Jeong Jeeyoon, Kim Dai-Sik, Park Hyeong-Ryeol
Department of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Gangwon 24341, Republic of Korea.
Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea.
Nanophotonics. 2022 May 25;11(13):3159-3167. doi: 10.1515/nanoph-2022-0214. eCollection 2022 Jun.
Metallic nanogaps are being widely used for sensing applications, owing to their ability to confine and enhance electromagnetic field within the hot spots. Since the enhanced field does not confine itself perfectly within the gap, however, fringe fields well away from the gap are of potential use as well in real systems. Here, we extend the concept of near field absorption enhancement by quantitatively analyzing terahertz absorption behavior of water molecules outside the hot spots of sub-20 nm-wide, ∼100 μm-long nanotrenches. Contrary to point-gaps which show negligible field enhancement at distances larger than the gap width, our extended nanogap act as a line source, incorporating significant amount of absorption enhancement at much longer distances. We observe absorption enhancement factors of up to 3600 on top of a 5 nm-wide gap, and still well over 300 at 15 nm away. The finding is well supported by theoretical analyses including modal expansion calculations, Kirchhoff integral formalism and antenna theory. Our results provide means to quantitatively analyze light-matter interactions beyond the hot spot picture and enable application of nanogaps for sensitive surface analyses of various material systems.
金属纳米间隙因其能够在热点区域内限制和增强电磁场而被广泛应用于传感领域。然而，由于增强后的场并不能完美地局限于间隙内，因此在实际系统中，远离间隙的边缘场也具有潜在用途。在此，我们通过定量分析宽度小于20nm、长度约100μm的纳米沟槽热点区域外水分子的太赫兹吸收行为，扩展了近场吸收增强的概念。与点间隙在距离大于间隙宽度时场增强可忽略不计不同，我们扩展后的纳米间隙充当线源，在更长距离处仍具有显著的吸收增强。我们观察到在5nm宽的间隙上吸收增强因子高达3600，在15nm远处仍远超过300。包括模态展开计算、基尔霍夫积分形式和天线理论在内的理论分析很好地支持了这一发现。我们的结果提供了超越热点图景定量分析光与物质相互作用的方法，并使纳米间隙能够应用于各种材料系统的灵敏表面分析。
