非对称MIM波导的双Fano共振与高灵敏度传感器

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本文主要探讨了一种由非对称金属-绝缘体-金属(Metal-Insulator-Metal, MIM)波导结构引发的新型双Fano共振现象。作者Bing-Hua Zhang、Ling-Ling Wang、Hong-Ju Li、Xiang Zhai和Sheng-Xuan Xia来自湖南大学物理与电子科学学院，他们使用有限差分时域(Finite-Difference Time-Domain, FDTD)数值模拟方法对这种结构进行了深入研究。非对称MIM波导结构的设计包括一个肩部耦合的矩形腔，其不对称性导致了在腔体内支持的新离散模式。这种新出现的离散模式与原有的连续谱态之间产生了强烈的干涉效应，从而形成了双Fano共振的独特特性。双Fano共振的特点在于，它包含了两种不同的配置——对称和反对称配置，这两种配置下的共振行为具有显著的不同。耦合模理论(Coupled-Mode Theory, CMT)进一步证实了这种双Fano共振的起源，它揭示了腔体模式与波导之间的不同耦合条件是形成两个Fano轮廓的关键因素。通过这种方式，作者不仅观察到了共振现象，还探讨了该结构的传感性能。令人印象深刻的是，研究者发现这种非对称MIM波导结构能够展现出极高的灵敏度，双Fano共振的响应达到了57，这意味着它在传感应用中具有潜在的优势。此外，FOM（ figure of merit，衡量传感器性能的重要指标）的高值表明，这种结构具有优良的信噪比和分辨率，这对于诸如光子学、生物传感或纳米技术等领域有着重要的实际价值。这篇研究论文深入剖析了非对称MIM波导结构如何驱动双Fano共振，并展示了其在传感领域的潜力，为未来设计和优化纳米光子学器件提供了新的理论依据和技术方向。

Two kinds of double Fano resonances

induced by an asymmetric MIM waveguide

structure

Bing-Hua Zhang, Ling-Ling Wang, Hong-Ju Li, Xiang Zhai and

Sheng-Xuan Xia

School of Physics and Electronic, Hunan University, Changsha 410082, People’s Republic of China

E-mail: kele1110@hnu.edu.cn

Received 22 January 2016

Accepted for publication 2 February 2016

Published 5 April 2016

Abstract

Asymmetric plasmonic waveguides with a shoulder-coupled rectangle cavity are proposed and

investigated numerically by using the ﬁnite-difference time-domain (FDTD) method. The

symmetry breaking of the structure results in a new discrete mode supported by the cavity. The

extreme interference between two discrete states and an intrinsic wide continuous state gives rise

to novel double Fano resonances with symmetric and anti-symmetric conﬁgurations. Coupled-

mode theory (CMT) further conﬁrms that two Fano proﬁles originate from the different coupling

conditions of the cavity modes with the waveguides. Moreover, the sensing characters are

performed. The Fano responses with the higher sensitivity and ﬁgure of merit (FOM) up to 57

are realized. Undoubtedly, the studied structure will play an important role in the nano-integrated

plasmonic devices for optical switching and sensing.

Keywords: plasmonics, Fano resonance, sensor, asymmetry

(Some ﬁgures may appear in colour only in the online journal)

1. Introduction

Fano resonance initially realized in an atomic system origi-

nates from the quantum interaction between a discrete state

and a continuous state [1]. A Fano-based sharp and asym-

metric line proﬁle has been achieved in classical systems

including metamaterials [2–7] and plasmonic structures [8].

The strong sensitivity of Fano resonance to local media brings

about a high ﬁgure of merit (FOM), which promises extensive

applications in optical devices such as sensors [ 9 –11], optical

switches [12 , 13], modulators [14–16], and demultiplexers

[17]. Surface plasmon polaritons (SPPs), the electromagnetic

waves trapped on metal-dielectric interfaces and coupled to

propagating free electron oscillations in metals, are con-

sidered as a promising method for realizing integrated optical

circuits due to unique properties of overcoming optical dif-

fraction limit and controlling light at the nanoscale. SPPs

supported by the metal-insulator-metal (MIM) waveguide are

widely investigated and applied due to the deep sub-wave-

length optical conﬁnements and low band loss. A great

number of MIM waveguide structures with various resonant

cavities hence have been proposed theoretically and numeri-

cally to realize Fano resonance in the past decades [

19–23].

The multiple Fano resonances are also realized in the MIM

waveguide structures through the coupling between resonant

cavities and the interference between the multiple modes in a

single cavity [20, 21]. Previous works focus on changing the

parameters of the structure to vary the Fano resonance peaks

but seldom pay attention to the symmetry of the Fano proﬁles

and how the asymmetric degree of the structure impacts the

sensing property.

In this paper, two kinds of double Fano resonances are

numerically realized in a compact asymmetric plasmonic

system, which consists of a rectangle insulator cavity with

two half-inﬁnite long MIM waveguides connecting on the

two sides of it. By altering independently the vertical posi-

tions of two waveguides to breaking the structure symmetry,

both symmetric and anti-symmetric waveguide modes can be

excited. Due to the interaction of the symmetric and anti-

symmetric waveguide modes, the transmission spectrum

Journal of Optics

J. Opt. 18 (2016) 065001 (7pp ) doi:10.1088/2040-8978/18/6/065001

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