◆
Transmission Enhancement in an Array
of Subwavelength Slits in Aluminum
Due to Surface Plasmon Resonances
Ho Bun Chan, Zsolt Marcet, Dustin Carr, John Eric Bower,
Ray Cirelli, Ed Ferry, Fred P. Klemens, John F. Miner,
Chien-Shing Pai, and J. Ashley Taylor
The coupling of light to surface plasmons through periodic subwavelength
metallic structures could strongly modify the optical properties of a metal
film. We demonstrate that the optical transmission through an array of
subwavelength slits is as high as 80% at resonance, even though the width
of each slit is almost 10 times smaller than the wavelength and the slits
occupy only 25% of the area of the metal. Numerical calculations suggest
that the field intensity is strongly enhanced near the metal surface. The
field enhancement could be used for generating nonlinear optical effects
and for high sensitivity detection of nanomechanical displacement.
© 2005 Lucent Technologies Inc.
Introduction
Surface Plasmons on a Metallic Surface
Advances in fabrication technologies have made it
possible to create artificial structures with features
smaller than the wavelength of light. The interaction
of light with matter at the subwavelength scale has led
to new optical devices and continues to generate fascinating science. For example photonic crystals [13]
utilize periodic variations in the dielectric function at
the scale of wavelength to manipulate light. Another
example involves near-field optical microscopy where
light is channeled through subwavelength apertures
to overcome the diffraction limit in optical microscopy
[6]. Here, we focus on surface plasmon subwavelength optical devices [3] in which the surface of
metal is artificially structured to tailor the properties of
surface plasmons and their interaction with light. We
will discuss basics of surface plasmons on a metal surface and describe the transmission properties of
nanostructured metallic films that we fabricated.
Surface plasmons are collective excitations of the
electron density guided by a metal-dielectric interface
[23]. They are solutions to Maxwell’s equations in
which the charge oscillations are localized at the metal
boundary (see Figure 1). The charge oscillation takes
place in the longitudinal (x) direction parallel to the
wavevector, as shown in Figure 1a. The charge density fluctuation is associated with an electromagnetic
field that is maximum at the surface and decays exponentially away from it. On a smooth metal surface, the dispersion relation for surface plasmons is
given by
k⫽
e1e2 1兾2
v
b ,
a
c e1 ⫹ e2
(1)
where e1 and e2 are the dielectric functions of the
metal and the dielectric, respectively. Figure 1b is a
Bell Labs Technical Journal 10(3), 143–150 (2005) © 2005 Lucent Technologies Inc. Published by Wiley Periodicals,
Inc. Published online in Wiley InterScience (www.interscience.wiley.com). • DOI: 10.1002/bltj.20109
sketch of the dispersion relation of surface plasmons.
For a smooth metal surface, light does not couple to
surface plasmons because surface plasmons have a
longer wavevector than light waves of the same frequency propagating along the surface [24]. However,
the presence of a periodic structure on the metal surface can make up for this momentum difference and
enable the direct coupling of light to surface plasmons.
Surface plasmons have been a subject of intense research due to their potential applications in a variety
of disciplines, including miniature photonic circuits
[14, 17], biochemical sensing [11], microscopy, and
high density optical data storage [26].
Surface plasmonic devices typically involve the
excitation of surface plasmons by light on a metal surface. The surface plasmons are then guided to specific
locations by structures that are artificially fabricated
on the metal surface. At the final step, the surface
plasmons are converted back into light. Therefore, instead of controlling light directly, these optical devices
operate by manipulation of surface plasmons. By
varying the wavelength, incident angle, and polarization of the incident light, surface plasmons can be
launched in controllable directions [9]. A number
of different structures, such as waveguides [4, 7],
reflectors [4], nanoparticle chains [16], nanoshells
[22], and hole arrays [8], have been created for
manipulating surface plasmons. One remarkable
kz
Panel 1. Abbreviations, Acronyms, and Terms
1D—One-dimensional
NJNC—New Jersey Nanotechnology
Consortium
RCWA—Rigorous coupled wave analysis
TM—Transverse magnetic
property of surface plasmons is their ability to channel
light and concentrate the optical energy in subwavelength structures, resulting in dramatic enhancement
of the local electromagnetic field. For instance, in surface-enhanced Raman spectroscopy [20], the roughness of the metal surface enables incident light to
excite surface plasmon resonance at certain wavelengths, leading to enormous enhancement of the
Raman cross section for molecules adsorbed on the
surface.
The discovery of extraordinarily large transmission through subwavelength hole arrays in optically
thick metal films by Ebbesen and coworkers [8] has
sparked additional interests in the field of surface plasmons. Even at wavelengths as large as ten times the
diameter of an individual hole, transmission efficiencies were observed to be orders of magnitudes larger
than predictions from standard aperture theory.
In some cases, the total transmission exceeds the
␻
Dielectric ␧2
ckx
E
kx
⫺⫺⫺ ⫹⫹⫹
⫺⫺⫺
⫹⫹⫹ ⫺⫺⫺
SP
Metal ␧1
kx
(a)
(b)
(a) Electromagnetic field and charge of surface plasmons propagating
along a metal-dielectric interface in the x direction.
(b) Dispersion relation of surface plasmons on a smooth metal surface.
Figure 1.
Surface plasmons propagating along a metal-dielectric interface and the corresponding dispersion relations.
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Bell Labs Technical Journal
fractional area of the holes. While surface plasmons
are believed to play a crucial role, the origin of
the large enhancement in transmission remains a
subject of intense research. In a simple picture,
light is coupled to surface plasmons through the periodic structure at the incident surface. The surface
plasmons are Bragg scattered by the array of holes to
set up localized surface plasmons modes in the vicinity of the holes. The evanescent field of the surface
plasmon modes tunnels through the holes to the other
side of the metal film and re-radiates into light of
the same frequency and momentum as the incident
wave. Since the pioneering experiment of Ebbesen
and coworkers on subwavelength hole arrays, a number of other geometries, including 1D slit arrays [2, 5,
12, 21], annular apertures [1], and a single hole or slit
surrounded by periodic corrugations [15], have been
shown by various researchers to possess extraordinary
transmission and beaming characteristics.
Transmission Properties of Fabricated
Nanostructured Metallic Films
Using the fabrication facilities at the New Jersey
Nanotechnology Consortium (NJNC), we have fabricated nanostructured aluminum layers that couple to
surface plasmons. Post-processing of the structures
and measurement of the optical properties are performed at University of Florida. The structures consist
of a 1D array of slits in an aluminum layer that is
completely surrounded by silicon oxide. The silicon
oxide and metallic layers are initially fabricated on a
silicon substrate and subsequently transferred to a
transparent quartz substrate, as we will describe in
the next paragraph. Starting with a blank silicon
wafer, a layer of silicon oxide is deposited by chemical vapor deposition, followed by a layer of aluminum
deposited by sputtering. The aluminum layer is patterned by deep ultraviolet or electron beam lithography and etched to create the slits. Another layer of
silicon oxide is then deposited. This layer of silicon
oxide fills the trenches and ensures that the aluminum is completely surrounded by silicon oxide.
The silicon oxide layer is then planarized by chemical
mechanical polishing to eliminate any topography resulting from the underlying aluminum slits. The metal
layer is therefore completely surrounded by silicon
oxide, ensuring that the surface plasmon resonances
for the upper and lower interfaces occur at the same
wavelength. In Figure 2, 2a shows a cross-section of
such a structure (sample A) with a period of 2 mm, slit
width of 0.4 mm and aluminum thickness of 0.41 mm.
After completion of the fabrication steps on the
silicon substrate, the top surface of the structure is
glued onto a transparent quartz substrate with an optical adhesive. The silicon wafer is then completely
removed by a combination of wet etching and mechanical polishing. The aluminum layers are hence
transferred onto the quartz substrate in the final structure, with the topmost layer in the final structure corresponding to the first silicon oxide layer deposited
on the silicon substrate.
Figure 2b shows the zero order transmission spectrum of sample A with transverse magnetic (TM) polarization (electric field perpendicular to slits). A
transmission peak can be identified at wavelength of
⬃3.2 mm. The maximum transmission is as large as
38% even though the fractional area of the slits is
only 20% and the peak wavelength is much larger
than the slit widths. Taking into account the refractive
index of silicon oxide (⬃1.53), the wavelength of light
in silicon oxide at which peak transmission occurs is
close to the periodicity of the structure (2 mm). In
Figure 3, 3a shows the transmission spectrum of sample B with different parameters: metal thickness of
0.3 mm, a different slit width of about 0.25 mm and
periodicity of 1 mm. The transmission peak has shifted
to a lower wavelength of ⬃1.7 mm. The peak transmission wavelength in silicon oxide remains comparable to the periodicity of the grating. The peak
transmission is even more pronounced than sample
A, reaching 80% for TM polarization.
The transmission peaks in samples A and B are
associated with the coupled surface plasmons mode of
transmission through narrow slits [21] in a metal
layer. Some authors refer to such resonance as horizontal surface plasmon resonances [5]. At these resonances, the incident light excites surface plasmons
on the first surface through the periodic structure.
Surface plasmons on the first surface then couple to
surface plasmons on the other surface through the
slits, which in turn re-radiate into light of the same
frequency and momentum as the incident wave. The
Bell Labs Technical Journal
145
Silicon oxide
Aluminum
Silicon oxide
Silicon
1
1
0.8
0.8
Transmission
Transmission
(a)
0.6
0.4
0.2
(b)
0
0.6
0.4
0.2
(c)
1.5
2
2.5
3
3.5
Wavelength (␮m)
4
0
1.5
2
2.5
3
3.5
Wavelength (␮m)
4
RCWA—Rigorous coupled wave analysis
TM—Traverse magnetic
(a) Scanning electron micrograph of the cross section of sample A. The thickness of
aluminum is 0.4 ␮m. Each slit is about 0.4 ␮m wide and the periodicity is 2 ␮m.
(b) Measured transmission for TM polarized light for sample A. The peak transmission
exceeds the fractional area of the slits by almost a factor of 2.
(c) Numerical calculation of the transmission using RCWA.
Figure 2.
Scanning electron micrograph and measured and calculated optical transmission for sample A.
wavelength at peak transmission depends largely on
the periodicity of the slit array. As the thickness of
the metal layer increases, the transmission peak shifts
to longer wavelengths and eventually transforms into
waveguide resonances (or vertical surface plasmons
modes) for deep slits [2, 10, 21].
We compare our preliminary experimental results
to numerical simulations using rigorous coupled wave
analysis (RCWA). RCWA is a numerical method that
analyzes transmission and reflection of periodic structures [18, 19]. In RCWA, the structure is divided into
multiple layers. The calculation involves spatial
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Bell Labs Technical Journal
Fourier expansion of the electromagnetic field and dielectric functions in each layer. The electromagnetic
field determined by RCWA satisfies Maxwell’s equation within each layer as well as the boundary conditions between adjacent layers. The numerical accuracy
is only limited by the number of orders used in the
Fourier expansion. Detailed descriptions of the RCWA
algorithms can be found in [18 and 19]. Figures 2c
and 3b plot the calculated transmission through sample A and sample B using RCWA, showing peaks at
⬃3.5 mm and ⬃1.7 mm, respectively, in reasonable
agreement with our measurement.
1
0.8
0.8
Transmission
Transmission
1
0.6
0.4
0.2
0
0.6
0.4
0.2
1.5
2
3
2.5
Wavelength (␮m)
3.5
4
(a)
0
1.5
2
2.5
3
Wavelength (␮m)
3.5
4
(b)
(a) Measured transmission for TM polarized light for sample B. The thickness of aluminum is 0.3 ␮m. Each slit is
about 0.25 ␮m wide and the periodicity is 1 ␮m. The peak transmission exceeds the fractional area of the slits
by almost a factor of 3.
(b) Numerical calculation of the transmission using RCWA.
RCWA—Rigorous coupled wave analysis
TM—Transverse magnetic
Figure 3.
Measured and calculated transmission for sample B.
Figure 4 shows the RCWA calculation of the
electric field for a structure with parameters similar to
sample B at resonance. For simplicity, the silicon oxide
layers are assumed to extend through all space above
and below the metal layer. The magnetic fields are directed out of the page and the magnitude is represented by the grayscale, while the arrows represent
the electric fields. The electric field is highly concentrated at the surface of the metal layer and in the slits.
At these locations, the electric field intensity is
enhanced by up to a factor of 44 compared to the
far-field regions.
Potential Applications
Apart from the remarkable transmission properties of these metallic nanostructures, the strongly
enhanced local fields on the metal surfaces have potential in other novel applications. One possibility is to
reduce the optical power required to produce nonlinear optical effects. The enhanced electromagnetic
4.5
4
3.5
3
2.5
2
1.5
1
0.5
Calculated electromagnetic field for sample B at peak
transmission. Electric fields are denoted by arrows.
Magnetic fields are directed out of the page, with
magnitude represented by the gray scale.
Figure 4.
Electromagnetic field distribution for sample B at
resonance.
Bell Labs Technical Journal
147
fields of the surface plasmons on rough metal surfaces
have already found numerous uses. For instance, in
surface-enhanced Raman spectroscopy [20], molecules adsorbed on the rough surface of metals display
huge enhancement of the cross section due to the
local field enhancement. The subject of nonlinear optics is concerned with the effects arising from the nonlinear response of a medium to electric fields in the
optical frequency range. Examples of nonlinear effects include frequency conversion, higher harmonic
generations, parametric amplifications, and optical
bistability. Nonlinear optics has become a major focus
of scientific research and engineering because of its
potential for applications in optical computing and
optical devices such as electro-optic modulators, optical parametric amplifiers, switches, filters, and frequency conversion devices. The main barrier to
applications of nonlinear optics is the small optical
nonlinearity of most materials. In fact, intense pulsed
lasers are usually required to reveal the nonlinear
properties. By placing nonlinear optical material on
the surface of the subwavelength metallic structures,
the enhanced local electromagnetic fields could be
used to reduce the optical power required for nonlinear optical effects.
In addition, surface plasmons can be used to improve the sensitivity and versatility of optical detection
of displacement in micromechanical devices. The surface plasmons channel the optical energy through the
slits that are much smaller than the wavelength,
building up intense local fields. Nanomechanical
movement in the vicinity of these fields is expected to
lead to large changes in the transmitted light intensity.
Mechanical transducers involving surface plasmons
could have far-reaching impacts in inertial sensing
and ultra-sensitive force detection. Furthermore, the
electromagnetic fields and the induced charges associated with the surface plasmons exert forces on the
metallic elements. Due to the enhancement of electromagnetic fields, such forces could be strong enough
to create motion in micromechanical components.
The coupling of surface plasmons with mechanical
degrees of freedom could lead to new schemes for actuating micromechanical components with lasers.
Subwavelength metallic structures possess remarkable transmission properties that are accompanied by
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Bell Labs Technical Journal
strong enhancement of electromagnetic field near the
surface. Understanding the propagation of surface
plasmons in complex metallic nanostructures is of
fundamental interest and practical importance in designing surface plasmon photonic devices that could
become important building blocks in future nanooptical systems.
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(Manuscript approved July 2005)
HO BUN CHAN is an assistant professor in the
Department of Physics at the University
of Florida in Gainesville, where his job
responsibilities include research and student
education. He holds A.B. and Ph.D. degrees
in physics from Princeton University in New
Jersey and the Massachusetts Institute of Technology in
Cambridge, respectively. Dr. Chan’s professional
interests include microelectromechanical systems,
Casimir forces, gravitation at nanoscale, fluctuations in
nonlinear systems, scanning probe microscopy, surface
plasmons polaritons, integrated optics, and tunneling
spectroscopy of quantum Hall systems.
ZSOLT MARCET is an undergraduate senior in the
Department of Physics at the University
of Florida in Gainesville. He assists with
research in the department, and he has a
special interest in plasmonic devices and
numerical simulations.
DUSTIN CARR is a principal member of technical staff at
Sandia National Laboratories in
Albuquerque, New Mexico. He holds a B.S.
degree in mathematics from Oklahoma
State University in Okmulgee and M.S. and
Ph.D. degrees in physics from Cornell
University in Ithaca, New York. Dr. Carr, whose
professional interests include nanomechanical systems,
optical microelectromechanical systems, surface
plasmonics, and integrated sensor systems utilizing
optical readout, was selected by MIT Technology
Review Magazine as one of the top 100 young
innovators in technology in 2005.
Bell Labs Technical Journal
149
JOHN ERIC BOWER is a member of technical staff in the
Nanofabrication Research Department of
the New Jersey Nanotechnology Consortium
in Murray Hill, New Jersey. He received a
B.S. degree in physics from Pennsylvania
State University at University Park and M.S.
and Ph.D. degrees, also in physics, from Stevens
Institute of Technology in Hoboken, New Jersey. At Bell
Labs, Dr. Bower’s responsibilities include fabrication of
MEMS- and nano-devices with an emphasis on
metalization as well as nanotechnology project
management. His professional interests include clusters,
nanomaterials, and nanostructures.
RAY CIRELLI is a distinguished member of technical
staff in the Nanofabrication Research
Department of the New Jersey
Nanotechnology Consortium in Murray Hill,
New Jersey. He has authored over 70
technical papers and has been awarded
numerous patents in the optical lithography field. He
has made critical contributions to various aspects of
deep-ultraviolet lithography in the areas of off-axis
illumination, reflectivity control, and proximity effect
correction. The current emphasis of his work is in new
lithography techniques for MEMS and nanotechnology
research.
ED FERRY works in the Nanofabrication Research
Department of the New Jersey Nanotechnology
Consortium in Murray Hill, New Jersey.
FRED P. KLEMENS works in the Nanofabrication
Research Department of the New Jersey
Nanotechnology Consortium in Murray Hill,
New Jersey. With over 25 years of
experience in plasma processing research,
he has a special interest in ultra-thin
polysilicon and deep silicon as well as silicon oxide etch
processing.
JOHN F. MINER is a member of technical staff in the
Nanofabrication Research Department of
the New Jersey Nanotechnology Consortium
in Murray Hill, New Jersey, where his
current responsibilities include chemical
vapor deposition of oxides and nitrides and
planarization of all materials at all levels. He holds a
B.S. degree in mechanical engineering from the New
Jersey Institute of Technology in Newark and an M.S.
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Bell Labs Technical Journal
degree in project management from Stevens Institute
of Technology in Hoboken, New Jersey.
CHIEN-SHING PAI is a distinguished member of
technical staff in the Nanofabrication
Research Department at Bell Labs in Murray
Hill, New Jersey. He received a B.S. degree
in electrophysics from Chiao-Tung University
in Hsin-Chu, Taiwan, and a Ph.D. degree
from the University of California at San Diego. At Bell
Labs, his early research focused on advanced electronic
devices. He was involved in the research and
development of both devices and processing for
advanced CMOS technologies for ULSI applications. His
work included selective-epi and silicide for front-end
CMOS devices and low-k materials and multilevelinterconnect integration for sub-100 nm ULSI
applications. In recent years, his work has expanded
to include MEMS technology for communication and
biotech applications. As a VLSI Technology Symposium
committee member since 1996, Dr. Pai has organized
and chaired short courses and rump sessions for the
symposium for several years.
J. ASHLEY TAYLOR is a member of technical staff in the
Nanofabrication Research Department of
the New Jersey Nanotechnology Consortium
in Murray Hill, New Jersey. He received his
Ph.D. in physical chemistry, with a
concentration in photoionization of
gaseous molecules, from the University of Houston in
Texas. As a postdoctoral fellow, also at the University
of Houston, he investigated the reactions of ions with
surfaces. His past job responsibilities at Bell Labs
include conducting studies characterizing materials
used for the manufacturing of integrated circuits;
developing dielectric plasma etch processes; developing
plasma etch processes for low-k dielectric materials,
which played an important part in the development
of the 0.13 mm integrated-circuit technology; and
developing processes for deep silicon plasma etch for
optical MEMS devices. Currently, he is studying the
interaction of liquids with nanotextured silicon
surfaces. Dr. Taylor has authored more than 30
publications dealing with materials characterization
and plasma etch, and he has several patent
applications pending. ◆