Ultra-compact on-chip slot Bragg grating
structure for small electric field detection
WENTAO QIU,
1
HUIHUI LU,
1,3
FADI ISSAM BAIDA,
2,
* AND MARIA-PILAR BERNAL
2
1
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Department of Optoelectronic Engineering,
Jinan University, Guangzhou, China
2
Département d’Optique P.M. Duffieux, Institut FEMTO-ST, UMR 6174 CNRS Université Bourgogne Franche-Comté, 15B Avenue des
Montboucons, 25030 Besançon Cedex, France
3
e-mail: thuihuilu@jnu.edu.cn
*Corresponding author: fadi.baida@femto‑st.fr
Received 9 January 2017; revised 21 March 2017; accepted 21 March 2017; posted 22 March 2017 (Doc. ID 282945); published 18 April 2017
In this paper, we present an ultra-compact 1D photonic crystal (PhC) Bragg grating design on a thin film lithium
niobate slot waveguide (SWG) via 2D- and 3D-FDTD simulations. 2D-FDTD simulations are employed to tune
the photonic bandgap (PBG) size, PBG center, cavity resonance wavelength, and the whole size of PhC. 3D-
FDTD simulations are carried out to model the real structure by varying different geometrical parameters such
as SWG height and PhC size. A moderate resonance quality factor Q of about 300 is achieved with a PhC size of
only 0.5 μm ×0.7 μm ×6 μm. The proposed slot Bragg grating structure is then exploited as an electric field
(E-field) sensor. The sensitivity is analyzed by 3D-FDTD simulations with a minimum detectable E-field as
small as 23 mV∕m. The possible fabrication process of the proposed structure is also discussed. The compact
size of the proposed slot Bragg grating structure may have applications in on-chip E-field sensing, optical
filtering, etc.
© 2017 Chinese Laser Press
OCIS codes: (130.3730) Lithium niobate; (130.6010) Sensors; (130.3120) Integrated optics devices.
https://doi.org/10.1364/PRJ.5.000212
1. INTRODUCTION
Highly sensitive electric field (E-field) sensing has an increasing
demand in many areas ranging from automotive, avionic, mili-
tary to EEG or ECG signal detection [1]. Oftentimes, the
conventional E-field sensor is made of CMOS-compatible
materials such as silicon, where a large quantity of metallic
materials is used for the design of electrodes. On the one hand,
the employed material, silicon, lacks the linear electro-optic
(EO) effect. Therefore, the mechanism exploited to detect
the E-field is a free-carrier-induced refractive index variation,
which brings limits in high-frequency and high-sensitivity
E-field sensing applications. On the other hand, the common
structure employed in the conventional E-field sensor is typi-
cally based on Mach–Zehnder interferometric configurations.
In order to obtain high sensitivity, a length of several milli-
meters to centimeters is needed; thus, it contains a large quan-
tity of metal that acts as antennas, which will bring distortions
to the E-field to be detected. As for the choice of material, lith-
ium niobate (LN) is suitable for developing a photonic E-field
sensor due to its large EO coefficient [2]. Thanks to the com-
mercially available thin film lithium niobate (TFLN) [3], ultra-
compact nano-waveguiding of different configurations on LN
can be possible [4]. Among them, a slot waveguide (SWG)
provides high light enhancement; therefore, it is a promising
waveguide (WG) configuration for developing highly sensitive
E-field sensors.
Since the first report of the SWG by Lipson [5], world-wide
efforts have been made in investigating SWG for various ap-
plications, notably in nonlinear effect enhancement [6,7]
and highly sensitive sensors [8,9]. Most often, SWG is
combined with active materials such as EO polymers or with
photonic crystals (PhCs) in order to obtain the desired func-
tionalities. For example, using frequency domain OCT,
Caër et al. experimentally demonstrated that slotted PhC
WG (PWG) can focus light to an effective area down to
0.02 μm
2
, which is about tenfold smaller than the diffraction
limit [6]. The Kerr/TPA balances in both standard PCW and
polymer-filled slot PCW are investigated in which a free-carrier
penalty on the nonlinear performance tends to decrease with
the increase of the slowing-down factor in slotted PCW [7].
Due to the nanometric size, SWG oftentimes combines with
1D PhC Bragg gratings to design active optical components. In
[8], a gold film slot Bragg grating on a silicon WG is proposed
as a refractive index sensor with a sensitivity of 730 nm∕RIU.
Wang et al. experimentally demonstrated a phased-shifted
Bragg grating with a quality factor up to 3 × 10
4
fabricated
212
Vol. 5, No. 3 / June 2017 / Photonics Research
Research Article
2327-9125/17/030212-07 Journal © 2017 Chinese Laser Press