double-layer hollow "L"-shaped graphene-based metasurface was designed,and the asymmetric transmission properties were further improved by changing the relative rotation angle of the two layers.By changing the relative rotation angle of the twolayer structure,the asymmetric transmission property is further improved to a maximum value of 0.38,which is at the advanced level among the graphene-based metasurfaces reported so far.By analysing the theory of circular dichroism symmetry breaking,this paper proposes concise design ideas that can achieve circular dichroism metasurface.A symmetrically distributed graphene strip-rectangular gold open-ring resonator chiral metesurface is designed,and the metesurface achieves two strong circular dichroism of-0.73 and 0.65 at the dual frequency bands of 8.45 THz and 9.75 THz,respectively,through the modulation of graphene chemical potential.Physical mechanisms for the generation of circular dichroism by excitation of asymmetric surface plasmons excitations in graphene are determined by analysing the distribution of electric fields and surface currents at two operating frequencies.By varying the magnitude of the graphene chemical potential,the value of circular dichroism of this metasurface can be dynamically modulated over a wide range and continuously,and this modulation process is accompanied by a blue shift of the operating frequency.We also demonstrate the application of the relevant devices for near-field imaging.In addition,the above design scheme is clearly universal: similar optical circular dichroism can still be observed in the proximity band range after the metal structure has been changed,which lays the foundation for the practical application of the related devices.更多还原