Room temperature, continuous wave lasing in microcylinder and microring quantum dot laser diodes

Abstract

We demonstrate room temperature, continuous wave lasing of laser diodes based on AlGaAswhispering gallery mode(WGM)resonators (microcylinder and microring) embedding a quantum dot(QD)active layer. Using InGaAlAs QDs, high-Q (>60 000) lasing modes are observed around 910 nm, up to 50 °C. Lasing with similar performance is obtained around 1230 nm, using InAsQDs. Furthermore, we show that the current injection in the active part of the device is improved in ring resonators, leading to threshold currents of approximately 4 mA for a device with 80 μm diameter. This geometry also suppresses WGMs with a high radial order, thus simplifying the lasing spectra. In these conditions, stable single-mode and two-color lasing can be obtained.

Full text

Room temperature, continuous wave lasing in microcylinder and microring
quantum dot laser diodes
M. Munsch, J. Claudon, N. S. Malik, K. Gilbert, P. Grosse et al.
Citation: Appl. Phys. Lett. 100, 031111 (2012); doi: 10.1063/1.3678031
View online: http://dx.doi.org/10.1063/1.3678031
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Room temperature, continuous wave lasing in microcylinder and microring
quantum dot laser diodes
M. Munsch,
1
J. Claudon,
1,a)
N. S. Malik,
1
K. Gilbert,
2
P. Grosse,
2
J.-M. Ge´rard,
1
F. Albert,
3
F. Langer,
3
T. Schlereth,
3
M. M. Pieczarka,
3
S. Ho¨ fling,
3
M. Kamp,
3
A. Forchel,
3
and S. Reitzenstein
3,4
1
CEA-CNRS-UJF group “Nanophysique et Semiconducteurs,” CEA, INAC, SP2M, F-38054 Grenoble, France
2
CEA, LETI, MINATEC Campus, 17 rue des Martyrs, 38054 Grenoble, France
3
Technische Physik, Physikalisches Institut, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
4
Institut fu¨r Festko¨ rperphysik, Technische Universita¨t Berlin, Hardenbergstraße 36, D-10623 Berlin, Germany
(Received 9 November 2011; accepted 28 December 2011; published online 20 January 2012)
We demonstrate room temperature, continuous wave lasing of laser diodes based on AlGaAs
whispering gallery mode (WGM) resonators (microcylinder and microring) embedding a quantum
dot (QD) active layer. Using InGaAlAs QDs, high-Q (>60 000) lasing modes are observed around
910 nm, up to 50 C. Lasing with similar performance is obtained around 1230 nm, using InAs
QDs. Furthermore, we show that the current injection in the active part of the device is improved in
ring resonators, leading to threshold currents of approximately 4 mA for a device with 80 lm
diameter. This geometry also suppresses WGMs with a high radial order, thus simplifying
the lasing spectra. In these conditions, stable single-mode and two-color lasing can be obtained.
V
C2012 American Institute of Physics. [doi:10.1063/1.3678031]
Over the last two decades, whispering gallery mode
(WGM) optical resonators made of semiconductor material
have attracted a considerable interest. Such microcavities
confine optical fields over tiny volumes (a few cubic wave-
lengths), while displaying a photon storage time approaching
10
5
optical cycles.
1
They have led to the demonstration of
fundamental quantum electrodynamics effects at cryogenic
temperature, using a quantum dot (QD) as an integrated
quantum light source.
2–4
On the application side, these reso-
nators are promising candidates for the development of
microlasers. They can be integrated in planar photonic cir-
cuits,
5
and various strategies have been explored to obtain a
directive far-field emission.
6
For practical applications, elec-
trical pumping and room temperature (RT) operation are
highly desirable. To date, most WGM laser diodes have
exploited a quantum well gain medium, in III-V materials
with low surface losses.
7,8
Nevertheless, self-assembled QDs
offer an alternative with several advantages. In particular,
the gain curve which is inhomogeneously broadened by the
QD size dispersion allows for two-color lasing. This is a key
ingredient of a recently proposed integrated THz source
9
based on intra-cavity difference frequency generation.
Until recently, most QD WGM microlasers were based
on a suspended l-disk supported by a low diameter
pedestal.
1,10–14
This geometry maximises the confinement of
the optical field but suffers from poor heat sinking, a severe
drawback to achieve continuous wave (CW) lasing.
2,10
Moreover, it does not allow for a direct injection of charge
carriers at the location of the WGM. Consequently, RT CW
lasing in a QD-l-disk system has only been achieved under
optical pumping.
1,13
In this context, AlGaAs l-cylinders
have recently emerged as a new class of high performance
optical WGM resonators.
15,16
Compared to suspended
l-disks, l-cylinders provide efficient heat sinking and
straightforward carrier injection. Taking advantage of these
assets, cryogenic CW microlasers were rapidly demon-
strated, using optical
17,18
or electrical
19
carrier injection.
In this letter, we demonstrate RT CW laser diodes based
on high-Q AlGaAs l-cylinder and l-ring resonators. We
show results obtained from two families of devices, embed-
ding InGaAlAs QDs and InAs quantum dot-in-a-well
(DWELL) that were optimized for operation at elevated tem-
peratures. The first device family exhibits WGM lasing
around 910 nm with Q-factors exceeding 60 000. In the sec-
ond device family, lasing occurs around 1230 nm, with simi-
lar performance. We furthermore show that ring resonators
exhibit smaller lasing thresholds (4 mA) and a simpler las-
ing spectrum. Under these conditions, stable single-mode
and two-color lasing can be obtained.
Our devices are made out of planar structures which are
grown by molecular beam epitaxy on a (001) n-doped GaAs
wafer. The active layer of sample 1 (S1) consists in a high
gain, single layer of self-assembled InGaAlAs QDs emitting
at about 900 nm.
20
This layer is inserted in a 260 nm-thick
AlGaAs layer, whose Al content increases from 18% to 30%
as the distance to the active layer increases. To ensure verti-
cal photonic confinement, this central layer is sandwiched by
Al
0.3
Ga
0.7
As claddings. Sample 2 (S2) employs a 6 layer
InGaAs DWELL design
21
with an emission peak at about
1270 nm. The active layer is inserted in a 250 nm-thick GaAs
waveguide which is embedded in Al
0.4
Ga
0.6
As claddings.
The RT modal gain of S1 (63 cm
1
) and S2 (17 cm
1
) were
estimated by defining a series of ridge waveguide lasers with
various resonator lengths. The bottom and top AlGaAs layers
are, respectively, n- and p-doped with reduced doping con-
centration close to the central part of the waveguide. Lateral
photonic confinement is achieved by etching l-cylinders or
l-rings in the samples. The etching mask (Ti-Pt-Au-Ni) is
first deposited on the epitaxial structure using electron beam
lithography, metal evaporation, and acetone lift-off. After
a)
Electronic mail: julien.claudon@cea.fr.
0003-6951/2012/100(3)/031111/4/$30.00 V
C2012 American Institute of Physics100, 031111-1
APPLIED PHYSICS LETTERS 100, 031111 (2012)
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partial removal of the top Ni layer during the reactive ion
etching (RIE) step, it further acts as top electrode. The n-
contact (Ni-Ge-Au-Ni-Au) is deposited onto the substrate,
prior to a 2 min annealing at 400 C.
Optical and electrical characterizations are performed at
RT; the experimental configuration is sketched in Fig. 1.
Electrical contacts are established using two sharp metallic
tips (curvature radius of 2 lm). The devices are connected to
a CW current source, also used to measure the intensity-
voltage characteristics. The emission of the devices is par-
tially collected by the cleaved facet of a multimode optical
fiber (core diameter: 50 lm, numerical aperture ¼0.2). To
optimize the collection of light, the fiber is approached close
to the resonator (20 lm), making a small angle (10)
with the equatorial plane of the sample. The collected light is
then analysed with an optical spectrum analyser (OSA), with
a maximum resolution of 10 pm.
Figures 2(a) and 2(c) show the spectra of two laser
diodes based on l-cylinder resonators, for several injection
currents I
inj.
They are made out of S1 and S2 and feature a
diameter D¼35 lm and 80 lm, respectively. In both cases, a
clear evidence of multimode lasing under CW pumping at
room temperature is found. Lasing modes are observed
around k¼905 nm for S1 and around k¼1230 nm for S2.
For both devices, the number of simultaneously lasing modes
increases with I
inj.
The integrated intensity of the brightest
mode of S1 is plotted in Fig. 2(b), and we determine a
threshold current of 7.5 mA. For S2, the threshold current is
larger (24 mA), owing to the larger surface of active material
[Fig. 2(d)]. The total optical output power of this multimode
device is about 1 mW at I
inj
¼100 mA (not shown).
The dipole associated with the fundamental optical tran-
sition of the QDs is perpendicular to the growth axis, which
coincides with the resonator axis. The QD gain medium,
thus, feeds optical modes with a TE polarization, which are
well defined in a l-cylinder resonator.
16
A WGM resonance
is then identified by a set of three integers (z,r,m) which are,
respectively, the vertical, radial, and azimuthal indices.
22
We
calculate the resonance wavelength of these modes in the
frame of the so-called effective index approximation, which
decouples the description of the vertical and lateral photonic
confinements. Here, only the z¼1 modes are confined. For
the device defined in S1 (S2), the TE
1,1,m
modes coupled to
the QD luminescence exhibit maround 390 (670). A com-
plete and unambiguous identification of rand min the l-cyl-
inders resonance spectra is difficult, because of the relatively
large spectral density of TE
1,r,m
modes. In addition, using the
simple model of Ref. 17, we estimate a spontaneous emis-
sion coupling b-factor of a few 10
4
for the QDs coupled to
the lasing modes. This order of magnitude is consistent with
the resonator modal volume and the homogeneous linewidth
of the emitters (5 nm at room temperature
23
).
WGMs with a low radial index (r¼1) are located at the
periphery of the structure, with a typical lateral extension of
one wavelength. Thus, the central part of the gain medium
does not participate to WGM lasing and current flowing to
this area is lost, only generating additional heat. We thus
fabricated microlasers with a ring shaped cross-section out
of S1 and S2. In S2, the l-rings were defined directly by the
RIE step. In S1, the l-rings are defined with a focus ion
beam (FIB) milling, starting from plasma-etched l-cylinders.
The ring is isolated from the central part by a trench which
ends at the QD active layer. In addition, the top of the central
part has been etched over half a micron. This geometry
allows contacting tiny rings (wall thicknesses as low as
3lm) with the tip, while avoiding spurious contact to
the central part. In addition, this structuration combines
selective injection of the holes at the periphery of the resona-
tor with a minimal degradation of its electrical and thermal
resistances.
Figure 3(a) shows the threshold currents of a series of
l-ring laser diodes made out of S2. They feature an outer di-
ameter of 80 lm and decreasing wall thicknesses (20, 15,
and 10 lm). We observe a significant drop of the lasing
threshold down to 5.3 mA as the central part of the l-cylin-
der is removed. The threshold dependence versus the temper-
ature of another microlaser with a wall thickness of 10 lmis
illustrated in Fig. 3(b). CW lasing is observed up to 50 C,
and a threshold current as low as 4.2 mA for 10 C was
achieved.
FIG. 1. (Color online) Sketch of a l-ring laser diode and experimental
configuration.
FIG. 2. (Color online) Room temperature, CW lasing of two l-cylinder
laser diodes. They are fabricated from S1 and S2 and feature a diameter of
35 lm and 80 lm, respectively. (a) and (c) Waterfall plot of the spectra of
the laser diodes for increasing injection current I
inj.
(b) and (d) Output power
of the brightest lasing mode vs I
inj.
A SEM picture of a representative 35lm-
diameter device is shown in the inset.
031111-2 Munsch et al. Appl. Phys. Lett. 100, 031111 (2012)
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In addition, the patterning into a l-ring helps suppress-
ing the modes with a high radial order. The lasing spectrum
is then dominated by the TE
1,1,m
modes, for which mis
determined with an uncertainty of 1. Figure 4(a) shows the
lasing spectra of a ring resonator defined in S1, for increas-
ing injection current. The device features an outer diameter
D¼35 lm and a wall thickness of 3 lm [inset in Fig. 4(c)].
A clean single-mode lasing of the TE
1,1,390
mode is observed
for I
inj
between 4 mA and 7 mA. Note the lasing threshold in
the l-ring is nearly a factor 2 smaller than the original
l-cylinder laser. Figure 4(b) illustrates the high spectral pu-
rity of the laser emission, with Q-factors exceeding 60 000.
The mode is split, a commonly observed feature in high-Q
WGM resonators.
1,17,18,24
Indeed, a very small perturbation
introduced by a residual sidewall roughness
24
or due to the
refractive index inhomogeneity associated with the QDs
(Refs. 18 and 25) is sufficient to couple counter-propagating
6mmodes and to generate a sizeable splitting.
As shown in Figs. 4(a) and 4(c), the simultaneous lasing
of the TE
1,1,392
and TE
1,1,390
modes is obtained for I
inj
between 7.0 and 8.5 mA. Similar lasing intensities in the two
modes are obtained for I
inj
¼7.7 mA. Such a two-color las-
ing is particularly relevant for a recently proposed scheme
for THz generation,
9
where THz is generated by non-linear
frequency difference between two lasing WGMs in the near
infrared. Above 10 mA, the TE
1,1,391
appears and dominates
the spectrum for I
inj
>11 mA. The free spectral range Dk
between the modes TE
1,1,390
,TE
1,1,391
, and TE
1,1,392
is meas-
ured at 1.98 nm. This value is in very good agreement with
the predicted value of 1.97 nm, obtained with the expression
Dk¼k
2
/(pDn
g
), where n
g
¼3.820 is the group index of the
TE
z¼1
mode guided in the planar (unetched) structure.
The ring patterning increases the series resistance from
8Xto 38 X, but this does not compromise the lasing
performance. In fact, the ring geometry still provides a very
efficient heat management. From lasing measurements con-
ducted between 20 C and 50 C (not shown), we estimate a
temperature shift for the mode wavelength of þ65 pm/C.
When I
inj
is increased from 4 mA to 8 mA, we measure a
redshift as low as 480 pm, corresponding to a temperature
increase of 7 C.
In conclusion, we have demonstrated RT WGM laser
diodes using l-cylinder and l-ring resonators, with a QD
gain medium at approximately 900 nm and 1230 nm. Ring
resonators provide the best performance in terms of lasing
threshold and lasing spectrum. Thanks to the inhomogene-
ously broadened gain provided by the QDs, the lasing can be
turned from single-mode to two-color, simply by changing
the injection current.
The authors warmly thank J. F. Motte, M. Terrier, and
M. Emmerling for expert technical assistance, and A. Tchel-
nokov for his support. The authors acknowledge the financial
support of the Future and Emerging Technologies (FET) pro-
gramme within the 7th Framework Programme for Research
of the European Commission, under the FET-Open
"TREASURE" project (Grant No. 250056).
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