[论文解读] Unraveling the temperature dynamics and hot electron generation in tunable gap-plasmon metasurface absorbers

Larousse Khosravi Khorashad, Christos Argyropoulos|arXiv (Cornell University)|Mar 29, 2022

Metamaterials and Metasurfaces Applications参考文献 82被引用 15

一句话总结

本研究利用双温模型（TTM）和流体非局域模型，研究了可调谐间隙等离子体超表面吸收体中的超快热电子动力学与热响应，以捕捉亚2 nm纳米间隙中电子的行为。结果表明，电子温度在880 fs内飙升至约880 K，显著改变银的介电函数，从而实现超快、可调谐吸收，这对设计高效光电探测器、光催化剂及非线性纳米光子器件至关重要。

ABSTRACT

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 non-equilibrium 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.

研究动机与目标

• 理解在超快激光激发下，间隙等离子体超表面中热电子的非平衡温度动力学。
• 量化从飞秒到纳秒时间尺度的热电子生成速率与温度演化过程。
• 利用双温模型模拟从非热载流子到热化载流子的过渡过程。
• 评估电子温度对材料性能的影响以及吸收光谱可调性的潜力。
• 确定在脉冲激光照射下，等离子体吸收体的损伤阈值，以支持非线性光学的实际应用。

提出的方法

• 采用双温模型（TTM）模拟从飞秒到纳秒时间尺度的电子与晶格温度动力学。
• 应用流体非局域模型，准确捕捉在非局域效应占主导地位的超薄（2 nm）金属纳米间隙中电子的行为。
• 使用COMSOL Multiphysics，结合电磁场与热力模块，求解具有温度依赖介电常数的频域麦克斯韦方程。
• 通过将电子温度与介电常数实部和虚部分量关联的方程，引入温度依赖的银介电函数。
• 施加全宽半高为130 fs的飞秒高斯激光脉冲（强度为5 GW/cm²），以驱动超快加热与吸收变化。
• 通过与体材料铁和金薄膜的解析基准结果对比，验证了模型的准确性。

实验结果

研究问题

• RQ1 在间隙等离子体超表面中，热电子温度从飞秒到纳秒时间尺度如何演化？
• RQ2 非局域效应在亚2 nm纳米间隙中增强电场与热电子生成的作用是什么？
• RQ3 瞬态电子温度如何改变银的复介电常数，从而影响吸收光谱？
• RQ4 在何种最大输入激光强度下，这些超表面会发生结构损伤？
• RQ5 超快温度上升在多大程度上实现了等离子体纳米光子器件中吸收的可调谐性？

主要发现

• 在5 GW/cm²激光激发下，银纳米条带中的电子温度在880 fs内达到约880 K，显著改变了材料响应。
• 流体非局域模型预测的电场增强与热电子生成速率高于局域模型，尤其在2 nm间隙中更为显著。
• 温度依赖的介电常数使银在共振波长（818 nm）处虚部增加了20%，导致吸收增强与光谱可调性提升。
• 由于电子温度动力学，吸收光谱表现出超快瞬态变化；在4 nm间隙结构中未观察到光谱位移，表明其对间隙厚度具有依赖性。
• 通过追踪晶格温度上升，估算出损伤阈值，预测在高强度脉冲照射下将发生熔化。
• 在纳米间隙处观察到空间分辨的介电常数变化，其最大变化发生在飞秒脉冲的峰值时刻。