Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides
Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides
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DOI：
10.1103/physrevb.99.075413
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发表时间：
2019-02-11
影响因子：
3.7
通讯作者：
Argyropoulos, Christos
中科院分区：
文献类型：
--
作者：
Li, Ying;Argyropoulos, Christos
开放（非厄米）有源光子系统中异常点（EPs）和光谱奇点的有趣物理现象最近引起了研究界越来越大的兴趣。这些谱简并已经在非对称主动和被动光子组态中得到，但它们在对称主动等离子体结构中的证明仍然是难以实现的。在本文中，我们提出了一种纳米级有源等离子体波导系统，该系统由一系列周期性狭缝组成，这些狭缝可以表现出特殊的点和光谱奇点，从而导致几种功能。所提出的对称有源系统在其截止波长附近工作，并表现为有效的epsilon-near-zero （ENZ）介质。我们展示了以非常低的增益系数值访问EP的形成，这是所提出的纳米级对称等离子体配置的独特特征。在与所提出的有源等离子体波导阵列有效ENZ共振波长重合的极电位处，实现了ENZ的无反射传输和完美的损耗补偿。当我们进一步增加加载在狭缝中的介质材料的增益系数时，ENZ共振处出现谱奇点，导致正反向的超散射（激光）响应。这些有趣的效应是由具有非常小的实际值增益系数的材料实现的，并且在亚波长尺度上，由于ENZ共振的有源狭缝内强而均匀的场增强导致光-物质相互作用增强。我们利用传输线模型从理论上分析了得到的极电位和发散谱奇异性，并研究了二次入射波的加入和所提出的有源ENZ等离子体系统响应的非线性。我们的发现为有趣的纳米光子应用提供了一条途径，如无反射主动ENZ介质，单向相干完美吸收器，纳米激光器，强光双稳性和全光开关纳米器件。
The intriguing physics of exceptional points (EPs) and spectral singularities in open (non-Hermitian) active photonic systems has recently sparked increased interest among the research community. These spectral degeneracies have been obtained in asymmetric active and passive photonic configurations but their demonstration with symmetric active plasmonic structures still remains elusive. In this paper, we present a nanoscale active plasmonic waveguide system consisting of an array of periodic slits that can exhibit exceptional points and spectral singularities leading to several functionalities. The proposed symmetric active system operates near its cutoff wavelength and behaves as an effective epsilon-near-zero (ENZ) medium. We demonstrate the formation of an EP that is accessed with very low gain coefficient values, a unique feature of the proposed nanoscale symmetric plasmonic configuration. Reflectionless ENZ transmission and perfect loss compensation are realized at the EP which coincides with the effective ENZ resonance wavelength of the proposed array of active plasmonic waveguides. When we further increase the gain coefficient of the dielectric material loaded in the slits, a spectral singularity occurs at the ENZ resonance leading to superscattering (lasing) response at both forward and backward directions. These interesting effects are achieved by materials characterized by very small gain coefficients with practical values and at subwavelength scales due to the strong and homogeneous field enhancement inside the active slits at the ENZ resonance leading to enhanced light-matter interaction. We theoretically analyze the obtained EP, as well as the divergent spectral singularity, using a transmission line model, and investigate the addition of a second incident wave and nonlinearities in the response of the proposed active ENZ plasmonic system. Our findings provide a route towards interesting nanophotonic applications, such as reflectionless active ENZ media, unidirectional coherent perfect absorbers, nanolasers, and strong optical bistability and all-optical switching nanodevices.