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Transparent Long-Pass Filter with Short-Wavelength Scattering
Based on Morpho Butterfly Nanostructures
Niraj N. Lal,*
,†,§,∥
Kevin N. Le,
‡
Andrew F. Thomson,
†
Maureen Brauers,
†
Thomas P. White,
†
and Kylie R. Catchpole
†
†
Research School of Engineering and
‡
Department of Physics, Australian National University, Acton, ACT 0200, Australia
§
Faculty of Engineering, Monash University, Clayton, VIC 3800, Australia
∥
CSIRO Manufacturing Flagship, Clayton, VIC 3168, Australia
*
SSupporting Information
ABSTRACT: We combine the principles of moth-eye
antireflection, Bragg scattering, and thin-film interference to
design and fabricate a short-wavelength scattering/long-pass
filter with sharp cutoff, high transmission of infrared light, and
strong reflection of visible light into high angles. Based on the
lamellae-edge features on Morpho didius butterfly wings,
nanostructures are self-assembled via sequential one-chamber
chemical vapor deposition, metal nanoparticle formation, and
wet-chemical etching. Finite-element modeling demonstrates
strong (>45%) reflection into the first diffracted order for
short wavelengths, while retaining >80% transmission for
longer wavelengths. Fabricated nanostructures couple more than 50% of reflected light into angles of >10°while enabling
broadband long-pass transmission. Such structures have potential applications in light trapping for tandem solar cells, stealth, and
signals processing.
KEYWORDS: Morpho butterfly, light trapping, tandem solar cell, optical scattering, optical filter
Controlling the transmission and reflection of light from a
surface is important for a wide range of technologies
including photovoltaics,
1
lighting,
2
stealth,
3
architecture,
4
anticounterfeiting,
5
and signal processing.
6
Each of these applications has specific optical requirements in
terms of scattering direction and intensity, wavelength,
bandwidth, and angular dependence. Traditionally, optical
components that scatter light are designed to either be highly
selective, with a narrow scattering response centered on a
particular wavelength,
7
or broadband, with wavelength-
independent scattering across the spectrum.
8,9
But for more
nuanced applications how can we scatter light? Tandem solar
cells, for example, require the selective reflection and scattering
of short-wavelength light to a top cell (Figure 1a, d) while
ensuring transmission of long wavelengths to an underlying
solar cell.
10
Such an optical response is difficult to achieve with
either strictly planar or completely irregular surfaces.
We present in this Letter a short-wavelength scattering long-
pass filter inspired by Morpho didius butterfly wing nanostruc-
tures (Figure 1b,c) where we combine the optical principles of
moth-eye antireflection, short-wavelength scattering from
butterfly wing lamellae, and long-pass transmission from planar
dielectric Bragg filters in a single large-area optical component
of self-assembled nanostructures (Figure 1d).
Structural color mechanisms inspired by nature have
demonstrated a wide range of optical responses including
complete broadband absorption,
11
broadband white reflec-
tion,
12
metallic reflection,
13
and structural iridescence.
14
Opaque Morpho butterfly wing structures have previously
been replicated via nanoscale self-assembly,
15,16
achieving
strong selective scattering and the striking blue iridescence
found on Morpho wings. These structures, however, are
opaque
17
and selectively scatter light in a narrow range of
wavelengths. Broadband response is simple to achieve with
planar long- and short-pass optical filters consisting of stacked
dielectric multilayers,
18,19
but these afford only specular
reflection and transmission.
For tandem solar cells, our initial focus, the condition on
angular scattering of reflection is strict:
10
a >2% increase in
absolute solar cell efficiency is possible with optimal light
trapping, but a sharp decrease in performance is observed with
non-wavelength-selective reflection at the interface between top
and bottom cells.
10,20−22
Strictly planar reflection is undesirable
due to increased out-coupling of light from the top of the cell
that could otherwise generate photocurrent in the bottom
cell.
10
Three-dimensional inverse-opal photonic crystals are
able to provide selective angular reflection, but with nonoptimal
transmission in the infrared.
23−25
Received: December 18, 2016
Published: March 29, 2017
Letter
pubs.acs.org/journal/apchd5
© XXXX American Chemical Society ADOI: 10.1021/acsphotonics.6b01007
ACS Photonics XXXX, XXX, XXX−XXX
Motivated by these requirements, we target the three
necessary features of a short-wavelength scattering long-pass
filter, namely, (1) low long-wavelength reflection; (2) scattering
and nonspecular reflection of short wavelengths; and (3) high
transmission of long wavelengths.
We achieve them through the combination of various
nanostructured elements: moth-eye tapering for antireflec-
tion;
26−28
Bragg scattering from butterfly lamellae nanostruc-
tures;
16,29−31
and Bragg long-pass transmission from periodic
dielectric multilayers.
18,19
The concept of selective transparency is well established for
planar optical elements. We demonstrate here the first
selectively transparent optical scattering component by
incorporating tapered butterfly lamellae structures within the
layers of a Bragg long-pass filter, achieving strong forward
scattering at narrow angles of long wavelengths and strong
backscattering at high angles of short wavelengths.
■METHODS
Motivated by recent work on perovskite−silicon tandem solar
cells where the top cell has a band gap of ∼1.55 eV, we design
the structure with a base consisting of a standard Bragg long-
pass filter centered on λ0= 600 nm, which enables a reflection/
transmission cutoffat the top-cell band gap (∼800 nm). The
layer structure is (Air (0.5H) L (HL)
4
Glass),
19
where H and L
correspond to quarter-wave layers of the alternating high- and
low-index materials, respectively. We first model with ideal
high-contrast materials SiO2(n= 1.46)
32
and TiO2(n= 2.4)
33
and subsequently with SiO2(n= 1.56) and SiNx(n= 2.12)
layers (collectively labeled SiON), which can be deposited
sequentially via plasma-enhanced chemical vapor deposition
(PECVD) (see Supporting Information Figure S1).
34
The
quarter-wave layers for the initial high and low materials are
62.5 nm (TiO2) and 102.7 nm (SiO2) respectively.
The butterfly structure on top of the Bragg filter (inset of
Figure 2a) also employs an alternating low−high structure, with
layer thickness informed by the structures fabricated by
Siddique et al.,
31
at 150 nm for the SiO2layer and 40 nm for
the TiO2layer. The base width of these structures was set to
500 nm, with a ledge width of 50 nm from the structures
fabricated by Aryal et al.
16
Four pairs of HL layers were
employed in the scattering structure, which had a total height of
the structure of 1350 nm, including a final 400 nm thick low
index to help elicit the moth-eye antireflection effect for long
wavelengths via a gradual refractive-index change.
26
The ledges
generate strong Bragg scattering of short-wavelength light as in
normal iridescent (though opaque) M. didius wing structures,
15
while maintaining the transmission of long-wavelength light
(Figure 1d).
We model the structure using two-dimensional (2D)
COMSOL finite-element simulations. For reflection and
transmission spectra, we use periodic boundary conditions
Figure 1. (a) Schematic of a transparent long-pass scatterer with cutoffwavelength λc. (b) Morpho didius butterfly (male).
41
(c) Morpho didius
butterfly wing nanostructure SEM [reproduced with permission from Proc. R. Soc. London B.
15
(d) Design of transparent long-pass scatterer
incorporating moth-eye antireflection, long-pass Bragg filter, and short-wavelength scattering from butterfly wing lamellae.
Figure 2. (a) Reflection, transmission, and absorption plots of modeled nanostructures with TiO2and SiO2high- and low-index layers. (b, c) Polar
scattering plots of reflected and transmitted light at 600 and 900 nm, respectively. (d, e) |E|2intensity profiles at λ= 600 nm and λ= 900 nm,
respectively.
ACS Photonics Letter
DOI: 10.1021/acsphotonics.6b01007
ACS Photonics XXXX, XXX, XXX−XXX
B
with varying periodicity from 600 to 1200 nm. E-field scattering
intensity and polar plots for single scattering elements are
simulated with perfectly matched layer (PML) boundary
conditions with a simulation width of 4 μm to minimize edge
effects.
The butterfly structure base width is maintained at 500 nm,
with layer thicknesses adjusted for the varying refractive indices.
Reflection and transmission spectra are averaged over 20
periods spanning the period range to both simulate the broad
power spectrum of periods present in the self-assembled
structure
35,36
and to eliminate artifacts arising from the periodic
boundary conditions, as discussed further below. Complete
modeled data for each individual period are included in
Supporting Information Figures S2−S7.
■RESULTS AND DISCUSSION
In this section we first present numerical modeling results to
demonstrate the theoretical feasibility of the approach. We then
describe the fabrication process and compare experimental
measurements with simulation results.
We first investigate reflection and transmission spectra from
a periodic array of scatterers and find excellent, long-pass
transmission alongside strong (>95%) short-wavelength
reflection, similar to optimized standard Bragg long-pass filters.
Spikes in the reflection and transmission spectra are parasitic-
reflection artifacts due to linear combinations of Bloch modes
within the periodic boundary conditions.
37−39
The strong and
broad reflection feature between 450 and 800 nm remains
constant across all periods, with the artifacts red-shifting with
increasing period (Figures S2−S4). We find high levels of
scattering into the first diffracted order (Figure 2a) with strong
nonplanar reflection (>45% of reflected light for each period).
The tapered scattering structure enhances long-wavelength
transmission with strong transmission (>90%) for wavelengths
above 800 nm.
The strong nonspecular reflection is directly related to the
scattering properties of individual elements. On modeling
single scatterers with PML boundary conditions, the polar plot
of the time-averaged Poynting vector (with background
incident light subtracted) shows the directionality of the
scattered and transmitted light at λ= 600 nm (Figure 2b) and λ
= 900 nm, respectively (Figure 2c). Field |E|2intensity profiles
demonstrate both strong transmission for long wavelengths
(Figure 2e) and scattering of reflected light at short
wavelengths (Figure 2d).
Inspired by the modeling, we fabricated structures using SiO2
and SiNxlayers on microscope glass slides cleaned via
sonication in distilled water, acetone, and 2-propanol for 10
min each. Layers were deposited sequentially via PECVD
simply by varying gas composition (T= 400 °C, P=20W,
pressure = 650 mT, SiNx:N
2= 980, NH3= 14, SiH4= 22 sccm,
n= 2.17 (@600 nm), SiO2:N
2= 161, N2O = 710, SiH4=9
sccm, n= 1.515 (@600 nm)), with deposition rates of 2.4 and
8.7 Å/s for high- and low-index layers, respectively. The layer
structure is adjusted from the TiO2/SiO2modeled structure
with layer thicknesses (Air |Tapered: 400 nm L |Butterfly: (38
nm H, 160 nm L)
5
|Bragg-planar: (69 nm H, 96.2 nm L)
4
|
34.5 nm H |Glass) (Figure S2).
Dielectric layer deposition is followed by metal evaporation
of 22 nm of Ag (Angstrom thermal evaporator) and subsequent
annealing at 250 °C under N2to self-assemble nanoparticles on
the surface (Figure 3a,i). These nanoparticles serve as an etch
mask for ICP-RIE etching (Figure 3a,ii) (P= 400 W, bias = 60
W, CHF3= 50 sccm, He cooling, t= 1000 s), before chemical
etching in HF (HF:H2O = 0.02:1 (48% HF), 40 s) selectively
etches the SiO2layers to form individual lamellae ledges
approximately 30 nm in depth (Figure 3a,iii). Silver nano-
particles are removed (HNO3:H2O = 0.1:1 (70% HNO3), 38
min) (Figure S9) to reveal large-area self-assembled Bragg
butterfly nanostructures (Figure 3b). These nanostructures are
fabricated without the use of lithography, nanoimprinting, or
other common cleanroom nanofabrication techniques, but
instead by inherently scalable self-assembly.
UV−vis reflection and transmission spectra were measured
using a spectrometer with an integrating sphere (PerkinElmer),
demonstrating a characteristic Bragg long-pass filter response
centered at 600 nm with strong reflection about λ0and high
transmission beyond ∼650 nm (Figure 4a).
Angular-resolved reflectance spectra were also collected for
wavelengths of 400−820 nm using the same spectrometer with
an ARTA attachment, with a detector height of 7 mm, a width
of 32 mm, and a distance of 120 mm from the sample (Figure
4b). Integrating the scattered reflectance in the solid angle, we
find more than half of all reflected light below the cutoff
wavelength of the Bragg filter is scattered into angles of >10°,
providing the first demonstration of a short-wavelength
scattering long-pass filter fabricated via large-area self-assembly.
More than 95% of transmitted light is forward scattered at
angles of <5°from normal (Figure S8b). The broadband
absorption observed in Figure 4a is caused by silicon-rich SiON
layers
34
and can be mitigated through the replacement of SiON
with a less absorbing high-refractive index material such as
TiO2. The strong nonspecular reflection is expected to be
further enhanced through (i) a deeper HF etch of the lamellae,
Figure 3. (a) Schematic of nanofabrication self-assembly: (i) PECVD of SiO2/SiNxlayers with formation of Ag nanoparticles on the surface, (ii)
ICP-RIE etching, (iii) chemical wet etch with HNO3and then HF. (b) Large-area SEM of nanostructures. Inset: isolated transparent scatterer with a
scale bar of 100 nm.
ACS Photonics Letter
DOI: 10.1021/acsphotonics.6b01007
ACS Photonics XXXX, XXX, XXX−XXX
C
(ii) use of materials with higher refractive-index contrast, (iii)
taller nanostructures, and (iv) use of materials with less optical
absorption.
We demonstrate long-pass transmission with short-wave-
length scattering for visible light, but expect the principle to be
widely applicable across the electromagnetic spectrum. Our
initial motivation was for light trapping in tandem solar cells,
10
but we identify here the possibilities for a wide range of future
nuanced applications for the transmission and scattering of light
including those as varied as scalable anticounterfeiting,
5
optical
camouflage while allowing long-wavelength communication,
3
batch signals processing,
6
and architectural windows with a
matte blue exterior allowing red-light transmission.
4,40
■CONCLUSION
By combining the optical principles of moth-eye antireflection,
Bragg scattering, and planar long-pass dielectric filters, we are
able to design and fabricate a short-wavelength scattering/long-
pass filter with sharp cutoff, high transmission of infrared light,
and strong reflection of visible light into high angles. Based on
the lamellae-edge features on M. didius butterfly wings,
nanostructures are self-assembled via sequential one-chamber
chemical vapor deposition, metal nanoparticle formation, and
wet-chemical etching.
Finite-element modeling demonstrates strong (>45%)
reflection into the first diffracted order for short wavelengths,
while retaining >80% transmission for longer wavelengths.
Fabricated nanostructures couple more than 50% of reflected
light into angles of >10°while enabling broadband long-pass
transmission. While these principles are demonstrated in the
visible regime, they are expected to be widely applicable across
the electromagnetic spectrum. Such structures have potential
applications in light trapping for tandem solar cells, stealth,
architecture, and signals processing.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsphoto-
nics.6b01007.
Complete modeling results for varying periods, modeling
results for SiON structures, measured reflection spectra
for five batch samples, measured angular transmittance
spectra, additional details of optical measurement and
nanostructure fabrication, and additional scanning
electron micrographs (PDF)
■AUTHOR INFORMATION
Corresponding Author
*E-mail: niraj.lal@anu.edu.au.
ORCID
Niraj N. Lal: 0000-0002-2393-176X
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors acknowledge the Australian Renewable Energy
Agency for funding and the facilities of the Australian
Microscopy & Microanalysis Research Facility at the Australian
National University. This work was performed in part at the
ACT node of the Australian National Fabrication Facility, a
company established under the National Collaborative
Research Infrastructure Strategy to provide nano- and micro-
fabrication facilities for Australia’s researchers.
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