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Light-matter interaction in low-dimensional materials. A theoretical study

机译：低维材料中的光物质相互作用。理论研究

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In this thesis we have investigated the optical properties of layered and two-dimensional materials for application in the field of plasmonics and metamaterials using Density Functional Theory (DFT). Both of these fields, if successful in their goals, promise new technologies for small scale photonics beyond the diffraction limit. A technological breakthrough of such a caliber would have far reaching consequences such as enabling a practical interface to nano scale integrated electronic circuits or enable the construction of novel devices like a superlens with resolution well beyond diffraction limit. However, the progress of these fields is currently inhibited by large losses that can only be resolved through the discovery of new materials. Using linear response time-dependent DFT we calculate the optical properties of several experimentally known layered transition metal dichalcogenides (TMDs) with the chemical formula of MX2 where M is a transition metal and X is a chalcogen atom (S, Se, Te). The TMDs constitute an interesting class of materials due to their diverse range of properties including both metals and semi-conductors. We find that the TMDs with group 5 transition metal atoms in the H monolayer exhibit a special bandstructure in which metallic bands are separated from other valence and conduction bands by finite energy gaps which has the potential to minimize the optical losses by reducing the density of states for scattering. The size of the energy gaps are, however, not sufficiently large to completely eliminate optical losses. We therefore propose a new class of layered materials with the chemical formula 2H-MXY where M is a group 4 transition metal atom, X is a chalcogen atom, and Y is a halogen atom (Cl, Br, I) which increases the size of the energy gaps and significantly reduces optical losses. This entails improved plasmonic normalized propagation lengths and superior lifetimes compared to the best plasmonic material, namely, silver.We show that all of the TMDs are natural hyperbolic materials, which means that they exhibit a strongly anisotropic dielectric response reflected by a sign-difference in their dielectric tensor resulting in hyperbolic isofrequency contours. Hyperbolic metamaterials obtain their anisotropic response from an artificial sub-wavelength structuring and are limited in their performance by the period of the structuring - the smaller, the better. In contrast, natural hyperbolic materials suffer no such limitation due to their lack of artificial structuring, and indeed, we find a much greater performance for all applications of hyperbolic materials.The possibility of stacking individual two-dimensional materials into so-called van der Waals heterostructures is perhaps one of the most interesting technological developments in the field of two-dimensional materials. We show that effective medium theory for the dielectric properties of graphene and hexagonal boron nitride heterostructures, which treat the heterostructure as a continuous medium, break down for atomically thin layers due to quantum mechanical effects but also for thick components due to multiple reflection effects. We propose an extended version of effective medium theory to account for the interface layers and show that the effective medium description is improved. The determination of the quality of materials for application within plasmonics and metamaterials requires an accurate calculation of the optical properties of materials which can be computationally demanding. To reduce the computational costs, the linear tetrahedron method and the employment of symmetries have been implemented which in some cases can reduce the computational costs by a factor of 200.
机译：在本文中，我们使用密度泛函理论（DFT）研究了层状和二维材料在等离子和超材料领域的光学特性。如果这两个领域的目标取得成功，它们都有望为超越衍射极限的小规模光子学提供新技术。这种口径的技术突破将产生深远的影响，例如实现与纳米级集成电子电路的实用接口，或者使诸如超透镜之类的新型器件的构造远远超过衍射极限。但是，这些领域的进展目前受到巨大损失的限制，这些损失只能通过发现新材料来解决。使用线性响应随时间变化的DFT，我们计算了几种化学上为MX2的化学上已知的层状过渡金属二硫代双金属卤化物（TMD）的光学性质，其中M为过渡金属，X为硫族原子（S，Se，Te）。由于TMD具有包括金属和半导体在内的多种特性，因此构成了一类有趣的材料。我们发现，在H单层中具有第5组过渡金属原子的TMD表现出特殊的能带结构，其中金属带通过有限的能隙与其他价带和导带分隔开，这有可能通过降低态密度来最小化光损耗用于散射。然而，能隙的大小不足以完全消除光学损失。因此，我们提出了一种新的化学式为2H-MXY的层状材料，其中M为第4组过渡金属原子，X为硫属元素原子，Y为卤素原子（Cl，Br，I），增加了分子的尺寸。能隙，并大大减少了光损耗。与最佳等离子材料银相比，这需要改进的等离子归一化传播长度和更长的寿命。我们证明，所有TMD都是天然的双曲线材料，这意味着它们表现出强烈的各向异性介电响应，这反映在硅的符号差异上。它们的介电张量导致双曲等值线轮廓。双曲线超材料是通过人工亚波长结构获得其各向异性响应的，并且其性能受到结构周期的限制-越小越好。相比之下，天然双曲线材料由于缺乏人工结构而不受此类限制，实际上，我们发现双曲线材料的所有应用都具有更高的性能。将单个二维材料堆叠到所谓的范德华斯中的可能性异质结构也许是二维材料领域中最有趣的技术发展之一。我们表明，石墨烯和六方氮化硼异质结构的介电特性的有效介质理论将异质结构视为连续介质，由于量子力学效应而分解为原子薄层，但由于多次反射效应而分解为厚组分。我们提出了有效介质理论的扩展版本来说明界面层，并表明有效介质描述得到了改进。确定用于等离子和超材料的材料的质量需要对材料的光学特性进行准确的计算，这可能需要进行计算。为了降低计算成本，已实施了线性四面体方法并采用了对称性，在某些情况下，可以将计算成本降低200倍。

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