量子光学概念在强耦合纳米光子学中的应用
On the applicability of quantum-optical concepts in strong-coupling nanophotonics
摘要：
植根于量子光学并受益于其良好的基础，近年来纳米光子学中的强耦合越来越受欢迎。由于纳米光子学是一个实验驱动的领域，缺乏适当的理论方法来描述突破性的进展往往成为一个重要的问题。为了解决这个问题，直接转移和扩展量子光学已有概念的诱惑是强烈的，即使严格的理由并不总是可用的。在本审查报告中，我们讨论了我们认为这一战略确实超越其界限的情况。我们专注于激子-等离子体相互作用，特别是在计算耦合中所涉及的激子的数量的想法。我们分析，从一个毫无根据的解释出现在不同层次的复杂性的理论描述中的术语N/V，一个可能会受到诱惑，使独立的假设数N和体积V是什么，并试图分别计算它们。这样的方法可以导致不同的、通常矛盾的结果，这取决于初始假设（例如，通过将$V$作为等离子体中模糊的模式体积的不同处理）。我们认为，这种矛盾的根源是问题本身-有多少激子耦合？，其忽视了系统的耦合组件的真实性质，没有意义，并且通常甚至没有任何实际重要性。如果人们对验证系统的量子性质感兴趣--这似乎是追求小N的强耦合的动机--人们可以转而关注光子发射率或二阶关联函数等量。
英文摘要：
Rooted in quantum optics and benefiting from its well-established foundations, strong coupling in nanophotonics has experienced increasing popularity in recent years. With nanophotonics being an experiment-driven field, the absence of appropriate theoretical methods to describe ground-breaking advances has often emerged as an important issue. To address this problem, the temptation to directly transfer and extend concepts already available from quantum optics is strong, even if a rigorous justification is not always available. In this Review we discuss situations where, in our view, this strategy has indeed overstepped its bounds. We focus on exciton--plasmon interactions, and particularly on the idea of calculating the number of excitons involved in the coupling. We analyse how, starting from an unfounded interpretation of the term N/V that appears in theoretical descriptions at different levels of complexity, one might be tempted to make independent assumptions for what the number N and the volume V are, and attempt to calculate them separately. Such an approach can lead to different, often contradictory results, depending on the initial assumptions (e.g. through different treatments of $V$ as the -- ambiguous in plasmonics -- mode volume). We argue that the source of such contradictions is the question itself -- How many excitons are coupled?, which disregards the true nature of the coupled components of the system, has no meaning and often not even any practical importance. If one is interested in validating the quantum nature of the system -- which appears to be the motivation driving the pursuit of strong coupling with small N -- one could instead focus on quantities such as the photon emission rate or the second-order correlation function.
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