Emission studies on photonic crystals fabricated using dyed polystyrene
colloids
Rajesh V. Nair, R. Vijaya, Keiji Kuroda, and Kazuaki Sakoda
Citation: J. Appl. Phys. 102, 123106 (2007); doi: 10.1063/1.2825636
View online: http://dx.doi.org/10.1063/1.2825636
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v102/i12
Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 102, 123106 共2007兲
Emission studies on photonic crystals fabricated using dyed
polystyrene colloids
Rajesh V. Nair and R. Vijayaa兲
Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Keiji Kuroda
Quantum Dot Research Center, National Institute for Materials Science, Tsukuba, Japan
Kazuaki Sakoda
Quantum Dot Research Center, National Institute for Materials Science, Tsukuba, Japan
and Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan
共Received 22 September 2007; accepted 24 October 2007; published online 21 December 2007兲
Three dimensionally ordered photonic crystals are fabricated with rhodamine B dyed polystyrene
colloidal spheres using the inward growing self-assembling method in less than 3 h. This avoids the
difficulties due to infiltration of active materials into passive photonic crystals. The superior optical
quality of the photonic crystals fabricated using this method results in high reflectance values even
at large angles of incidence. The study of emission characteristics on these functionalized photonic
crystals shows a clear trend dependent on the angle of emission, wavelength, and the angle-resolved
transmittance. The dip observed in the emission spectrum clearly matches the photonic stop band
position at different angles of observation. The emission spectrum measured at different angles was
found to follow a simple functional form related to the intrinsic emission of the dye and the stop
band effect due to the photonic crystal environment. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2825636兴
I. INTRODUCTION
Photonic crystals are artificial structures with a periodic
variation of refractive index and are useful for controlling,
manipulating, and localizing the propagation of light.1–3 In
order to explore the utility of photonic crystals for applications in low-threshold laser design, specially functionalized
photonic crystals with active media are required. The active
media used for emission studies with photonic crystals
should have a narrow emission linewidth, large quantum efficiency for emission, and be easy to infiltrate into the photonic crystals without disturbing the ordering in the initial
photonic crystal template. Important light sources that have
been used for emission studies are lanthanides or rare-earth
atoms, organic dyes, and quantum dots. All these active media can be infiltrated into the photonic crystals using different techniques.4–6 The emission spectra of lanthanides have
characteristic sets of sharp peaks.7 Since the linewidth of
their emission spectrum is small, the effect of the photonic
stop band will be felt more strongly, but the quantum efficiency of lanthanides is very low. In contrast, organic dyes
have broad emission spectra.8 They also have high quantum
efficiency and are strongly dependent on chemical interactions and environment. The main drawback of dyes as emission probes in photonic crystals is the process of photobleaching, which limits their usage with time.
The inhibition of spontaneous emission from an organic
dye infiltrated in a photonic crystal was studied in Refs. 9
and 10. Modified spontaneous emission has been observed
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-8979/2007/102共12兲/123106/5/$23.00
for inverse photonic crystal made of TiO2 infiltrated with
organic dye.11,12 The directional dependence of spontaneous
emission has been analyzed quantitatively for a gravity sedimented polystyrene 共PS兲 photonic crystal doped with an organic dye and for quantum dot infiltrated titania inverse photonic crystals.13 In that work, the so-called “escape function”
was proposed, and this function was found to follow the
non-Lambertian distribution for the photonic crystals and the
Lambertian distribution function for a random media. Directional fluorescence spectra of the laser dye in opal and inverse opal have been analyzed in Ref. 14 in the first and
second order photonic stop band, and an enhancement of
emission in the blue side of the spectrum for a given direction of emission was reported. In all these earlier works,
photonic crystals are fabricated using the gravity sedimentation method followed by the infiltration of the active species
共dyes兲 using chemical routes. During the infiltration of these
materials, the initial photonic crystal template may get disturbed by the solvent used or result in incomplete infiltration,
thus decreasing the efficiency of controlled emission. Therefore, it will be better if these active species are attached to
the building blocks using chemical methods before the fabrication of photonic crystals thus avoiding spatial diffusion
during emission studies.10 Then, using these modified building blocks of submicron spheres, high-quality three dimensionally ordered 共functionalized兲 photonic crystals can be
fabricated, and the effect of the photonic stop band on the
emission of active species can be probed. In Ref. 7, the dye
is homogeneously distributed in a layer inside the sphere,
and the photonic crystals are fabricated using these spheres
by the gravitational sedimentation method. Photoluminescence modification has also been observed recently for a
102, 123106-1
© 2007 American Institute of Physics
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123106-2
Nair et al.
J. Appl. Phys. 102, 123106 共2007兲
FIG. 1. 共Color online兲 共a兲 SEM image
for a photonic crystal made using PSRhB colloidal spheres of diameter 302
nm 共scale bar is 2 ␮m兲. 共b兲 AFM image of the photonic crystal shows the
共111兲 plane of the fcc lattice 共scan area
1 ␮m ⫻ 1 ␮m兲. 共c兲 CLSM image of
the photonic crystal shows the size of
the domains 共scale bar is 20 ␮m兲.
photonic crystal fabricated using these fluorescent
microspheres.15 It may be noted that gravitational
self-assembly16 suffers from the disadvantage of a large
number of unwanted and uncontrolled defects in the fabricated crystals and requires more time for the growth of crystals.
We have used dyed PS spheres 共Micro Particles, Germany兲 as the building blocks for the fabrication of photonic
crystals. The dye selected was rhodamine-B, which has a
broad emission spectrum in the visible region. The dye is
homogeneously distributed throughout the interior of the
sphere with a concentration of approximately 0.09 wt. %.
Since the dye is entrapped inside the sphere, its photostability can be greatly modified as compared with dyes infiltrated
through solution chemistry,17 and hence this limitation is
overcome. This is very important for designing lasers using
the photonic band gap concept. The quality of the fabricated
photonic crystal is ascertained through standard structural
and optical characterizations. The dyed photonic crystal
sample is excited with a frequency-doubled Nd: YAG laser,
and the emission characteristics due to the photonic crystal
environment are measured at different angles. The focus of
this paper is mainly on the use of dye-polymer colloids as the
building blocks avoiding the infiltration process for the active species, adopting a recently reported method of photonic
crystal fabrication resulting in samples of superior optical
quality, clear observance of selective suppression in the
angle-resolved emission, and the analysis of the experimental results through a model related to the angle-dependent
transmittance leading to a simple functional form for the
emission.
II. FABRICATION OF DYED PHOTONIC CRYSTALS
One of the most celebrated methods of fabrication of
three dimensionally ordered photonic crystals in the visible
region is the self-assembling route using colloidal suspensions of submicron size spheres made of organic or inorganic
materials. A modified form of gravitational self-assembly,
which is based on the capillary force to organize the colloids,
gives samples of very high optical quality but requires more
time and is limited by the diameter of the spheres.18 In contrast to the common self-assembly methods for the fabrication of photonic crystals, we have used the recently reported
inward growing self-assembling method19 for fabricating the
photonic crystals using dyed PS spheres, which has yielded
high-quality photonic crystals in less than 3 h. The building
blocks used were rhodamine-B dyed PS 共PS-RhB兲 spheres
with a mean diameter of 302 nm with a polydispersity index
of 0.17. First, a substrate 共glass兲 with a dimension of
2.5 cm⫻ 2.5 cm was treated with chromic acid overnight,
washed with deionized water and ethanol, and dried in an
oven. The colloidal solution of known concentration and volume is dropped on the substrate and spread uniformly using
a 1 ml syringe pipette and left to dry in the ambient atmosphere. After 1 min, depending on the sphere diameter, beautiful colors start appearing, and these colors move toward the
center of the film as growth takes place. The crystallization is
completed within 3 h for these samples. All the prepared
samples are heated at 75 ° C 共below the glass transition temperature of polymers兲 for 2 h after the growth to remove any
solvent used in the fabrication from the pores and also to
enhance the mechanical stability of the samples. The success
of this method depends highly on the concentration, the volume of the solution used 共0.05–0.1 ml兲, the temperature, and
any type of air flow in the fabrication room. The air flow can
force the crystallization toward a particular direction instead
of toward the center of the substrate. Under optimized conditions, a uniform film forms on the entire substrate with a
central void of millimeter size.
III. RESULTS AND DISCUSSION
A. Structural characterization
The structural characterization of the photonic crystals is
carried out using a scanning electron microscope 共SEM兲 and
an atomic force microscope 共AFM兲. The domain sizes are
mapped using a confocal laser scanning microscope
共CLSM兲. The samples analyzed in a SEM are coated with a
thin layer of gold to avoid any charging effect during the
experiment, and AFM images are taken in tapping mode. The
microscope images are taken in the reflection mode. Figure 1
shows the SEM, AFM, and CLSM images of the photonic
crystals fabricated using the PS-RhB spheres having a mean
diameter of 302 nm. The hexagonal ordering in Figs. 1共a兲
and 1共b兲 confirms the 共111兲 plane of the face-centered-cubic
共fcc兲 lattice parallel to the substrate.20 It has been observed
previously that the absolute value of the zeta potential of
dyed colloidal suspension decreases as compared to a nondyed colloidal suspension, and this will affect the stability of
the colloids.21 In spite of this aspect, the present method
produces photonic crystals with good structural quality. The
microscope image 关Fig. 1共c兲兴 shows the domains separated
by grain boundaries, and the uniform color for every domain
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123106-3
Nair et al.
FIG. 2. Absorption spectra for PS-RhB colloidal suspension 共line 1兲 and for
bare PS 共nondyed兲 colloidal suspension 共line 2兲. The peak shown using an
arrow indicates the absorption due to the dye, which is absent for the bare
colloidal suspension. The fluorescence spectrum for the PS-RhB colloidal
suspension 共line 3, dashed-dotted line兲 shows the emission peak at 580 nm
共marked with a vertical line兲 when excited at 532 nm.
is an indication that each of the domains has the same orientation. The SEM, AFM, and CLSM images confirm that good
quality photonic crystals can be prepared from dyed PS
spheres using this method.
B. Optical characterization
The absorption and fluorescence spectrum for the dyed
colloidal suspension was measured using an absorption spectrophotometer and a spectroflurometer. The results are shown
in Fig. 2. The absorption spectrum for the PS-RhB colloidal
suspension is shown as line 1. A small peak appears around
560 nm 共shown using a downward arrow on line 1兲 for the
PS-RhB colloidal suspension corresponding to the absorption of the dye, and it is absent in the case of a bare PS
colloidal suspension whose absorption spectrum is shown as
line 2. In both of these cases, the absorption increases toward
the ultraviolet region due to the absorption of PS. Line 3
shows the fluorescence spectrum for PS-RhB colloidal suspension at an excitation wavelength of 532 nm 共dasheddotted line兲. The emission wavelength is centered at 580 nm
J. Appl. Phys. 102, 123106 共2007兲
shown using a vertical line. The absorption and fluorescence
spectra of the dyed colloidal suspension give the excitation
and the expected peak emission wavelength of the dyed colloids required for the laser-induced emission studies on the
photonic crystals prepared from them 共to be discussed in the
next section兲.
The reflection/transmission measurements are routinely
used for probing the photonic stop band characteristics of
photonic crystals. A Perkin-Elmer Lambda 950 spectrophotometer with a halogen lamp as the source, having beam
dimensions of 12.5 mm⫻ 5 mm for unpolarized light, is
used for measuring the photonic stop band. Figure 3共a兲
shows the transmittance measured from the 共111兲 plane of a
bare PS photonic crystal 共dashed-dotted line兲 and a PS-RhB
photonic crystal as well as the reflectance spectra of a PSRhB photonic crystal 共black line兲. It is clear that the high
reflection wavelength region is accompanied by a wellresolved minimum in transmission for PS-RhB photonic
crystal. The reflection peak is slightly shifted to the shorter
wavelength region as compared with the transmission minimum in the PS-RhB photonic crystal. This is because the
reflection spectrum is measured at 8°, while the transmission
spectrum is measured at normal incidence. The reflection
spectrum for a nondyed PS photonic crystal also shows reflectance values as high as 50%, and the photonic stop band
position is centered at 611 nm 共not shown here兲. The dip in
transmission spectrum at 560 nm, shown using a downward
arrow in Fig. 3共a兲, is due to the absorption of RhB dye as
expected from the absorption measurement on the colloidal
suspension. This dip is absent in the transmission spectrum
of the nondyed PS photonic crystal. The photonic stop band
is centered at 609 nm with a peak reflectance of 60% for the
PS-RhB photonic crystal with a gap size 共⌬␭ / ␭兲 of 8%. The
thickness of the photonic crystal, and hence the number of
layers, can be estimated22 from the Fabry-Pérot oscillations
on either side of the stop band as 4.84 ␮m 共the number of
layers is 20兲 for the PS-RhB photonic crystal.
The photonic band structure calculation for the PS photonic crystal23 shows that the first order photonic stop band
should center at 0.6 a / ␭, where a is the fcc lattice constant,
and ␭ is the free space wavelength. From our measurements,
FIG. 3. 共Color online兲 共a兲 The reflection 共black line兲 and transmission 共blue line兲 spectra for a photonic crystal fabricated using PS-RhB colloids having a
diameter of 302 nm show the photonic stop band at 609 nm. The dashed-dotted line is the transmission spectrum for the photonic crystal made from nondyed
PS spheres having a sphere diameter of 280 nm. 共b兲 Photonic stop band at angles of incidence of 8° 共at the extreme right兲, 15°, 30°, 35°, 45°, and 60° 共at the
extreme left兲 for PS-RhB photonic crystal. The photonic stop band shifts steadily toward shorter wavelengths with an increase in the angle of incidence, as
shown by the arrow.
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123106-4
Nair et al.
FIG. 4. Schematic of the experimental arrangement used in the emission
studies
the first order stop band position is obtained at 0.65 a / ␭ in
the nondyed PS photonic crystal, and at 0.7 a / ␭ for the dyed
PS photonic crystal. This implies a slight shift to the highfrequency region for the dyed PS when compared with the
bare PS, due to the change in the dielectric constant of the
polymer after the dye is incorporated in it.
We have done the angle-resolved reflection spectroscopy
using unpolarized light to map the photonic stop band at
different angles of incidence. This study will enable us to
ensure that the stop band position overlaps with the emission
profile of the dye in the experiment described in the next
section. The results of angle-resolved reflectance measurements are shown in Fig. 3共b兲. With an increase in the angle
of incidence 共␪兲, the photonic stop band shifts toward the
shorter wavelength region as governed by the modified
Bragg’s law.6 The angle-resolved reflection spectra recorded
at different angles from 8° to 45° show that the full width at
half maximum and the peak reflectance remain constant.
This is a direct indication of the superior quality of the photonic crystals fabricated using this self-assembling method.
At an angle of incidence of 60° 关shown in Fig. 3共b兲, extreme
left兴 the peak reflectance reduces, and the shape of the spectrum changes drastically. This is due to multiple Bragg diffractions involving many crystal planes at higher angles of
incidence.24 It may be seen that, in the case of 60° incidence,
a photonic stop band is not present in the emission wavelength range of 550–650 nm of the RhB dye.
C. Emission studies with functionalized photonic
crystals
For the clear features of the photonic crystal environment to emerge in the emission studies, the excitation wave-
J. Appl. Phys. 102, 123106 共2007兲
length should be outside of the photonic stop band, while the
emission wavelength should be within the stop band. Hence
the excitation wavelength is chosen to be 532 nm, which is
outside the stop band of the photonic crystal, in order to
penetrate well into the crystal and interact with the dye. By
varying the angle of incidence, as shown earlier in Fig. 3共b兲,
it has been possible to shift the photonic stop band wavelength across the peak emission wavelength of 580 nm for
this dye. Figure 4 illustrates our experimental setup for the
study of the emission characteristics of the functionalized
photonic crystals. The second harmonic 共532 nm兲 of a quasicontinuous-wave Nd: YAG laser 共SOLAR TII: LF2210兲 was
used for excitation. The pulse duration and repetition rate of
the laser were approximately 30 ns and 4 kHz, respectively.
The laser beam with a diameter of 4 mm at the exit mirror
and a power of 5 mW was focused on the sample with a lens
of focal length 400 mm, so as to result in a spot size of
50 ␮m. The emission from the dyed photonic crystal was
collected at several angles 共at a distance of 100 mm from the
sample surface兲 and detected by a spectrometer 共Ocean Optics, USB4000兲. This angle-resolved emission has been recorded by moving the detector at different angles 共␪兲 with
respect to the direction of the incident excitation beam. The
sample position and the excitation beam direction 关normal to
the 共111兲 plane兴 were kept fixed. The advantage of this setup
is that the same position of the sample is probed throughout
the measurements with the probed sample volume being a
constant. This enables a direct comparison between the emission spectra measured at different angles of the detector position.
In Fig. 5共a兲, the emission spectra obtained at five different angles 共␪ = 4°, 15°, 30°, 45°, and 60°兲 of the detector are
shown by solid lines. The effect of the dark current of the
detector has been subtracted. We can see that the emission
profile changes when the detection angle is varied. Reflectance data R␪共␭兲 measured at ␪ = 8°, 15°, 30°, and 45° in the
same sample are also plotted by dotted lines 共here, the reflectance at 8° is plotted with the emission at 4°兲. It should
be noted that the positions of the reflectance peaks coincide
with those of the dips in the emission, indicating that the
emission spectra are affected by the presence of the band
gap. At an angle of incidence of 4° and 15°, the dip in emission spectrum is at the red end of the spectrum where the
photonic stop band is present for those incident angles. At
FIG. 5. 共Color online兲 共a兲 Experimentally measured emission intensity
共solid line兲 and percentage of reflection 共dotted line兲 as a function of
wavelength, at incident angles of 4°,
15°, 30°, 45°, and 60°. 共b兲 Detection
angle dependence of emission obtained from the model.
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123106-5
J. Appl. Phys. 102, 123106 共2007兲
Nair et al.
the incident angle of 30°, the dip in emission shifts toward
the blue side of the spectrum where the photonic stop band is
present, while the emission spectrum has recovered in the red
part. This clearly shows that the dip observed is due to the
photonic band gap effect, and the light emitted from the photonic crystals has a directional nature.
In order to explain the detection angle dependence of the
emission, we apply two assumptions: 共a兲 The emission at 60°
shows an intrinsic profile without being affected by the photonic band gap because the band gap moves to a wavelength
range much lower than the emission wavelength and 共b兲
transmittance T␪共␭兲 can be obtained simply as 1 − R␪共␭兲. In
Fig. 5共b兲, the function F␪共␭兲 given by
F␪共␭兲 = k1 + k2 ⫻ F60共␭兲 ⫻ 共1 − R␪共␭兲兲
共1兲
is plotted for ␪ = 4°, 15°, 30°, and 45°. Here, k1 and k2 are
free parameters that describe the background signal, and the
intensity of the dye emission. F60共␭兲 is the emission profile
at 60°. It can be seen that the detection angle dependence of
the emission is well reproduced by Eq. 共1兲, which is a product of the intrinsic emission with the angle-resolved transmittance. Since the active species are uniformly present even
in the depth of the photonic crystal sample, the transmittance
is a better parameter for this fit rather than the reflectance. It
may be emphasized that the angle ␪ is used with the same
meaning in the emission and transmittance data since the
light in the emission band originates inside the crystal and
gets measured by the detector placed at an angle ␪ to the
incident beam direction, which is normal to the sample surface. The unintended defects in the self-assembled photonic
crystals can cause random scattering of the light emitted by
the functionalized building blocks. However, the directional
property of the photonic crystal environment is strong
enough in the samples studied here enabling a significant
modification in the angle-resolved emission. The thicknesses
of the photonic crystals in this case are significantly lower
than those obtained by the sedimentation method. This is an
advantage leading to lesser scattering events during the passage of the emitted light through the crystal.
IV. CONCLUSIONS
Good-quality photonic crystals are fabricated using the
inward growing self-assembling method from PS-RhB
spheres in a short time span of less than 3 h. The optical
characterization results show a first order photonic stop band
with a peak reflectance of more than 50%. With a laser excitation at 532 nm, which is in the absorption band of this
dye, the emission characteristics were measured at different
angles with respect to the direction of excitation. The emission profile could be analyzed from the measured angle dependence of the reflectance of these samples and was found
to follow a simple functional form, which is a product of the
intrinsic emission with the angle-resolved transmittance.
ACKNOWLEDGMENTS
RVN and RV thank the Department of Science and Technology and the University Grants Commission, Government
of India, for financial support and are grateful for the use of
the characterization facilities at the MEMS Department and
the FIST-IRCC central facility at the Department of Physics,
IIT Bombay. RVN and RV also thank Professor S. S. Major
at the Physics Department of IIT Bombay for the use of the
spectrophotometer.
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