446IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 6, MARCH 15, 2008
InAs–InP (1.55-?m Region) Quantum-Dot
Martin T. Hill, S. Anantathanasarn, Y. Zhu, Y.-S. Oei, P. J. van Veldhoven, M. K. Smit, and R. Nötzel
Abstract—In this letter, we demonstrate electrically pumped
continuous-wave lasing at room temperature in microring lasers,
which employ a quantum-dot gain medium. Lasing occurs in the
important 1.55-?m telecom wavelength range. The 2-?m-wide
ring waveguides are made from InGaAsP–InP (100) material
suitable for active–passive photonic integrated circuits. Lasing in
rings down to 22 ?m in diameter is found, with a threshold current
of 12.5 mA.
Index Terms—Integrated optics, microring lasers, quantum dots
(QDs), semiconductor lasers.
sibility of compact single-mode light sources, suitable for high
density photonic integration. Microring lasers, although not as
small as microdisk lasers, have a number of advantages: They
allowing easy integration with passive waveguide components
, . They have good thermal characteristics and allow elec-
trical pumping. There is no central region where carriers are
wasted or the formation of unwanted higher order lasing modes
can occur .
Quantum-dot (QD) semiconductor gain material offers a
number of advantages over conventional quantum-well (QW)
or bulk material. In particular, QD material offers low threshold
current densities, wide gain bandwidth, and reduced problems
from surface recombination. The use of QD material for mi-
crodisk lasers has been demonstrated for low temperatures,
and/or for pulsed optical pumping –. The use of QD ma-
terial in ring lasers has only been shown for large (millimeter
length) ring lasers , . Here we demonstrate lasing in the
important 1.55- m telecom wavelength region for microring
lasers that are continuous-wave electrically pumped at near
room temperature (RT).
ICRORING and disk lasers have received considerable
II. DEVICE FABRICATION
Metal–organic vapor-phase epitaxy was employed to grow
the QD material on n-type InP (100). The QD active regioncon-
Manuscript received July 23, 2007; revised December 15, 2007. This work
was supported by the NRC Photonics Program of the Dutch Ministry of Eco-
The authors are with COBRA, Eindhoven University of Technology, 5600
MB, Eindhoven, The Netherlands (e-mail: email@example.com).
Digital Object Identifier 10.1109/LPT.2008.916963
bandgap at 1.25
in the center of a 500-nm-thick
nominal InAs amount for QD formation was 3.5 monolayers
(MLs). One ML GaAs interlayer was inserted underneath each
QD growth to tune the QD emission into the 1.55- m region
. The photoluminescence (PL) of the QDs taken from the
surface peaked at 1580 nm with a full-width at half-maximum
(FWHM)of 200nmand Gaussian shape. The
core was clad below by 500-nm n-InP, and above by 1500-nm
p-InP, finalized by a 100-nm-thick p-InGaAsP contact layer.
Electron beam lithography and liftoff were used to form a
60-nm-thick Ti waveguide mask on top of a 50-nm SiN layer
deposited on the wafer. The SiN was dry etched with reactive
ion etching (RIE), to form the final waveguide mask consisting
of the Ti and SiN layers. H –CH RIE was also used to etch
the InP–InGaAsP and QD layer structure to a depth of approx-
m, well into the bottom n-InP cladding. The ridge
sidewall angle was less than 2 from vertical and care was taken
to minimize surface roughness for maximum quality ( ) factor
Polyimde was used to planarize the waveguide structure. The
top p-type electrical contact was made with optical lithography,
evaporation of Ti–Pt–Au, and liftoff. The n-type electrical con-
tact was made on theback side of thewafer also with Ti–Pt–Au.
Fig. 1 shows a scanning electron microscopy (SEM) picture
of a ring laser structure with the output coupling waveguide
(without polyimide and contact metallization). Rings of the fol-
lowing outside diameters were fabricated: 82, 42, 22, 12, and
8 m.Theringwaveguidewidthwas2 m.Thegapbetweenthe
ring waveguide and the waveguide going to the chip facets was
approximately 0.5 m. The curved section of waveguide going
to the chip facet had the same radius of curvature and width as
the ring to which it was coupled. Simulations (two-dimensional
finite-difference time-domain) indicate that about 0.1%, 0.07%,
and 0.04% of the light in the lasing mode of the ring is cou-
pled to the output waveguide with this separation between the
waveguides, for the 82-, 42-, and 22- m diameter rings, respec-
tively. Good coupling has been shown to be possible with such
a ring-waveguide system . Other waveguide-ring coupling
schemes will likely perform better than the one used here .
However, this scheme and gap size was chosen mostly for pro-
cessing ease, and to degrade the
A small length of the bent section of the waveguide going to
the chip edges was pumped with the same p-contact as the ring
This length was approximately 32, 27, and 22
82-,42-, and 22- m diameterrings, respectively.The rest of the
m ( 1.25). The five QD layers were placed
1.25 waveguide core. The
as little as possible.
m for the
1041-1135/$25.00 © 2008 IEEE
HILL et al.: InAs–InP (1.55- m REGION) QD MICRORING LASERS 447
Fig. 1. (a) SEM image of ring laser structure (without polyimde and metalliza-
tion), the scale bar is 10 ?m. (b) Schematic of ridge waveguide structure and
layer stack for the ring lasers.
Low absorption in the QD active layer  permits light from
the ring to travel along the waveguides to the chip facets and be
The cleaved chips are mounted on a temperature-controlled
copper chuck. A single-mode fiber with a lensed tip captures
light from one of the waveguides at the chip facet and directs it
to an optical spectrum analyzer (OSA). The devices are electri-
cally pumped from a dc current source. The temperature of the
copper chuck is 283 K. The larger devices also lased at higher
temperatures, with increased thresholds. The particular QD ma-
terial has a low characteristic temperature , which needs to
be improved. We chose 283 K to be able to convincingly show
lasing in a range of devices.
Measured spectrafrom the82-,42-, and 22- m diameterring
lasers are shown in Fig. 2 (smaller devices did not lase). It can
i.e., single-mode lasing, which occurred for all currents tested.
However, there was a jump in lasing wavelength by one res-
onator free-spectral range (FSR), as the current was increased.
The 42- and 82- m devices show multiple lasing modes,
likely due to the inhomogeneous nature of the QD gain
medium. The larger FSR of the 22- m ring coupled with the
finite bandwidth of the QD gain medium is believed to be the
reason behind the single-mode operation for this device. The
measured FWHM of the lasing peaks was
by the resolution of the OSA. Narrow laser line-width is typical
for lasers with moderate or high
0.05 nm, limited
Fig. 2. (a) Spectrum for 82-?m diameter ring laser at 50 mA. (b) Spectrum
for 42-?m diameter ring laser at 27.5 mA. (c) Spectrum for 22-?m diameter
ring laser at 22 mA. The measured FWHM of the lasing peaks is limited by the
resolution of the spectrum analyzer of 0.05 nm.
The optical power measured at the chip facet for one end of
the output waveguide versus injection current is shown in Fig. 3
for the 42- m diameter laser. The toe in the power curve in-
dicates a threshold current of approximately 16 mA, giving a
threshold current density of 6.4 kA/cm . For the 22- and 82- m
diameter lasers, the spontaneous emission from the small sec-
tion of the waveguide going to the chip facets which is pumped
was the case for most devices. Thus, a clear increase of the op-
devices. However, by plotting the optical power contained in
the lasing modes  versus injection current, some measure of
the threshold current can be found. These plots for the 22- and
82- m lasers are also shown in Fig. 3. They indicate threshold
currents of 12.5 mA (9.5 kA/cm ) and 35 mA (7 kA/cm ) for
the 22- and 82- m lasers, respectively. For the reported 42- m
laser, we believe that the p-contact to the output waveguide was
poor, resulting in the lower spontaneous emission levels.
Such threshold current densities are somewhat higher than
those for long Fabry–Pérot and ring lasers constructed using
similar QD material . Furthermore, the lasing occurs at
shorter wavelengths. This is typical for smaller devices ex-
hibiting higher losses due to the QD size, i.e., wavelength
distribution . Here, however, a contribution from excited
state lasing cannot be excluded due to the lasing occurring at
the high-energy side of the PL spectrum, though well within
the FWHM. Lower threshold currents may be obtained by
better material design (higher barrier heights and improved
characteristic temperature), as mentioned in .
Switching between clockwise (CW) and counterclockwise
(CCW) lasing modes, as sometimes seen in QW ring lasers,
was not observed. This can be seen from the increase of the
light from one side oftheoutput waveguidewithincreasing cur-
rent. However, it has been seen in larger QD ring lasers that
448IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 6, MARCH 15, 2008 Download full-text
Fig. 3. Light–currentcharacteristics for microring lasers.?)82-?m,o)42-?m,
and ?) 22-?m diameter lasers. For the 42-?m device, total light power at the
output waveguide is shown. For the 22- and 82-?m devices, power in the lasing
mode is given. The power levels are scaled for the 22- and 82-?m diameter
lasers to fit the results of the different devices on the same graph (the scaling
factors are shown).
at higher current levels there may be a partial redistribution of
power between CW and CCW modes . Further experiments
are planned to search for possible modal power redistribution in
operating regimes other than those reported here.
The use of QD material in microlasers is not immediately ob-
vious as the maximal modal gain available is limited. In similar
to be of theorder of 15cm
. To obtain lasing requires low
loss and high quality factors in microring resonators of the size
reported here is possible. The significant loss factor is wave-
guide surface roughness . Here we show that indeed it is pos-
sible to construct electrically pumped CW QD microring lasers,
operating at near RT. Such lasers are compact and with suffi-
ciently small dimensions they can be single-mode. The use of
QD material here provides low surface recombination and its
inhomogeneous nature may be advantageous for some applica-
However, to provide useful light sources in photonic in-
tegrated circuits, the lasers demonstrated here will require
stronger coupling to passive output waveguides. An upper
limit on the out-coupling
can be found given
, the intrinsic ring resonator
refractive indexto be
, the ring
, and, wavelength
devices with diameters of a few tens of micrometers should be
of the coupling waveguide  and reducing the distance be-
ring and waveguide will require improved dry etching methods.
Forexampleinductivelycoupled plasmaRIE has beenshown to
be able to produce the narrow deep trenches required between
is possible for infinite
. Finitewill reduce
cm, however, useful
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