302 CHINESE OPTICS LETTERS / Vol. 7, No. 4 / April 10, 2009
Metal-dielectric-metal plasmonic waveguide devices for
manipulating light at the nanoscale
Invi ted Paper
Georgios Veronis
1∗
, Zongfu Yu
2
, S¸¨ukr¨u Ekin Kocaba¸s
2
, David A. B. Miller
2
,
Mark L. Brongersma
3
, and Shanhui Fan
2
1
Department of Electrical and Computer Engineering and Center for Computation
and Technology, Louisiana State University, Baton Rouge, LA 70803, USA
2
Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA
3
Geballe Laboratory of Advanced Materials, Stanford University, Stanford, CA 94305, USA
∗
E-mail: gveronis@lsu.edu
Received December 23, 2008
We review some of the recent advances in the development of subwavelength plasmonic devices for ma-
nipulating light at the nanoscale, drawing examples from our own work in metal-dielectric-metal (MDM)
plasmonic waveguide devices. We introduce bends, splitters, and mod e converters for MDM waveguides
with no additional loss. We also demonstrate that optical gain provides a mechanism for on/off switch-
ing in MDM plasmonic waveguides. Highly efficient compact couplers between dielectric waveguides and
MDM waveguides are also introduced.
OCIS codes: 130.2790, 240.6680, 260.2110.
doi: 10.3788/COL20090704.0302.
1. Introduction
Light-guiding structures which allow subwavelength
confinement of the optical mode ar e important for
achieving compact integrated photonic devices
[1−7]
. The
minimum confinement of a guided optical mode in di-
electric waveguides is set by the diffraction limit and
is o f the order of λ
0
/n, where λ
0
is the wavelength in
free space and n is the refractive index. As opposed to
dielectric waveguides, plasmonic waveguides have shown
the p otential to guide subwavelength optical modes , the
so-called surface plasmon pola ritons, at metal-dielectric
interfaces.
Several different plasmonic waveguiding structures
have bee n proposed, such as metallic nanowires
[2,3]
and
metallic nanoparticle arrays
[4,5]
. Most of these s truc-
tures support a highly-confined mode only near the
surface plasmon fr equency. In this regime, the optical
mode typically has low group velocity and short propa-
gation length. It has been shown however that a metal-
dielectric-metal (MDM) structure with a dielectric region
thickness of ∼100 nm supports a propag ating mo de with
a nanoscale modal size at a waveleng th range extend-
ing from zero-frequency (DC) to visible
[8]
. Thus, such a
waveguide could be potentially important in providing
an interface betwe en conventional optics and subwave-
length electronic and optoelectronic devices. Because
of the predicted attractive properties of MDM waveg-
uides, their modal structure has been studied in great
detail
[8−12]
, and people have also started to explore such
structures experimentally
[13−15]
. Recent resear ch work
has therefore focused on the development of functional
plasmonic devices, including active devices, for nanoscale
plasmonic integrated circuits.
Here we provide a review of some of our own rec e nt re-
search activities aiming to advance the state of the art of
plasmonics through the introduction of novel MDM plas-
monic waveguide dev ic e s for manipulating light at the
nanoscale
[16−19]
. We first briefly review the simulation
method used in our studies, then introduce bends, split-
ters, and mode converters for MDM waveguides with no
additional loss. We also demonstrate that optical gain
provides a mechanism for on/off switching in MDM
plasmonic waveguides. Finally, we introduce highly
efficient compact couplers betwe en dielectric waveguides
and MDM waveguides.
2. Simulation method
We study the pro perties of MDM plasmonic waveguide
devices using a two-dimensional (2D) finite-difference
frequency-domain (FDFD) method
[20,21]
. This method
allows us to directly use experimental data for the
frequency-dependent dielectric constant of metals such
as silver
[22]
, including both the real and imaginary parts,
with no further approximation. Perfectly matched layer
(PML) absorbing boundary conditions are used at all
boundaries of the simulation domain
[23]
.
Due to the rapid field variation at the metal-dielectric
interfaces, a very fine grid re solution of ∼1 nm is re-
quired at the metal-dielectric interfaces to adequately
resolve the local fields. On the other hand, a grid res-
olution of ∼ λ/20 is s ufficient in other regions of the
simulation domain. For example, the required grid size
in air a t λ
0
= 1.5 5 µm is ∼7 7.5 nm, which is almost
two orders of magnitude larger than the required gr id
size at the metal-dielectric interfaces. We therefore use a
nonuniform orthogonal grid
[24]
to avoid an unnecessary
computational cost. We fo und that by using such a grid
our results are accurate to ∼0.05 %.
3. Bends and splitters
In this section, we investigate the performance o f
bends and power splitters in MDM plasmo nic waveg-
1671-7694/2009/040302-07
c
2009 Chinese Optics Letters
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