IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 22, NOVEMBER 15, 2015 2395
High Confinement Factor Ridge Slot
Waveguide for Optical Sensing
Huang Zengzhi, Yong Zhang, Cheng Zeng, Danping Li, Muhammad Shemyal Nisar,
Jinzhong Yu, and Jinsong Xia
Abstract—A ridge slot waveguide on 220 nm silicon-on-
insulator platform is proposed and theoretically investigated. The
confinement factor of the ridge slot waveguide is much higher
than the fully etched slot waveguide in the case of air-cladding.
The sensitivity of sensors based on ridge slot waveguides is higher
than that of the fully etched slot waveguides when the cladding
refractive index is in the range of 1.0–1.18.
Index Terms—Semiconductor waveguides, silicon devices,
optical sensors.
I. INTRODUCTION
A
SLOT waveguide is formed by two ridges of high index
material, sandwiching by a narrow trench, which is
void or filled with a lower-index material. Slot waveguides,
proposed in 2004 [1], [2], have been intensively studied.
A variety of optical devices have been proposed using the
slot waveguide structures, such as directional couplers [3], [4],
microring resonators [5], [6], beam splitters [7] and polar-
ization converters [8]. For a vertical slot waveguide, the
fundamental TE optical mode is enhanced in the gap region,
making slot waveguide a suitable configuration to realize light
and matter interaction on silicon-on-insulator platform.
Air-cladding slot waveguides are commonly adopted in
optical sensing, in which the refractive index change of the
cladding medium will cause a shift to the optical resonant
peak of the microcavities [9]–[11]. In order to achieve high
sensitivity, the proportion of the modes distributed in the
cladding medium should be as large as possible, thus, the
confinement factor of the waveguide should be high. However,
the confinement factor of the fully-etched slot waveguide is
relatively low, especially when the top silicon layer is thin.
Manuscript received May 21, 2015; revised July 16, 2015; accepted
August 7, 2015. Date of publication August 11, 2015; date of current version
September 30, 2015. This work was supported in part by the Major State Basic
Research Development Program of China under Grant 2013CB632104 and
Grant 2013CB933303, and in part by the National Natural Science Foundation
of China under Grant 61177049 and Grant 61335002. (Corresponding author:
Jinsong Xia.)
H. Zengzhi, Y. Zhang, and D. Li are with the Wuhan National
Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan 430074, China, and also with the School of Optical
and Electronic Engineering, Huazhong University of Science and
Technology, Wuhan 430074, China (e-mail: huangzengzhi@hust.edu.cn;
yonglea@126.com; lidanpinghust@qq.com).
C. Zeng, M. S. Nisar, J. Yu, and J. Xia are with the Wuhan National
Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan 430074, China (e-mail: zengchengwuli@yeah.net;
mshemyalnisar@gmail.com; zyu@semi.ac.cn; jinsongxia@gmail.com).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2015.2466595
Fig. 1. Ridge slot waveguide.
In this article we propose a ridge slot waveguide on 220 nm
SOI platform with the slot not fully etched to the buried SiO
2
(as shown in Fig.1.). This ridge slot waveguide maintains high
confinement factor when the refractive index of the cladding
varies from 1.0 to 1.46, which is preferable for on-chip
gas sensing. The ridge slot waveguide is also beneficial for
nonlinear optic application, such as electro-optic modulation
and generating Kerr effect, since confinement factor is also an
important parameter in these applications.
The article will be organized like this. First, the optical
modes of the ridge slot waveguide and fully-etched slot
waveguide are simulated. A detailed analysis and comparison
shows that ridge slot waveguide has better ability to confine
light mode in the slot and cover region than the fully-
etched slot waveguide. Second, the confinement factor of the
ridge slot waveguide is calculated, so is the case of fully-
etched slot waveguide. Its application in homogenous gas
sensing is also analyzed in this part. In the third part, we talk
about the limitations in formation of the ridge slot waveguide,
especially the concern that how deep the waveguide should be
etched to maintain high confinement factor. Parameters sweeps
are performed to verify this prediction. Finally, we believe
that ridge slot waveguide can be useful in many other on-chip
applications as it improves confinement of light and minimizes
the whole device and thus the integrated package.
II. M
ODAL INVESTIGATION AND CALCULATION
We use commercial finite element method software
(Comsol Multiphysics 4.2a) to calculate the optical energy
distribution in the slot waveguides. The top silicon layer
thickness h = 220 nm, ridge width W = 200 nm, slot width,
g = 100 nm and the thickness of the silicon planar layer,
s = 20 nm.
Fig. 2.(a) shows the field distribution of the major com-
ponent Ex(x, y) for the quasi-TE fundamental mode of the
ridge slot waveguide with air cladding. Fig. 2.(b) and Fig. 2.(c)
are Ex(x, 0) and Ex(0, y), respectively. From the Fig. 2.(b),
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