ZTE Communications

S pecial Topic
Design of a Silicon-Based High-Speed Plasmonic Modulator
Mu Xu, Jiayang Wu, Tao Wang, and Yikai Su
Design of a Silicon-Based
High-Speed Plasmonic Modulator
Mu Mu Xu, Xu, Jiayang Jiayang Wu, Wu, Tao Tao Wang, Wang, and and Yikai Yikai Su Su
(State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiaotong University, Shanghai 200240, P.R.China)
Abstract
In this paper, we propose a silicon-based high-speed plasmonic modulator. The modulator has a double-layer structure with a 16 μm long
metal-dielectric-metal plasmonic waveguide at the upper layer and two silicon single-mode waveguides at the bottom layer. The
upper-layer plasmonic waveguide acts as a phase shifter and has a dielectric slot that is 30 nm wide. Two taper structures that have
gradually varied widths are introduced at the bottom layer to convert the photonic mode into plasmonic-slot mode with improved coupling
efficiency. For a modulator with two 1 μm-long mode couplers, simulation shows that there is an insertion loss of less than 11 dB and a
half-wave voltage of 3.65 V. The modulation bandwidth of the proposed modulator can be more than 100 GHz without the carrier effect
being a limiting factor in silicon. The fabrication process is also discussed, and the proposed design is shown to be feasible with a hybrid of
CMOS and polymer technology.
Keywords
plasmonic phase modulator; gradually varied taper; high speed
A1 Introduction
modulator is a critically important device in
optical communication systems and
short-reach electro-optical interconnects.
Modulation speed continues to improve and is
now well above 100 GHz, which rivals the
processing speed of state-of-the-art CMOS electronic
devices. The silicon optical modulators described in this
paper are capable of having a modulation bandwidth of more
than 40 GHz [1], depending on whether carrier accumulation
[2], carrier injection [3], or carrier depletion [4] is used.
To enable carrier accumulation in silicon, a metal-oxide
semiconductor (MOS) structure is preferred. In such a
structure, the metal acts as the electrode, and the oxide acts
as the barrier. When voltage is applied, free carriers
accumulate at the interface between the oxide and the
semiconductor. In 2004, A. Liu et al. fabricated the first
silicon-based carrier accumulation modulator [2]. Although
the device only had 1 GHz bandwidth, it paved the way for
future silicon modulators.
Carrier injection can be achieved by using a
forward-biased silicon P-I-N diode, and depletion can be
achieved by using a reverse-biased silicon P-N diode. In
2007, Q. Xu et al. fabricated a silicon-ring-based
carrier-injection modulator with a bandwidth of more than
12.5 GHz [3]. In 2008, S. J. Spector et al. increased the
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ZTE COMMUNICATIONS
March 2012 Vol.10 No.1
bandwidth of a silicon-based carrier-injection modulator to
more than 26 GHz, even reaching up to 40 GHz [1]. In 2010,
Po Dong et al. used silicon carrier-depletion modulators
integrated on 0.25 μm long silicon-on-insulator (SOI)
waveguides with VπL of 1.4 V-cm to show that low energy
consumption in a silicon modulator is feasible [4].
One of the limitations of silicon is two-photon absorption
(TPA), which makes it hard to further increase modulation
bandwidth [5]. To overcome this limitation, a silicon-organic
hybrid (SOH) approach has been suggested. By combining
silicon waveguides with highly nonlinear electro-optical (EO)
organic materials or high-absorption multilayer III-V
Quantum well materials, the modulation speed can be
increased to more than 30 GHz [6],[7]. In 2011, L. Alloatti et
al. developed an SOH-platform-based electro-optic
modulator that operated at 42.7 Gb/s and had a VπL of about
9 V-mm [6].
Although the response speed of x (2) nonlinearity in organic
material can be at least 165 GHz [8], the speed of the
practical modulator is still limited because of the weak
confinement, which causes a considerable proportion of light
to leak into the transparent silicon electrodes. Recent
progress in plasmonic silicon photonics has greatly improved
confinement in the waveguides, and this is promising for the
design of future ultracompact photonic devices. However,
although confinement is improved, optical loss increases
because of Ohmic loss in the metal and scattering at the grain