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Proceedings ArticleDOI

Development of photonic crystal structures for on-board optical communication

02 May 2014-Proceedings of SPIE (SPIE)-Vol. 9127, pp 912705

AbstractWe present designs for sharp bends in polymer waveguides using colloidal photonic crystal (PhC) structures. Both silica (SiO2) sphere based colloidal PhC and core-shell colloidal PhC structures having a titania (TiO 2) core inside silica (SiO2) shells are simulated. The simulation results show that core-shell Face Centered Cubic (FCC) colloidal crystals have a sufficient refractive index contrast to open up a bandgap in the desired direction when integrated into polymer waveguides and can achieve reflection >70% for the appropriate plane. Different crystal planes of the FCC structure are investigated for their reflection and compared with the calculated bandstructure. Different techniques for fabrication of PhC on rectangular seed layers namely slow sedimentation; spin coating and modified doctor blading are discussed and investigated. FCC and Random FCC silica structures are characterized optically to show realisation of (001) FCC. © 2014 SPIE.

Topics: Photonic crystal (57%), Colloidal crystal (56%), Refractive index contrast (53%), Crystal (50%)

Summary (1 min read)

1. INTRODUCTION

  • Single mode polymer waveguides can be used to guide information-carrying light from one component to another on a backplane.
  • The authors have designed the different photonic crystal structures to be integrated with 5 µm wide polymer waveguides.
  • The Face Centre Cubic (FCC) structure is formed only if the exact positioning of each individual horizontal layer of spheres repeats itself at every third layer in vertical direction.
  • Defects in the sedimentation process can change the repetition from say the third to every second layer resulting in AB, BC or AC stacking.
  • FCC structures fabricated using both natural sedimentation and the new proposed technique are simulated to show that pure FCC structures having sufficient index contrast can be used to achieve in-plane bending of light at sharp angles.

2. FCC STRUCTURES FOR IN-PLANE BENDING

  • The optical properties of a FCC structure with 1000nm diameter silica spheres is modelled and simulated.
  • The larger refractive index contrast = 0.67 between the core-shell particles and the background air is very helpful for the photonic crystal to act as a reflector.
  • It can be observed in Fig. 7(b) that the reflection peak for 45° incidence is situated at larger values of frequencies which is as per predictions from the bandstructure.

3. MANUFACTURING AND CHARACTERIZATION OF COLLOIDAL CRYSTALS

  • The crystal structure and orientation of the particles in the colloidal crystals determine the reflection and transmission of the light and thus the properties of the photonic crystal.
  • The preferred crystal form of mono-dispersed particles is the FCC stacking.
  • Fabrication of FCC structures without any external field or force will result in a hexagonal ground plane as shown in Fig. 8(A).
  • The template is manufactured by nano-imprint lithography.
  • The cracks, due to FCC stacking adband white sample.

4. CONCLUSIONS

  • The authors have simulated different FCC structures that can be integrated into polymer waveguides to show that core-shell colloidal photonic crystal structures can be used for achieving sharp bends in polymer waveguides.
  • Air-clad silica spheres photonic crystal structures can only provide reflections 15% as compared to more than 50% reflections from core-shell structures.
  • Truncated core-shell photonic crystal structures on rectangular seed layer are calculated to reflect more than 70% of the incident light.
  • Different fabrication techniques for fabrication of silica spheres FCC structures on top of rectangular seed layer have been investigated and implemented.
  • The authors initial characterization results show that FCC structures with (001) FCC ground plane can be fabricated which can be distinguished from (111) FCC or RFCC structures using optical characterization.

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Development of photonic crystal structures for on-board optical
communication
Muhammad Umar Khan*
a
, John Justice
a
, Arjen Boersma
b
, Maurice Mourad
b
, Renz van Ee
b
, Alfons
van Blaaderen
c
, Judith Wijnhoven
c
, Brian Corbett
a
a
Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland;
b
TNO, De Rondom 1
5612AP, Eindhoven, The Netherlands;
c
Soft Condensed Matter, Ornstein Laboratory, Utrecht, The
Netherlands.
ABSTRACT
We present designs for sharp bends in polymer waveguides using colloidal photonic crystal (PhC) structures. Both silica
(SiO
2
) sphere based colloidal PhC and core-shell colloidal PhC structures having a titania (TiO
2
) core inside silica (SiO
2
)
shells are simulated. The simulation results show that core-shell Face Centered Cubic (FCC) colloidal crystals have a
sufficient refractive index contrast to open up a bandgap in the desired direction when integrated into polymer
waveguides and can achieve reflection >70% for the appropriate plane. Different crystal planes of the FCC structure are
investigated for their reflection and compared with the calculated bandstructure. Different techniques for fabrication of
PhC on rectangular seed layers namely slow sedimentation; spin coating and modified doctor blading are discussed and
investigated. FCC and Random FCC silica structures are characterized optically to show realisation of (001) FCC.
Keywords: Photonic Crystal, Silica, Face Centre Cubic, Core-shell, Bandstructure, Doctor blading, Sedimentation, Spin
coating.
1. INTRODUCTION
Single mode polymer waveguides can be used to guide information-carrying light from one component to another on
a backplane. The low cost and ease of fabrication associated with polymer waveguides makes them attractive but the low
index contrast inherent in polymeric materials requires large bending radii to reduce radiation loss making it difficult to
achieve a high component density. Thus, a means to have more compact bends in polymer optical waveguides is
required. Photonic crystals are artificial periodic structures which have been used to manipulate the properties of light.
Some applications for photonic crystal are to confine/trap light at resonance
1
, compress a light pulse by slowing it down
2
,
split light into different polarizations
3
, optical trapping
4
and to enhance nonlinear effects
5
. Periodicity and refractive
index contrast are the main parameters that determine the functionality of these structures. In this paper, we present the
design and fabrication of photonic crystal structures that can act as an in-plane reflector for polymer waveguides at
wavelengths in the 1550 nm range as used in telecommunications.
We have designed the different photonic crystal structures to be integrated with 5 µm wide polymer
waveguides. The photonic crystal structures need to be of at least the same size as that of the waveguide for efficient
bending of the light. Thus, the mature two dimensional photonic crystal structures cannot be used. Rather, three
dimensional structures need to be implemented. The optical response of the different structures are simulated and
compared where we use the commercially available Lumerical Solutions software based on the Finite Difference Time
Domain (FDTD) method.
Natural sedimentation of mono-dispersed particles on a flat surface is the easiest way to realise three
dimensional photonic crystal structures. The multi-layer structures fabricated in this manner can be expected to have
horizontal layers of hexagonally packed spheres stacked above each other. The Face Centre Cubic (FCC) structure is
formed only if the exact positioning of each individual horizontal layer of spheres repeats itself at every third layer in
vertical direction. Three different positioning, repeating every third layer, can be designated as A, B and C arrangements
and the stacking of repeated ABCABC... is the FCC stacking as shown in Fig.
1. The FCC lattice is formed only if
everything goes right in the natural sedimentation process with the spheres positioning themselves at the right locations
Photonic Crystal Materials and Devices XI, edited by Sergei G. Romanov, Gabriel Lozano,
Dario Gerace, Christelle Monat, Hernán Ruy Míguez, Proc. of SPIE Vol. 9127, 912705
© 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2052214
Proc. of SPIE Vol. 9127 912705-1
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FCC (ABC)
A
HCP (AB)
RFCC (ABC+AB)
without any defects. Defects in the sedimentation process can change the repetition from say the third to every second
layer resulting in AB, BC or AC stacking. Structures having alternating layers of spheres like ABAB…are called
Hexagonal Closed Pack (HCP) structures. So, any deviation from ideal sedimentation will result in changing of FCC into
HCP and a mix of FCC and HCP structures results. This mix of FCC and HCP structures can be named as a Random
Face Centered Cubic (RFCC) structure. In this paper, we propose to realise FCC structures using a starting surface with a
rectangular seed layer which may be less prone to defect generation. FCC structures fabricated using both natural
sedimentation and the new proposed technique are simulated to show that pure FCC structures having sufficient index
contrast can be used to achieve in-plane bending of light at sharp angles.
Fig. 1: Three different relative arrangements of spheres: A, B and C are represented in White, Red and Blue colors respectively to
show the stacking of Face Centered Cubic (FCC), Hexagonal Closed Packed (HCP) and Random Face Centered Cubic (RFCC)
structures.
2. FCC STRUCTURES FOR IN-PLANE BENDING
The optical properties of a FCC structure with 1000nm diameter silica spheres is modelled and simulated. The
photonic crystal structure is ‘integrated’ at the end of a 5 µm high polymer waveguide with the intention to bend the light
travelling inside the waveguide. The FCC structure of silica spheres will act as a reflector if the index contrast (Δn)
between the silica spheres (n
silica
=1.45) and the background air (n
air
=1.0) is sufficient to open up a band-gap at the
incident direction. In our example, the photonic crystal is tilted at 45° to the incident light in order to bend the light at
90°. The reflected light will be guided by a polymer waveguide placed at a right angle to the incident waveguide. In the
simulations, air acts as the cladding of the waveguides and background for the stacked silica spheres. The modelled
photonic crystal structure with the waveguide arrangement and the calculated 90° and back reflected optical powers are
shown in Fig.
2.
Proc. of SPIE Vol. 9127 912705-2
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20
15
10
-- 5
-90° Reflection
- Back Reflection
d 20
d
ir. 15
10
5
400 1500 1600 1700 1800 1900 2000
Wavelength (nm)
60-
G''
= 40-
o
20-
- 90° Reflection
- Back Reflection
1500 1600 1700 1800 1900
Wavelength (nm)
2000
Fig. 2: (a) Modelled natural sedimentation FCC photonic crystal structure embedded in polymer waveguides for 90° bending of light.
(b) 90° and back reflected optical powers are plotted in red and green colors respectively.
Reflections from the structure can be expected at a wavelength of 1500 nm using Bragg’s Law for an effective
refractive index of

=1.33. The effective refractive index of the FCC structure is calculated from

=

0.74 +

∗0.26 where 0.74 is the packing factor of the silica spheres in air. The red plot in Fig. 2(b) confirms 90°
reflections around 1500 nm. The small reflection intensity shows that the index contrast between silica and air is not
sufficient to open up a wide bandgap which results in most of the light transmitting through the structure. The green plot
in Fig.
2(b) shows negligible reflections in the backward direction.
A larger refractive index contrast is required for higher reflectivity and can this be achieved by increasing
refractive index of the spheres. Core-shell spheres having core of higher refractive index material TiO
2
(n
titania
= 2.4)
inside a silica (n
silica
= 1.45) shell can be considered. Such core-shell particles embedded into polymer waveguides are
modelled and simulated to see whether the index contrast is sufficient for such structures to be efficient reflectors at
telecom wavelengths. The effective refractive index of the core-shell particles having

=2.4 and

=1.45 is
calculated to be

=1.67 using

=[



]+[

(

−

)]. The effective refractive
index of core-shell FCC structure in air background is calculated to be 1.5 using

=0.74

+0.26∗

=1.5 where 0.74 is the sphere packing factor in FCC. The larger refractive index contrast  = 0.67
between the core-shell particles and the background air is very helpful for the photonic crystal to act as a reflector. The
calculated reflection intensities from core-shell particles crystal are shown in Fig. 3. The red plot shows that more than
50% reflection at 90° can be obtained around telecom wavelengths. These reflections from photonic crystal reflector also
follow Bragg’s law and move to smaller wavelengths with an increase in angle of incidence.
Fig. 3: Red plot shows calculated optical power in 90° waveguide. Green plot shows calculated optical power reflected backwards in
the incident waveguide from TiO
2
-SiO
2
core-shell photonic crystal structure.
The possibility of forming RFCC structures using the natural sedimentation process will make it difficult to
obtain a pure FCC structure. This lack of control in stacking might be overcome if the spheres are forced to position
(a)
(b)
Proc. of SPIE Vol. 9127 912705-3
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uncated
heces
themselves a
t
desired locat
i
layer, in whi
c
shown in Fi
g
arrange the
m
subsequent l
a
make a well
s
A t
h
FCC unit cel
l
4
(a) shows t
incoming lig
h
selected refl
e
we truncate
t
p
article struc
b
ending of li
g
the incident
l
shown by th
e
results in bet
t
Fig. 4: (a) Re
c
b
lack, red and
The
band-gaps in
plane is sho
w
p
lane of FC
C
on top of t
h
b
andstructur
e
direction mo
v
W points in t
h
of band-gap
moving to la
r
t
some partic
i
ons which c
o
c
h spheres ar
e
g
. 8(B) with
t
m
selves in a r
e
a
yer of spher
e
s
tacked recta
n
h
ree dimensi
o
l
and the sele
c
h
at FCC (00
1
ht
if the red
u
e
cting plane
d
t
he photonic
c
tures fabricat
g
ht. Truncate
l
ight. Simula
t
e
plot in Fig.
t
er efficiency.
(
c
tangular seed
yellow colors
r
calculated b
a
different ori
w
n in Fig.
6
(a
)
C
is represent
e
h
e ban
d
-gap
e
dispersion i
n
v
es to higher
h
e reciprocal
to higher fre
r
ger values w
i
ular points.
A
o
uld ultimate
l
e
stacked ove
r
t
he base of t
h
e
ctangular ar
r
e
s will fit the
m
n
gular seed la
y
o
nal sphere st
r
c
ted FCC refl
e
1
) reflecting
p
u
ndant spher
e
d
ecrease the r
e
c
rys
t
al struct
u
ed on rectan
g
d core-shell
s
t
ions show th
a
5
(a). Truncat
i
(
a)
layer structure
r
espectively. (b
)
a
ndstructure
o
entations of
t
)
. The reflecti
o
e
d by Γ-L dir
e
with Full
W
n
the Γ-L dir
e
frequencies a
space is equi
v
quencies is i
n
i
th an increas
e
A
ny pits or p
i
l
y result in b
e
r
a rectangula
r
h
e FCC unit
c
r
angement in
m
selves in th
e
y
er structure.
r
ucture
b
ased
e
cting plane
a
p
lane is at 4
5
e
s indicated
b
e
flection effi
c
u
re at 45° to
g
ular seed la
y
s
tructures are
a
t more than
i
ng the struct
u
showing FCC
)
Structure is tr
u
o
f the FCC c
r
t
he crystal la
t
o
n spectru
m
i
e
ction in reci
p
W
idth Half
M
e
ction is line
a
s we move fr
o
v
alent to cha
n
n
agreement
w
e
in the angle
o
i
llars on an
u
e
tter stacking
r
grid of sma
l
c
ell identifie
d
contrast to
a
e
grooves for
m
on a rectan
g
a
re marked in
b
5
° to the inci
d
b
y the yellow
c
iency due to
access the d
e
y
er are simul
a
simulated to
70% of inci
d
u
re makes F
C
unit cell, FC
C
u
ncated at 45°
t
r
ystal is sho
w
t
tice in recip
r
i
s plotted on t
p
rocal space.
M
aximum (F
W
a
r and behave
o
m L to W d
i
n
ging the ang
l
w
ith the Bra
g
o
f incidence.
u
nderlying su
r
of the spher
e
l holes as sh
o
d
. This seed
l
a
hexagonal
a
m
ed from firs
t
g
ular seed lay
e
b
lack and red
d
ent light w
h
triangle are
them scatteri
n
e
sired FCC p
l
a
ted to show
t
calculate am
o
d
ent light can
C
C (001)
p
lan
e
C
reflecting pla
t
o expose appr
o
w
n in Fig.
5
(
b
r
ocal space.
T
op of the ban
d
The reflectio
n
W
HM) equal
s like a simp
l
i
rections in t
h
l
e of incidenc
e
g
g’s law of
d
r
face can for
c
e
s. We
p
ropo
s
o
wn in Fig. 8(
l
ayer is inten
d
a
rrangement
w
t
layer of sph
e
er
is depicte
d
colors respe
c
h
ich can only
removed. Th
n
g the light i
n
l
ane as is sho
t
hese structur
e
o
unt of optica
be reflected
e
at 45° to th
(b
)
ne and the ex
c
o
priate FCC pl
a
b
). The bands
t
T
he calculate
d
d
structure in
F
n
peak from t
h
to the open
l
e Bragg refl
e
h
e reci
p
rocal
s
e
of light on (
d
ispersion wi
t
c
e the sphere
s
s
e using a re
c
B). The FCC
d
ed to force
t
w
ithout the s
e
e
res and thus
d
in Fig.
4
. T
h
c
tively. The s
t
be exposed
d
ese spheres i
n
n
different di
r
wn in Fig.
4
(
e
s can be us
e
l power refle
c
from truncat
e
e incident ac
c
)
c
ess spheres to
a
ne to the incid
e
t
ructure sho
w
d
reflection
fr
F
ig.
6
(b) to s
h
h
e (111)
p
la
n
n
ing of the
b
e
ctor. This ba
n
s
pace. Move
m
111) plane.
T
t
h the resona
n
s
to sit on th
e
c
tangular see
d
(001) plane i
s
t
he spheres t
o
e
ed laye
r
. Th
e
should help t
o
h
e
b
ase of th
e
ructure in Fi
g
d
irectly to th
e
n
front of th
e
r
ections. Thu
s
(
b). Core-she
l
e
d for in-
p
lan
e
c
ting at 90° t
o
e
d structure a
s
c
essible whic
h
be truncated i
n
e
nt light.
w
s openings o
fr
o
m
the (111
h
ow that (111
n
e falls exactl
y
b
an
d
-gap. Th
e
nd
-gap in Γ-
L
m
ent from L t
o
T
his moveme
n
n
ce frequenc
y
e
d
s
o
e
o
e
g
.
e
e
s
l
l
e
o
s
h
n
f
)
)
y
e
L
o
n
t
y
Proc. of SPIE Vol. 9127 912705-4
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80
60
40
20
f;
m
60
40
20
1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800
Wavelength (nm)
1
i
-
---_ f
-
_
-- _ tr
- -
_
+
+
# 'k
- F`
-_ --
- - ~__` -
.
-
_--_
Gamma
L W X K
k (Gamma-L-W-K-L-U )
L U
60
50
40
30
20
10
0.9-
0.8-
0.7-
O.6
R
; 0.5
L.
1
-<111> Reflection
d 0.4
- 0.3
0.2
0.1
0
1400 1600 1800 2000 2200 2400 2600 2800 3000
Wavelength (nm)
0.5
ma
L W X K
k (Gamma-L-W-K-L-U)
30
20
10
(a) (b)
Fig. 5: (a) 90° reflection response from truncated rectangular seed layer structure. (b) Bandstructure of core-shell FCC crystal.
Fig.
5(b) shows that band-gap in L-U and L-K directions follow the same trend so it can be concluded that L-U
and L-K directions in the reciprocal space are identical. Reflections from (001) plane of FCC unit cell is mapped to the
middle point of W-K directions which is named as X. The calculated reflection response for normal incidence of light on
(001) plane of core-shell FCC photonic crystal shows a reflection peak around 1.6 THz. The reflection spectrum from the
(001) plane of core-shell FCC is plotted on top of the calculated bandstructure in Fig.
7(a) to show that reflection peak
falls in the band-gap frequencies at X point which is right in the middle of W-K direction. It can be observed in the
bandstructure that band-gap at X moves to larger frequency values on moving towards W or K point from X.
(a) (b)
Fig. 6: (a) Calculated reflection spectrum at normal incidence to (111) plane of the FCC crystal. (b) Reflection spectrum from (111)
FCC plane plotted against bandstructure showing band-gap in the Γ-L direction.
Fig.
5(b) shows inter-crossings of optical bands near K/U and W points in the bandstructure. The crossings of
the bands result in transfer of energy between the optical bands present for those frequencies. A small reflection peak in
the reflection response appears just next to the strong reflection peak at the start of the intercrossing. This new peak gets
energy from the stronger peak in the neighborhood in result of the energy transfer. As a result, the initial peak before the
start of the crossing transfers its complete energy to the new born peak and disappears. This generation of two peaks in
the reflection response because of the intercrossing can be observed on changing the angle of incidence on (001) plane of
FCC. The relatively small peak next to the larger reflection peak in Fig. 5(a) for 45° incidence of light on truncated core-
shell FCC is because of the intercrossing of bands. Reflection response for 45° incidence on (001) plane of FCC is
plotted on the bandstructure in Fig.
7(b) to show the agreement between the bandstructure and calculated reflection
spectrum. It can be observed in Fig.
7(b) that the reflection peak for 45° incidence is situated at larger values of
frequencies which is as per predictions from the bandstructure. Two peaks at 1.6 THz and 1.85 THz are observed
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2 citations


Journal ArticleDOI
Abstract: A polymer waveguide is fabricated on top of an inverted opal photonic crystal structure to demonstrate an air clad functioning waveguide for the first time to the best of our knowledge. An optically thick layer with a refractive index (n = 1.01) close to air was realized on a silicon wafer by first co-forming a self-assembled 3-D photonic crystal structure with PMMA spheres and a silica backbone. Following the fabrication of polyimide waveguides on this surface by micro-molding, the PMMA spheres were removed to leave behind an inverted opal structure underneath the waveguides with refractive index close to air. Broadband, polarization independent fundamental mode optical waveguiding from 1300 nm to 1600 nm wavelengths was obtained. This original approach overcomes some of the drawbacks associated with conventional polymer waveguides and can be the basis for a range of optical interconnection and sensing applications.

2 citations


References
More filters

Journal ArticleDOI
01 Jan 1997-Nature
Abstract: Colloidal crystals are three-dimensional periodic structures formed from small particles suspended in solution. They have important technological uses as optical filters1–3, switches4 and materials with photonic band gaps5,6, and they also provide convenient model systems for fundamental studies of crystallization and melting7–10. Unfortunately, applications of colloidal crystals are greatly restricted by practical difficulties encountered in synthesizing large single crystals with adjustable crystal orientation11. Here we show that the slow sedimentation of colloidal particles onto a patterned substrate (or template) can direct the crystallization of bulk colloidal crystals, and so permit tailoring of the lattice structure, orientation and size of the resulting crystals: we refer to this process as 'colloidal epitaxy'. We also show that, by using silica spheres synthesized with a fluorescent core12,13, the defect structures in the colloidal crystals that result from an intentional lattice mismatch of the template can be studied by confocal microscopy14. We suggest that colloidal epitaxy will open new ways to design and fabricate materials based on colloidal crystals and also allow quantitative studies of heterogeneous crystallization in real space.

1,124 citations


Journal ArticleDOI
TL;DR: If all-optical devices using photonic crystal designs promise to be smaller than the wavelength of light, and to operate with bandwidths that are very difficult to achieve electronically, operation at single-photon power levels could be feasible.
Abstract: The quest for all-optical signal processing is generally deemed to be impractical because optical nonlinearities are usually weak. The emerging field of nonlinear photonic crystals seems destined to change this view dramatically. Theoretical considerations show that all-optical devices using photonic crystal designs promise to be smaller than the wavelength of light, and to operate with bandwidths that are very difficult to achieve electronically. When created in commonly used materials, these devices could operate at powers of only a few milliwatts. Moreover, if these designs are combined with materials and systems that support electromagnetically induced transparency, operation at single-photon power levels could be feasible.

737 citations


Journal ArticleDOI
Abstract: We demonstrate how slow group velocities of light, which are readily achievable in photonic-crystal systems, can dramatically increase the induced phase shifts caused by small changes in the index of refraction. Such increased phase sensitivity may be used to decrease the sizes of many devices, including switches, routers, all-optical logical gates, wavelength converters, and others. At the same time a low group velocity greatly decreases the power requirements needed to operate these devices. We show how these advantages can be used to design switches smaller than 20 µm×200 µm in size by using readily available materials and at modest levels of power. With this approach, one could have ∼105 such devices on a surface that is 2 cm×2 cm, making it an important step towards large-scale all-optical integration.

629 citations


Journal ArticleDOI
Abstract: One of the main obstacles for putting into practice the interesting optical and structural properties of colloidal crystals in actual devices is the incompatibility of the time-consuming and unclean self-assembly crystallization techniques commonly used to make colloidal crystals with the fast and dirtfree technology required to fabricate devices. With this in mind, different approaches have been taken and outstanding improvements affecting both the crystalline quality and the ease of fabrication have been achieved. However, most techniques developed to date are very sensitive to small variations of ambient humidity or temperature, which affect the lattice thickness and degree of crystallinity, and make it difficult to tailor the properties of the material. Also, it takes a few days for crystals to deposit on the substrate, or at least a few hours if precise temperature control is achieved. Finally, increasing the area on which the crystal is deposited up to at least the size of a wafer would require the use of a large amount of spheres and large baths, which, in turn, would increase the length of the process and complicate the fine control of the different parameters involved. Recently, a new approach to colloidal crystallization of submicrometer diameter spheres that overcomes some of the obstacles mentioned above has been proposed: Jiang et al. developed a procedure to prepare thin colloidal silica–polymer composite films based on spin-coating. In brief, silica colloidal spheres are first dispersed in a mixture of a viscous triacrylate monomer and a photoinitiator. The non-volatile dispersion is then spin-coated on a silicon wafer to obtain a thin layer. As the dispersion smears on the substrate, shearing induces 3D ordering of the particles in the monomer matrix, which is photopolymerized later on, providing mechanical stability for the structure. Selective removal of either the polymer matrix or the particles results in the formation of a direct silica or inverse polymer colloidal-crystal structure, respectively. Although it constitutes a formidable step forward in terms of colloidal-crystal processing, this technique still presents a series of drawbacks that may limit its application. First, it employs a viscous, dense monomer solution as the dispersion medium, which implies that lengthy and thorough stirring is needed to ensure the suspension is aggregate-free before spin-coating. Second, this monomer solution must be photopolymerized to be stabilized; thus, the resulting composite presents no porosity and a very low refractive-index contrast, which makes it useless for most foreseen applications. Only after further selective etching is porosity recovered, and the dielectric contrast is high enough to present an intense diffraction peak. Finally, this method cannot be easily extended to the crystallization of the different types of monodisperse latex particles usually employed in the field, since the common organic nature of both particles and matrix would make almost impossible the final selective elimination of one of them by plasma, thermal, or most organic liquid etchings. In this communication, we present a simple and reliable method to crystallize submicrometer monodisperse silica and latex colloids using a mixture of volatile solvents as dispersion media, allowing one to attain a strongly diffracting opal-like structure within minutes without further processing. A thorough study of the influence of the different relevant parameters, namely particle concentration and relative concentration of each solvent in the dispersion medium, was carried out. In the course of our investigations, we found that it was possible not only to attain planarized colloidal crystals with controlled thickness and good optical quality, but also to determine the crystal growth direction with respect to the substrate, which implies a major qualitative improvement with respect to previous techniques. Evidence of such control is obtained from both electron microscopy and optical spectroscopy. Colloidal suspensions of two types (Stober SiO2 and sulfonated polystyrene) were prepared by dispersing monodisperse sediments in different mixtures of ethanol, distilled water, and ethylene glycol (EG). These compounds were chosen attending to the ease of dispersion of the colloidal particles in them, their viscosity, and their volatility. Colloidal particles were first suspended in ethanol, distilled water, or mixtures of both, and then EG was added to decrease the vapor pressure of the suspension media. By doing so, our intention was to obtain a medium that required a long time to evaporate allowing particles to order by shearing, as reported in the literature, but volatile enough so that it would practically disappear after a few minutes. C O M M U N IC A TI O N S

275 citations


Journal ArticleDOI
22 Jul 2010-Langmuir
TL;DR: A simple, roll-to-roll compatible coating technology for producing 3D highly ordered colloidal crystal-polymer nanocomposites, colloidal crystals, and macroporous polymer membranes and it is demonstrated that the doctor blade coating speed can be significantly increased by using a dual-blade setup.
Abstract: This article reports a simple, roll-to-roll compatible coating technology for producing 3D highly ordered colloidal crystal-polymer nanocomposites, colloidal crystals, and macroporous polymer membranes. A vertically beveled doctor blade is utilized to shear align silica microsphere-monomer suspensions to form large-area nanocomposites in a single step. The polymer matrix and the silica microspheres can be selectively removed to create colloidal crystals and self-standing macroporous polymer membranes. The thickness of the shear-aligned crystal is correlated with the viscosity of the colloidal suspension, and the coating speed and the correlations can be qualitatively explained by adapting the mechanisms developed for conventional doctor blade coating. We further demonstrate that the doctor blade coating speed can be significantly increased by using a dual-blade setup. The optical properties of the self-assembled structures are evaluated by normal-incidence reflection measurements, and the experimental results agree well with the theoretical predictions using Bragg's law and a scalar wave approximation model. We have also demonstrated that the templated macroporous polymers with interconnected voids and uniform interconnecting nanopores can be directly used as filtration membranes to achieve size-exclusive separation of particles.

122 citations


Frequently Asked Questions (1)
Q1. What are the contributions in "Development of photonic crystal structures for on-board optical communication" ?

The authors present designs for sharp bends in polymer waveguides using colloidal photonic crystal ( PhC ) structures. Different techniques for fabrication of PhC on rectangular seed layers namely slow sedimentation ; spin coating and modified doctor blading are discussed and investigated.