Room temperature passive mode-locked laser based on InAs/GaAs quantum-dot superlattice

Article (PDF Available)inNanoscale Research Letters 7(1):545 · October 2012with14 Reads
DOI: 10.1186/1556-276X-7-545 · Source: PubMed
Abstract
Passive mode-locking is achieved in two sectional lasers with an active layer based on superlattice formed by ten layers of quantum dots. Tunnel coupling of ten layers changes the structural polarization properties: the ratio between the transverse electric and transverse magnetic polarization absorption coefficients is less by a factor of 1.8 in the entire electroluminescence spectrum range for the superlattice.
N AN O E X P R E S S Open Access
Room temperature passive mode-locked laser
based on InAs/GaAs quantum-dot superlattice
Mikhail Sobolev
*
, Mikhail Buyalo, Idris Gadzhiev, Ilya Bakshaev, Yurii Zadiranov and Efim Portnoi
Abstract
Passive mode-locking is achieved in two sectional lasers with an active layer based on superlattice formed by ten
layers of quantum dots. Tunnel coupling of ten layers changes the structural polarization properties: the ratio
between the transverse electric and transverse magnetic polarizatio n absorption coefficients is less by a factor of 1.8
in the entire electroluminescence spectrum range for the superlattice.
Keywords: Mode-locking, Laser, Polarization, Quantum dots, Superlattice, In(Ga) As/GaAs
Background
In recent years, intense efforts have been devoted to the
studies of effects of tunneling coupling between electron
states in semiconductor heterostructures with quantum
dots (QDs), which offer much promise in the develop-
ment of high-speed lasers [1], optical modulators [2],
and amplifiers [3]. For optical amplifiers and modula-
tors, it is desirable to have polarization-independent
characteristics. Thus, dependencies of gain and absorp-
tion have been studied in quantum well structures [4]
and QDs [5]. However, in standard uncoupled QD struc-
tures, the absorption coefficient at the lasing wavelength
for transverse electric (TE)-polarized light differs by an
order [2]. It is known that in structures with coupled QDs,
the intensity of transverse magnetic (TM) polarization
increases with the number of QD layers [2,5,6].
Direct current modulation of semiconductor lasers does
not meet the needs of modern high-speed communication
lines, so systems consisting of a laser and modulator are
used. As more broadband alternative to the direct current
modulation can be laser with integrated electro-optical
modulator based on the Stark effect, high-speed perform-
ance of the Stark modulator is fundamentally limited by
physical processes, namely, carrier escape from QDs and
carrier removal from the p-n junction area. Because the
same processes are crucial for the passive mode-locking
(PML) regime, the modulation frequency ceiling can be
determined by the largest feasible PML frequency in a laser
fabricated from the same structure. It should be noted that
the implementation of two sectional PML lasers is technic-
ally easier than creating a high-speed modulator, because
there is no need to eliminate parasitic capacitance and in-
ductance. The modulation frequency ceiling can be deter-
mined by the largest feasible frequency of the of the PML
regime in a laser fabricated from the same structure.
In this communication, we report on a room-
temperature study a ten-layer system of tunnel-coupled
In(Ga)As/GaAs QD. As shown in [7,8], the structure
with ten tunnel-coupled layers of In(Ga)As/GaAs QDs
exhibits the Wannier-Stark effect and is a quantum dot
superlattice (QDSL). We have observed the EL and ab-
sorption spectra for light polarized in the plane perpen-
dicular to the growth axis (x and y) in the same spectral
range as that for light polarized along the growth direc-
tion (z) of the structure. No transitions involving light holes
were observed in the electroluminescence and absorption
spectra. The observed behavior of the measured signals
allows one to conclude that the optical transitions for light
polarized in the plane perpendicular to the growth axis and
in the plane along the structure growth direction involve
ground states of heavy holes, whose wave functions have, in
addition to the x and y components, a z component. In this
system, the ratio between the light absorption coefficients
for TE and TM polarizations is close to 1 in contrast to
structures with unbounded QDs, where the ratio is about
10. This makes it a promising structure for optical
polarization-independent modulators used in fiber-optic
communication lines (FOLs). Two sectional PML laser
diodes with an absorbing section acting as modulator were
* Correspondence: m.sobolev@mail.ioffe.ru
Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg
194021, Russia
© 2012 Sobolev et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Sobolev et al. Nanoscale Research Letters 2012, 7:545
http://www.nanoscalereslett.com/content/7/1/545
made from the SLQD structure. It shows the fundamental
possibility of implementing a laser and modulator in a
monolithically integrated design.
Methods
Laser structures were grown by molecular beam epitaxy
on n
+
-GaAs (001) substrate and are similar to the struc-
ture described in [6,8]. The structure consisted of an n-
doped bottom Al
0.35
Ga
0.75
As layer with a thickness of
1.5 μm, a waveguide undoped GaAs layer with a thick-
ness of 480 nm containing ten layers of In(Ga)As QD, a
p-doped upper Al
0.35
Ga
0.75
As layer with a thickness of
1.5 μm, and a p
+
-doped contact GaAs layer. QD ensem-
bles were grown ten times by InAs 2.3 monolayer depos-
ition with GaAs barrier layers with a thickness of 6 nm
between QD layers. Thus, layers of self-organized In(Ga)
As QDs were built into the central part of the undoped
GaAs matrix. The refractive index of the upper and
lower Al
0.35
Ga
0.75
As layers differed from that of the
central layer, which confined light within the central part
of the undoped SLQD-containing region. The vertical
alignment of QDs was observed by transmission electron
microscopy (see Figure 1) [6,8].
Two sectional lasers were fabricated from SLQD struc-
tures. Standard lithography techniques were used to
make a 5-μm strip forming a single-mode waveguide.
The cavity length was 3.5 mm, the absorber length was
10% of the cavity length, and the se ctions were electric-
ally isolated by the gap in the contact. This laser design
is in fact standard and is described in various publica-
tions [2,8-10] but differs from them in that the active
layer is SLQD, formed by ten QD lay ers and thin barrier
layers between them. The devices were mounted on a
copper heat sink; all measurements were performed at
room temperature.
Absorption measurements were provided as described
in [8,9] using this device. The experimental setup is
shown on Figure 2b,c. A sample with two equal sections
was used. The emission in waveguide wa s excited by the
current injection in one of the sections; the pumping
current is far below the threshold current. On the first
stage (Figure 2b), the emission spectrum (I
0
) from the
closest section to the monochromator section (the right
setion on Figure 2b) is measured, nothing is applied to
the other section. Thus, the spectrum of source light is
obtained. Next, the closest section is reverse-biased, and
the other section is pumped with the same current as
the right section in the first stage (Figure 2c). In this
waveguided setup, radiation from the left section pene-
trates into the right section almost without loss, then
experiences partial absorption by SLQDs in the right
section, and reaches measuring setup through low-
reflectanc e facet. Hence, the emission spectra of pa ssed
light through the absorption se ction (I
0
e
-k(V)l
)is
obtained. Since both sections have the same length and
the optical scheme of the experimental setup was not
changed, one can assume that the intensity of the emis-
sion reaching the absorber section on the second stage is
approximately equal to the intensity measured on the first
stage. This allows the derivation of the SLQD absorption
spectra in absolute values.
PML investigation was under pulsed current injection
(pulse duration 1 μs) and direct current (DC) reverse
bias. An autocorrelation setup based on a Michelson
interferometer was used for pulse duration measurements,
controlled by an oscilloscope with a 50-GHz bandwidth, an
electrical spectrum analyzer with a 22-GHz bandwidth, and
a 20-GHz photodetector. The devices were mounted to
copper heat sink; all measurements were done at room
temperature.
Results and discussion
The devices were pumped by DC in light-current and
absorption experiments. A clear, rigid switching-on effect
is observed, which is eliminated only when significant for-
ward current I
r
is applied to the absorber section (Figure 3).
Threshold current I
th
decreases with forward bias applied
to the absorber section, increasing with a minor change in
differential efficiency. Rigid switching on is related to the
optical bistability effect induced by absorber bleaching be-
cause the carrier escape speed is not high enough. This
rigid switching-on effect is a characteristic phenomenon of
two sectional QD lasers with PML.
Figure 4 shows the light absorption spectra for the emis-
sion and absorption sections with an In(Ga)As/GaAs
SLQD. The spectra were measured for two polarization
directions: in the plane perpendicular to the growth axis
Figure 1 Transmission electron micrographs of the cross
section of the sample. The sample has ten InAs QD layers and
iswith a GaAs spacer layer 6.0-nm thick between them.
Sobolev et al. Nanoscale Research Letters 2012, 7:545 Page 2 of 5
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(x and y planes) and along the structuresgrowthdirection
(z axis). Commonly, by these polarization directions are
meant the TE and TM modes, respectively.
At the lasing wavelength in the PML regime, the absorp-
tion coefficient for TM-polarized light is only 1.6 times
smaller than that for TE polarization (Figure 4). The max-
imum ratio of absorption coefficients reaches 1.8 and at
energies less than 1.012 eV and more than 1.156 eV;
absorption for TM polarization is more (Figure 4, lines 1
and 2). The electroluminescence spectrum width in the
laser structure at a current density of J =0.3J
th
is about
130 nm, where J
th
is the threshold current density. It is
due to two factors: the QD size dispersion and energy
level splitting with QD coupling. The absorption coeffi-
cient front is about 90 nm, which is comparable to the
electroluminescence spectrum width.
Lasing spectra lay in the range 1,160 to 1,170 nm (Figure 4,
line 3) and shift to the longwave region with reverse bias in-
crease. The FWHM of the spectrum at V
r
= 2Vis5.2nm.
PML was observed in wide injection current range at re-
verse bias from 1to3 V (Figure 4) with a repetition fre-
quency of 12.5 GHz. At low reverse biases the carrier
lifetime in the absorber section τ
abs
is much more than the
roundtrip time τ
R
, so there is no mode-locking. With re-
versebiasincreasingthecarrierescaperategrowsupand
τ
abs
becomes comparable with the round-trip time, so short
pulse observation becomes possible. The smallest pulse dur-
ation is achieved at injection currents near the threshold
and revers e bi as at 2 V. The pulse width at half maximum
was derived from the measured autocorrelation function
(Figure 5), and the assumption of the Gaussian pulse profile
is 10 ps. This gives a time-bandwidth product value of about
Figure 2 Schematic of a double-section laser design and the experimental setup for SLQD absorption measurements. (a) Schematic of a
double-section laser design with amplifying and absorbing sections, used to measure the electroluminescence, and absorption spectra for two
light polarization directions: in the plane perpendicular to the growth axis (?)(xy plane) and along the growth direction (||) (z axis) of the
structure. (b) and (c) The experimental setup for SLQD absorption measurements: (b) direct detection of electroluminescence spectra and
(c) detection of electroluminescence passed through absorbing section.
Sobolev et al. Nanoscale Research Letters 2012, 7:545 Page 3 of 5
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10. Such large value is due to the high injection current and
the fact that measurements were done in pulsed mode. An
increase in reverse bias leads to PML collapse and the laser
emits in the cw mode. It is due to the shift of laser spectra
to the longer wave-length region where absorption modula-
tion of saturated and unsaturated states of the saturable ab-
sorber is not enough for mode-locking.
Conclusions
In conclusion, based on a structure containing ten layers
of coupled QDs, two sectional lasers were created in
which PML realization needs a rather small reverse bias
on the absorber section. The absorption value both for
TM and TE polarizations exceeds 50 cm
1
, which is
sufficient for modulators used in FOLs. In contrast to
0,90 0,95 1,00 1,05 1,10 1,15 1,20
0
20
40
60
K, cm
-1
Energy, eV
1
2
3
0,0
0,2
0,4
0,6
0,8
1,0
Intensity, a.u.
Figure 4 The absorption laser spectra. The absorption spectra for TM and TE mode (lines 1 and 2) and of laser spectra in PML regime (line 3).
0,0 0,1 0,2 0,3 0,4 0,5
0
10
20
30
40
50
Power, mW
Current, A
1
2
3
Figure 3 Watt-ampere characteristics of the laser at various currents in absorber section, the optical power from one mirror. 1isI
r
=
0mA and I
th
= 313 mA; 2 is I
r
= 0.08 mA and I
th
= 262 mA; 3 is I
r
= 15 mA and I
th
= 186 mA.
Sobolev et al. Nanoscale Research Letters 2012, 7:545 Page 4 of 5
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the structure with uncoupled QDs, where TM
polarization can be neglected, the luminescence intensity
and absorption coefficients for TE and TM polarizations
in SLQD are comparable.
Competing interests
The authors declare that they have no competing interests.
Authors contributions
MS supervised the project and provided laser structures and drafted the
manuscript. MB carried out experimental studies, analyzed and interpreted
the data and participated in drafting of the manuscript. IM carried out
samples characterization and experimental studies, analyzed and interpreted
the data. IO carried out absorption measurements. YuZ fabricated the
samples from laser structures. EP provided critical review and final approval
of the article. All authors discussed the results and implications and
commented on the manuscript at all stages. All authors read and approved
the final manuscript.
Acknowledgments
This study was supported by the Russian Foundation for Basic Research (12-
02-00388-а), grant of Russian Academy of Sciences and grant for young
researchers of SPb government.
Received: 10 July 2012 Accepted: 22 September 2012
Published: 2 October 2012
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Cite this article as: Sobolev et al.: Room temperature passive mode-
locked laser based on InAs/GaAs quantum-dot superlattice. Nanoscale
Research Letters 2012 7:545.
-400 -300 -200 -100 0 100 200 300 400 500
0,0
1,5
3,0
4,5
6,0
7,5
9,0
ACF, a.u.
Time Delay, ps
x10
-5
1
2
3
Figure 5 Autocorrelation functions of the laser at different reverse bias: 1 - V
r
=-1V, 2 - V
r
=-2V, 3 - V
r
=-3V.
Sobolev et al. Nanoscale Research Letters 2012, 7:545 Page 5 of 5
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