基于手性超表面的太赫兹波偏振转换与圆二色性研究

【作者】 张鑫；

【导师】 高亚臣；

【作者基本信息】 黑龙江大学 ， 物理电子学， 2025， 硕士

【摘要】 太赫兹（Terahertz,THz）波是一种频率介于微波和红外波之间的电磁辐射（0.1-10 THz）,广泛应用于波谱、成像及通信等多个领域。由于THz波的独特特性,自然界中的大多数材料在该频段缺乏显著的电磁响应。随着超材料和超表面技术的发展,这一局限性得到了有效突破。超材料和超表面能够与入射的THz波发生共振耦合,从而实现自然界材料无法呈现的独特电磁响应。通过合理设计,这些人工微结构能够灵活调控THz波的关键特性,如振幅、相位、偏振、传播方向以及共振特性等。手性超表面作为超表面的一种特殊形式,通过引入手性结构,能够充分调控THz波的偏振态,且在圆偏振波的调控上具有独特的优势。借助手性超表面的独特特性,可以实现对THz波偏振转换与圆二色性（Circular Dichroism,CD）功能的调控。目前,该领域在功能集成度、频谱响应范围以及品质因数（Quality Factor,Q）等方面仍存在提升空间。针对这些问题,本文相对应提出了三种新型手性超表面,并采用有限元数值仿真方法及相关物理机制对其性能进行了系统的分析与讨论。具体而言,本文的工作内容如下:（1）由于目前大多数手性超表面功能单一、不可调谐,难以满足器件小型化和多功能集成的发展需求。为此,本文提出了一种基于石墨烯手性超表面的可调谐多功能偏振转换器。通过调节石墨烯费米能级（E_f）,该手性超表面实现了x偏振转y偏振的线偏振转换,线偏振转圆偏振转换和圆偏振转换（Circular Polarization Conversion,CPC）诱导的单频CD功能的可切换和可调谐。此外,基于CPC诱导的单频CD响应特性,本文还提出并仿真验证了一种能够检测三种代表性禽流感病毒的传感策略。（2）由于现有手性超表面研究大多集中在CPC诱导的单频CD响应,为了进一步拓展频谱响应范围并增强其实用性,本文提出了一种可产生CPC诱导的双频CD响应及波前整形应用的石墨烯手性超表面。该手性超表面在特定频率点能够实现可调谐的CPC诱导的双频CD响应,并能实现互补近场成像应用。此外,利用Pancharatnam-Berry相位原理,通过调节石墨烯E_f,能够实现对异常反射,涡旋光束以及聚焦波前整形应用的动态调控。（3）由于在THz波段中手性超表面实现的CD响应大多为二维（Two Dimension,2D）手性,且Q值相对较低,这在手性发射和偏振波检测等领域的应用中具有一定的局限性。为了解决这些问题,本文提出了一种全介质手性超表面。首先,在保持超表面镜面对称性条件下分析了相对应连续谱束缚态（Bound States In The Continuum,BIC）的特性,并通过远场偏振图深入探讨了其独特的拓扑性质。随后,通过打破全部的镜面对称性,构建了全介质手性超表面,从而实现了三维（Three Dimension,3D）内禀手性BIC。基于光学手性密度的分析,进一步探讨了其手性响应的物理机制,并结合多级子分解理论揭示了高Q值共振的产生。最后,研究了不同圆偏振波激发下的三次谐波（Third Harmonic Generation,THG）转换效率。结果表明,设计的3D内禀手性BIC能够实现超高的非线性CD_THG效应。初步验证表明,这种全介质手性超表面在非线性手性发射领域具有一定的应用潜力。更多还原

【Abstract】 Terahertz（THz）waves are electromagnetic radiation with frequencies between microwaves and infrared waves（0.1-10 THz）.They are widely used in spectroscopy,imaging,and communication,among other fields.Due to the unique characteristics of THz waves,most natural materials exhibit negligible electromagnetic response in this frequency range.With the development of metamaterials and metasurfaces,this limitation has been effectively overcome.Metamaterials and metasurfaces can resonate and couple with incident THz waves,enabling unique electromagnetic responses that are not exhibited by natural materials.Through careful design,these artificial microstructures can flexibly manipulate key properties of THz waves,such as amplitude,phase,polarization,propagation direction,and resonant characteristics.Chiral metasurfaces,as a special class of metasurfaces,incorporate chiral structures to effectively control the polarization state of THz waves and offer distinctive advantages in manipulating circularly polarized waves.By leveraging the unique properties of chiral metasurfaces,it is possible to control polarization conversion and circular dichroism（CD）functionalities of THz waves.Currently,there is still room for improvement in terms of functional integration,spectral response range,and quality factor（Q）in this field.To address these issues,this paper proposes three novel chiral metasurfaces and systematically analyzes and discusses their performance using finite element numerical simulation methods and relevant physical mechanisms.Specifically,the content of this paper is as follows:（1）Currently,most chiral metasurfaces have single functionalities,are non-tunable,and cannot meet the development requirements of miniaturization and multifunctional integration of devices.To address this,this paper proposes a tunable multifunctional polarization converter based on a graphene chiral metasurface.By adjusting the Fermi level（E_f）of graphene,the chiral metasurface achieves switchable and tunable functions,including linear polarization conversion from x-polarized to y-polarized,linear polarization to circular polarization conversion,and circular polarization conversion（CPC）-induced single-frequency CD functionality.In addition,based on the CPC-induced single-frequency CD response,this paper also proposes and simulates a sensing strategy capable of detecting three representative avian influenza viruses.（2）As most existing research on chiral metasurfaces focuses on CPC-induced single-frequency CD responses,in order to further extend the spectral response range and enhance their practicality,this paper proposes a graphene chiral metasurface capable of generating CPC-induced dual-frequency CD responses and wavefront shaping applications.This chiral metasurface can achieve a tunable CPC-induced dual-frequency CD response at specific frequency points and enable complementary near-field imaging applications.Furthermore,by utilizing the Pancharatnam-Berry phase principle and adjusting the graphene E_f,dynamic control of anomalous reflection,vortex beams,and focused wavefront shaping applications can be realized.（3）In the THz frequency range,the CD responses achieved by chiral metasurfaces are mostly two-dimensional（2D）chiral,with relatively low Q factors,which limits their application in areas such as chiral emission and polarization wave detection.To address these issues,this paper proposes a dielectric chiral metasurface.First,the characteristics of the corresponding bound states in the continuum（BIC）are analyzed while maintaining the mirror symmetry condition of the metasurface,and its unique topological properties are further explored through far-field polarization diagrams.Subsequently,by breaking all mirror symmetries,a dielectric chiral metasurface is constructed,achieving three-dimensional（3D）intrinsic chiral BIC.Based on the analysis of optical chiral density,the physical mechanism of its chiral response is further investigated,and the generation of high Q-factor resonances is revealed using multi-level subdecomposition theory.Finally,the third harmonic generation（THG）conversion efficiency under different circularly polarized wave excitations is studied.The results show that the designed 3D intrinsic chiral BIC can achieve an ultra-high nonlinear CD_THG effect.Preliminary validation indicates that this dielectric chiral metasurface has certain application potential in the field of nonlinear chiral emission.更多还原