MB2.3 (Invited)
11:30 AM – 11:00 AM
Red, Green, and Blue Colloidal Quantum Dot-based Optically
Pumped Distributed Feedback Lasers
Kwangdong Roh1, Joonhee Lee1, Cuong Dang1, Craig Breen2, Jonathan S. Steckel2, Seth Coe-Sullivan2, and
Arto Nurmikko1*
1
School of Engineering, Brown University, Providence, Rhode Island 02912, USA
2
QD Vision Inc., 29 Hartwell Ave., Lexington, Massachusetts 02421, USA
Tel: +1 (401) 863-2869; Fax: +1 (401) 863-1387; Email: Arto_Nurmikko@Brown.edu
Abstract— Red, green and blue distributed feedback lasers
based on dense colloidal CdSe quantum dot thin films were
optically pumped in quasi-steady state. The red lasers showed
32% internal quantum efficiency and 400 µW output power.
film are main consideration for the design and optimization
of the resonant DFB structures.
Index Terms – Colloidal quantum dot, RGB, Single exciton
gain, DFB lasers, Steady-state
I. INTRODUCTION
Advanced colloidal synthesis methods are producing
colloidal quantum dots (CQD) as high efficient fluorescent
materials covering full-visible spectrum for many
applications such as solid state lighting [1] and display [2].
High performance projection displays will benefit from full
color, high intensity coherent emitters which currently
require at least three different materials with distinct
technologies. CQDs are emerging as a potential nanomaterial for full color, single material lasers. By enabling
single exciton gain mechanism [3] in CdSe/ZnCdS core/shell
CQD films, we recently solved the major obstacle of nonradiative multiexciton Auger recombination [4] while
maintaining the high quantum efficiency, optical oscilation
strength of type-I CQDs. The first CQD vertical cavity
surface emitting lasers demonstrated the high performance
level of the CQD gain media. Here, we present a major step
toward red, green and blue (RGB), practical CQD lasers for
high performance display applications. The RGB distributed
feedback (DFB) lasers with a compact, quasi-continuous
optical pump source and crisp collimated laser beam outputs
are demonstrated with this new type CQD gain media.
Fig. 1: Configuration of the second order CQD-DFB lasers. (a)
Schematic of a CQD-DFB laser in which a densely-packed solid
CQD film is deposited on second order quartz gratings. The pump
beam direction is vertical (i.e. perpendicular to grating plane). The
second order diffraction provides in-plane feedback for the CQD
optical gain media while the first order diffraction provides two
output coupling channels for the laser, each vertical (perpendicular)
to the grating surface. (b) Cross-sectional SEM image of a quartz
grating with a pitch of 400 nm. (c) An angled view of a complete
CQD-DFB laser structure under SEM.
The CQD-DFB structures were pumped through a
cylindrical lens in stripe excitation configuration
(perpendicularly oriented to the grating grooves). Fig. 2a-f
present close-up images of excitation stripe for RGB lasers
with pumping levels are below and above threshold,
respectively. These visually demonstrate threshold behavior
of laser action in CQD-DFB structures whereupon a bright
center area suddenly appears on a uniform photoluminescent stripe when increase pumping intensity. The
intensity of laser outputs as a function of pumping level in
Fig. 2g shows well defined laser threshold for all CQD-DFB
lasers. The thresholds are 250 μJ/cm2, 290 μJ/cm2 and 350
μJ/cm2 for the RGB CQD-DFB lasers, respectively. These
values present significant reduction from comparison cavityless laser (ASE), as expected for the low loss, resonant high
Q cavity of the DFB structures. The spectra of these CQDDFB lasers in the inset of Fig. 2g clearly show the single
mode, narrow laser lines. Optimizing DFB gratings to match
the optical gain peak (ASE spectra) need to be done
carefully to achieve high performance lasers. Unlike earlier
CQD (red) laser demonstrations with ultrashort
pulse/powerful pump lasers in the literature, our pumping
source employed a double frequency (532 nm for red laser)
II. EXPERIMENTS AND RESULTS
The CQD-DFB lasers were fabricated by spin casting an
extra high concentration CQD solution on second order
quartz gratings which were designed and made by either
holographic interference lithography (adequate for red
lasers) or focused ion beam milling (to reach the green and
blue). The second order gratings ease the fabrication process
(double pitch of first order gratings). The square profile of a
typical quartz grating achieved by inductive plasma coupling
reactive ion etching technique is shown in cross-sectional
scanning electron microscopy image (Fig. 1b). Fig. 1c shows
a complete CQD-DFB structure, where an epitaxial like
CQD layer conformally covers the grating and also forms an
optically smooth top surface. A spin cast film on planar
surface showed a densely packed structure with effective
refractive index of 1.75-1.8, which implies a filling factor of
50%. This refractive index and the gain spectra of the CQD
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and triple frequency (355 nm for green and blue lasers) from
a compact, sub-nanosecond, solid state laser (1064 nm, 1
kHz, PowerChip laser from Teem Photonics). The full width
at half maximum of the optical pump pulse is 270 ps, which
is 2.25 times longer than perennially inhibitive non-radiative
multiexciton Auger recombination (Au = 120 ps), thus
enabling access to quasi-steady state regime. This highlights
the role of single exciton mechanism for optical gain of our
densely packed CQD thin film as elucidated in Ref [3].
performance of the red CQD-DFB lasers is somewhat
similar to that of e.g. semiconductor laser pointers in term of
power, collimation, and efficiency.
Fig. 2: Performance of quasi-steady state CQD-DFB laser. a-f)
Micro-photograph images of pumped stripe (400 μm) on the RGB
CQD-DFB lasers with below and above threshold pump levels,
respectively. e) RGB laser intensities as a function of pump pulse
energy density. (Inset) Single mode RGB laser spectra.
Fig. 3: Photographic images of RGB quasi-steady state CQD-DFB
lasers. a) Direct, vertical blue-laser output on the remote screen
with blue CQD photoluminescence background. b) Top view of a
red laser beam under bright room light. c-d) Red and green well
collimated CQD-DFB laser beams on remote screens when using
the second cylindrical lens for collimation.
Fig. 3 shows spatially coherent outputs of the RGB CQDDFB lasers where filters were used to remove residue pump
laser. Well-defined beams (average power 400 μW at 1 kHz)
were observed on remote screens or by direct view e.g. of
the red lasers. The laser output is well collimated in
horizontal dimension (Fig. 3a) due to the second order DFB
laser. Fig. 3b shows a robust red laser output in bright room
light. With a cylindrical lens to collimate the laser beam
outputs, Fig. 3c-d show well-defined, spatialy coherent red
and green laser beams with their laser spots projected on a
remote screen, respectively.
ACKNOWLEDGMENT
The research was funded by NSF (ECCS-1128331) and US.
DOE/BES (DE-FG02-07ER46387). We are very grateful to
H. Kim, S. Ahn, and Prof. Heonsu Jeon of Seoul National
University for the holographic gratings for red lasers.
REFERENCES
These proof-of-concept red CQD-DFB lasers presently
degrade in the order of an hour as we have applied no heat
management or encapsulant protection. In the red internal
energy conversion is 28% which corresponds to a 32% of
quantum efficiency by the single exciton quantum efficiency.
The biexciton gain CQD media in past literature with fast
nonradiative multiexciton Auger recombination simply
cannot produce such high quantum efficiency. The
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