Khosravi Khorashad Larousse, Argyropoulos Christos
Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln 68588, NE, USA.
Nanophotonics. 2022 Apr 12;11(17):4037-4052. doi: 10.1515/nanoph-2022-0048. eCollection 2022 Sep.
Localized plasmons formed in ultrathin metallic nanogaps can lead to robust absorption of incident light. Plasmonic metasurfaces based on this effect can efficiently generate energetic charge carriers, also known as hot electrons, owing to their ability to squeeze and enhance electromagnetic fields in confined subwavelength spaces. However, it is very challenging to accurately identify and quantify the dynamics of hot carriers, mainly due to their ultrafast time decay. Their nonequilibrium temperature response is one of the key factors missing to understand the short time decay and overall transient tunable absorption performance of gap-plasmon metasurfaces. Here, we systematically study the temperature dynamics of hot electrons and their transition into thermal carriers at various timescales from femto to nanoseconds by using the two-temperature model. Additionally, the hot electron temperature and generation rate threshold values are investigated by using a hydrodynamic nonlocal model approach that is more accurate when ultrathin gaps are considered. The derived temperature dependent material properties are used to study the ultrafast transient nonlinear modification in the absorption spectrum before plasmon-induced lattice heating is established leading to efficient tunable nanophotonic absorber designs. We also examine the damage threshold of these plasmonic absorbers under various pulsed laser illuminations, an important quantity to derive the ultimate input intensity limits that can be used in various emerging nonlinear optics and other tunable nanophotonic applications. The presented results elucidate the role of hot electrons in the response of gap-plasmon metasurface absorbers which can be used to design more efficient photocatalysis, photovoltaics, and photodetection devices.
在超薄金属纳米间隙中形成的局域表面等离子体激元可导致对入射光的强烈吸收。基于这种效应的等离子体超表面能够在受限的亚波长空间中压缩和增强电磁场，从而有效地产生高能电荷载流子，即所谓的热电子。然而，准确识别和量化热载流子的动力学过程极具挑战性，主要原因在于它们的超快时间衰减特性。它们的非平衡温度响应是理解间隙等离子体超表面的短时间衰减和整体瞬态可调吸收性能所缺失的关键因素之一。在此，我们利用双温度模型系统地研究了从飞秒到纳秒等不同时间尺度下热电子的温度动力学及其向热载流子的转变。此外，我们还使用了一种流体动力学非局部模型方法来研究热电子温度和产生率阈值，在考虑超薄间隙时该方法更为精确。所推导的与温度相关的材料特性被用于研究在等离子体激元诱导的晶格加热建立之前吸收光谱中的超快瞬态非线性变化，从而实现高效可调谐纳米光子吸收器的设计。我们还研究了这些等离子体吸收器在各种脉冲激光照射下的损伤阈值，这是推导可用于各种新兴非线性光学和其他可调谐纳米光子应用的最终输入强度极限的一个重要量。所呈现的结果阐明了热电子在间隙等离子体超表面吸收器响应中的作用，这可用于设计更高效的光催化、光伏和光探测器件。
