Awad Ehab
Electrical Engineering Department, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia.
Nanomaterials (Basel). 2022 Jun 21;12(13):2131. doi: 10.3390/nano12132131.
Graphene is well-known for its extraordinary physical properties such as broadband optical absorption, high electron mobility, and electrical conductivity. All of these make it an excellent candidate for several infrared applications such as photodetection, optical modulation, and optical sensing. However, a standalone monolayer graphene still suffers from a weak infrared absorption, which is ≅2.3%. In this work, a novel configuration of graphene metamaterial embedded inside Bundt optical-antenna (optenna) is demonstrated. It can leverage the graphene absorption up to 57.7% over an ultra-wide wavelength range from 1.26 to 1.68 µm (i.e., Bandwidth ≅ 420 nm). This range covers the entire optical communication bands of O, E, S, C, L, and U. The configuration mainly consists of a Bundt-shaped plasmonic antenna with a graphene metamaterial stack embedded within its nano-wide waveguide that has a 1.5 µm length. The gold average plasmonic loss is ≅25%. This configuration can enhance graphene ultra-broadband absorption through multiple mechanisms. It can nano-focus the infrared radiation down to a 50 nm spot on the graphene metamaterial, thus yielding an 11.5 gain in optical intensity (i.e., 10.6 dB). The metamaterial itself has seven concentric cylindrical graphene layers separated by silicon dioxide thin films, thus each layer contributes to the overall absorption. The focused infrared propagates tangential to the graphene metamaterial layers (i.e., grazing propagation), and thus maximizes the light-graphene interaction length. In addition, each graphene layer experiences a double-face exposure to the nano-focused propagating spot, which increases each layer's absorption. This configuration is compact and polarization-insensitive. The estimated maximum absorption enhancement compared to the standalone monolayer graphene was 25.1 times (i.e., ≅4 dB). The estimated maximum absorption coefficient of the graphene stack was 5700 cm, which is considered as one of the record-high reported coefficients up to date.
石墨烯以其非凡的物理特性而闻名，如宽带光吸收、高电子迁移率和电导率。所有这些特性使其成为多种红外应用的理想候选材料，如光电探测、光调制和光学传感。然而，单独的单层石墨烯仍存在红外吸收较弱的问题，吸收率约为2.3%。在这项工作中，展示了一种嵌入邦特光学天线（光天线）内部的新型石墨烯超材料结构。在1.26至1.68微米的超宽波长范围内（即带宽约为420纳米），它能将石墨烯的吸收率提高到57.7%。这个范围覆盖了O、E、S、C、L和U的整个光通信波段。该结构主要由一个邦特形状的等离子体天线组成，其纳米宽的波导内嵌入了石墨烯超材料堆栈，波导长度为1.5微米。金的平均等离子体损耗约为25%。这种结构可以通过多种机制增强石墨烯的超宽带吸收。它可以将红外辐射纳米聚焦到石墨烯超材料上一个50纳米的光斑上，从而使光强度提高11.5倍（即10.6分贝）。超材料本身有七个由二氧化硅薄膜隔开的同心圆柱形石墨烯层，因此每层都对整体吸收有贡献。聚焦的红外光沿与石墨烯超材料层相切的方向传播（即掠入射传播），从而使光与石墨烯的相互作用长度最大化。此外，每个石墨烯层都双面暴露于纳米聚焦的传播光斑，这增加了每层的吸收。这种结构紧凑且对偏振不敏感。与单独的单层石墨烯相比，估计的最大吸收增强倍数为25.1倍（即约4分贝）。石墨烯堆栈的估计最大吸收系数为5700厘米，这被认为是迄今为止报道的最高记录系数之一。
