2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)
The International Conference Centre Birmingham
20-23 August 20112, Birmingham, United Kingdom
Red, Green, and Blue Laser Action in Solid
Colloidal Quantum Dot Films
Cuong Dang
\ Kwangdong Roh2, Joonhee Lee \ Craig Breen3, Jonathan S. Steckee , Seth Coe-Sullivan3, and
12
Arto Nurmikko , *
'School of Engineering, Brown University, Providence, Rhode Island
02912, USA
02912, USA
3QD Vision Inc., 29 Hartwell Ave., Lexington, Massachusetts 02421, USA
2Department of Physics, Brown University, Providence, Rhode Island
Email: Cuong Dang@Brown.edu,
* Arto
Nurmikko@Brown.edu
We report on wavelength-variable lasing from
previous reports. Here, we leverage the high performance of
optical gain media composed of colloidal quantum-dots (CQD)
this high-density single exciton nanocomposite gain media
thin films across the visible spectrum. Exploiting single exciton
towards practical laser applications where the optical pump
Abstract
-
gain enables amplified spontaneous emission (ASE) and vertical
cavity lasing at very low optical pump thresholds. The average
number of exciton per CQD at the ASE threshold is <N>- 0.8,
which significantly reduces losses from enhanced nonradiative
multiexciton
Auger
recombination
in
nanometer
laser performance in densely packed nanocomposites.
are
II. EXPERIMENTS AND RESULTS
The epitaxial-like CQD films
(250 - 300 nm thick on
with extraordinarily high concentration initial CQD solution
(up to
123 mg/ml in Toluene solvent) as the starting
material. The CdSe/ZnCdS core/shell CQD structure and its
I. INTRODUCTION
dots
at least two-orders-Ionger than the nonradiative Auger rate.
fused silica substrate) are prepared by spin-casting technique
Index Terms - Quantum dot, Red green blue laser, Single
exciton gain, Vertical cavity laser
quantum
nanoseconds - i.e. on the order of single exciton lifetime and
sized
semiconductor particle, enabling quasi-continuous-wave CQD
Colloidal
sources are compact and delivering pulses up to many
surface aromatic ligands are engineered so that the spin-cast
attractive
fluorescent
materials because of their ability to emit full color spectrum
by tuning their size in a simple synthesis procedure
[1].
films are very high density (at least
50% packing density)
80% from their
yet still preserve their high quantum yield of
initial solution environment.
Their use at low excitation as luminescent media is well
established due to high quantum efficiency and stability.
But, at higher excitation levels for achieving population
inversion for a laser, earlier work has found that when
generating more than one electron hole pair (exciton) per
CQD, the radiative efficiency drops dramatically because of
enhanced non-radiative multiexciton Auger recombination in
the O-dimensional quantum confined system
[2]. This has
been a challenging problem for over a decade: how to extend
such CQDs to laser applications when relying on optical
amplification from bi-exciton states. In the laboratory, the
approaches to build up the population inversion quickly have
relied on using ultrashort (sub-psec) optical pulse pumping
sources to overcome the very fast Auger recombination,
typically on the order of hundreds of ps
conditions
are
impractical
for
any
[3]. Such extreme
compact
device
applications such as, say, RGB projector displays.
One
recent
approach
has
evoked
type-II
colloidal
quantum dots as a potential solution because large positive
biexciton binding energy
[4] enables single exciton gain in
these core-shell engineered nanocrystals. This scheme for
optical
gain
potentially
decreases
the
lasing
threshold;
however, the reduced interband optical oscillator strength
significantly reduces the optical gain and luminescent rate.
We have very recently demonstrated single exciton
optical gain from epitaxial-like type-I colloidal QD thin
films in the red, green, and blue (RGB)
[5], where the
threshold is more than an order of magnitude lower than
Fig. 1. Red, green and blue CQD films in stripe excitation with ultrashort
pulsed excitation. a) Plan view photographs of emission from excited RGB
stripes with pumping levels are below and above ASE thresholds,
respectively. b) Coherent edge emission as function of pump energy
density with arrows indicating the ASE energy density thresholds for RGB
CQD films, respectively.
The red, green, and blue (RGB) CQD films demonstrated
single exciton gain with an average number of excitons per
CQD of <N>
�
0.80, 0.76, 0.73, respectively, at ASE
thresholds. The ASE for RGB films was achieved at record
low pumping energy density of
90, 145 and 800 IlJ cm-2,
respectively (Fig. 1 and Ref. [5]) with a frequency-doubled
amplified Ti:sapphire ultrashort pulsed laser (400 nm, 100 fs,
100 kHz) as a pumping source. These values are more than
an order of magnitude improvement from other reports but,
as mentioned, are impractical for compact laser RGB
applications. Here we focus specifically on the red emitting
quantum dots while increasing the pump pulse duration to a
steady state (quasi-continuous) regime not accessed before,
to our knowledge with CQD lasing.
Figure 2a shows the intensity of edge emission in stripe
excitation configuration as a function of pumping energy
density with a compact sub-nanosecond-pulsed solid state
laser as a pumping source. The threshold behavior is clearly
presented the ASE with abrupt increase of emission intensity
and spectral narrowing. The ASE threshold for the red QD
films is 720 /!J cm-2, which is somewhat higher than
previous value when pumped by an ultrashort pulsed « psec)
laser. The key difference is mainly due to absorption
coefficient of CQD film at two different pumping
wavelengths and to lesser degree due to the possible residual
competition from the Auger decay process in this "quasi­
steady-state". In a model calculation, we input all
experimentally measured parameters into ladder rate
equations: absorption coefficient, multi-exciton and single
exciton recombination rates. Then, the average number of
generated exciton per CQD «N» reaches the maximum of
0.86 at the ASE threshold. This is in very good agreement
with <N>=0.80 in ultrashort pulse excitation.
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400
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600
700
800
Pumping Energy Density
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900
1000
(�J/cm2)
ASE @ 25 mJ/cm'
ASE @ 1.5 mJ/cm
-. - Pumping Laser
0.6
0.4
0.2
density and fmally reaches the pumping laser pulse width
which is more than twice of Auger time constant. The result
directly shows quasi-steady-state ASE from red QD film
exploiting the operation in the single exciton gain regime
where non-radiative Auger process is not the limitation.
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30
•
20
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•
2.0
2.5
3.0
Pumping Energy Density (mJ/cm')
Fig. 3. Intensity of edge emission as a function of pump energy density in a
stripe excitation configuration for the red CQD film pumping by a 10nanosecond pulsed laser (second harmonic at 532 nm from Nd:YAG laser).
Further investigation of rate equations solution for a
longer pulsed pumping laser of 10 ns, accessible to many
compact YAG-Iaser optical pump sources, enabled us to
predict the required pumping level of 1.6 mJ cm-2 to
generate <N> = 0.80 excitons per QD on average, the
threshold for ASE of the red QD film. For comparison, if the
ASE threshold were <N> = 1.5 [2] as reported in the bi­
exciton gain CQD literature, the threshold pump level would
be 30 mJ cm-2, an impractical condition including that most
of absorbed energy is converted into from non-radiative
Auger recombination. Our demonstration of the first optical
gain media from colloidal QD thin film working with
nanosecond-pulse pumping laser source is presented in Fig.
3. The ASE threshold is in very good agreement with our
model prediction which highlights the important role of
single exciton gain in practical laser applications. The
repetition rate was 20 Hz and the ASE remained stable
during experiment without any added thermal management
(an obvious need for improvement).
In conclusion, single exciton gain mechanism enables
the quasi-steady-state ASE in the CQD thin films. The
results suggest a potential CQD laser working in continuous­
wave mode. Our current research is on thermal management
for CQD thin films and practical pumping sources for green
and blue CQD lasers to enable a full color CQD laser screen.
�
ACKNOWLEDGMENT
0.0
o
200
400
600
Time
800
1000
1200
(ps)
Fig. 2. ASE of the red CQD film pumped by a compact
rate, 532-nm wavelength, 270-ps pulse laser (PowerChip
Photonics). a, Intensity of edge emission as a function
density per pulse. b, Transient ASE at two different pump
timing referenced to the pumping laser.
The research was funded by U.S. DOE/BES and NSF.
REFERENCES
I-kHz repetition
laser from Teem
of pump energy
energy densities,
The dynamics of optical gain and stimulated emission
are shown in Fig. 2b, where the time resolved ASE and laser
pulse were measured by a fast photodiode (25 GHz) and a
digital sampling oscilloscope (50 GHz). The pulse width and
intensity of ASE output is increasing with pumping energy
[ 1] B. O. Dabbousi et aI., "(CdSe)ZnS Core-Shell Quantum Dots:
Synthesis and Characterization of a Size Series of Highly Luminescent
Nanocrystallites," J. Phys. Chem. BIOI, 9463 ( 1997).
[2] S. L. Sewall, R. R. Cooney, E. A. Dias, P. Tyagi, P. Kambhampati,
"State-resolved observation in real time of the structural dynamics of
muItiexcitons in semiconductor nanocrystals," Phys. Rev. B 84, 235304
(20 1 1).
[31 V. I. Klimov et aI., Optical Gain and Stimulated Emission in
Nanocrystal Quantum Dots. Science 290, 3 14 (2000).
[4] V. I. Klimov et aI., "Single-exciton optical gain in semiconductor
nanocrystals," Nature 447, 44 1 (2007).
[51 C. Dang et al. "Red, green and blue lasing enabled by single-exciton
gain in colloidal quantum dot films". Nature Nanatech., in press
978-1-4673-2200-3/12/$31.00 ©2012 IEEE