Parametric Resonances in Nonlinear Plasmonics
Artist depiction of the parametric excitation of plasmonic modes of metallic nanospheres embedded in a nonlinear host medium.
Localized surface plasmon polaritons are characteristic modes that arise from the resonant interaction of photons with the free-charge oscillations in metallic particles. Plasmonic resonances have attracted great attention as a means to localize and enhance optical fields over subwavelength regions. Such unique properties have fostered the advent of the field of nonlinear plasmonics, relying on plasmonic field enhancement to boost otherwise weak nonlinear effects. One open challenge in nonlinear plasmonics is the selective optical excitation of high-order resonances, as those would offer higher quality factors and field confinement compared with the readily accessible dipolar modes. In addressing such issue, researchers at the University of Kansas (KU) have explored an uncharted path in nonlinear plasmonics: the parametric excitation and amplification of localized surface plasmon polaritons. In the article "Parametric Resonances in Nonlinear Plasmonics" published in Chinese Optics Letters, Vol 17, Issue 12, 2019 (Shima Fardad, Eric Schweisberger, Alessandro Salandrino. Parametric resonances in nonlinear plasmonics [Invited][J]. Chinese Optics Letters, 2019, 17(12): 122402), the Authors review the physical principles of plasmonic parametric resonance (PPR) and discuss potential applications of PPR.
In contrast with conventional localized plasmonic resonances, in which modes are excited directly by an external field of frequency and spatial profile matching those of a given mode of the plasmonic particle, PPR is a form of amplification in which a pump field transfers energy to a mode in an indirect way. In PPR in fact the modes of a plasmonic structure are amplified by means of a temporal modulation of the background permittivity caused by an appropriate pump field. Such permittivity variation translates into a modulation of the modal resonant frequency. Under specific pump conditions amplification can occur. Among the unique characteristics of PPR is the possibility of accessing modes of arbitrarily high order with a simple spatially uniform pump, provided that such pump exceeds a certain intensity threshold – a characteristic of all parametric resonances. The unique characteristics of PPR have prompted the author to introduce the concept of Plasmonic Parametric Absorbers (PPA). PPAs exhibit a reverse saturable absorption behavior whereby an incident field that is parametrically resonant with one or more of the modes of a plasmonic particle experiences a strongly enhanced absorption whenever its intensity exceeds the relevant PPR threshold. Such effect makes PPAs very promising candidates for optical limiting applications, in addition of being of fundamental interest in the emerging field of nonlinear plasmonics.
"We have barely scratched the surface of the promising science and applications that could emerge from parametric effects in nonlinear plasmonics," commented Eric Schweisberger, a graduate student working with Prof. Salandrino and Prof. Fardad at the University of Kansas. The University of Kansas team is currently exploring the PPR theory in the context of propagating surface plasmons and is working towards an experimental demonstration of optical limiting exploiting plasmonic parametric absorbers.
非线性等离子体中的参数共振
艺术图：嵌入在非线性介质中的金属纳米球的等离子体模式参数激励。
局域表面等离激元是光子与金属粒子中自由电荷振荡共振相互作用产生的特征模。等离子体共振作为一种在亚波长范围内定位和增强光场的手段，引起了人们的极大关注。这种独特的性质使得依靠等离子体场的增强来增强原本微弱的非线性效应成为可能，这促进了非线性等离子体电子学的出现。在非线性等离子体电子学中，高阶共振的选择性光激发是一个难点，因为与容易获得的偶极模相比，高阶共振将提供更高的质量因子和场约束。为了解决这个问题，堪萨斯大学（KU）的研究人员探索了非线性等离子体电子学中的一个全新领域：局域表面等离激元的参数激发和放大。在文中，作者回顾了等离子体参数共振（PPR）的物理原理，并讨论了PPR的潜在应用，相关成果发表于Chinese Optics Letters第17卷第12期上（Shima Fardad, Eric Schweisberger, Alessandro Salandrino. Parametric resonances in nonlinear plasmonics [Invited][J]. Chinese Optics Letters, 2019, 17(12): 122402）。
在局域等离子体共振中，模式外部场直接激发，该外部场的频率和空间分布与等离子体的给定模式相匹配。PPR与传统的局域等离子体共振不同，PPR是一种放大形式，其中的抽运光场以间接方式将能量转移到模式中。在PPR中，等离子体结构的模式实际上是通过适当的抽运场引起的背景介电常数的时间调制而放大的。这种介电常数的变化转化为模态共振频率的调制。在特定的条件下便会发生放大。PPR的独特特性之一是，只要这种抽运超过一定的强度阈值（这是所有参数共振的特征），就可以用一个简单的空间均匀的抽运获得任意高阶的模式。基于这样的特性，本文引入了等离子体参数吸收器（PPA）的概念。PPAs表现出一种反饱和吸收行为，即当入射场的强度超过相应的PPR阈值时，与等离子体的一个或多个模式参数共振的入射场将经历一次强增强吸收。这种效应使得PPAs在光限幅方面有着非常广阔的应用前景，同时在非线性等离子体电子学的新兴领域也有着重要的应用前景。
与Salandrino教授和Fardad教授合作的堪萨斯大学研究生Eric Schweisberger指出：“未来非线性等离子体中的参数效应会有力地推动科学和应用的发展，我们正在为此努力”。堪萨斯大学的研究小组目前正在研究传播表面等离子体的PPR理论，并致力于利用等离子体参数吸收体进行光限幅的实验演示。