The theory of space-time varying metamaterials (Ref. 4659)

时空变化超材料理论 (参考文献 4659)

• 批准号: 2859646
• 金额: --
• 依托单位: UNIVERSITY OF EXETER
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2859646
• 关键词: theory; space; time; varying; metamaterials

When we first learn about wave propagation we are taught about the refractive index, n. This is first simply a number; about 1.5 for glass, and a large complex number for metals. Next we learn the refractive index can depend on position. The spatial dependence of the refractive index leads to reflection, where the wave vector changes sign, one of the most everyday of wave phenomena. Metamaterial research is commonly concerned with designing sub-wavelength scale structures with an effective refractive index that can be varied across space in a controlled way.But what about a refractive index that varies in time? An example could be where n is the same throughout space, but at some moment in time its value changes. Again there is reflection, but unlike reflection from a spatial interface, the reflected wave occurs after it encounters the temporal 'interface', as required by causality [1]. In this case the frequency changes sign rather than the wave vector. Varying the refractive index in time we thus alter the frequency (e.g. colour, in the case of visible light) of the wave. If we change the refractive index in both space and time we then have the ability to change the wave in a way that would be otherwise impossible, modifying both its frequency and direction of propagation. Space-time varying material parameters have been shown to lead to extreme wave amplification without requiring gain [2], and laboratory analogues of astrophysical phenomena such as Hawking radiation [3].However, until recently it has been challenging to make materials where the parameters vary in time. Recent experiment in optics [4] and acoustics [5] have made time varying material parameters a reality. Yet these materials typically have a complicated effect on an incident wave, that is a far cry from the simplified picture described above. Not only is the refractive index not changed instantaneously, but it also depends on frequency. At present there is no agreed theoretical approach to the calculation of fields within these materials. This project will develop the theory of space time varying metamaterials, building on the theoretical approach derived in [6] where the material parameters are replaced with operators.In this project we will:(1) Investigate the physics of space time varying metamaterials, considering a variety of experimental platforms, from acoustics, to optics, and radio frequency materials.(2) Further develop the theory in [6], treating non-planar materials, non-electromagnetic waves, and more exotic material parameters, e.g. anisotropy or bianisotropy. Attempt to develop analytical solutions to these operator equations that have so far been treated only numerically.(3) Apply the theory to understand how the effects reported in e.g. [1,2] change when moving from theoretical idealizations to more realistic experimental platforms.(4) Use the theory to explore new and unforeseen wave phenomena in space-time varying metamaterials.References:[1] E. Galiffi, R. Tirole, S. Yin, H. Li, S. Vezzoli, P. A. Huidobro, M. G. Silveirinha, R. Sapienza, A. Alu, and J. B. Pendry "Photonics of Time-Varying Media" Adv. Phot. 4, 014002 (2022).[2] J. B. Pendry, E. Galiffi, and P. A. Huidobro, "Gain mechanism in time-dependent media", Optica 8, 636-637 (2021).[3] R. Anguero-Santacruz and D. Bermudez, "Hawking radiation in optics and beyond", Phil. Trans. Roy. Soc. A 378, https://doi.org/10.1098/rsta.2019.0223 (2020).[4] J. Bohn, T. S. Luk, C. Tollerton, S. W. Hutchings, I. Brener, S. A. R. Horsley, W. L. Barnes, and E. Hendry, "All-optical switching of an epsilon-near-zero plasmon resonance in indium tin oxide", Nat Commun. 15 1017 (2021).[5] C. Cho, X. Wen, N. Park, et al. "Digitally virtualized atoms for acoustic metamaterials" Nat. Commun. 11, 251 (2020).[6] S. A. R. Horsley, E. Galiffi, and Y. T. Wang, "Eigenpulses of dispersive time-varying media" arXiv:2208.11778 (2022).

当我们第一次学习波的传播时，我们学到了折射率n，这首先是一个简单的数字；玻璃为1.5左右，金属为大的复数。接下来我们学习折射率可以依赖于位置。折射率的空间依赖性导致反射，其中波矢量改变符号，这是最常见的波现象之一。超材料研究通常关注的是设计具有有效折射率的亚波长尺度结构，该结构可以在可控的方式下跨空间变化。但是随时间变化的折射率呢？例如，n在整个空间中是相同的，但在某个时刻它的值发生了变化。同样也有反射，但与空间界面的反射不同，反射波是在遇到时间“界面”后发生的，这是因果关系[1]所要求的。在这种情况下，频率改变的是符号而不是波矢量。随时间改变折射率，我们就改变了波的频率（如可见光的颜色）。如果我们同时改变空间和时间上的折射率，那么我们就有能力以一种不可能的方式改变波，改变它的频率和传播方向。时空变化的物质参数已被证明可以在不需要增益b[2]的情况下导致极端的波放大，以及实验室类似的天体物理现象，如霍金辐射[3]。然而，直到最近，制造参数随时间变化的材料一直具有挑战性。最近在光学[4]和声学[5]上的实验使时变材料参数成为现实。然而，这些材料通常对入射波有复杂的影响，这与上面描述的简化图像相去甚远。折射率不仅不会瞬间改变，而且还取决于频率。目前还没有统一的理论方法来计算这些材料内部的场。本项目将发展时空变化的超材料理论，建立在[6]中导出的理论方法的基础上，其中材料参数被算子取代。在这个项目中，我们将：(1)研究时空变化的超材料的物理学，考虑到各种实验平台，从声学、光学到射频材料。(2)进一步发展[6]理论，处理非平面材料、非电磁波和更多的奇异材料参数，如各向异性或双各向异性。尝试开发这些算子方程的解析解，这些方程到目前为止只被数值处理过。(3)应用该理论来理解例[1,2]中报道的效应在从理论理想化到更现实的实验平台时是如何变化的。(4)利用该理论探索时空变化的超材料中新的不可预见的波动现象。参考文献：[1]E. Galiffi， R. Tirole， S. Yin， H. Li， S. Vezzoli， P. A. Huidobro， M. G. Silveirinha， R. Sapienza， A. Alu， J. B. Pendry，“时变介质的光子学”，光子学报，40 (4):2023 - 2023李建军，李建军，李建军，“时间依赖介质的增益机制”，光学学报，36 (6):636-637 (2013). bbb10R. angero - santacruz和D. Bermudez，“光学中的霍金辐射”，Phil。反式。罗伊。Soc。A 378, https://doi.org/10.1098/rsta.2019.0223 (2020).[4]J. Bohn， T. S. Luk， C. Tollerton， S. W. Hutchings， I. Brener， S. A. R. Horsley， W. L. Barnes， E. Hendry，“氧化铟锡中epsilon-近零等离子体共振的全光开关”，物理学报，15 (10)(2021). b[5]赵志强，文新，朴宁，等。“声学超材料的数字虚拟原子”，声学学报，11,251 (2020). b[6]王玉涛，王玉涛，王玉涛，“色散时变介质的本征脉冲”，物理学报，39(4):1177 - 1177（2022）。