Surface-enhanced Raman spectroscopy (SERS) and related spectroscopies are powered primarily by the concentration of the electromagnetic (EM) fields associated with light in or near appropriately nanostructured electrically-conducting materials, most prominently, but not exclusively high-conductivity metals such as silver and gold. This field concentration takes place on account of the excitation of surface-plasmon (SP) resonances in the nanostructured conductor. Optimizing nanostructures for SERS, therefore, implies optimizing the ability of plasmonic nanostructures to concentrate EM optical fields at locations where molecules of interest reside, and to enhance the radiation efficiency of the oscillating dipoles associated with these molecules and nanostructures. This review summarizes the development of theories over the past four decades pertinent to SERS, especially those contributing to our current understanding of SP-related SERS. Special emphasis is given to the salient strategies and theoretical approaches for optimizing nanostructures with hotspots as efficient EM near-field concentrating and far-field radiating substrates for SERS. A simple model is described in terms of which the upper limit of the SERS enhancement can be estimated. Several experimental strategies that may allow one to approach, or possibly exceed this limit, such as cascading the enhancement of the local and radiated EM field by the multiscale EM coupling of hierarchical structures, and generating hotspots by hybridizing an antenna mode with a plasmonic waveguide cavity mode, which would result in an increased local field enhancement, are discussed. Aiming to significantly broaden the application of SERS to other fields, and especially to material science, we consider hybrid structures of plasmonic nanostructures and other material phases and strategies for producing strong local EM fields at desired locations in such hybrid structures. In this vein, we consider some of the numerical strategies for simulating the optical properties and consequential SERS performance of particle-on-substrate systems that might guide the design of SERS-active systems. Finally, some current theoretical attempts are briefly discussed for unifying EM and non-EM contribution to SERS.

中文翻译：

---

表面增强拉曼光谱的电磁理论

表面增强拉曼光谱（SERS）和相关的光谱学主要由与光相关的电磁场（EM）集中在适当的纳米结构的导电材料中或附近提供能量，最主要但不是唯一的是高导电性金属（如银）和黄金。考虑到纳米结构导体中表面等离子体激元（SP）共振的激发而发生这种场集中。因此，优化用于SERS的纳米结构意味着优化等离子体等离子体纳米结构的能力，以将EM光场集中在目标分子所处的位置，并增强与这些分子和纳米结构相关的振荡偶极子的辐射效率。这篇综述总结了过去四个十年中与SERS相关的理论的发展，特别是那些有助于我们当前对与SP相关的SERS理解的理论。特别强调了用于优化具有热点的纳米结构的显着策略和理论方法，这些有效方法是用于SERS的高效EM近场集中和远场辐射基板。描述了一个简单的模型，据此可以估算SERS增强的上限。几种可能允许或接近该极限的实验策略，例如通过分层结构的多尺度EM耦合级联增强局部和辐射EM场，以及通过将天线模式与等离激元波导腔混合来产生热点模式，讨论了会导致局部场增强的增加。为了将SERS的应用广泛扩展到其他领域，尤其是材料科学，我们考虑了等离子体纳米结构和其他材料相的混合结构，以及在此类混合结构中所需位置产生强大的局部EM场的策略。在这种情况下，我们考虑了一些数值策略，用于模拟基体上颗粒系统的光学性质和相应的SERS性能，这些策略可能会指导SERS活性系统的设计。最后，简要讨论了一些当前的理论尝试，以统一EM和非EM对SERS的贡献。我们考虑了等离激元纳米结构和其他材料相的混合结构，以及在此类混合结构中所需位置产生强大的局部EM场的策略。在这种情况下，我们考虑了一些数值策略，用于模拟基体上颗粒系统的光学性质和相应的SERS性能，这些策略可能会指导SERS活性系统的设计。最后，简要讨论了一些当前的理论尝试，以统一EM和非EM对SERS的贡献。我们考虑了等离激元纳米结构和其他材料相的混合结构，以及在此类混合结构中所需位置产生强大的局部EM场的策略。在这种情况下，我们考虑了一些数值策略，用于模拟基体上颗粒系统的光学性质和相应的SERS性能，这些策略可能会指导SERS活性系统的设计。最后，简要讨论了一些当前的理论尝试，以统一EM和非EM对SERS的贡献。我们考虑了一些数值策略，用于模拟基体上颗粒系统的光学性质和相应的SERS性能，这些策略可能会指导SERS活性系统的设计。最后，简要讨论了一些当前的理论尝试，以统一EM和非EM对SERS的贡献。我们考虑了一些数值策略，用于模拟基体上颗粒系统的光学特性和相应的SERS性能，这些策略可能会指导SERS活性系统的设计。最后，简要讨论了一些当前的理论尝试，以统一EM和非EM对SERS的贡献。