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The hybrid concept for realization of an ultra-thin
plasmonic metamaterial antireflection coating and
plasmonic rainbow
M. Keshavarz Hedayati,a S. Fahr,b C. Etrich,b F. Faupel,c C. Rockstuhlb and M. Elbahri*ad
We report on the design, simulation, fabrication, and characterization of a novel two layer anti-reflective
coating (ARC) based on a plasmonic metamaterial and a dielectric. Promoted by the strong material
dispersion of the plasmonic metamaterial, our novel concept (called hybrid ARC) combines two possible
Received 6th January 2014
Accepted 12th March 2014
arrangements for layers in an anti-reflection coating into a single structure; albeit at two different
wavelengths. This, however, causes a broadband reduction of reflection that is less sensitive against
oblique incidence when compared to traditional antireflective coatings. Furthermore, we show that the
DOI: 10.1039/c4nr00087k
current metamaterial on a metal reflector can be used for the visualization of different coloration such
www.rsc.org/nanoscale
as plasmonic rainbow despite its sub-wavelength thickness.
The nite reectivity from the interface of two disparate media
with dispersive material properties is an obstacle that oen
denies the design of efficient photonic and opto-electronic
devices.1 Traditionally, the problem can be diminished while
incorporating anti-reection coatings (ARCs)2 that are based on
graded index layers,3–6 gradient-index coatings,7,8 or nanostructured textures.9,10 However, these approaches usually suffer
from one or multiple severe drawbacks such as a narrow spectral domain of operation, sensitivity against oblique incidence,
complexity, or a lack of applicability to extremely thin lms.
Here, we mitigate these problems by introducing and verifying a
new class of ultrathin two-layer anti-reection coating with a
metamaterial as the top and a dielectric material as the second
layer; demonstrating therewith an entire novel concept named
as “hybrid ARC”. The key feature of this hybrid ARC is using
(quasi) two arrangements for the dielectric layers in one design
where the refractive indices ascend or descend in consecutive
layers with a descending order, albeit at a different wavelength.
This is only possible by exploiting the strongly dispersive
character of metamaterials. High ARC performance on silicon
substrate is shown to be possible by plasmonic nanocomposites
with a strong dispersion in the permittivity around its plasmonic resonance. Below the plasmon resonance wavelength the
a
Nanochemistry and Nanoengineering, Institute for Materials Science, Faculty of
Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstrasse 2, 24143 Kiel,
Germany
b
Institute of Condensed Matter Theory and Optics, Abbe Center of Photonics, FriedrichSchiller-Universität, Jena, Max-Wien-Platz 1, 07743 Jena, Germany
c
Chair for Multicomponent Materials, Institute for Materials Science, Faculty of
Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstrasse 2, 24143 Kiel,
Germany
d
Institute of Polymer Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1,
21502 Geesthacht, Germany. E-mail: me@tf.uni-kiel.de
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layer acts as a traditional graded-index coating while it performs
as a Fabry–Perot interferometer at longer wavelengths. This
provides the opportunity to considerably lower the reection
across a broad range of wavelengths with only a marginal
angular sensitivity. Moreover, the hybrid concept can be applied
on metals where the tunability of the plasmonic nanocomposite
enables realization of plasmonic rainbow colors by a subwavelength coating.
The consideration of anti-reective coatings (ARCs) as
being very important is justied from their integration in
nearly all photonic devices.11–15 Optical elements where they
nd use range from ordinary lenses over any laser system up
to advanced photonic devices for disruptive technologies.
Traditional ARCs made from an individual non-absorbing
layer can usually be optimized to operate perfectly at an isolated design wavelength. Then, the refractive index (RI) of the
ARC (being directly linked to the square root of the permittivity for non-magnetic, homogenous, isotropic, local materials as considered here) has to be the geometric mean of the
RIs of the materials on both sides of the respective interface
from which the spurious reection is encountered, i.e. herein
called a substrate and the incident medium. By no means of
restriction, we consider in the following the RI of the substrate
to be larger than the RI of the incident material (coating). The
thickness of the ARC ought to be a quarter of the desired
wavelength. However, and quite detrimental, the vanishing
reectivity only occurs at normal incidence and only at the
isolated design wavelength. Nevertheless, wider-band ARCs are
possible by relying on innovative designs,16,17 plasmonic and
metamaterial layers18,19 or multilayer coatings.20 For instance,
two-layer ARCs,21 i.e. ‘V’ coat, could result in a wide-band ARC
by a proper selection of lms. In such a case the RI of the top
layer should be smaller than the second layer and each layer
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thickness shall equal a quarter of the desired wavelength. The
rst suggestion for such a traditional arrangement of the layer
materials is linked to the name of Lord Rayleigh; hence
hereaer we call it the Rayleigh conguration. In fact, the
order of the ARC lm is very crucial in such a technique and
once the order of layers is inversed (i.e. low and high RI
placing as the spacer and the top layer, respectively), the
reection of the device increases and could even turn the
substrate (in certain circumstances) into a mirror (Braggmirror).22,23 We wish to call such an arrangement in the
following the reverse-Rayleigh conguration. Note that the in
the reverse-Rayleigh conguration at least two reection dips
surround the central reection peak.24
Recently, a new class of ARCs has been introduced, specically for metallic substrates where the coating acts as an
absorbing element to reduce the light reection. In such a
conguration, the reection drops not only due to the interference25 but also by exploiting the absorbing character of the
coating combined with losses in the metallic substrate.26–34 The
strong optical attenuation through the highly absorbing coating
or the strong resonant behavior in layers29 gives rise to low
reection from the metal substrate, although the thickness of
the coating is much less than the wavelength of light. This
concept works well on gold lms which could sustain a plasmonic response under certain conditions as well as a broad
intrinsic absorption (i.e. minimum reection) in the visible
range because of interband transition. The question arises
whether this concept is equally applicable to semi-conductors
as the substrate material. It is known that semi-conductors like
silicon exhibit strongly dispersive material properties that
complicate the design of efficient ARCs. Therefore, it remains a
challenge to perceive an ARC that operates over the entire range
of the visible and near-IR for semi-conductors.35
Here, we introduce an ultra-thin bi-layer coating as an ARC.
The key-feature of our coatings is the use of a material with a
high RI for the layer that faces the incident medium, i.e. the top
layer. Moreover, the thicknesses of all involved layers are
considerably thinner than a half or even a quarter of a wavelength. By a systematic analysis we show that an excellent antireection performance is possible. While demonstrating the
concept in the rst stage with a pair of dielectric materials
possessing only a weak dispersion, i.e. TiO2/SiO2, we exploit the
strong dispersive nature of metamaterials in the second stage to
demonstrate the hybrid-concept and eventually achieve a broadband ARC with only a marginal angular sensitivity.
The metamaterial we will use consists of an ultrathin plasmonic nanocomposite made from ultra-ne metallic nanoparticles (diameter D < 5 nm (ref. 26)). Fig. 1a and b show the
cross-sectional and top view TEM images of the sample. It is
apparent that the particles' diameters are around 5 nanometers
and they are randomly distributed in the matrix. It possesses a
dispersive permittivity with a Lorentzian prole centered at the
particle plasmon resonance. The homogenous isotropic metamaterial is characterized by a strongly dispersive RI that takes
high values at long wavelengths and small values at short
wavelengths, taken with respect to the particle plasmon resonance. Such a material can therefore benecially be used to
6038 | Nanoscale, 2014, 6, 6037–6045
Paper
perceive an ARC that combines the reverse-Rayleigh and the
Rayleigh ARC in the same structure, albeit at different wavelengths. Therefore, we call this structure a “Hybrid-Antireection” structure. We postulate that the broadband anti-reection
performance of the presented metamaterial is facilitated by the
anomalous material dispersion around the plasmon resonance.
The dispersive refractive index of the composite varies in a way
that at wavelengths longer than the resonance the coating
serves in a reverse-Rayleigh conguration but at smaller wavelengths the composite's RI is smaller than the second layer and
hence the Rayleigh condition is satised. In other words, by
overlapping the reection dip of the traditional ARC with that of
a Fabry–Perot interferometer, the corresponding reection dip
of our two layer coating is very broad despite its low thickness.
This unique dispersive RI of the presented metamaterial leads
to the opportunity to observe a hybrid wide-band ARC encompassing the Rayleigh/reverse-Rayleigh congurations. Full wave
electro-magnetic simulations of the metamaterial verify that it
acts as a homogenous medium rather than an ensemble of
plasmonic absorbers. This entails their description in terms of
effective material properties which paves the way to consider
such a structure in the design of many high efficient ARC
devices.
To start with, we consider a bi-layer ARC where the top layer
facing air as the incident medium is an ultrathin lm of a high
RI material. With the nal device in mind, the thickness is
chosen to be 20 nm. This adheres to the desire to have an ultrathin and compact ARC. We leave here the exact value of the RI as
a free parameter. The second layer shall be made from a low RI
material. Silicon dioxide is selected as the low RI lm since it is
a common material in silicon industry and it can be either
deposited or grown on the silicon substrate with good adhesion.
We leave as a degree of freedom the thickness of this layer. In
such a scheme, the substrate, i.e. silicon wafer, has the highest
RI in the stack.
To identify on analytical grounds the conditions where the
ARC operates optimal, a thin-lm transfer matrix technique is
applied to calculate the reection. Results are shown in Fig. 1c.
There, the reectivity at a design wavelength (in this case at 600
nm) is calculated depending on the RI of the top layer and the
thickness of the SiO2 layer. It is apparent that the reection is
suppressed for a rather high value of RI of the ultrathin top layer
and a thin SiO2 layer. To suppress reectivity at longer design
wavelengths, the RI should be approximately the same but the
thickness of the SiO2 layer should be slightly increased.
However, it can always be assured that the reection can be
reduced to a negligible quantity, even though the coating is subwavelength in its thickness, i.e. far below the quarter of the
design wavelength.
According to the calculated reection contour (Fig. 1c), a
high RI material which suitably matches the required RI is TiO2
(its average RI in the visible region is 2.4 (ref. 36)). The oxide
lms were prepared by sputtering of a dielectric target (namely
SiO2 or TiO2) and the thicknesses were measured with a prolometer. Based on the simulation, 20 nm TiO2 layer and a 50
nm SiO2 coated on silicon could realize low reection at 570
nm. Indeed, the fabricated stacks with the mentioned geometry
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(a) Cross-sectional image of plasmonic nanocomposite in which the arrows indicate each of the samples. Light blue, blue, green and red
arrows represent the platinum (top adhesion layer for cutting of the sample), nanocomposite, SiO2 layer and silicon wafer, respectively. (b) Top
view TEM image of the plasmonic nanocomposite wherein the dark spheres are the silver particles. (c) Reflection of a 20 nm film with varying RI
on a SiO2 film with varying thickness on top of a silicon substrate at a wavelength of 600 nm. (d) (black) Reflection spectrum of a typical reverseRayleigh configuration (20 nm TiO2 atop of 50 nm SiO2 coated Silicon) in comparison with bare silicon (blue). The inset shows the reflection
spectrum of 50 nm SiO2 atop of 20 nm TiO2 coated silicon (Rayleigh configuration).
Fig. 1
and thicknesses provide a broad reection reduction with a
reection minimum at 570 nm wavelength for silicon as shown
in Fig. 1d which agrees well with simulation. By resorting to a
traditional order for the layers, i.e. an ordinary Rayleigh
conguration where the materials are arranged in the
ascending order of their RI, it was observed that the reection
minimum occurs at 410 nm [Fig. 1d (inset)], in agreement with
Rayleigh's postulation but it does not vanish totally since its
thickness is far below the quarter-wavelength which is required
for anti-reectivity.
It can be seen that the ARC in the Rayleigh conguration is
spectrally narrower than the reverse-Rayleigh conguration and
its remaining reection almost doubled. However, since those
results might be affected by experimental uncertainties, we
would like to stress the major advantage of the reverse-Rayleigh
concept that can be better appreciated while comparing the
angular dependency of both congurations (i.e. Rayleigh and
reverse-Rayleigh). The average reection at higher incidence
angles for the case of the Rayleigh conguration is almost twice
the intensity of a similar lm but in reverse order (Fig. 2a). In
fact, the reection drop considerably red-shis upon changing
the geometry from traditional to the reverse-one which proves
that the performance of ultra-thin reverse-Rayleigh ARC is more
promising for an operation in the visible spectrum.
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The more pronounced reection drop and red-shi of the
curve in reverse-Rayleigh compared to the Rayleigh conguration can be well explained by interference.37 In principle,
destructive interference of the direct reected light and the light
reected at consecutive interfaces requires a phase accumulation of p by the wave traversing the layers back and forth. This
easily explains the dogma on using quarter wavelength layers
with a small RI as the rst (top) layer. However, in the case of a
top-lm with high RI (reverse-Rayleigh), the light which travels
through the low RI layer and the reected wave have a p phase
difference38 which ends up with a destructive interference. In
other words, in the case of a top-lm with high RI, p phase
accumulation comes for free as the impinging light reected at
the various interfaces experiences p–0–p phase shis. The
phase difference of the incoming and reected light in the
2p
cavity is39 DF ¼
ð2ndÞ þ p where n and d are the RI and
l
thickness of the spacer layer, respectively. By inserting the
values of thickness and RI of SiO2 in the mentioned equation, it
3p
ends up with
, which provides the condition of destructive
2
interference. Color changes of the coating by changing the
thickness of the spacer layer (i.e. SiO2 lm) from 10 to 50 nm,
further prove the interference role in the reverse-Rayleigh
coating. Fig. 2b shows the true color photograph of the silicon
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Fig. 2 (a) Reflection spectra of 20 nm TiO2 atop of 50 nm SiO2 coated on silicon (solid line) and 50 nm SiO2 on 20 nm TiO2 coated on silicon
(dashed line) at different angles of incidence. "RR" and "R" stand for "reverse-Rayleigh" and "Rayleigh" configurations, respectively. (b) True
photograph of silicon samples coated with 30 nm TiO2 film atop of (from left to right) 10, 30 and 50 nm SiO2 films, respectively. (c) Complex
refractive index of the Ag–SiO2 nanocomposite with 15% (black), 20% (red) and 40% (blue) filling factors. Solid and dotted-lines represent the real
and imaginary parts of RI, correspondingly. (d) Geometry of the anti-Rayleigh hybrid ARC which is composed of the 20 nm nanocomposite atop
of SiO2 coated silicon.
samples coated with 30 nm TiO2 lm atop of 10, 30 and 50 nm
SiO2 on the silicon substrate.
Nevertheless, neither ARC based on reverse-Rayleigh concept
nor Rayleigh provide the desired properties of a wide-band ARC
(i.e. whole solar spectrum range) owing to high reection that
appears at short and long wavelengths of the visible for both
congurations. Additionally, for shiing the reection dip to
NIR, increasing the layer thickness and/or using a high RI
material is needed. Note that there are only a limited number of
materials with high refractive index which could fulll the
required RI contrast for the reverse-Rayleigh conguration.
In our opinion, and from technological point of view, the
eld is revolutionized if a coating is used that would enable
both Rayleigh and reverse-Rayleigh congurations simultaneously, at different wavelengths though. Suppressing the
reections at multiple wavelengths would automatically enable
a broadband ARC based on ultrathin lms. However, this
requires the use of strongly dispersive materials in the design of
the ARC. Ideally, the geometrical dispersion that degrades the
anti-reection action beyond the target wavelength in an ARC
design where non-dispersive materials are used has to be
compensated by a suitable material dispersion. This perfect
balancing, however, requires an anomalous dispersion in the
material properties, which is always accompanied by absorption. Nonetheless, motivated by the recent work on ARC
6040 | Nanoscale, 2014, 6, 6037–6045
coatings on metals using weakly absorbing materials,29 we may
conclude that if the ARC coating is sufficiently thin and
contains ultrane plasmonic nanoparticles, the absorption
might not be detrimental. To evaluate the potential of this idea,
we sought out a way to design a new concept for a coating that
considers articial materials (metamaterial) in their design that
possesses a strong chromatic anomalous dispersion.
Accordingly, we consider a plasmonic nanocomposite of tiny
metallic nanoparticles embedded in a dielectric host as a metamaterial with the required highly dispersive refractive index.
The properties of the nanocomposite can be tuned by many
parameters which constitute a great degree of freedom. They
constitute an extraordinary material platform with many
intriguing advantages. The fabrication of these nanocomposites
is based on self-assembly processes using sputter techniques,
which is well established,40–42 and they can be deposited on a
large surface in a short time and at low costs (for more details
see the Methods section).
This metamaterial derives its unique properties from the
excitation of localized plasmon polaritons in the metallic
nanoparticles.43 The nanoparticles are sufficiently small and
arranged sufficiently dense, such that the material can be
considered as effectively homogenous and isotropic. The
material is characterized by a Lorentzian resonance in the
effective permittivity which is centered at the plasmon
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resonance wavelength. Due to the isotropy of the material and
the vanishing of any magnetic response, the permittivity
uniquely denes the effective refractive index. The material can
be perceived as a strongly dispersive dielectric with some nite
absorption in resonance.
It is known that, with lling fractions for the metallic
nanoparticles between 20% and 40%, the effective properties
cannot be derived from canonical effective medium theories
such as Clausius–Mossotti.44 Instead, we used nite-difference
time-domain (FDTD) simulations of a sufficiently large supercell and calculated the dispersive complex reection and
transmission coefficient45 (cf. Methods section). From these
coefficients effective properties were retrieved for the
composite. These parameters were aerwards fueled into a
thin-lm transfer matrix technique to simulate all quantities of
interest. Selected congurations were equally simulated by the
FDTD method to cross-check the predictive power of the effective properties. Identical results were always predicted, justifying the treatment of the nanocomposite as an effective
medium (for more details see the Methods section). The
dispersive RIs of the nanocomposites with different lling
factors are shown in Fig. 2c. It is apparent from this dispersion
graph that at resonance the absorption is maximal and the real
part of the RI undergoes anomalous dispersion. At wavelengths
longer than the resonance wavelength the material is characterized by a large RI (large permittivity) and hence would be
suitable to serve in the reverse-Rayleigh conguration as the top
Nanoscale
material (ntop > nspacer). In contrast, at wavelengths smaller than
the resonance wavelength, the medium is characterized by a
rather small RI (small permittivity) and accordingly would be
appropriate to be used in the Rayleigh conguration as the top
material (ntop < nspacer). Therefore, plasmonic nanocomposites
can be considered as the hybrid ARC that meets the condition of
both Rayleigh and reverse-Rayleigh geometries resulting in a
broad-band ARC coating.
We demonstrate the hybrid concept by coating a polished
silicon wafer with a 20 nm silver–silicon dioxide nanocomposite
separated from the substrate by a thin layer (50 nm) of silicon
dioxide as shown schematically in Fig. 2d. Such a stack gives
rise to the realization of a black silicon with a homogeneous
ultrathin layer coating. Fig. 3a shows the reection spectra of
20% and 30% nanocomposites deposited on 50 nm SiO2 coated
silicon and the inset is the true color photograph of the sample
with 30%, which looks black indeed.
Angular reectance measurement of the coating with 30%
lling factor (Fig. 3b) shows the marginal angular and polarization dependency of the plasmonic hybrid ARC.46 More details
on the angular behavior,33 especially for the angular domain
that is not shown here, can be found in the literature.26
The spectra possess two main dips in the reection spectra.
The small wavelength dip is attributed to the graded AR (i.e. in
analogue to a conguration where 50 nm SiO2 is deposited atop
of 20 nm TiO2 (cf. Fig. 1d (inset))) and the second reection dip
originates from the destructive interference of the reected eld
Fig. 3 (a) Reflection spectra of 20 nm Ag–SiO2 with 20% (red) and 30% (blue) filling factors deposited atop of 50 nm SiO2 coated silicon. The inset
shows the true color photograph of the silicon coated with the optimized film, which turns black in its appearance. (b) Reflection spectra of the
film as in Fig. 2c measured at different angles of incidence with s-(solid lines) and p-polarization (dotted lines), (c) reflection spectra of hybrid ARC
with 30% filling fraction of metal in the top layer deposited on spacer layers with three different thicknesses. The ARC is deposited again on
silicon. Black, red and blue curves showing the spectra of 50, 70 and 100 nm thick spacer layers, correspondingly. (d) Reflection contour of the 20
nm film with various RIs on the SiO2 film with different thicknesses at (left) 800 nm and (right) 900 nm wavelengths.
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(i.e. in analogy with the TiO2 lm atop of SiO2 (cf. Fig. 1d (black
curve))).
In spite of the expected wavelength shis of the antireection dips [cf. Fig. 3b (inset)], one peak at 450 nm (i.e. where the
plasmon of the nanocomposite arise) is also revealed that is
invariant against the angle of incidence and hence we attribute
it to the plasmon resonance of the composite. Indeed, the
wavelength of the peak remains unchanged by angle variation,
which conrms the localized nature of the (particle plasmon)
resonance.
To demonstrate the tunability of the hybrid ARC coating and
meanwhile to gain more understanding about the role of plasmon in the observed phenomena, the effect of the spacer layer
on the optical responses was examined. Keeping the top layer
thickness and composition constant while increasing the
thickness of SiO2 from 50 nm to 100 nm, a red-shi of the
reverse-Rayleigh as well as Rayleigh antireection dips was
observed (Fig. 3c). This behavior is in good agreement with our
simulation (Fig. 3d) which shows that an increase of the spacer
results in a broad ARC in NIR (800–900 nm) for the case of the
reverse-Rayleigh conguration. On the other hand, ARC
performance of the graded conguration (small wavelength
regime) in the hybrid is deteriorated by thickening the spacer
Fig. 4 (a) True photograph of 25 nm SiO2 coated gold film covered by
20 nm Au–SiO2 nanocomposite with different filling factors. From left
to right the filling factor of the nanocomposite increased (13%, 20%,
30%, 40%, 50% and 60%, correspondingly) while the first and second
samples are bare gold and 45 nm SiO2 coated gold, respectively. (b)
Calculated transmission enhancement of silicon by plasmonic coating,
which is calculated by normalization of the transmission of coated
silicon by that of the bare one. The black line shows the normalized
transmission of bare silicon while the red curve is the normalized
transmission of the Ag–SiO2 nanocomposite deposited on SiO2
coated silicon.
6042 | Nanoscale, 2014, 6, 6037–6045
Paper
layer. Indeed, the origin of the mentioned high reection with a
thicker inter-layer could be attributed to two phenomena; rstly
the constructive interference of the incident and the reected
light, and secondly, the spectral overlap of the plasmonic
resonance of the nanocomposite and the Rayleigh antireection
dip. Such an overlap occurred because of the red-shi of the
Rayleigh originated dip via thickening of the spacer layer.
Indeed, integrating the plasmonic structure as a hybrid ARC
coating provides an additional degree of freedom for tuning the
performance of the ARC coating. In other words, by using the
hybrid plasmonic ARC, the designer can reach the desired
optical properties not only by alteration of the layer thickness
but also by adjusting the lling factor (RI) and type of the
metallic constituents of the nanocomposite. Generally, the
performance of the hybrid ARC depends on the contrast of the
RIs between the layers. At the wavelengths where the top layer
shows higher RI, the reverse-Rayleigh condition is satised but
the traditional Rayleigh would be realized once the top lm
possesses the lower RI in the stack. The presented plasmonic
anti-reector shows such a low and tunable reectivity due to
the extreme dispersive RI of the metamaterial (cf. Fig. 2c), which
has been shown formerly.47–50 The RI of the nanocomposite with
40% lling factor changes from 0.9 up to 3.15 from small to
long wavelengths. In other words, the relative RI changes of the
host matrix before and aer embedding of the nanoparticles
can vary from 30% to +140%.
In fact, the tunability of the plasmon resonance and correspondingly the reection change29 by changing the lling factor
of the nanocomposite provide the possibility for the coloring of
metals using our hybrid concept. Fig. 4a shows a true photograph of samples of 25 nm SiO2 coated gold lm which is
covered with 20 nm gold–SiO2 nanocomposite with a variety of
lling factors, creating a spectrum of colors including yellow,
orange, blue and green. The colors originated from the different
reection drops associated with each lling factor demonstrating the potential of the hybrid concept for realization of
plasmonic rainbow colors.51 Note that demonstration of various
colors on the silicon substrate is not possible under the same
conditions (i.e. constant thickness of layers while varying the
lling factor). It seems that, the surface plasmon of the gold
lm (which is absent on silicon) in parallel with the hybrid ARC
contributes to the rainbow colors.
We believe that the hybrid concept could pave the way for
new highly efficient ARCs for a variety of applications ranging
from photovoltaics52 and optics to solar absorbers and stealth
technology53,54 as well as other elds where high reection is
undesired. But for energy applications, not only the low reectance rather high transmission is desired. Calculation showed
that the light transmission into the substrate (silicon) is
enhanced by using the plasmonic coating. Fig. 4b shows the
light absorbed by silicon (i.e. light transmitted into silicon) by
means of current plasmonic coating. The light reaching the
substrate is apparently increased through the coating, which
shows the potential of such an approach for energy harvesting
purposes. However, the preliminary results on photocurrent
measurements of presented coating on p-silicon showed some
current loss that we attributed to the poor interface of the
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prepared lms which acts as the sites for electron–hole
recombination. Nevertheless, a more sophisticated design is
required to better explore the role of the present ARC for electron–hole generation which is beyond the scope of this
manuscript.
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Conclusions
In summary, we demonstrated a new concept for antireection
coatings and experimentally showed an ultra-thin tunable
plasmonic anti-reector by using a routine technique of MEMS/
NEMS technology. The developed hybrid ARC, which is based on
a continuous plasmonic medium, acts as a consolidation of the
graded-index and interferometer (reverse-Rayleigh) providing a
wide-band reection drop throughout the visible spectrum. Due
to the simplicity of our production approach, the concept can be
further extended to other substrates/applications where broadband ultra-thin anti-reective coatings are required.
Nanoscale
company was used. To extract the absolute value of reection, the
measured reection spectra of the samples were normalized to
the tabulated data of the mirror provided by the manufacturing
company (PerkinElmer). In all types of measurements, the scan
step was xed to 4 nm and the base line was collected twice by a
full sweep across the spectral domain of interest while the integration and acquisition times were kept constant.
Polarization-dependent and variable angle spectroscopic
Ellipsometry reection measurements of the lms was carried
out with J.A. Woollam Co., Inc. M2000 UI (spectroscopic
ellipsometer) with a dual lamp system with deuterium and
Quartz Tungsten Halogen (QTH) lamps as light sources (data
provided by LOT catalogue Europe). The angle sweep step was
selected to be 5 or 10 and the angle variation from 45 to 85
was performed. In order to have a comparable study and achieve
the best signal-to-noise, 5 second acquisition time was applied
for all experiments. Accordingly, the measurement did not take
more than few seconds. For analyzing the data, CompleteEASE®
soware package provided by the company was used.
Methods
Fabrication
Simulations
All depositions were carried out in a cylindrical vacuum
chamber, which was primarily evacuated to 10 6 mbar. We used
an RF magnetron for sputtering of SiO2 and a DC magnetron
sputter source for silver and gold. Both sources were oriented in
reverse directions relative to the sample holder at 50 angle to the
substrate plane. All the coating was done while the rotatable
sample holder was spinning in order to end up with a uniform
thickness and composition (details in ref. 40–42). In order to
keep the lling factor below the percolation threshold, the
deposition rate of the metal was adjusted to be less than that of
the dielectric during co-deposition. In other words, to avoid the
coalesce of nanoparticles in the matrix, i.e. the formation of
fractal aggregates of NPs, the deposition rate of SiO2 was set to be
10 nm s 1 while the rate of silver was adjusted to be around 3 nm
s 1. However, for the creation of the plasmonic rainbow, the
mentioned constrain was actually not necessary. Therefore, the
gold deposition rate was varied between 3 and 13 nm s 1. This
enabled us to fabricate a gold nanocomposite with a wider range
of lling factors (below and above the percolation threshold).
Nevertheless, we increased the rate of deposited gold only to an
extent such that we avoid the formation of a continuous gold lm
instead of particles. By formation of a gold lm no localized
plasmon resonance would appear which has to be avoided. From
the nal TEM investigation data we could eventually observe that
the particles do not coalesce and they are mainly spherical which
further simplied the theoretical modeling (see Simulation).
The thickness of the lms was measured with a prolometer
(Dektak 8000 surface prole measuring system) and the thickness of dielectric was further measured with an ellipsometer
(M2000 (J.A. Woollam Co., Inc.)). Optical properties of the
samples at normal incidence were measured with a UV/vis/NIR
spectrometer (Lambda900, Perkin Elmer). For transmission
measurements, the base line was collected by measuring the
empty compartment (i.e. air considered as the reference) while
for reection measurements, the mirror provided by the
FDTD simulations were made with an in-house developed code
on a sufficiently large cluster.45 The simulated structure
compares entirely to the experimental geometry. For this
purpose a random arrangement of spherical metallic nanoparticles with a diameter corresponding to the mean diameter
as extracted from the TEM samples has been generated. The
lling fraction has been adjusted according to the experimental
values and we only enforced an isolation of all spherical nanoparticles, i.e. their interpenetration has been excluded. The
spatial domain considered in the simulation was 100 nm 100
nm in lateral directions. In these directions periodic boundaries
were enforced to eventually mimic an innitely extended space.
The chosen spatial domain was sufficiently large to exclude any
notable effect from the periodicity. In the propagation direction
the sequence of layers and their respective thicknesses have
been considered in full analogy with the experimental situations. Permittivity of SiO2 has been taken as non-dispersive and
equal to 2.25. Permittivity of gold was taken as tabulated in the
literature55 but with an additional correction term to accommodate the nite and small size of the nanoparticles.56 The
intrinsic dispersion of the material has been fully taken into
account by performing at each wavelength an individual
simulation and adjusting the free parameters in a Drude model
to provide a material with the respective properties at the
considered wavelength. Spatial discretization in the FDTD was 1
nm and perfectly matched layers were used in the propagation
direction. To retrieve the effective properties the complex
reection and transmission coefficients have been extracted
from the FDTD simulations and a parameter retrieval has been
applied.57
This journal is © The Royal Society of Chemistry 2014
Acknowledgements
M.K.H., F.F. and M.E. gratefully acknowledge the nancial
support by the German Research Foundation (DFG) through the
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Nanoscale
projects EL 554/1-1 and SFB 677 (C1,C9). M.E. would like to
thank the Initiative and Networking Fund of the Helmholtz
Association (grant no. VH-NG-523) for providing the nancial
base for the start-up of his research group. The authors also
gratefully acknowledge Dr U. Schürmann and Prof. Kienle for
TEM measurements. This work was supported by the German
Federal Ministry of Education and Research (PhoNa) and by the
Thuringian State Government (MeMa). We thank Karsten Verch
(http://www.karstenverch.com) for his artistic view of the
concept in Fig. 2d.
Notes and references
1 D. Bouhafs, A. Moussi, A. Chikouche and J. Ruiz, Design and
simulation of antireection coating systems for
optoelectronic devices: Application to silicon solar cells,
Sol. Energy Mater. Sol. Cells, 1998, 52, 79–93.
2 L. Rayleigh, On reection of vibrations at the connes of two
media between which the transition is gradual, Proc. Lond.
Math. Soc., 1879, 1, 51–56.
3 J. Cox, G. Hass and A. Thelen, Triple-layer antireection
coatings on glass for the visible and near infrared, J. Opt.
Soc. Am., 1962, 52, 965–968.
4 A. Turner, Some current developments in multilayer optical
lms, J. Phys. Radium, 1950, 11, 444–460.
5 W. Lowdermilk and D. Milam, Graded-index antireection
surfaces for high-power laser applications, Appl. Phys. Lett.,
1980, 36, 891–893.
6 J.-Q. Xi, et al. Optical thin-lm materials with low refractive
index for broadband elimination of Fresnel reection, Nat.
Photonics, 2007, 1, 176–179.
7 S. Mukherjee and W. Lowdermilk, Gel-derived single layer
antireection lms, J. Non-Cryst. Solids, 1982, 48, 177–184.
8 M. Minot, The angular reectance of single-layer gradient
refractive-index lms, J. Opt. Soc. Am., 1977, 67, 1046–1050.
9 P. Clapham and M. Hutley, Reduction of lens reexion by the
“Moth Eye” principle, Nature, 1973, 244, 281–282.
10 Y.-F. Huang, et al. Improved broadband and quasiomnidirectional anti-reection properties with biomimetic
silicon nanostructures, Nat. Nanotechnol., 2007, 2, 770–774.
11 R. Peterson and J. Ramsey, Thin lm coatings in solarthermal power systems, J. Vac. Sci. Technol., 1975, 12, 174–
181.
12 J. Zhao and M. A. Green, Optimized antireection coatings
for high-efficiency silicon solar cells, IEEE Trans. Electron
Devices, 1991, 38, 1925–1934.
13 J. K. Kim, et al. Light extraction enhancement of GaInN light
emitting diodes by graded refractive index Indium Tin Oxide
anti reection contact, Adv Mater., 2008, 20, 801–804.
14 P. Spinelli, M. Verschuuren and A. Polman, Broadband
omnidirectional
antireection
coating
based
on
subwavelength surface Mie resonators, Nat. Commun.,
2012, 3, 692.
15 Y. Wang, Y. Liu, H. Liang, Z. Mei and X. Du, Broadband
antireection on the silicon surface realized by Ag
nanoparticle-patterned black silicon, Phys. Chem. Chem.
Phys., 2013, 15, 2345–2350.
6044 | Nanoscale, 2014, 6, 6037–6045
Paper
16 A. Kabiri, F. Capasso and E. Girgis, Buried Nanoantenna
Arrays: Versatile Antireection Coating, Nano Lett., 2013,
13(12), 6040–6047.
17 P. Spinelli, et al. Optical impedance matching using coupled
plasmonic nanoparticle arrays, Nano Lett., 2011, 11, 1760–
1765.
18 X.-R. Huang, R.-W. Peng and R.-H. Fan, Making metals
transparent for white light by spoof surface plasmons,
Phys. Rev. Lett., 2010, 105, 243901.
19 R.-H. Fan, et al. Broadband antireection and light-trapping
enhancement of plasmonic solar cells, Phys. Rev. B: Condens.
Matter Mater. Phys., 2013, 87, 195444.
20 D. M. Braun and R. L. Jungerman, Broadband multilayer
antireection coating for semiconductor laser facets, Opt.
Lett., 1995, 20, 1154–1156.
21 B. Moys, The theory of double-layer antireection coatings,
Thin Solid Films, 1974, 21, 145–157.
22 Y. Fink, et al., A dielectric omnidirectional reector, Science,
1998, 282, 1679–1682.
23 M. F. Schubert, J.-Q. Xi, J. K. Kim and E. F. Schubert,
Distributed Bragg reector consisting of high-and lowrefractive-index thin lm layers made of the same material,
Appl. Phys. Lett., 2007, 90, 141115–141113.
24 M. Ettenberg, A new dielectric facet reector for
semiconductor lasers, Appl. Phys. Lett., 1978, 32, 724.
25 H.-T. Chen, Interference theory of metamaterial perfect
absorbers, Opt. Express, 2012, 20, 7165–7172.
26 M. K. Hedayati, et al. Design of a perfect black absorber at
visible frequencies using plasmonic metamaterials, Adv
Mater., 2011, 23, 5410–5414.
27 M. K. Hedayati, F. Faupel and M. Elbahri, Tunable
broadband plasmonic perfect absorber at visible
frequency, Appl. Phys. A: Mater. Sci. Process., 2012, 109,
769–773.
28 M. A. Kats, et al. Ultra-thin perfect absorber employing a
tunable phase change material, Appl. Phys. Lett., 2012, 101,
221101–221105.
29 M. A. Kats, R. Blanchard, P. Genevet and F. Capasso,
Nanometre optical coatings based on strong interference
effects in highly absorbing media, Nat. Mater., 2012, 12,
20–24.
30 X. Xiong, S. C. Jiang, Y. H. Hu, R. W. Peng and M. Wang,
Structured metal lm as a perfect absorber, Adv Mater.,
2013, 25, 3994–4000.
31 J. Hao, et al. High performance optical absorber based on
a plasmonic metamaterial, Appl. Phys. Lett., 2010, 96,
251104.
32 N. Liu, M. Mesch, T. Weiss, M. Hentschel and H. Giessen,
Infrared perfect absorber and its application as plasmonic
sensor, Nano Lett., 2010, 10, 2342–2348.
33 M. Hedayati, A. Zillohu, T. Strunskus, F. Faupel and
M. Elbahri, Plasmonic tunable metamaterial absorber as
ultraviolet protection lm, Appl. Phys. Lett., 2014, 104,
041103.
34 M. K. Hedayati, F. Faupel and M. Elbahri, Review of
Plasmonic
Nanocomposite
Metamaterial
Absorber,
Materials, 2014, 7(2), 1221–1248, DOI: 10.3390/ma7021221.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Open Access Article. Published on 20 March 2014. Downloaded on 09/07/2014 09:48:46.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
35 E. T. Hamden, et al., Ultraviolet antireection coatings for use
in silicon detector design, Appl. Opt., 2011, 50, 4180–4188.
36 M. Radecka, K. Zakrzewska, H. Czternastek, T. Stapiński and
S. Debrus, The inuence of thermal annealing on the
structural, electrical and optical properties of TiO2 x thin
lms, Appl. Surf. Sci., 1993, 65–66, 227–234, DOI: 10.1016/
0169-4332(93)90663-V.
37 H.-T. Chen, et al. Antireection coating using metamaterials
and identication of its mechanism, Phys. Rev. Lett., 2010,
105, 073901.
38 D. L. Caballero, A theoretical development of exact solution
of reectance of multiple layer optical coatings, J. Opt. Soc.
Am., 1947, 37, 176–178.
39 K. Renk and L. Genzel, Interference lters and Fabry-Perot
interferometers for the far infrared, Appl. Opt., 1962, 1,
643–648.
40 F. Faupel, V. Zaporojtchenko, T. Strunskus and M. Elbahri,
Metal polymer nanocomposites for functional applications,
Adv. Eng. Mater., 2010, 12, 1177–1190.
41 U. Schürmann, W. Hartung, H. Takele, V. Zaporojtchenko
and
F.
Faupel,
Controlled
syntheses
of
Ag–
polytetrauoroethylene nanocomposite thin lms by cosputtering from two magnetron sources, Nanotechnology,
2005, 16, 1078.
42 H. Takele, H. Greve, C. Pochstein, V. Zaporojtchenko and
F. Faupel, Plasmonic properties of Ag nanoclusters in
various polymer matrices, Nanotechnology, 2006, 17, 3499.
43 S. A. Maier, Plasmonics: Fundamentals and Applications,
Springer, 2007.
44 T. C. Choy, Effective Medium Theory: Principles and
Applications, Oxford University Press, 1999, vol. 102.
45 A. Taove and S. C. Hagness, Computational Electrodynamics:
the Finite-difference Time-domain Method, Artech House,
Incorporated, 2005.
This journal is © The Royal Society of Chemistry 2014
View publication stats
Nanoscale
46 C. Etrich, et al., Effective Optical Properties of Plasmonic
Nanocomposites, Materials, 2014, 7, 727–741.
47 U. Schürmann, Eigenschaen von Polymer-SilberNanokompositen hergestellt durch Co-Sputtern, PhD
thesis, University of Kiel, 2006.
48 J. De Sande, et al., Refractive index of Ag nanocrystals
composite lms in the neighborhood of the surface
plasmon resonance, J. Appl. Phys., 2002, 91, 1536–1541.
49 S. G. Moiseev, Composite medium with silver nanoparticles
as an anti-reection optical coating, Appl. Phys. A: Mater. Sci.
Process., 2011, 103, 619–622.
50 S. Dutta Gupta, Strong-interaction—mediated critical
coupling at two distinct frequencies, Opt. Lett., 2007, 32,
1483–1485.
51 T. Huang and X.-H. N. Xu, Synthesis and characterization of
tunable rainbow colored colloidal silver nanoparticles using
single-nanoparticle
plasmonic
microscopy
and
spectroscopy, J. Mater. Chem., 2010, 20, 9867–9876.
52 S. Pillai, K. Catchpole, T. Trupke and M. Green, Surface
plasmon enhanced silicon solar cells., J. Appl. Phys., 2007,
101, 093105.
53 J.-B. Brückner, et al., Metamaterial lters at optical-infrared
frequencies, Opt. Express, 2013, 21, 16992–17006.
54 N. Engheta, in Antennas and Propagation Society International
Symposium, 2002, IEEE. 392–395 (IEEE).
55 P. B. Johnson and R.-W. Christy, Optical constants of the
noble metals, Phys. Rev. B: Solid State, 1972, 6, 4370.
56 S. Kawata, Near-eld Optics and Surface Plasmon Polaritons,
Springer, 2001, vol. 81, p. 210.
57 C. Menzel, C. Rockstuhl, T. Paul, F. Lederer and T. Pertsch,
Retrieving effective parameters for metamaterials at
oblique incidence, Phys. Rev. B: Condens. Matter Mater.
Phys., 2008, 77, 195328.
Nanoscale, 2014, 6, 6037–6045 | 6045