All-optical switching in silicon photonic
waveguides with an epsilon-near-zero
resonant cavity [Invited]
ANDRES D. NEIRA,GREGORY A. WURTZ,* AND ANATOLY V. Z AYATS
Department of Physics, King’s College London, Strand, London WC2R 2LS, UK
*Corresponding author: g.wurtz@unf.edu
Received 5 September 2017; revised 19 December 2017; accepted 20 December 2017; posted 22 December 2017 (Doc. ID 305271);
published 27 March 2018
Strong nonlinearity of plasmonic metamaterials can be designed near their effective plasma frequency in the
epsilon-near-zero (ENZ) regime. We explore the realization of an all-optical modulator based on the Au non-
linearity using an ENZ cavity formed by a few Au nanorods inside a Si photonic waveguide. The resulting modu-
lator has robust performance with a modulation depth of about 30 dB/μm and loss less than 0.8 dB for switching
energies below 600 fJ. The modulator provides a double advantage of high mode transmission and strong non-
linearity enhancement in the few-nanorod-based design.
© 2018 Chinese Laser Press
OCIS codes: (240.6680) Surface plasmons; (160.3918) Metamaterials; (230.4320) Nonlinear optical devices; (190.4390) Nonlinear
optics, integrated optics.
https://doi.org/10.1364/PRJ.6.0000B1
1. INTRODUCTION
Photonic integrated circuits (PICs) provide passive and active
functionalities to contro l the flow of optical signals, such as
spectral filters, optical multiplexers, electro-optical modulators,
photodetectors, and interferometers [1– 6]. Silicon photonics
has proven to be an excellent platform for photonic integration
as silicon has a high refractive index (n ∼ 3.4) and low optical
losses at telecom wavelengths, while its nanofabrication is
compatible with CMOS industry standards, allowing for
direct compatibility with electronics [7–10]. Several electro-
optical modulators have been demonstrated on a Si platform
using resonant structures such as Mach–Zehnder interferome-
ters [9,10], waveguide-ring-resonator geometries [7], and a
hybrid Si-plasmonic platform [11,12] based on the application
of an electric field via electrical interconnections, thus limiting
the maximum modulation speed to a few tens of gigahertz.
All-optical modulation overcomes this barrier since it relies
on ultrafast nonlinear optical processes such as two-photon
absorption, free carrier heating, or the Franz–Keldysh effect,
achieving modulation speeds exceeding 100 Gbit/s [3–5,
12–16]. Nevertheless, nonlinear optical processes are generally
weak requiring a high energy per bit to be practically obser v-
able. Recently, surface plasmon excitations in metallic struc-
tures have been used to reduce the energy requirement of
nonlinear optical processes, since the electric field is enhanced
in plasmonic nanostructures at sub-wavelength scales, allowing
efficient light–matter interactions [17]. Several approaches have
already demonstrated the use of such an enhancement to
achieve nonlinear switching in various geometries including
plasmonic waveguides, particles, and nano-antennas, as well
as metamaterials [18–22]. In particular, hyperbolic plasmonic
metamaterials [23] have been shown to provide high-speed and
low-energy all-optical modulation in both free-space and inte-
grated geometries [24–28] owing to strong nonlinearity and
high sensitivity to refractive index changes. The metamaterial
waveguides also provide important opp ortunities for guided
mode dispersion engineering [28–31].
In hyperbolic metamaterials, the nonlinearity is particularly
enhanced in the so-called ε-near-zero (ENZ) reg ime near the
effective plasma frequency [27], where one component of the
effective permittivity tensor of the metamaterial is vanishingly
small [Reε ∼ 0]. Not only is the nonlinear optical response of
the metamaterial in this regime strong, but it can also be tuned
through the metamaterial’s geometrical design to the required
operating wavelength [25, 32].
In this context, all-optical modulators using the concept of
hyperbolic metamaterials based on plasmonic nanorods
integrated with silicon photonics have been proposed [
26].
In order to further reduce losses associated with the use of met-
als and subsequently reduce switching energy requirements and
modulation efficiency, the reduction of the number of metallic
meta-atoms [33] in a metamaterial modulator is desirable. This,
however, needs to be achieved while preserving the conditions
required for efficient modulation.
Research Article
Vol. 6, No. 5 / May 2018 / Photonics Research B1
2327-9125/18/0500B1-05 Journal © 2018 Chinese Laser Press