Two-photon pumped lasing in single-crystal organic nanowire exciton polariton resonators

Article abstract of DOI:10.1021/ja200549v

Single-crystal organic nanowires were fabricated with a soft-template-assisted self-assembly method in liquid phase. These nanowires with rectangular cross section can serve as resonators for exciton-photon coupling, leading to a microcavity effect and a

Full text of DOI:10.1021/ja200549v

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pubs.acs.org/JACS
Two-Photon Pumped Lasing in Single-Crystal Organic Nanowire
Exciton Polariton Resonators
Chuang Zhang,† Chang-Ling Zou,‡ Yongli Yan,† Rui Hao,† Fang-Wen Sun,‡ Zheng-Fu Han,‡
Yong Sheng Zhao,*,† and Jiannian Yao*,†
†Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
‡Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui 230026, China
S Supporting Information
b
structures.13 Following our continuous investigations on
ABSTRACT: Single-crystal organic nanowires were fabri-
cated with a soft-template-assisted self-assembly method in
liquid phase. These nanowires with rectangular cross section
can serve as resonators for excitonꢀphoton coupling, lead-
ing to a microcavity effect and a relatively low threshold of
laser actions. Two-photon-pumped blue lasing was ob-
served in these organic waveguiding nanostructures above
a threshold of 60 nJ, excited with a 750 nm near-infrared
femtosecond pulse laser at 77 K.
intermolecular interactions and successful fabrication of 1D
organic structures,14 herein well-defined organic nanowires
of TPA molecules are designed and synthesized with a
surfactant-assisted self-assembly method. These nanostruc-
tures can work as efficient resonators for the strong exci-
tonꢀphoton coupling and are capable of emitting laser
around 475 nm at low-power excitation according to our
loss/gain calculation in the EP model. When a single wire was
excited locally with a near-infrared pulse laser, blue TPP
lasing was observed in the resonance modes of EPs above a
threshold of 60 nJ. This low-threshold frequency-upconver-
sion nanowire laser is expected to be integrated with the
traditional downconversion elements to compose integral
photonic circuits at nanoscale.15
In this work, 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-di-
methylpyrazol-1-yl)-1,3,5-triazine (DPBT, Figure 1) was de-
signed and synthesized for its high quantum yield in two-
photon excitation fluorescence16 and preponderant molecular
packing along the c crystal axis. Single-crystalline nanowires
were prepared by injecting a tetrahydrofuran (THF) solution of
DPBT into aqueous solutions of cetyltrimethylammonium
bromide (CTAB) at different temperatures (see Supporting
Information). As shown in the photoluminescence (PL) image
in Figure 1A, the as-prepared DPBT nanostructures have uni-
form 1D wire-like morphology and emit strong blue PL with
bright spots at the wire ends (see inset) under unfocused UV
excitation. This characteristic of optical waveguiding indicates
that PL energy can propagate efficiently along the axis direction,
which is attributed to the smooth surface and defect-free
structure of the nanowires, as shown in the scanning electron
microscopy (SEM) image in Figure 1B. The flatness of the
surface and the rectangular shape of cross section are clearly
illustrated in the atomic force microscopy (AFM) image
(Figure S1). The transmission electron microscopy (TEM)
image and selected area electron diffraction (SAED) pattern in
Figure 1C indicate that the wire is a single crystal growing along
the [001] direction with flat end facets. The flat end facets can
efficiently reflect the guided PL, which is essential for optical gain
and amplification of stimulated emission.17 Figure 1D displays the
molecular packing in a DPBT crystal, revealing strong πꢀπstacking
and primary growth along the c crystal axis. The preferential
rganic dyes exhibiting two-photon pumped (TPP) lasing
O
based on two-photon absorption (TPA) molecular struc-
tures have received increasing attention due to the broad range of
possible applications in wavelength upconversion, 3D fluores-
cence imaging, and 3D microfabrication.1 Unlike common
second-harmonic generation in second-order nonlinear crystals,
TPP lasing provides a possible way to accomplish frequency
upconversion of coherent light within a broad spectral range but
without phase-matching requirements.2 However, traditional
organic TPP lasers in liquids, films, solid rods, optical fibers,
and artificial optical microcavites often require relatively high
pump power due to the nonlinear nature of TPA. To create low-
power TPP lasers, tiny devices, e.g., organic nanowire TPP lasers,
are desired to minimize the energy required to transport the
electrons to their upper states.3 In addition, single-crystalline
organic nanowires can be synthesized4 and integrated5 readily,
which exhibit excellent performances of photon confinement and
propagation6 and high charge-carrier mobilities7 for future
electrically pumped laser devices.
The strong coupling between excitons and photons can result
in the formation of exciton polaritons (EPs) in semiconductor
microcavities,8 which are mixed light-matter quasiparticles that
combine the best features of photonics and excitonic systems.9 In
organic materials, excitons and EPs are highly stable because of their
large binding energies, showing long-distance exciton migration and
propagation of EPs in organic dye nanofibers.10,11 Therefore, TPP
lasers in organic crystalline nanowires may offer a low excitation
threshold according to the stable existence of EPs confined in
nanowire microcavities.12 To our knowledge, the fabrication of
single-crystalline 1D nanostructures from TPA molecules remains
challenging because of their specific donorꢀπ-bridgeꢀacceptor
Received: January 19, 2011
Published: April 25, 2011
r
2011 American Chemical Society
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DPBT nanowires. Figure 2A shows the light guiding of two-photon
excited PL signals from the excited right wire tip to the left tip, which
indicates that the energy of EPs can propagate along the wire axis and
outcouple as PL photons at tips. Interestingly, the guided PL spot at
77 K is much brighter than that at 300 K, although the excitation
intensity and guiding distance are kept constant. This enhanced
guided PL at low temperature was more accurately determined
with the emission spectra as shown in Figure 2B, which reveals
that the optical loss in the guiding process at 77 K is only half of
that at 300 K. More importantly, the significant decrease in
optical loss at 77 K induces a series of PL modulations formed
from FabryꢀPꢀerot (FP) modes between the two end facets. The
re-absorption loss in light guiding from the overlaps between
emission and absorption can be significantly reduced in EP
coupling because of the increased stability of excitons at low
temperature. In organic materials, the stability of Frenkel ex-
citons and the coupling between excitons and photons is
determined by the exciton binding energy.11,19 The thermal
disturbance at liquid nitrogen temperature is kT77K ≈ 7.7
meV, which is much lower than that at room temperature
(kT300K ≈ 30 meV). Accordingly, Frenkel excitons become
stable enough at 77 K to form EPs with photons that can travel
back and forth along the crystalline wire axis coherently, which
may help in the FP-type resonance in the 1D nanostructure.
The PL spectra of the outcoupling light at 77 K from the tips in
DPBT nanowires with different lengths (L = 6.6, 10.0, 15.2 μm)
were measured to study the relationship between the FP-type
resonance modes and the size of the nanowire microcavity. As
shown in Figure 2C, the modulated PL spectra of the outcoupled
emission present an increasing number of FP modes with
increasing cavity length in the EP resonators. For the PL spectra
of FP-type resonance, the mode spacing is given by Δλ = λ2/
2Lng, where L is the length of the nanowire resonator, λ is the
light wavelength, and ng is the group refractive index as a function
of the wavelength.20 The modes emerge at the lower energy side
but do not appear at shorter wavelengths because the exciton
character of the EPs changes through coupling with acoustic
phonons. During the propagation of EPs, continuous absorption
and re-emission of PL occurs, leading to the guided distance-
dependent PL intensities at different wavelengths (Figure S3). As
shown in Figure 2D, the PL intensity decays exponentially at
shorter wavelengths but much more slowly at longer wavelengths
as a function of propagation distance. The nearly linear decay at
longer wavelengths can be attributed to the weaker absorption of
lower energy EPs and continuous re-emission in EP
propagation.6 These results indicate that a lower propagation
loss is expected at longer wavelengths within the emission range
in these EP resonators.
Figure 1. (A) PL image of the DPBT nanowires excited with the UV
band (330ꢀ380 nm) light source; scale bar is 25 μm. Inset: magnified
image of several wire ends; scale bar is 5 μm. (B) SEM image of the
nanowires with uniform diameter and smooth surfaces; scale bar is 2 μm.
(C) TEM image of a single DPBT wire; scale bar is 500 nm. Inset: SAED
pattern of the wire. (D) Theoretically predicted growth morphology of a
single crystal of DPBT. Inset: molecular structure.
Figure 2. (A) Bright-field microscopy image and PL images of a single
DPBT nanowire taken at room temperature (300 K) and liquid nitrogen
temperature (77 K), respectively; scale bar is 2 μm. (B) Corresponding
PL spectra of the excited spots (black dashed lines) and guided spots
(blue lines) in the PL images in (A). (C) Modulated PL spectra collected
at guided tips of three DPBT nanowires with different lengths. (D)
Decay of the guided PL intensity at different wavelengths as an
exponential function of the propagation distance.
To verify the existence of EP resonance in the obtained
nanowires, we calculated the accurate complex refractive index
of DPBT crystals in the propagation direction of [001] from the
obtained PL data. First, the group refractive index ng along the
axis is calculated from ng = λ2/2LΔλ by measuring the peak
spaces Δλ in modulated PL spectra in Figure 2C. Plots of ng
versus λ for the three lengths are shown in Figure 3A, which
indicates that ng is about 2.1 at longer wavelength in the emission
band and suddenly increases at shorter wavelength. The PL
intensity at guided spots is then given by I = I0 exp[ꢀ2ωIm(n)X/c],
where I0 is the PL intensity at the excited spot, ω is the frequency
of the light, Im(n) is the imaginary part of the refractive index, X
is the propagation distance, and c is the speed of light. The
Im(n)ꢀλ relationship can be plotted as shown in Figure 3B from
growth along the [001] direction can be induced by the CTAB
micelles, which leads to the formation of 1D nanostructures.18
The excellent crystallinity can further improve the excitonꢀpho-
ton coupling and the migration of excitons in the light guiding
process.
The TPA cross section of DPBT is as large as 160 GM (1 G€oppert-
Mayer unit, 1 GM = 10ꢀ50 cm4 s photonꢀ1 moleculeꢀ1) around
750 nm (Figure S2), so a focused 750 nm femtosecond pulse laser
was adopted to locally excite the two-photon fluorescence of the
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Figure 3. (A) Group refractive index ng and (B) imaginary part of the
complex refractive index Im(n) versus wavelength along the [001]
direction in single-crystalline DPBT nanowires at 77 K. The fitted
curves are calculated according to the EP model in optical waveguiding.
(C) Simulated loss/gain versus wavelength in the 6.6 μm long DPBT
nanowire at 77 K. (D) Simulated electric field intensity distribution (λ =
475 nm, n = 1.767) in the DPBT nanowire at 77 K which is 6.6 μm long,
500 nm wide, and 500 nm high.
Figure 4. (A) PL spectra recorded at the tip of the 6.6 μm long DPBT
nanowire excited at different energies (25, 60, and 100 nJ) at 77 K. Inset:
power dependence profile of PL intensities at the tip of the same
nanowire. (B) PL decay profiles of the same nanowire excited at different
energies (10, 25, 40, and 60 nJ) at 77 K. The PL lifetime shows a sharp
decrease at 60 nJ pump energy.
the optical loss versus propagation distance in Figure 2D. The
Im(n) shows a dependence on PL wavelength identical to that of
n, especially when the photon energy is larger than 2.7 eV. The
complex refractive index shows strong dispersion in high photon
energy bands, which is characteristic of EP from the strong
coupling between excitons and photons.21 The dispersion curve
shows distinguishing features of propagation of EP in 1D organic
nanostructures, very different from those of either uncoupled
light or exciton. This result confirms that the formed EP may
cause resonance and amplification of the PL emission in these
single-crystalline nanowires.
The optical loss and gain in the nanowire EP resonators are
calculated to study the possibility of optically pumped TPP
lasing. It is well known that the laser action is possible only when
the optical gain is larger than the optical loss. Three major factors
were taken into account in the determination of the loss/gain
ratios of different FP modes: the outcoupling and reflection at the
end facets, the optical gain of the dye DPBT, and the absorption
loss in the guiding process. The reflectivity at the end facets can be
evaluated from the Fresnel equation with normal incidence R =
((nꢀ 1)/(n þ 1))2, where n is the phase refractive index (see
Supporting Information), and the corresponding reflection loss can
be estimated as γ = ꢀ(lnR)/L.22 The gain is determined by the
spatial spectra and the spectral overlap between the resonance and
the gain material. The absorption loss as light travels is derived
directly from the decay of the guided PL intensity at different
wavelengths in Figure 2D. In this work, the scale of the nanowire
waveguide is nearly the same as the PL wavelength, so almost all
the radiative energy can be confined in the nanowire microcavity.
In our numerical simulation, the overlap of the cavity mode with
the gain medium is >95% and insensitive to the wavelength in the
range of 380ꢀ650 nm. Considering all these factors, we estimate
the loss/gain curve versus PL wavelength in Figure 3C, which
illustrates the possibilities of lasing action over the emission
spectra range. A minimum value is noted in the loss/gain curve at
∼475 nm, which theoretically predicts that the FP modes nearby
might be pumped at relatively low threshold energies.
The properties of the FP resonance in the nanowire are
investigated with the three-dimensional finite element method.
The electric field intensity at 475 nm (n = 1.767) in the selected
single wire (6.6 μm long, 500 nmwide, and500 nm high) on a silica
substrate (n = 1.45) is described in Figure 3D. The electric field
intensity at the cross section reveals that the energy of the electric
field is well confined in the nanostructures as EP resonators, and the
scattering into the air and the substrate is quite limited. The mode
profiles along the nanowire clearly show the efficient light guiding,
along with the reflection and leak-out at the end facets, which
indicate a typical FP-type EP resonance. Here we do not consider
the transverse magnetic polarization modes, as they are poorly
confined andreflected at the end facets in the nanowires. According
to the calculation of the possible laser action in the EP resonators,
the pump power was increased to locally excite the 6.6 μm long
DPBT nanowire. As displayed in Figure 4A, PL peaks in the spectra
strongly depend on the increase of pump energy of the pulse laser
(λ = 750 nm, ∼200 fs). At a low excitation power of 25 nJ, the PL
spectrum displays broad FP modes over the entire emission band,
almost the same as the spectra in Figure 2C. However, a sharp
cavity mode at 475 nm above the onset power as an amplified
spontaneous emission was observed at excitation power of 60 nJ. At
a higher excitation power of 100 nJ, the full-width at half-maximum
(fwhm) at 475 nm narrows to 2.9 nm, and the sharp peaks at ∼465
and 485 nm can also be clearly observed, which justifies the
calculated curve of laser action in Figure 3C.
The inset in Figure 4A illustrates the energy dependence of
PL intensity, showing an obvious energy threshold at ∼60 nJ,
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which reveals a nonlinear gain and a threshold characteristic of
a laser. To verify the TPP laser action above the threshold, we
measured the PL decay profiles at different pump energies,
which indicate that the PL lifetime gradually decreases as the
pumped power increases (below the threshold) and sharply
falls to 0.58 ns at 60 nJ (above the threshold), as shown in
Figure 4B. These results demonstrate that the resonance of EPs
not only plays an important role in light propagation but also
leads to the interference and enhancement of PL emission in
the nanowires as miniaturized optical microcavties. The quality
factor Q in this microcavity is as high as 57.0, attributed to the
optically flat surface and the existence of EPs, which agrees well
with the estimation from end facet reflectivity, Q = 2nπ/λγ =
60.1. The nanowires show great potential in fabricating laser
devices, considering its relatively high Q-factor among organic
nanostructures. The significant spectral line narrowing (fwhm
= 2.9 nm), the existence of laser cavity resonances, the
clear indication of a threshold (Eth = 60 nJ), and the short
emission lifetime above threshold (τ = 0.58 ns) verify the TPP
lasing in this kind of organic single-crystalline nanowire EP
resonators.
’ AUTHOR INFORMATION
Corresponding Author
yszhao@iccas.ac.cn; jnyao@iccas.ac.cn
’ ACKNOWLEDGMENT
This work was supported by National Natural Science Foun-
dation of China (Nos. 51073164, 91022022), the Chinese
Academy of Sciences, and the National Basic Research 973
Program of China.
’ REFERENCES
(1) He, G. S.; Tan, L. S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008,
108, 1245–1330.
(2) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt,
J. C.; Kannan, R.; Yuan, L. X.; He, G. S.; Prasad, P. N. Chem. Mater. 1998,
10, 1863–1874.
(3) O’Carroll, D.; Lieberwirth, I.; Redmond, G. Nat. Nanotechnol.
2007, 2, 180–184.
Organic nanowires, as typical optical waveguiding structures,
can lead to FP-type PL modulation and some properties of laser
light because waveguides are common laser geometry.23 The
interaction between photons and excitons is determined by the
ratio of the oscillator strength to the mode volume. Therefore, it
is the very high oscillator strength of the Frenkel excitons rather than
any cavity effect that causes EPs to exist in the presented DPBT
nanowires. Although the distinctions between polariton-type and
photon-type lasing remain subtle in organic nanostructures,24,25 the
existence of EPs should play an essential role in optical waveguiding
and TPP lasing according to our experiments. In agreement with
calculation from the EP model in selected DPBT nanowires, the
strong dispersion and resonance of EP leads to an efficient optical
gain around 475 nm in the microcavity. In our experiments, TPP
lasing in the EP resonators can only be pumped at low tempera-
ture. However, this kind of model TPP nanolaser device based on
EP effects in organic waveguides might provide enlightenment
for other organic EP devices and find application in miniaturized
photonics integrations.
(4) Zhao, Y. S.; Fu, H. B.; Peng, A. D.; Ma, Y.; Xiao, D. B.; Yao, J. N.
Adv. Mater. 2008, 20, 2859–2876.
(5) Zhao, Y. S.; Wu, J. S.; Huang, J. X. J. Am. Chem. Soc. 2009,
131, 3158–3159.
(6) Takazawa, K.; Kitahama, Y.; Kimura, Y.; Kido, G. Nano Lett.
2005, 5, 1293–1296.
(7) Jiang, L.; Fu, Y. Y.; Li, H. X.; Hu, W. P. J. Am. Chem. Soc. 2008,
130, 3937–3941.
(8) Hopfield, J. J.; Thomas, D. G. Phys. Rev. Lett. 1965, 15, 22–25.
(9) Andreani, L. C.; Panzarini, G.; Gerard, J. M. Phys. Rev. B 1999,
60, 13276–13279.
(10) Chaudhuri, D.; Li, D.; Che, Y.; Shafran, E.; Gerton, J. M.; Zang,
L.; Lupton, J. M. Nano Lett. 2011, 11, 488–492.
(11) Takazawa, K.; Inoue, J.; Mitsuishi, K.; Takamasu, T. Phys. Rev.
Lett. 2010, 105, 067401.
(12) van Vugt, L. K.; Ruhle, S.; Ravindran, P.; Gerritsen, H. C.;
Kuipers, L.; Vanmaekelbergh, D. Phys. Rev. Lett. 2006, 97, 147401.
(13) Gao, F.; Liao, Q.; Xu, Z. Z.; Yue, Y. H.; Wang, Q.; Zhang, H. L.;
Fu, H. B. Angew. Chem., Int. Ed. 2010, 49, 732–735.
(14) Zhao, Y. S.; Fu, H. B.; Peng, A. D.; Ma, Y.; Liao, Q.; Yao, J. N.
Acc. Chem. Res. 2010, 43, 409–418.
In conclusion, single-crystalline nanowires were prepared
from an organic TPA compound, DPBT, via surfactant-assisted
self-assembly in aqueous solution. These nanowires can serve as
EP resonators at 77 K because of the increased stability of the
strong coupling between photons and Frenkel excitons. Single-
nanowire TPP lasing was observed around 475 nm, pumped with
a 750 nm pulse laser above a threshold of 60 nJ. The low driving
power can be attributed to the EP resonance in the nanowire
microcavity. The Q factor in this microcavity is as high as 57.0
because of the optically flat surfaces and the existence of EPs in
the nanostructures. This approach to organic crystalline TPP
nanolasers in 1D organic nanostructures may lead to a new series
of upconverted organic photonic devices.26
(15) Yan, R. X.; Gargas, D.; Yang, P. D. Nat. Photon. 2009, 3, 569–576.
(16) Fu, L. M.; Wen, X. F.; Ai, X. C.; Sun, Y.; Wu, Y. S.; Zhang, J. P.;
Wang, Y. Angew. Chem., Int. Ed. 2005, 44, 747–750.
(17) Yan, H. Q.; Johnson, J.; Law, M.; He, R. R.; Knutsen, K.;
McKinney, J. R.; Pham, J.; Saykally, R.; Yang, P. D. Adv. Mater. 2003,
15, 1907–1911.
(18) Lei, Y. L.; Liao, Q.; Fu, H. B.; Yao, J. N. J. Am. Chem. Soc. 2010,
132, 1742–1743.
(19) Scholes, G. D.; Rumbles, G. Nat. Mater. 2006, 5, 683–696.
(20) Sirbuly, D. J.; Law, M.; Yan, H. Q.; Yang, P. D. J. Phys. Chem. B
2005, 109, 15190–15213.
(21) van Vugt, L. K.; Piccione, B.; Agarwala, R. Appl. Phys. Lett. 2010,
97, 061115.
(22) Maslova, A. V.; Ning, C. Z. Appl. Phys. Lett. 2003, 83, 1237–1239.
(23) van Vugt, L. K.; Ru1hle, S.; Vanmaekelbergh, D. Nano Lett.
2006, 6, 2707–2711.
(24) Deng, H.; Weihs, G.; Santori, C.; Bloch, J.; Yamamoto, Y.
Science 2002, 298, 199–202.
(25) Zhao, Y. S.; Peng, A. D.; Fu, H. B.; Ma, Y.; Yao, J. N. Adv. Mater.
2008, 20, 1661–1665.
(26) Zhang, C.; Zheng, J. Y.; Zhao, Y. S.; Yao, J. N. Adv. Mater. 2011,
23, 1380–1384.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details; optical
b
characterization; discussion on EP model; AFM image and cross
section profile; TPA cross section of DPBT; and spatially
resolved spectra at the wire tip. This material is available free
of charge via the Internet at http://pubs.acs.org.
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