of 18
1
Hyper
-
Selective Plasmonic Color Filters
Dagny Fleischman
, Luke A. Sweatlock
†*
, Hirotaka Mura
kami
, Harry Atwater
California Institute of Technology, 1200 E California Blvd, Pasadena CA 91125
*Northrop Grumman NG Next, One Space Park, Redondo Beach CA
90250
Sony Corporation
1
-
7
-
1 Konan
,
Minato
-
ku, Tokyo, 108
-
0075 Japan
ABSTRACT:
The subwavelength mode volumes of plasmonic filters are well matched to the small size
of state
-
of
-
the
-
art active pixels (~ 1
μ
m) in CMOS image sensor arrays used in portable
electronic
devices. Typical plasmonic filters exhibit broad (> 100 nm) transmission bandwidths. Dramatically
reducing the peak width of filter transmission spectra would allow for the realization of CMOS
hyperspectral imaging arrays, which demand the FWHM
of transmission peaks to be less than 30 nm.
We
find that
the
design of
5 layer metal
-
insulator
-
metal
-
insulator
-
metal structures
give
s
rise to multi
-
mode interference phenomena
that
suppresses spurious transmission features gives rise to a single
narrow
transmission
band with
FWHM as small as 17 nm. The transmission peaks of these multilayer
slot
-
mode plasmonic filters (MSPFs) can be
systematically vari
ed
through
out
the visible and near
infrared
spectrum
, so the same basic structure can
serve as a
filter
over a large range of wavelengths
.
Periodic arrays of subwavelength holes or nanoslits in metal films enable efficient conversion of
optical energy between incident photons and surface propagating two
-
dimensional charge density
waves, surface plasmon polar
itons (SPPs). Due to the permittivity discontinuity at metal
-
dielectric
surfaces, SPPs have an in
-
plane momentum k
SPP
greater than that of light in free space k
o
. Patterned
metal surfaces including gratings, or arrays of holes or slits, allow the matchin
g of momentum and
thereby enable efficient conversion of light into SPPs via scattering. The strength of interaction
between photons and SPPs can be tailored by changing geometric factors such as the shape of the
scattering elements, and the symmetry and
periodicity of the array as well as by selecting the
permittivity of the constituent materials
1
.
2
In particular, periodic arrays of subwavelength apertures passing through a metal film exhibit
enhanced transmission exclusively at conditions corresponding to
constructive mutual interference
between incident light and SPPs traveling along the surface between adjacent slits. In the case that the
metallic layer is thick enough to be substantially opaque to incident photons, the SPP mediated process
is the domin
ant mode of transmission and the surface acts as a band
-
pass color transmission filters.
Such aperture arrays have been the topic of substantial scientific interest due to these remarkable optical
properties and their utility as a testbed for studying fun
damental light
-
matter in
teractions in plasmonic
systems
2,3
.
Due to their simplicity, scalability, and durability, plasmonic slit gratings have become an
attractive route for development of technological applications including ultra
-
compact filters suita
bl
e for
cameras and displays
4,
5
.
The dispersion of plasmonic propagating modes can be further engineered using metal
-
clad slot
waveguides, often realized as multilayer stacks with a metal
-
insu
lator
-
metal (MIM) configuration
6
.
Such MIM stacks may support
a multitude of polaritonic modes which lie either inside or outside the
“light cone,” that is, with in
-
plane momentum either greater or less than that of a photon with equal
energy. This additional degree of freedom enables substantially more complex opti
cal transmission
filter spectra enabling narrow bandwidth suitable for multispectral and hyperspectr
al color filtering
applications
7
.
Designing Plasmonic Color Filte
rs
Finite difference time domain methods (FDTD) were used to determine the transmission
spectra
of different filter structures. Figure 1 illustrates the different types of transmission filters and their
spectral behavior. MIM have been
used to make RGB color filters
7
.
These structures can be optimized
to have narrowband transmission, but a
s the structure is optimized to minimize FWHM of the
transmission peak, the intensity of the next highest order mode increases. This trade
-
off can be lifted by
introducing a second MIM mode into the structure that couples with the original MIM mode,
leadi
ng to
3
the suppression of the spurious transmission.
The asymmetric nature of the coupled MIM modes plays
a role in the suppression of the spurious transmission, as illustrated in Supplemental Figure 1(a).
The
multilayer slot
-
mode plasmonic filter (MSPF)
investigated demonstrates a narrow transmiss
ion
bandwidth and spurious
peak suppression, as shown in Figure 1(b), and by changing the periodicity of
the slits, this filter can be swept across the entire visible spectrum.
The MSPFs were optimized using par
ameters sweeps that considered both the thicknesses and
optical indices of all the insulating and metallic layers, as well as the width and spacing of the milled
slits. The initial values for the thicknesses of the metallic layers were determined by consi
dering the
skin and penetration depths of various metals. For a successful filter, the top and bottom metallic layers
of the structure must be sufficiently thick to be opaque across the visible and near IR parts of the
electromagnetic spectrum. Using Rak
ic data for Ag as an example, the 1/e penetration depth (d
p
) of Ag
was calculated to range from 12.9 nm
16.8 nm across the visible spectrum. To prevent 98% of light
from penetrating the structure, the top and bottom layers must be 4 times d
p
, so 51.6 nm
to 67.2 nm.
Therefore 68 nm was used as the initial parameter sweep value when optimizing the system that utilized
Ag.
Likewise, the starting point for the thickness of the insulating layers was approximated by
considering the propagating modes guided
laterally within the structure. Numerically determined
dispersion curves derived from experimental optical constants of Ag and SiO
2
can be used to determine
the
available modes within an MIM
6
.
For
SiO
2
thicknesses of less than 100 nm, traditional photoni
c
waveguide modes are cut off in an Ag/
SiO
2
/Ag system, so the waveguide only supports high
-
momentum surface plasmon modes. Therefore, the parameter sweeps used 100 nm as the upper value
restriction for the
SiO
2
thickness of each waveguide.
Iterating ove
r the parameter sweep led to the final device structure, with alternating layers of Ag
and
SiO
2
. Both
SiO
2
layers were optimized to 70 nm, the top and bottom Ag layers are 70 nm and the
4
spacer layer is 50 nm. The width of the slit is 50 nm for all filter
s and the slit periodicities investigated
vary from 250 nm to 550.
The position of the transmission peak varies linearly with the periodicity of the slits and, as
shown in Figure 2(a), peak position can be swept across the visible and near IR spectrum. Th
erefore,
just by varying the inter
-
slit pitch, a series of MSPFs with the same layer materials and thicknesses can
be used as a color filter across a wide range of the spectrum. The FWHM of the transmission spectra
are about 20 nm on average with no peak
exceeding 28 nm, as shown in Figure 2(b). Additionally, the
overall transmission of the side
-
lobe peak does not exceed 11% of that of the primary peak in the visible
portion of the spectrum, and does not exceed 25% of the primary peak intensity in all fil
ters
investigated.
Analytical Analysis
A series of FDTD simulations were executed by sweeping over the visible spectrum using a
single frequency plane wave source. Complex vector field data was collected by finely meshed
monitors capturing the EM behav
ior over the span of each of the FDTD simulations. This data
contained the Cartesian space variations of the electric and magnetic field information over time, which
evolved as the plane wave injected into the simulation interacted with the MSPF. This ve
ry large dataset
can be compressed by taking a discrete
-
time Fourier transform at runtime to yield the field data in the
frequency domain.
A single electric field component from the compressed data set of a single simulation is plotted
in Figure 3(a). T
he spatial mapping of the electric field superimposed on the MSPF depicts multiple
modes that are active in the filter structure. These modes are active along both
SiO
2
layers as well as
the top and bottom Ag surfaces. The spatial mapping of the electric
field indicates that the modes in the
two
SiO
2
insulators are coupled, because they demonstrate a characteristic beating pattern that indicates
that power is being transferred between the two MIMs. This result was expected physically
the spacer
5
layer bet
ween the two insulating layers is thinner than the skin depth of Ag at the energies of the
generated electric fields.
To better determine the natures of the various modes within the MSPF, a second Fourier
Transform was performed. By taking an FFT over t
he propagation direction of the modes, the phasor
direct space dataset can be moved into momentum space (i.e. “k
-
space”). The results of this FFT can
be plotted, as shown in Figure 3(b), to reveal the spatially resolved intensity of the various modes wit
hin
a structure that was excited by a single frequency source. The profiles of these modes can be
determined by spatially mapping the intensity a given spectral frequency. For example, in Figure 3(b)
there is one hot spot at 2.7 cycles per micron that sp
ans the MSPF from 60 nm to 300 nm, one hot spot
on the top surface of the filter at 1.4 cycles per micron that spans 300 nm to over 400 nm, and a
collection of hot spots below the bottom surface of the MSPF (at spectral frequencies spanning 0.5 to
2.3 cycl
es per micron).
Closer inspection of the intensity patterns of the hot spots below 70 nm shows that those at the
lower spectral frequencies are unbound modes that do not contribute to filter behavior. Discarding those
hot spots and plotting the rest of
the linear intensity variations of the single spectral frequency hot spots
yields the plot shown in Figure 3(c). This plot indicates there are three active plasmon modes in this
structure, on the top surface of the filter, one on the bottom surface of the
filter, and one spanning the
insulating layers contained within the filter. The predominant surface mode, shown in blue in Fig 3(c),
corresponds to SPPs excited at the top Ag surface of the filter. The other excitation is a super
-
mode
corresponding to a
coupling of the two MIM modes generated within each of the two insulating layers
in the structure. This is the mode that was implied in the spatial field map in Figure 3(a) is now clearly
depicted in Figure 3(c), which reveals that the two MIM modes with
in the super
-
mode are coupled
because of strong field overlap within the 50 nm Ag spacer.
The behavior of the energy propagating through the filter can be determined by the FFT analysis,
but it does not indicate how these modes contribute to the overall
behavior of the filter. By normalizing
6
the transmission curves over the dispersion behavior of each mode and the pitches of the filter gratings,
we can reveal the exact ways the modes are contributing to the behavior of the unsuppressed
transmission peak
.
The dispersion behavior of each plasmon mode can be determined by constructing a dispersion
curve for the MSPF. The dispersion curve shown in Figure 4(a) was constructed by using the Fourier
Transformed k
-
space data sets and plotting the power of the mo
des at each spectral frequency as a
function of energy. The two branches on curve correspond to the bottom side SPP and the metal
-
insulator
-
metal
-
insulator
-
metal (MIMIM) super
-
mode, and can be mapped to the frequency values that
correspond to these modes
in Figure 3(c). The lower intensity signal plotted to the left of the branches
corresponds to the unbound quasi
-
modes bouncing off the surface of the filter. From the data contained
within this plot, the dispersion curve for each of the two modes can be
analytically determined.
The transmission behavior was first normalized over the pitch of the filter grating to lift the
dependence of the transmission curves on that characteristic of the
filters3. Figures 4(b) and S1(b
) show
the transmission curves no
rmalized by the SPP
and MIMIM
dispersion curve
s respectively
. The
transmission peaks normalized by the SPP dispersion curve are aligned between 0.5 and 1, with the
transmission minima collapsing at 1 on the normalized axis
3
. This behavior illustrates that
the SPP mode
satisfies the momentum matching conduction required for it to contribute to the transmission behavior
of the filter, with the MIMIM super
-
mode acting as a supplementary suppression to remo
ve second
highest order peak.
Experimental Verifica
t
i
on
MSPFs were fabricated by depositing alternating layers of Ag and
SiO
2
in an electron beam
evaporator and then subsequently milled using a focused ion beam (FIB). The 50 nm slit milled into a
330 nm structure is a prohibitively demanding aspect ratio
for a FIB trench mill. For a set of proof
-
of
-
concept filters, these demanding design conditions can be relaxed by considering filters only towards
7
the lower energy portion of the visible spectrum. For a slit width of 120 nm, the
suppression of the
spurio
us transmission
peak is retained and the FWHM of the primary transmission peak only takes a 25
nm hit.
When Ag is deposited on
SiO
2
in an electron beam evaporator, the Ag films grow w
ith a
columnar growth mechanism
8
.
These films are rough, which increas
es plasmonic loss, thereby reducing
overall trans
mission intensity of the filter
9
.
The roughness of Ag deposited on
SiO
2
is even more
problematic in a multilayer structure like the MSPF because the roughness of each Ag layer compounds.
A rough substrate
increases the roughness of the film deposited on it due to differences in atomic flux
received by areas of the film with positive and negative curvatures that are larger than can be
compensated for by surface diffusion
10
. Because the
SiO
2
conformally depo
sits on the underlying Ag
layer, each Ag layer sees a progressively rougher substrate, leading to a very rough top surface of the
MSPF.
By utilizing a seed layer of AgO deposited onto each
SiO
2
surface, a much smoother Ag film can
be deposited
11
. The AgO
is deposited by e
lectron
beam evaporating Ag in a cha
mber with an O2
pressure of 9.5x10
-
5
torr. Once 2 nm of AgO are on the surface of the
SiO
2
, the deposition is paused and
the AgO is held under vacuum. Because AgO is not vacuum stable, the oxygen is p
umped out of the
film, leaving a thin Ag layer on the surface of the
SiO
2
11
. The deposition is then resumed and the rest of
the Ag is deposited at in a chamber with pressure 2.3
x10
-
6
torr and no oxygen flow. The roughness of
Ag films deposited with this
method was measured to have an RMS of 2.56 nm and the top Ag surface
of a multilayer deposited with the AgO growth method has an RMS of 2.92 nm.
To further protect the integrity of the filter, a sacrificial layer was put on the top Ag surface.
First a thi
n layer of PMMA was then spun onto the top surface of the MSPF and then another 70 nm
layer of Ag was deposited on top of the PMMA. The sacrificial layer protects the top Ag film of the
MSPF by confining the worst of the ion beam damage to the surface of
the sacrificial layer, rather than
the surface of the MSPF. To utilize the best possible resolution of the ion beam, the slits are milled in
8
FEI Versa FIB, at 30 kV and 1.5 pA
12
. The high accelerating voltage and low beam current help
compensate for the
high aspect ratio of the filter structure. The sacrificial layer is then removed using a
heated solvent bath.
The fabricated filters are then measured using a supercontinuum laser with monocrometer set
-
up
that allows for the sample to be illuminated wit
h a narrow bandwidth of incident radiation. A 50X
objective takes the collimated light and focuses it down to a 10 um spot size that is shined on the 30 um
x 30 um MSPFs. Squares equal in size to the filters were milled 100 um away from each filter and a
re
used to determine the intensity of the laser. All transmitted power was collected by a Si photodiode that
was affixed behind the substrate in which the filters and normalization squares have been milled. The
experimental response of each filter was de
termined by normalizing the light transmitted through the
filter by that transmitted through its corresponding normalization square. The experimental
transmission response generally confirmed the expected narrowband width behavior, as shown in Figure
5(a)
.
Figure 5(a) shows the experimental transmission response of a prototype filter that has an inter
-
slit pitch of 475 nm. A cross section of the prototype filter was milled using the FIB. The micrograph
of the filter’s cross
-
section, shown in Fig 5(
c
)
reveals that there is a slight taper to the filter structure.
Using FDTD simulations, we can compute the transmission behavior of filters with a progressively
increasing sidewall taper. The results of these
simulations, shown in Figure 5(b
) illustrate t
he
importance of the slit sidewalls on the overall behavior of the structure. Using the information gathered
from the FDTD simulations, it was determined that to maintain filtering behavior with side lobe
suppression, the sidewalls of the slit could not p
ossess greater than a 5
taper. The side lobe in the
experimental transmission is due to the 13.7
taper in the fabricated filter.
The FFT analysis indicated that the MSPF super
-
mode is responsible for
the suppression of the
spurious
transmission peak, and the individual MIM modes are coupled together. As the slits are
tapered, the difference between the lengths of the two channels increases, which affects the interference
9
between the two modes, thereby reducing the filtering efficie
ncy of this mode and allowing multiple
orders of modes to propagate through the structure.
The polarization response was also experimentally confirmed to match the simulated
predications, as shown in Figures 6(a) and (b).
Conclusi
ons
A plasmonic color f
ilter
with a
single
narrowband transmission response was designed us
ing
FDTD
and fabricated to confirm the simulated response
.
The filter is readily amenable
to device
integration
, with a size well
-
matched to state of the art CMOS image sensors. The plas
monic filter
utilizes a geometry that flexibly allows for precise selection of the spectral bands of interest
, allowing
for portable electronic devices to
be capable of multi
-
and hyperspectral imaging
.
The behavior of this
filter was analytically determi
ned to arise from a co
mbination of SPP excitations
--
t
he surface SPP
mode leads to the enhanced transmission behavior associated with sub
wavelength plasmonic filters,
while t
he slightly asymmetric MIM super
-
mode leads to
the suppression of the spurious
tra
nsmission
peak that arises in other narrowband plasmonic filter geometries. The MSPF is inherently gated, and
this feature will be capita
lized on in future work by incorporating transparent conducting oxides into
this geometry to create tunable narrowband
color filters spann
ing both the visible and near infrared
parts
of the spectrum.
Fabrication Method
s
A fused silica slide was prepared by 5 minute sonication in acetone followed by a rinse with IPA.
The alternating metal and insulating layers were all
deposited via electron beam deposition in the same
chamber to maintain the integrity of the Ag/
SiO
2
interfaces. A silver oxide seeding method was used to
produce smooth Ag films
11
. 2 nm of Ag is deposited at a rate of 0.1 A/s in a chamber under a
pressure
of 9.5
x10
-
5
torr O2. The AgO film is then reduced to an Ag film under vacuum for 10 minutes to yield
10
a
n
Ag film on the surface of the silica substrate. The remaining 68 nm of Ag are deposited at a 0.5 A/s
deposition rate followed by a
SiO
2
deposi
tion deposited at 1.5 A/s under a pressure of 2.3
x10
-
6
torr. The
remaining Ag and
SiO
2
layers are deposited using this method. Once the depositions are completed, 90
nm of PMMA is spun onto the Ag surface before depositing another 70 nm of Ag as a sacrif
icial layer.
Ga+ ions at 30 kV and 1.5 pA are used to mill 130 nm wide slits into the MSPF and sacrificial layer
stack. Multi
-
pass milling is used to reduce the taper of the slits
first a rectangle is milled, followed by
a frame around the perimeter, to
better define the edges and clean off redeposition within the slit. After
milling the sacrificial layer is removed using first heated remover PG followed by submerging it in
acetone and spraying it with an acetone squirt bottle before rinsing in IPA.
A
c
knowledgme
nts
This work was supported by Sony Corporation, the Hybrid Nanophonics Multidisciplinary Unviersity
Research Initiative Grant (Air Force Office of Scienti
fic Research FA9550
-
12
-
1
-
0024),
Northrop
Grumman Corporation, and the facilities of the Ka
vli Nanoscience Institute (KNI) at Caltech.
LAS
acknowledges support from the Resnick Sustainability Institute at Caltech.
Helpful discussions with
Michelle Sherrott, Max Jones, and Matt Sullivan
are also
gratefully
acknowledged
.
Re
f
erences
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12
Figure 1
(a) Schematics of MIM and MIMIM filter structures. All
dark grey
metal
layers are Ag and 70 nm
thick, except for the 50 nm center metal layer of the MIMIM filter. All
light grey
insulating layers are
70 nm of SiO2 (b) Comparison between MIM and MIMIM transmission behavior for the structures
shown in (a) that shows similar FW
HM but enhanced suppression of the secondary peak in the MIMIM
case
13
Figure 2
(a) Super position of the transmission behavior of filters with varying slit pitches. As slit pitch increases
the narrowband transmission peak is controllably shifted to longer wavelengths (b) The relationship
between FWHM, peak position, and sideband to
peak ratio. The blue axis illustrates the ratio of FHWM
to the peak position. The dashed line sets the threshold of a 30nm FWHM, and the dotted line illustrates
the ratio of the transmission peak’s FWHM to the peak position. The dotted line is beneath t
he dashed
line for the entire visible spectrum, indicating that all filters fulfill the criteria for hyperspectral imaging.
The orange dotted line illustrates the ratio between the sideband and main intensity peaks, showing the
best filters are also in the
visible part of the spectrum
14
Figure 3
(a) Field behavior from a single slit depicts the coupled interactions of the two MIM modes confined to
their insulating layers (b) A FFT of the spatial field behavior of single frequency excitations
indicates
the presence of multiple modes at different spectral frequencies spanning the structure. (c) These modes
can be spatially resolved via a Power FFT
15
Figure 4
(a) By taking FFTs of a sweep of single frequency excitations, a dispersion c
urve can be constructed
that illustrates the behavior of both active modes (b) Universal curve analysis confirms that the SPP
mode
on the top surface of the MSPF
filter is predominantly responsible for the filters transmission
behavior
. The various colors
of the transmission curves correspon
d to different peak intensity positions
that have all been normalized by the SPP dispersion curve.
16
Figure 5
(a) Experimentally determined transmission of a single MIMIM filter (b)
Simulated dependence of
transmission on taper of slits (c)
Top down and c
ross
-
sectional SEMs of the MSPF filter (d)
TEM
micrograph showing the layer thicknesses and roughness of the
five layers of the
filter
17
Figure 6
(a) Simulated
polarization response varying from 0 degrees (blue) to 90 degrees (green) (b) The
simulated polarization response was confirmed experimentally, with a 0 degree measurement (blue), a
30 degree measurement (red), and a 45 degree measurement (orange)