of 6
Surface-wave-enabled darkfield aperture
for background suppression during
weak signal detection
Guoan Zheng
1
, Xiquan Cui, and Changhuei Yang
Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 91125
Edited by F. Javier García de Abajo, University of the Basque Country, Spain, and accepted by the Editorial Board March 17, 2010 (received for review No
vember
2, 2009)
Sensitive optical signal detection can often be confounded by the
presence of a significant background, and, as such, predetection
background suppression is substantively important for weak signal
detection. In this paper, we present a novel optical structure de-
sign, termed surface-wave-enabled darkfield aperture (SWEDA),
which can be directly incorporated onto optical sensors to accom-
plish predetection background suppression. This SWEDA structure
consists of a central hole and a set of groove pattern that channels
incident light to the central hole via surface plasmon wave and
surface-scattered wave coupling. We show that the surface wave
component can mutually cancel the direct transmission compo-
nent, resulting in near-zero net transmission under uniform normal
incidence illumination. Here, we report the implementation of two
SWEDA structures. The first structure, circular-groove-based SWE-
DA, is able to provide polarization-independent suppression of
uniform illumination with a suppression factor of 1230. The second
structure, linear-groove-based SWEDA, is able to provide a sup-
pression factor of 5080 for transverse-magnetic wave and can
serve as a highly compact (5.5 micrometer length) polarization sen-
sor (the measured transmission ratio of two orthogonal polariza-
tions is 6100). Because the exact destructive interference balance is
highly delicate and can be easily disrupted by the nonuniformity of
the localized light field or light field deviation from normal inci-
dence, the SWEDA can therefore be used to suppress a bright back-
ground and allow for sensitive darkfield sensing and imaging
(observed image contrast enhancement of 27 dB for the first
SWEDA).
surface plasmon
nanophotonics
cell imaging
biological and chemical sensing
T
he ability of a sensor to observe a weak optical signal in the
presence of a strong background can be significantly limited
even if the sensor is fully capable of measuring the same weak
signal in the absence of background (1, 2). There are several fac-
tors that can contribute to this degradation in sensitivity, and
their relative significance is dependent on the measurement sce-
narios involved.
The case of stars in the sky provides a good illustration of some
of these limitations. A bright star that is quite visible at night may
disappear from our sight during the day. This disappearing act is
attributable to two major factors. First, the bright daytime back-
ground can introduce a proportionate noise term that the bright-
ness of the star must exceed to be observable. Second, our eyes
naturally adjust their dynamic range to accommodate the bright
daytime background. As the bit depths of most measurement
systems (including our eyes) are finite, we necessarily view the
sky with a coarser brightness scale during the day. If the incre-
mental brightness of the star versus the background is smaller
than this gradation scale, the star is simply indistinguishable from
its background.
The approach of adding bit depth can address part of the pro-
blem; however, it is an
engineering
solution that comes at a
price of more sophisticated electronics and greater data volume.
Moreover, it does not eliminate the proportionate noise term
from the background. Interference arrangements can potentially
be employed to destructively interfere and cancel the background
(for situations where the light sources involved are coherent).
However, such schemes are understandably elaborate and non-
trivial to employ. A sensor that can intrinsically cancel a strong
background prior to signal detection would be a simpler solution
with broad applicability.
In this paper, we report a unique sensor structure that accom-
plishes this type of darkfield sensing for coherent light field in a
robust, compact, and simple format. This structure, termed sur-
face-wave-enabled darkfield aperture (SWEDA), generates a
surface-wave-enabled component that interferes destructively
with the direct light transmission component. As such, a SWE-
DA-bearing sensor will detect no signal when illuminated by a
uniform light field at normal incidence. A SWEDA-bearing sen-
sor is insensitive to the background normal incidence light field
but is, instead, highly sensitive to weak localized light field varia-
tions or light fields at nonzero incident angles that disrupt the
exquisitely balanced interference condition.
We will next describe the operating principle of the proposed
structure. Then, we will report on our simulations and experimen-
tal implementation of a polarization-independent SWEDA,
which utilizes a circular groove pattern for surface wave coupling.
We next extend the SWEDA concept to a polarization-sensitive
case by using a linear groove pattern for surface wave coupling.
Finally, we report on our experimental demonstration of the abil-
ity of the circular-groove-based SWEDA to detect weak signals in
the presence of a strong background; we also present a proof of
concept that, among other applications, shows that sensors based
on such structures can be used to implement a new class of dark-
field microscopes.
Results
SWEDA Concept.
The light interaction between the subwavelength
features on a metal-dielectric interface has been a subject of in-
tensive study in recent years (3
12). It has been shown that ap-
propriately patterned rings of metal corrugation around a hole
can significantly change the total amount of light transmission
through the aperture (3, 5, 7, 8, 10, 11). One primary component
involved in such a light interaction between the central hole and
the metal corrugation is the surface plasmon (SP) wave, the elec-
tromagnetic surface wave existing at the interface between a di-
electric and a noble metal (13). Recently, some theoretical and
Author contributions: G.Z. and C.Y. designed research; G.Z. performed research;
G.Z. analyzed data; and G.Z., X.C., and C.Y. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. F.D. is a guest editor invited by the Editorial Board.
Freely available online through the PNAS open access option.
1
To whom correspondence should be addressed. E-mail: gazheng@caltech.edu.
This article contains supporting information online at
www.pnas.org/lookup/suppl/
doi:10.1073/pnas.0912563107/-/DCSupplemental
.
www.pnas.org/cgi/doi/10.1073/pnas.0912563107
PNAS
May 18, 2010
vol. 107
no. 20
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APPLIED PHYSICAL
SCIENCES
experimental results (5
9, 12) also show that the SP wave is not
the only component involved in the light interaction of subwave-
length features on the metal-dielectric interface. A surface scat-
tered component also plays a role at the short range interaction.
Therefore, the mediated-transmission behavior of this corruga-
tion-based aperture can be intuitively explained as follows. When
light falls on a patterned groove structure on the metal, it couples
into the surface wave, including the SP wave and the surface scat-
tered wave. By choosing the groove periodicity such that the sur-
face wave launched at each groove adds up in phase, we can
generate a strong propagative surface wave that is directed to-
ward the hole. The surface wave can then be converted back
to a propagating optical wave at the central hole. In essence,
the groove structure serves as an antenna for light collection
and uses the surface wave to transport the collected optical power
to the hole. Using this approach, researchers have reported both
light transmission enhancement and suppression (3, 5, 7, 10, 14).
Our SWEDA design differs from previous surface-wave-modu-
lated aperture design by exactly balancing the direct transmission
component of the central hole and the surface wave component
induced from the grooves (for clarity, we refer to the center open-
ing as the central hole and the entire structure as the aperture).
This creates a situation where the two transmission components
can interact significantly
thus providing an additional means for
light manipulation. To be specific, we precisely control the ampli-
tude of the surface wave by changing the periodicity and depth of
the groove structure. Through judicious choice of the groove
structure and the central hole size, we can arrive at a situation
where the surface wave component is equal in strength to the di-
rect transmission component. Furthermore, we can adjust the
phase lag between the surface wave and the direct transmission
component by choosing the gap between the innermost groove
and hole appropriately. If we adjust this phase lag such that
the optical wave generated by the surface wave component is
180° out of phase with the direct transmission component, the two
components will destructively interfere and result in little or
no light transmits through the hole in the presence of uniform
normal-incidence illumination. Because this destructive interfer-
ence condition critically depends on an exact balance of the two
mentioned components, a small change in spatial distribution of
the input light field intensity or phase will disrupt the destructive
interference condition and permit significant light transmission
through the hole.
In the context of high-sensitivity optical signal detection, the
advantage of SWEDA can be easily appreciated. The structure
can effectively suppress a uniform normal-incidence background
from reaching the underlying sensor and instead only permit
highly localized light field variations or light fields at nonzero
incidence angles to pass through and be detected. As such, the
underlying sensor no longer needs to contend with a high back-
ground and its associated noise fluctuation terms. The bit depth
can also be optimized and devoted to the detection of the weaker
light field variations. Used in an appropriate manner, such de-
vices can potentially allow for greater signal detection sensitivity
in
weak signal buried in high background
scenarios. This meth-
od also enables a unique way to build darkfield microscopes on
the sensor level that does not rely on elaborate bulk optical ar-
rangements.
SWEDA with Circular Groove Pattern.
The first type of SWEDA
is shown in Fig. 1
A
. It adopts a circular pattern for the groove
design. We refer to this structure as the circular-groove-based
SWEDA. Due to its circular symmetry nature, this type of SWE-
DA provides a polarization-independent behavior for signal de-
tection and imaging. We began the implementation of such a
SWEDA by using a commercial software
CST Microwave Stu-
dio to perform a set of simulations to understand the interplay
between our design parameter choices and system characteristics.
The simulations were performed at a nominal wavelength of
738 nm. The permittivity of gold at this wavelength is
19
.
95
þ
1
.
48
i
(15). There are primarily 4 specific parameters that im-
pact the performance of the SWEDA: (
i
) Groove periodicity
and groove depth. The groove periodicity (defined here as the
p
-parameter in Fig. 1
A
) and groove depth can be adjusted to con-
trol the magnitude of the surface wave coupled into the structure.
Note that the exact match of the groove periodicity to the wave-
length of the surface wave is not necessarily desired, as this may
induce an overly strong surface wave component. (
ii
) The number
of grooves. The strength of the coupled surface wave increases as
a function of groove numbers. On the other hand, we desire a low
number of grooves for overall SWEDA structure compactness
considerations. (
iii
) Central hole size. This affects the strength of
the direct transmission component. We would additionally aim to
restrict the aperture size such that the light transmission is not
multimoded. Multimode light transmission significantly compli-
cates the destructive interference balancing act as we would need
to achieve destructive interference between the surface wave
component and the direct transmission component for all modes
involved. (
iv
) The distance between the innermost groove and the
rim of the central hole (defined here as the
s
-parameter in
Fig. 1
A
) determines the phase difference between the surface-
wave-induced and the direct-transmission components. To ac-
complish exact cancellation of the two components, we require
this phase difference to be 180°. Other parameters such as
the groove profile can also be used to tune the surface wave
component; however, from the fabrication point of view, the pro-
file of the groove is not as easy to control as the 4 parameters we
mentioned.
The simulation program allowed us to map out the interplay of
these parameters and the overall SWEDA system characteristics.
We define the darkfield suppression factor as the ratio of the total
power transmission through a simple hole (without grooves) to
the total power through a SWEDA. For good darkfield perfor-
mance, we desire this ratio to be as high as possible. We were
able to arrive at a design parameter set (Fig. 1
A
) that provides
a suppression factor of 6640 by the simulation program. The si-
mulated electric field distributions for this particular SWEDA de-
sign (Fig. 1
A
) and that of a corresponding simple hole (Fig. 1
B
)
are shown here. We can see that the SWEDA structure should
indeed be able to suppress light transmission through the central
hole significantly. We next simulated the translation of a cylind-
rical dielectric object (radius 300 nm, thickness 200 nm, displace-
ment height 300 nm, permittivity 2.25) across the top of the
SWEDA structure (Fig. 1
C
F
). We can see that the SWEDA
began to transmit light significantly when the object
s presence
directly above the central hole significantly perturbed the direct
transmission component and, consequently, disrupted the deli-
cately balanced destructive interference condition. We further
observed that the presence of the object above the groove struc-
tures did perturb the SWEDA to a certain extent as well. How-
ever, the impact was much less significant (Fig. 1
F
); this can be
well appreciated by noting that the generation of surface wave
occurred over the entire area associated with the ring grooves
and localized changes of the light field over the area had a dimin-
ished impact on the overall surface wave component. As a whole,
this simulation indicates that the SWEDA is maximally sensitive
to light field heterogeneity directly above the central hole.
We next fabricated a number of SWEDA structure based on
the parameters suggested by our simulation results. Fig. 2
A
shows
the SEM image of a typical SWEDA that we have created by
focus ion beam (FIB) milling (see
Methods
for more details).
We fabricated a set of 13 SWEDAs with different spacing
s
ran-
ging from 540 nm to 1020 nm. A single hole without the groove
structure was also fabricated to serve as a control. To characterize
the optical property of the SWEDA, we used a tunable wave-
length laser (Spectra
Physics Tsunami continuous wave Ti:
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Zheng et al.
Sapphire) as the illumination source. The transmissions through
the apertures were collected by an inverted microscope with a
20X objective lens.
Fig. 2
B
shows the optical transmission images of the 13 SWE-
DAs and the reference single hole at normal incidence for three
different wavelengths. We can see that the spacing parameter
s
does indeed have a significant impact on the transmission of
the SWEDA structures. The transmission intensity measured for
these SWEDA structures are plotted in Fig. 2
C
(wavelength of
738 nm); we used the unadorned simple hole for normalization.
The simulation prediction for each of the structures is also
plotted for comparison. From the plots, the implemented SWE-
DA structure with
s
-parameter of 780 nm exhibited the desired
near-zero transmission characteristics. The optimized SWEDA
s
structure parameters were a close match with our simulation
predictions
the
s
parameter differed by 6 nm (<
0
.
8%
). The
measured suppression factor for the optimized SWEDA was
1230. In other words, this SWEDA transmitted 1230 times less
light than an unadorned simple hole of size equal to that of
the central SWEDA hole.
We next measured the spectral response of the optimized
SWEDA over a spectral range of 700 nm to 790 nm. Because
SWEDA
s operation depends on the exact amplitude balance and
opposing phase relationship of the surface-wave-enabled trans-
mission component and the direct transmission component, we
can expect that the darkfield property of SWEDA to be optimized
for one single wavelength. Fig. 2
D
shows the experimentally mea-
sured and simulation-predicted spectral transmission of the
SWEDA. As expected, there is a single minimum over the range
of interest and the transmission increased monotonically away
from this point. It is also worth noting that the suppression factor
actually remained fairly high (
>
50
) for a bandwidth of approxi-
mately 10 nm.
For a given incident light field, we can decompose it into dif-
ferent plane wave component with respect to the transverses
wave vector (16). In Fig. 2
E
, we measure the transmission of the
SWEDA as a function of the normalized transverses wave vector
(k
x
k
0
). Fig. 2
E
represents the system transfer function of the
SWEDA: SWEDA rejects the normal incident plane wave com-
ponent and transmits other components with efficiency as dic-
tated by this transfer function.
The good agreement between the experimental and simulated
spacing, wavelength and transverses wave vector trends, as
evident in Fig. 2
C
,
D
, and
E
, is a proof of the SWEDA working
principle. The discrepancy in darkfield suppression factor is at-
tributable to fabrication imperfections associated with the FIB
milling process. We tend to end up with rounded structure edges
experimentally. Another contribution might be the variation in
groove depth due to the intrinsic roughness of the groove bottom
(metals mill nonuniformly as a function of grain orientation due
Fig. 1.
Simulations of the circular-groove-based SWEDA. Displayed is the
real part of the electric-field at
λ
¼
738
nm (equivalent to the time-domain
fields at the instant of time when the source phase is zero). (
A
) The optimized
SWEDA structure, where
s
¼
774
.
3
nm,
p
¼
560
nm, thickness of gold
¼
340
nm, diameter of hole
¼
300
nm, and groove depth
¼
140
nm and re-
fraction index of the dielectric substrate
¼
1
.
5
. The simulation predicts that
the darkfield suppression factor of this structure equals 6640. (
B
) Simulation
for the simple hole. (
C
E
) Simulations of a cylindrical scatterer (radius 300 nm,
thickness 200 nm, displacement height 300 nm, permittivity 2.25) transiting
across the SWEDA. (
F
) The transmission signal curve from the SWEDA as the
cylindrical scatterer (the same as
C
E
) moves across it. The full width at half
maximum was determined to be 395 nm.
Fig. 2.
Experimental characterization of the circular-groove-based SWEDA.
(
A
) The SEM image of a typical fabricated SWEDA. (
B
) The optical transmis-
sion images of the 13 SWEDAs and the reference single-hole under normal-
incidence illumination for three different wavelengths. (
C
) The measured op-
tical transmission signals from SWEDAs with different
s
ranging from 540 to
1020 nm (left to right). The signals from the SWEDA were normalized by sin-
gle hole (signal from the single hole at normal incidence was set to unity). The
simulated intensity is also shown for comparison. (
D
) The normalized optical
transmission signals of SWEDA (
s
¼
780
nm) with different incident wave-
lengths. (
E
) The normalized optical transmission signals of SWEDA with dif-
ferent normalized transverses wave vector (see
Methods
for more details).
Zheng et al.
PNAS
May 18, 2010
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APPLIED PHYSICAL
SCIENCES
to channeling). If exact matches of experiments and simulation
are desired in specific applications, such imperfections may be
mitigated by employing a sacrificial layer, as described in ref. 17,
during fabrication to help preserve the sharpness of edges.
SWEDA with Linear Groove Pattern.
The second type of SWEDA is
shown in Fig. 3
A
and
B
. It adopts a linear pattern for the groove
design (refer as the linear-groove-based SWEDA), and as such, it
is highly sensitive to the polarization state of the incident light. As
shown in Fig. 3
A
, incoming transverse-magnetic (TM) polarized
light (where the electric field is perpendicular to the groove struc-
ture) can be collected and converted into a surface wave by the
periodic grooves and then be recoupled into propagating light
through the central hole. On the other hand, the TE (with mag-
netic field perpendicular to the groove structure) coupling effi-
ciency of the SP wave, a major component of the total surface
wave, is much smaller than the TM case (13), and thus, the ab-
sence of interference with the SP wave permits significant TE po-
larized light transmission through the hole in Fig. 3
B
. Fig. 3
C
and
D
show the simulations of this SWEDA at a nominal wavelength
of 750 nm [the permittivity of gold at this wavelength is
20
.
96
þ
1
.
55
i
(15)]. We were able to arrive at a set of design parameters
that provide a darkfield suppression factor of 35400 for TM wave
in our simulations. The simulated magnetic field distributions for
this particular design are shown in Fig. 3
C
. In Fig. 3
D
, we also
show the electric field distributions for the TE wave, from which
we can see that the structure does transmit TE wave significantly.
The difference between the TM and TE cases also verifies the
surface-wave-enabled mechanism of the linear-groove-based
SWEDA, because the SP wave can only be induced efficiently
for TM polarization (13). We also note that, from the simulations
shown in Fig. 3
C
and
D
, the linear-groove-based SWEDA pro-
vides a polarization extinction ratio of 42500 for the two ortho-
gonal polarization states.
We next fabricated a number of linear-groove-based SWEDAs
with linear-groove patterns. Fig. 4
A
shows the SEM image of a
typical linear-groove-based SWEDA that we have created by FIB
milling. We fabricated a set of 9 linear-groove-based SWEDAs
with different spacing
s
ranging from 455 nm to 775 nm. The op-
tical transmission signals of linear-groove-based SWEDAs are
normalized and plotted as a function of spacing
s
and wavelength
λ
in
Fig. S1
. The measured darkfield suppression factor for the
optimized linear-groove-based SWEDA was 5080. In other
words, this structure transmitted 5080 times less TM light than
an unadorned simple hole of size equal to that of the central
SWEDA opening. In Fig. 4
B
, a light field is incident on the SWE-
DA with a polarization angle
θ
this geometry is used for our
subsequent measurements. The optical images with different po-
larization angles are shown in Fig. 4
C
, and the normalized signals
of SWEDAs are plotted in Fig. 4
D
. The measured polarization
extinction ratio is 6100, meaning that the amount of TE light
transmission through the linear-groove-based SWEDA is 6100
times higher than the TM case. Such a high extinction ratio posi-
tively indicates that the linear-groove-based SWEDA can serve as
a highly compact and highly efficient polarization sensor.
Demonstration of the Circular-Groove-Based SWEDA
s Ability to Boost
Detection Sensitivity.
Due to the polarization-independent nature
of the circular-groove-based SWEDA, it can be used to suppress a
bright normal-incidence background regardless of the incident
light field
s polarization state. The ability of such a SWEDA
to improve small signal detection is illustrated in the following
experiment. We prepared a sample comprised of an indium tin
oxide (ITO)-coated glass slide that was marked with shallow pits
of radius of 175 nm and 250 nm via the FIB (Fig. 5
A
and
B
). Next,
we transmitted a uniform light field of intensity about
0
.
2
W
cm
2
from a 738 nm laser through the sample. We then used a
1
1
relay
microscope to project a virtual image of the pits onto our opti-
mized circular-groove-based SWEDA (see
Methods
for more de-
tails). We next raster-scanned the sample and measured the light
transmission through the SWEDA at each point of the scan. We
then generated an image of the sample from the collected data.
As is evident in Fig. 5
C
and
D
, SWEDA allowed us to identify the
presence of the two pits with little difficulty. We next acquired
Fig. 3.
Working principle and simulation of the linear-groove-based
SWEDA. (
A
) The TM incident light is coupled into surface wave by the linear
groove pattern. The destructive interference between the surface-wave-
enabled component and the direct transmission component results in zero
transmission. (
B
) The TE incident light cannot be coupled into SP waves
(a primary component of the total surface wave), and transmission is induced
in the absence of destructive interference. (
C
) Simulation of the TM case. Dis-
played is the real part of the electric field at
λ
¼
750
nm. The parameters of
the optimized SWEDA are:
s
¼
658
nm,
p
¼
660
nm, thickness of gold
¼
340
nm, diameter of hole
¼
300
nm, groove depth
¼
140
nm, and refrac-
tive index of the dielectric substrate
¼
1
.
5
. The simulation predicts a TM dark-
field suppression factor of this structure versus a simple hole is 35400. (
D
)
Simulation of the TE case. Displayed is the real part of the magnetic field
at
λ
¼
750
nm. The simulation predicts the polarization extinction ratio of
the two orthogonal polarization states is 42500.
0 153045607590
1E-5
1E-4
1E-3
0.01
0.1
1
Normalized intensity
Polarization angle (deg)
Simulation
Experiment
A
E-field
H-field
θ
Direct transmission component
Surface-wave-enabled component
B
C
D
455
495
535
575
655
615
735
Spacing
s
(nm)
θ
=0 deg
Single hole
695
775
θ
=30 deg
θ
=60 deg
θ
=90 deg
No transmission
2
μ
m
s
p
Fig. 4.
Experimental characterization of the linear-groove-based SWEDA.
(
A
) The SEM image of a typical fabricated SWEDA. (
B
) The light is incident
on the SWEDA with a polarization angle
θ
.(
C
) The optical transmission
images of the SWEDAs for different polarization angles at
λ
¼
750
nm. (
D
)
The normalized optical transmission signals of SWEDA are plotted as a func-
tion of polarization angle. The measured polarization extinction ratio for TE
and TM incidence is 6100.
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Zheng et al.
images of the same pits (Fig. 5
E
and
F
) with a simple camera
(based on the same sensor chip). It is difficult to identify the pre-
sence of the two pits in this case. The total light fluence incident
on the sample for both the SWEDA and camera image acquisi-
tions was kept the same to allow for direct result comparisons.
Fig. 5
G
shows plots of signal traces across the images. The
SWEDA-acquired data were normalized on the same scale.
The camera image data were normalized versus the average back-
ground signal. The backgrounds associated with the SWEDA-
acquired data were low and the contributive signals from the pits
were well discernible. In fact, the contributive signals were suffi-
ciently well-resolved that we can use them to quantify their
relative strengths for the two pits. In comparison, the high back-
grounds in the camera images combined with the associated noise
masked the scattering contributions from the pits. The measured
contrast improvement was 25 dB for the 175 nm pit and 27 dB for
the 250 nm pit.
As pointed out in our introduction, circular-groove-based
SWEDA can potentially be employed to perform darkfield micro-
scopy imaging at the sensor level. The principle involved is
substantially different from that of a conventional darkfield mi-
croscope. Whereas a conventional system depends on oblique il-
lumination and a relatively small objective angle of collection to
screen out the uniform background via a fairly sophisticated bulk
optical arrangement, the ability of circular-groove-based SWE-
DA to screen out uniform background presents a more direct
approach. To demonstrate that such a system can indeed be im-
plemented, we employed our optimized circular-groove-based
SWEDA in the same experimental scheme to scan slides of star-
fish embryos in different developmental stages. The illumination
intensity was
0
.
2
W
cm
2
. Fig. 5
H
and
J
show the results. Similar
images of the specimens taken with a standard microscope are
shown in Fig. 5
I
and
K
for comparison. We can see that the SWE-
DA generated image has a dark background, as is consistent with
a darkfield microscope image. We can also see that the edge and
interior of the starfish embryo appeared brighter in the SWEDA
image and darker in the control image. This is again consistent
with our expectations of a darkfield image as sample locations
with substantial scattering should appear brighter in a darkfield
image and darker in a simple transmission image. We would like
to emphasize that this is a proof of concept experiment. A feasible
darkfield microscope can be implemented by employing a laser as
the light source in a standard microscope and using a sensor chip
patterned with a grid of tightly spaced circular-groove-based
SWEDA as the microscope camera.
Discussion
As our experimental findings indicate, SWEDA is a robust, struc-
turally simple and highly compact approach to accomplish optical
background suppression and/or polarized light field suppression.
It is also worth noting that, in principle, there is no theoretical
limit to how close the SWEDA darkfield suppression factor
can approach infinity; the practical limit is only set by the fabrica-
tion tolerance and the net transmission through the opaque me-
tal layer.
There are a few limitations associated with the SWEDA
structure that are worth discussing presently. First, the structure
is optimized for single wavelength operations. This limitation can
be overcome by using more complicated SWEDA-type structures
involving multibeam interference that can operate over a broad
range of wavelengths. Second, the amount of light transmitted is
largely limited by the size of the central hole. We believe that this
issue can be potentially addressed by replacing the central open-
ing of SWEDA with multiple C-shape apertures (18) to increase
light collection efficiency. Third, this structure works well at only
blocking uniform light field at normal incidence. Fortunately, the
general SWEDA concept can be extended with asymmetric struc-
ture designs to screen out light at other incidence angles.
SWEDA technology can potentially be used in a range of
different applications. The linear-groove-based SWEDA demon-
strated in the present work is a highly compact and highly sensi-
tive polarization sensor. Because the polarization state of light
will change during the interaction with chiral materials, this
SWEDA design may also find some applications in on-chip de-
tection of some chiral materials such as sugar, proteins, and DNA
(19, 20). The ability to fully suppress a coherent background as
exhibited by the circular-groove-based SWEDA can be useful for
small signal detections in metrology applications. It is especially
applicable in detection scenarios where the overall background
intensities fluctuate with time. As our background subtraction oc-
curs at the individual pixel level, SWEDA technology removes the
need for balanced detection schemes. The predetection back-
ground subtraction, which is a light cancellation process, is also
intrinsically more sensitive than postdetection cancellation
schemes that are susceptible to intrinsic detection statistical vari-
ations. The inclusion of chemical reagents in the central hole can
also turn such a SWEDA structure into a high-sensitivity sensor
that can react to small refractive index changes of the reagents.
From a practical implementation viewpoint, SWEDA struc-
tures can be fabricated directly on top of CCD or CMOS sensor
pixels. The small size and planar design of SWEDA make such
implementation particularly suited for foundry fabrication. Sen-
sor chips with broad-bandwidth SWEDA can then be used in
place of the standard camera sensor to accomplish sensor level
darkfield imaging. Such systems, in combination with a coherent
light source, can transform a standard microscope into a simpler
and cheaper darkfield microscope than current darkfield micro-
scopes. Such systems can also enable edge-detection imaging in
machine vision applications if the illumination source employed
is coherent. A SWEDA array can also replace the hole array in
the optofluidic microscope (OFM) (21)
a low-cost, lensless and
high resolution microscopy approach, to accomplish darkfield mi-
croscopy imaging on a chip. The use of SWEDA in this case is
especially appropriate as both the OFM and SWEDA implemen-
tation are well suited for semiconductor mass fabrication. In fact,
it is difficult to envision a more compact and cost-effective ap-
proach for incorporating darkfield ability in an OFM system.
500nm
G
A
500nm
C
D
E
B
F
20 μm
20 μm
I
K
H
J
-3
-2
-1
0
1
2
3
0.0
0.2
0.4
0.6
0.8
1.0
Intensity
(a
.
u
.
)
x (micron)
Fig. 5.
The sensitivity enhancement demonstration for the circular-groove-
based SWEDA. (
A
and
B
) The SEM images of the 175 and 250 nm pits on the
ITO-coated glass. (
C
and
D
) The SWEDA-based raster-scanned images of the
samples (
A
) and (
B
). (
E
and
F
) Microscope images of the samples (
A
) and (
B
)
under the same illumination condition as the SWEDA collected images using
a conventional camera with the same complementary metal oxide semicon-
ductor (CMOS) chip. (
G
) Center line traces of the images in (
C
F
). Please see
(
C
F
) for color reference guide. The observed image contrast (signal/back-
ground) enhancement is approximately 25 dB for the 175 nm pit and approxi-
mately 27 dB for the 250 nm pit. (
H
and
J
) The SWEDA-based raster-scanned
images of the starfish embryos. (
I
and
K
) Conventional bright field micro-
scope images.
Zheng et al.
PNAS
May 18, 2010
vol. 107
no. 20
9047
APPLIED PHYSICAL
SCIENCES
Finally, we would like to note that the general concept of exact
balancing the surface-wave-enabled component and direct light
transmission component in a destructive interference manner
is a unique idea that can inspire other surface-wave structures
with novel properties. Effectively, such structures are tiny inter-
ferometers (approximately 6 micrometers or less) that can be fab-
ricated on a single metal substrate and that have excellent
stability (our SWEDA structures exhibited no significant perfor-
mance drift over the entire duration of our experiments). Because
the structure is planar, it can be mass produced in a semiconduc-
tor foundry. The proposed structure can also be redesigned for
operation at longer wavelengths. As an example of other poten-
tial applications, we believe that the concept of SWEDA can be
applied to optical isolation in a ultracompact format, polarization
control in semiconductor lasers (22), wavefront detection, ex-
tending depth of field of the type II aperture-based imaging de-
vice (23), and perspective imaging (24) by customizing the optical
transfer function on the pixel level.
Methods
Simulation.
The simulations were performed in 3 dimensions. The calculation
domain was
12
λ
×
12
λ
×
3
λ
and contained approximately 12 million meshes.
The transmission of the SWEDA and single hole were calculated by integrat-
ing the Poynting vector over a
6
λ
×
6
λ
region (
0
.
85
λ
beneath the aperture).
For all simulations, we applied a perfect match layer at the outer boundaries.
Sample Preparation.
We started with a 2 nm thick titanium layer (adhesion
layer) and 340 nm thick gold layer that were coated on a 1 mm thick glass
substrate by an e-beam evaporator (Temescal BJD-1800). A focused ion beam
(FEI Nova200 dual-beam system using Ga
þ
ions with a 5 nm nominal beam
diameter) was employed to perform milling. A low ion beam current was
used (30 pA, 30 keV) in the milling process to accomplish the requisite fine
structure.
Optical Setup.
For Fig. 2
E
, the transmission of the SWEDA is collected by a 20X
objective and the whole detection setup is assembled on a motorized rota-
tion stage to measure the spatial response of the aperture with different
transverse wave vector of the incident light. The sensitivity enhancement de-
monstration for the circular-groove-based SWEDA (Fig. 5) was conducted in a
laboratory-built
1
1
relay microscope (25). The collection element employed
in the relay microscope was a 0.4 numerical aperture Olympus objective. We
used the relay microscope to project the virtual image of the sample onto our
optimized circular-groove-based SWEDA. We accomplished imaging by ras-
ter-scanning the sample. The scanning process was controlled by two motor-
ized actuators (Newport, LTA-HS) and a motion controller (Newport, ESP301).
The motion step was 200 nm for Fig. 5
C
and
D
and 1.5
μ
m for Fig. 5
H
and
J
.
ACKNOWLEDGMENTS.
We are grateful for the constructive discussions with
and the generous help from Professor Axel Scherer (Caltech); Dr. Xin Heng
(BioRad); and Dr. Meng Cui, Dr. Emily McDowell, Ms. Yingmin Wang, Dr. Ji-
gang Wu, Mr. Jian Ren, and Mr. Lap Man Lee (Caltech). We appreciate the
assistance of Kavli Nanoscience Institute at Caltech. This work is funded by
the Wallace Coulter Foundation, the National Science Foundation Career
Award BES-0547657, and the National Institutes of Health Grant
R21EB008867-01.
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