Gap induced mode evolution under the
asymmetric structure in plasmonic resonator
system
YONG-PAN GAO,
1
TIE-JUN WANG,
1
CONG CAO,
2
AND CHUAN WANG
1,
*
1
State Key Laboratory of Information Photonics and Optical Communications and School of Science, Beijing University of Posts and
Telecommunications, Beijing 100876, China
2
School of Ethnic Minority Education, Beijing University of Posts and Telecommunications, Beijing 100876, China
*Corresponding author: wangchuan@bupt.edu.cn
Received 25 October 2016; revised 20 January 2017; accepted 23 January 2017; posted 24 January 2017 (Doc. ID 278412);
published 28 February 2017
The modulation of resonance features in microcavities is important to applications in nanophotonics. Based on
the asymmetric whispering-gallery modes (WGMs) in a plasmonic resonator, we theoretically studied the mode
evolution in an asymme tric WGM plasmonic system. Exploiting the gap or nano-scatter in the plasmonic ring
cavity, the symmetry of the system will be broken and the standing wave in the cavity will be tunable. Based
on this asymmetric structure, the output coupling rate between the two cavity modes can also be tuned.
Moreover, the proposed method could further be applied for sensing and detecting the position of defects in
a WGM system.
© 2017 Chinese Laser Press
OCIS codes: (140.3945) Microcavities; (280.4788) Optical sensing and sensors; (140.4780) Optical resonators.
https://doi.org/10.1364/PRJ.5.000113
1. INTRODUCTION
Surface plasmon polaritons (SPPs) are the collective oscillations
of electrons on metal-dielectric interfaces [1,2]. During the
past decades, SPPs have been widely studied due to their
strong electromagnetic (EM) fields and sub-wavelength fea-
tures. In practical application, SPPs can be used as sensors
for chemical and biological analysis [3,4]. The whispering-
gallery modes (WGMs) are waves such as light and sound
that travel around a concave surface. Whispering-gallery waves
were first explained by Rayleigh [5]. With the characteristics
of high-quality factor and small-mode value, the application
and fabrication of optical WGMs turn into a hot point
[6–16]. The study of optical WGMs can be traced back to
the early part of the 20th century [6,7]. Because the quality
factor of the optical whispering-gallery cavity could reach as
high as 10
8
Q-factor [8–10 ], it could be widely applied in
modern optics, such as particle sensing [11,12], frequency
comb generation [13], and optomechanically induced trans-
parency [14,15]. As a special form of EM waves, much like
the optical modes of a microcavity, a broad continuum of
SPP modes can be observed in WGM modes. Recently, the
high-Q SPP whispering-gallery microcavity was also experi-
mentally achieved [17].
Due to its high quality factor, optical WGM resonators
are extensively studied in sensing technology. In 2007, the
experimental schemes of label-free single molecule detection
were demonstrated [18,19]. In the field of biomedical sensing,
detection on a single virus level was experimentally achieved
based on discrete changes in the resonance frequency/wavelength
of a WGM [20–23]. More recently, based on elastic Rayleigh
scattering, effective sensing on nanoparticles was experimentally
realized at the nanometer level [12,24]. Meanwhile, sensing
based on sub-wavelength plasmonic structures has been widely
studied, such as plasmonic-enhanced WGM sensing [25], plas-
monic chain ring resonators [26], microring plasmonic cavities
[4], and the WGMs of SPPs [27].
In this paper, we theoretically study the gap induced mode
evolution by analyzing the field dynamics of the asymmetric
WGM microresonator in a metal–insulator–metal (MIM)
structure [28–30]. This method can be applied in position
detection and spectrum modulation. Practically, when the
ring-type plasmonic WGM resonator is imperfectly fabricated,
Rayleigh scattering occurs due to the presence of the defect
or the gap. We find the electric field distribution will change
according to the position change of the defect or the gap, so the
position of the defect could be detected.
This paper is organized in four sections. In Section 2,we
present the model and the analytical expressions of the sym-
metric reduced plasmonic WGM system. In Section 3 ,we
numerically analyzed the electric field distribution and solved
Research Article
Vol. 5, No. 2 / April 2017 / Photonics Research 113
2327-9125/17/020113-06 Journal © 2017 Chinese Laser Press
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