March 1, 2002 / Vol. 27, No. 5 / OPTICS LETTERS
Enhancement of two-photon emission in photonic crystals
Przemyslaw Markowicz, Christopher Friend, Yuzhen Shen, Jacek Swiatkiewicz, and Paras N. Prasad
The Institute for Lasers, Photonics and Biophotonics, Departments of Chemistry and Physics,
State University of New York at Buffalo, Buffalo, New York 14260
Ovidiu Toader and Sajeev John
Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario M5S 1A7, Canada
Robert W. Boyd
The Institute of Optics, University of Rochester, Rochester, New York 14627
Received July 31, 2001
We report the influence of photonic stopgaps on two-photon excited emission from highly efficient nonlinear
chromophores infiltrated into high-quality photonic crystals.
fect) in emission within the frequency range and direction of the stopgap as well as sharp enhancement of
the two-photon excited emission associated with the stopgap’s edge.
development of low-threshold upconversion lasers.© 2002 Optical Society of America
We have observed a sharp decrease (filter ef-
This effect may be important for the
Three-dimensional periodic dielectric structures in
which the refractive index varies on length scales of
attracted muchtheoretical and experimental at-
An especially interesting class of such
structures are photonic bandgap crystals.
range of frequencies defined by the photonic bandgap,
no light can propagate through the crystal in any
direction.Instead, light in this frequency range is
This localization has a significant effect
on the spontaneous emission of excited molecules
placed inside the photonic crystal.7,8
frequency of excited molecules is adjusted to the fre-
quency range of the gap, spontaneous emission of light
into unwanted modes can be completely inhibited.
This effect may allow for the development of thresh-
A number of experiments with one-photon ex-
citation of dyes incorporated into ordered11–13and
disordered14photonic structures, which act as lasing
materials, have been undertaken.
ordered structures, the range of order associated with
the crystal building blocks and the refractive index
determine the optical properties of these structures.
Currently, polystyrene particles are the building
blocks with the highest refractive index, ?1.6, per-
mitting the production of photonic crystals with
long-range particle order.
properties of the crystals are still weak, they are
sufficiently strong to allow us to observe the influence
of the crystals on two-photon emission.
In this Letter we report experimental results of the
modification of two-photon excited, upconverted emis-
sion spectra from a highly efficient dye incorporated
into photonic crystals that exhibit stopgaps ranging
in size from 1% to 3% of the gap center frequency.
The photonic crystals were fabricated from polystyrene
spheres with a diameter of 200 nm and then infiltrated
with dye.Coumarin 503 dye was chosen for the in-
filtration because the emission spectrum of this dye
fits into the stopgap of the transmission spectrum of
If the transition
In the case of
Although the scattering
the polystyrene photonic crystal and because its high-
fluorescence quantum efficiency [0.84 (Ref. 15)] leads
to mostly radiative decay.
The method introduced by Park and Xia16was ap-
plied for the crystallization of polystyrene nanoparti-
cles.The transmission spectra of photonic structures
were measured with the aid of a Biorad MRC-1024
confocal scanning microscope mounted upon a Nikon
Eclipse-800 upright microscope.
port of the microscope was connected to a spectrome-
ter (HoloSpec, Kaisar Optical Systems, Inc.) with a
multimode fiber.The white light was focused upon
the surface of the sample, and the transmitted light
was collected by a Nikon Plan 103 objective with a
N.A. of 0.25.We observed the central wavelengths of
the stopgaps at ?500 and ?525 nm for polystyrene–
methanol and polystyrene–dimethyl sulfoxide (DMSO)
crystals, respectively (Fig. 1).
transmission larger than 80% outside the gap sug-
gests long-range particle order in the crystals.
The side output
Sharp attenuation with
and polystyrene–DMSO assemblies (without dye).
Transmission spectra of polystyrene–methanol
0146-9592/02/050351-03$15.00/0© 2002 Optical Society of America
OPTICS LETTERS / Vol. 27, No. 5 / March 1, 2002
photonic band structure of the polystyrene–methanol
crystal is shown in Fig. 2.
Scanning-electron microscope images of a dried
sample (not shown) confirmed the long-range particle
order in the crystals and the fact that polystyrene par-
ticles crystallize into an fcc structure with the (111)
face parallel to the surface.
surface with light at normal incidence.
refractive-index contrast is small, the stopgap position
is accurately approximated by the Bragg equation16
We illuminated the
where d111is the spacing between (111) planes in the
fcc crystal and neff? limk!0?ck?v? is the effective re-
fractive index obtained by considering the long-wave-
length limit of photon dispersion relation v?k? and
where c is the speed of light.
of polystyrene, methanol, and DMSO are npol? 1.6,
nmeth? 1.33, and nDMSO? 1.48, respectively.
There was good agreement between the theoretical
calculations of the position of the stopgap (501.8 nm)
calculations performed for the polystyrene–DMSO
structure showed a difference of 10 nm between
the theoretical and the experimental results, which
were 515 and 525 nm, respectively.
the gap was shifted does not seem to have had any
influence on our two-photon experiments because the
particles were still well ordered.
Photonic crystals with the range of order similar to
those whose transmission spectra are presented in
Fig. 1 were used in our two-photon experiments.
After the polystyrene particles had been assem-
bled during fabrication, the sample was infiltrated
with Coumarin 503 dissolved in methanol or in
DMSO.The concentrations of Coumarin 503 in
methanol and in DMSO were 2.9 3 1022and 11.77 3
1022M, respectively.Then the sample was illu-
minated with laser light (described below) and the
two-photon excited upconverted emission spectrum
We obtained two-photon excited fluorescence spec-
tra of the dye in a photonic crystal by exciting the dye
with a mode-locked Ti:sapphire laser at a wavelength
of 800 nm (80 fs).The direction of the laser light was
perpendicular to the surface of the sample.
lected spectra in the reflection mode (Nikon Plan 103
objective).Laser light with a power of ?20 mW was
focused onto the sample.
spot size was ?5 mm.Because the scanning-electron
microscope images of the crystal surface indicated that
the crystal domains extended over an area of at least
a few hundred square micrometers, it was possible to
confine the focused spot to a single crystal domain.
Coumarin 503 in our structure we noticed that,
besides filter effects similar to those reported for
transmission of external plane waves,13,17a sharp
maximum can also appear (Fig. 3).
concentration of solvated dye molecules was relatively
high, we did not observe any changes in spectral shape
The refractive indices
The reason why
The diameter of the focal
attributable to the dipole–dipole transfer of excitation.
The emission spectrum of the dye solution, without
a photonic crystal, overlapped well with the rescaled
spectrum taken for the solution with a 100-times
lower dye concentration.In both spectra the sharp
maximum did not appear.
be a result of amplification of light inside the photonic
crystal.The position of the sharp maximum depends
on the position of the stopgap.
the shapes of the emission spectra from Coumarin
and polystyrene–DMSO structures
spectra also overlapped with the rescaled reference
spectra outside the stop-gap region.
spectrum was measured for Coumarin 503 placed
in a cell with the same thickness as the polystyrene
structure ?8 mm?.
In our experiment, in contrast to those presented
in other reports,12,13the attenuation and amplification
of light in emission spectra exist simultaneously.
check the position of the maximum with respect to the
stopgap more precisely, we performed the following ex-
periment: before the two-photon excited upconverted
emissionspectrum of Coumarin 503 was measured, the
transmission spectrum of exactly the same region of
The maximum seems to
Outside the stopgap
(neff ? 1.529; stopgap, 2.96%) photonic crystal.
a is the lattice constant.
polystyrene–DMSO photonic structures.
Two-photon excited upconverted emission spec-
March 1, 2002 / Vol. 27, No. 5 / OPTICS LETTERS Download full-text
B, linear transmission spectrum measured at the same
spot in polystyrene–Coumarin 503 in a methanol photonic
A, Two-photon excited emission spectra and
the sample was recorded.
ment are shown in Fig. 4.
two-photon emission spectra appeared at the edge of
the attenuation in the transmission spectra of exter-
nal plane waves. This means that the amplification
of two-photon excited upconverted emission took place
at the edge of the (1,1,1) stopgap.
In our case, the threshold for laser emission was not
yet reached, as the magnitude of the maximum was
relatively small.However, the influence of the pho-
tonic stopgap on the emission spectrum is noticeable
because the maximum did not appear in the refer-
ence spectrum.It is exciting to see that, even with
the current refractive-index contrast, the amplifica-
tion of two-photon emission was observed.
more dramatic enhancements may be expected in more
strongly scattering photonic crystals.18
Although the lasing threshold was not reached, it is
useful to think of our results in terms of distributed
feedback in three-dimensional structures.
for the propagation of electromagnetic waves in a
periodic medium with absorption and gain in one-
dimensional distributed-feedback lasers were reported
by Kogelnik and Shank.19
these structures appears to be due to elastic Bragg
In our structure the feedback was realized as a re-
sult of coupling between the incident field and the
counterpropagating wave created by Bragg reflection
from the (111) crystal planes.
contrast with the Kogelnik–Shank theory,19there was
only one significant maximum.
one maximum in an emission spectrum can be due to
the gain characteristics of the dye medium.
tively, maxima in the emission spectra can appear as a
result of the enhanced fluorescence that is predicted to
occur near the photonic band edge20or can arise from
defects in a photonic crystal.
The results of that experi-
The amplification in the
The positive feedback in
However, in our case, in
The presence of only
In conclusion, we have demonstrated the influence
of well-ordered photonic crystals on the two-photon
excited upconverted emission of highly efficient non-
linear dye.Attenuation and an enhancementlike
maximum in the emission spectrum were observed.
The attenuation can be understood as a pure filter
effect in a way similar to that of the transmission
spectrum. The maximum can be interpreted in terms
of a distributed-feedback mechanism that appeared
as a result of elastic Bragg scattering at the stopgap’s
edge. Our results suggest that low-threshold upcon-
verted lasing may be attained near the band edges
of strongly scattering photonic crystals in which the
corresponding feedback effects are much stronger than
in other crystals.
This research was supported by U.S. Air Force Of-
fice of Scientific Research (USAFOSR) grant F49620-
00-1-0064, USAFOSR Defense University Research
Initiative on Nanotechnology grant F49620-01-1-0358
to the authors in Buffalo, and USAFOSR grant
F49620-00-1-0061 to the author in Rochester.
Markowicz’s e-mail address is firstname.lastname@example.org.
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