2. 2
Quantum Dot Laser
Quantum Dots:
Are semiconductor particles a few nanometres in size, having optical and electronic properties
that differ fromlarger particles due to quantum mechanics. They are a central topic in
nanotechnology. When the quantum dots are illuminated by UV light, an electron in the
quantum dot can be excited to a state of higher energy. In the caseof a semiconducting
quantum dot, this process corresponds to the transition of an electron fromthe valence band
to the conductanceband. The excited electron can drop back into the valence band releasing
its energy by the emission of light. This light emission (photoluminescence) is illustrated in the
figure on the right. The color of that light depends on the energy difference between the
conductance band and the valence band.
In the language of materials science, nanoscalesemiconductor materials tightly confine either
electrons or electron holes. Quantum dots are sometimes referred to as artificial atoms,
emphasizing their singularity, having bound, discrete electronic states, like naturally occurring
atoms or molecules. Itwas shown that the electronic wavefunctions in quantum dots
resembles the ones in real atoms. By coupling two or more such quantumdots an artificial
molecule can be made, exhibiting hybridization even at roomtemperature.
Quantumdots haveproperties intermediate between bulk semiconductors and discreteatoms
or molecules. Their optoelectronic properties change as a function of both sizeand shape.
Larger QDs of 5–6 nm diameter emit longer wavelengths, with colors such as orangeor red.
Smaller QDs (2–3 nm) emit shorter wavelengths, yielding colors like blue and green. However,
the specific colors vary depending on the exact composition of the QD.
Potential applications of quantum dots include single-electron transistors, solar cells, LEDs,
lasers, single-photon sources, second-harmonic generation, quantumcomputing, cell biology
research, and medical imaging. Their small sizeallows for someQDs to be suspended in
solution, which may lead to use in inkjet printing and spin-coating. They have been used in
Langmuir-Blodgett thin-films. These processing techniques resultin less expensive and less
time-consuming methods of semiconductor fabrication.
3. 3
Quantum Dot Laser:
Is a semiconductor laser that uses quantum dots as the active laser medium in its light
emitting region. Dueto the tight confinement of chargecarriers in quantum dots, they exhibit
an electronic structuresimilar to atoms. Lasers fabricated fromsuch an active media exhibit
device performancethat is closer to gas lasers, and avoid some of the negative aspects of
device performanceassociated with traditional semiconductor lasers based on bulk or
quantum well active media. Improvements in modulation bandwidth, lasing threshold, relative
intensity noise, linewidth enhancement factor and temperature insensitivity haveall been
observed. The quantumdot active region may also be engineered to operate at different
wavelengths by varying dot sizeand composition. This allows quantum dot lasers to be
fabricated to operate at wavelengths previously notpossibleusing semiconductor laser
technology.
Recently, devices based on quantum dot active media are finding commercial application in
medicine (laser scalpel, optical coherence tomography), display technologies (projection, laser
TV), spectroscopy and telecommunications. A 10 Gbit/s quantum dot laser that is insensitive
to temperature fluctuation for usein optical data communications and optical networks has
been developed using this technology. The laser is capable of high-speed operation at 1.3 μm
wavelengths, at temperatures from 20 °C to 70 °C. Itworks in optical data transmission
systems, opticalLANs and metro-access systems. In comparison to the performanceof
conventional strained quantum-well lasers of the past, the new quantum dot laser achieves
significantly higher stability of temperature.
Quantum Confinement Effect:
To understandtheQD concept, firstof all, weshouldconsider thequantumconfinement effects
on electrons. Quantum confinement occurs when one or more of the dimensions of a
nanocrystal approach the Exciton Bohr radius. The concepts of energy levels, bandgap,
conduction bandand valence band still apply.However,theelectron energylevels can no longer
be treated as continuous - they must be treated as discrete.
4. 4
Comparisons of quantumwells, wires, rods and dots a. Geometries of the different structures.
b. Plots of Eg (the increase in the bandgap over the bulk value) against d (the thickness or
diameter) for rectangular quantumwells, cylindricalquantum wires and sphericalQDs
obtained fromparticle-in-a-box approximations. The grey area between the dot and wire
curves is the intermediate zone corresponding to quantumrods. The vertical dotted line and
points qualitatively representthe expected variation in the bandgap for InAs quantumrods of
varying length/diameter ratio, as studied by Kan et al.1. c. A plot of Eg againstlength/diameter
ratio for the InAs quantumrods synthesized by Kan et al., showing the dependence of the
bandgap on the shapeof the quantum rods. The dotted line represents the variation expected
froma particle-in-a-box approximation
Quantumwell, or quantum wireconfinements give the electron at least one degree of
freedom. Although this kind of confinement leads to quantization of the electron spectrum
which changes the density of states, and results in one or two-dimensionalenergy subbands, it
still gives the electron at least one direction to propagate. On the other hand, today’s
technology allows us to create QD structures, in which all existing degrees of freedom of
electron propagation arequantized. We can think this confinement as a box of volume
d1d2d3. Theenergy is therefore quantized to
E = Eq1 + Eq2 + Eq3 where Eqn = h2(q1_/dn)2 / 2mc
5. 5
q1, q2 and q3 are the quantum numbers associated with an energy subband.
Since the allowed energy levels are discrete and separated, we can represent the density of
states as delta functions. (Figure 3) The energy levels of a QD can be adjusted with a proper
design accordingto the needs of theapplication. Forinstance, the addition or subtractionof just
a few atoms to the QD has the effect of altering the boundaries of the bandgap. Changing the
geometry of the surfaceof the QD also changes the bandgap energy, owing again to the small
size of the dot, and the effects of quantum confinement.
Development of Quantum Dot Lasers:
The laser operation is based on producing radiative emission by coupling electrons and holes at
nonequilibrium conditions to an optical field. The advantages of quantum well lasers on
traditional lasers first predicted in 1970s (Dingle and Henry 1976), and first quantum well lasers
which were very inefficient were demonstrated at those dates (van der Ziel et al. 1975). The
advantages recognized were:
The confinement and nature of the electronic density of states result in more efficient
devices operating at lower threshold currents than lasers with bulk active layers. The laser
threshold current density can be reduced by decreasing the thickness of the active layer.
6. 6
Discrete energy levels provide a means of "tuning" the resulting wavelength of the material.
Since the thickness ofthe quantum well-depends on the desired spacing between energy levels,
tuning can be done by changing the quantum well dimensions or thickness. For energy levels
of greater than a few tens of meV’s, the critical dimension is approximately a few hundred
angstroms.
Since the quantumconfinement in a QD is in all three dimensions, tunability of a quantum dot
laser (QDL) is higher than a quantumwell laser (QWL). The concept of semiconductor QDs was
proposed for semiconductor laser applications by Arakawa and Sakakiin 1982, predicting
suppression of temperaturedependence of the threshold current. Henceforth, reduction in
threshold currentdensity, reduction in total threshold current, enhanced differential gain and
high spectralpurity/no-chirping weretheoretically discussed in 1980’s (Asada etal. 1986).
At this point, we should examine the basics of laser operation. A laser utilizes
stimulated that is triggered by an incident photon of the sameenergy. This occurs when a
medium has morepopulation of electrons in the excited quantumlevel than in the ground
level. This artificially situation, called population inversion, is produced by either electrical
stimulation (electroluminescence) or optical stimulation and is different fromthe spontaneous
emission, whereby the electron returns to the ground state in the natural course(within the
lifetime of the excited states), even in the absenceof any photon to stimulate it.
In Figure3, stimulated recombination of electron-hole pairs takes place in the GaAs quantum
well region, wherethe confinement of carriers and the confinement of the optical mode
enhance the interaction between carriers and radiation. In Figure 4, we can observethe
changes in density of states for different dimensionalities. The population inversion necessary
for lasing occurs moreefficiently as the active layer material is scaled down frombulk (3D) to
QDs (0D). However, theadvantages in operation depends both on the absolute size of the
nanostructures in the active region, and on the uniformity of size. A broad distribution of sizes
smears the density of states, producing behavior similar to that of bulk material.
8. 8
Figure 4 - Density of electronic states as a function of structure size
QD lasers acquired more importanceafter significant progress in nanostructuregrowth in the
1990’s such as theself-assembling growth technique for InAs QDs. Thefirstdemonstration of a
quantum dot laser with high threshold density was reported by Ledentsov and colleagues in
1994. Bimberg et al. (1996) achieved improved operation by increasing the density of the QD
structures, stacking successive, strain-aligned rows of QDs and thereforeachieving vertical as
well as lateral coupling of the QDs. In addition to utilizing their quantum sizeeffects in edge
emitting lasers, self-assembled QDs havealso been incorporated within vertical cavity surface
emitting lasers. QD lasers are not as temperature dependent as traditional semiconductor
lasers. This theory was utilized by applications and in 2004; temperature-independent QD
lasers were invented in Fujitsu Laboratories.
9. 9
Basic Characteristics of QD Lasers:
In a laser, the stimulated emission is amplified by passing the emitted photons to stimulate
emission at other locations. (Figure 5)
The basic components of a laser are:
An active medium (gain medium which is the QD in our case) wherepopulation
inversion is created by a proper pumping mechanism. The spontaneously emitted
photons at some site in the medium stimulate emission at other sites as it travels
through it.
An energy pump source(electric power supply for QDLs)·
Two reflectors (rear mirror and output coupler) to reflect the light in phase(determined
by the length of the cavity) so that the light will be further amplified by the active
medium in each round-trip (multipass amplification). The output is partially transmitted
through a partially transmissiveoutputcoupler wherethe output exits as a laser beam
(R= 80% in the figure)
Figure3 shows schematic view of the band structureof a typical quantum dot laser. An ideal
QD laser consists of a 3D-array of dots with equal sizeand shape(middle of the figure),
surrounded by a higher band-gap material which confines the injected carriers. The whole
structureis embedded in an optical waveguideconsisting of lower and upper cladding layers
(n-doped and pdoped shields).
10. 10
Figure 5 - Schematic band structure of a quantum dot laser with self-organized dots under forward bias. A 3D
array of dots vertically aligned along the growth direction, which is formed during the growth, of multiple QD
layers is illustrated schematically. Typically the dot area density in the (100)plane is 4×1010 cm−2 and the dot
size distribution is around 10%. The distance between the dot layers is 5 nm and the real dot density in the
recombination volume with a thickness of 200 nm is 6×1015 cm−3 for three QD layers.
Figure 6 - Schematic of a laser cavity
11. 11
Figure 7 - Scheme of double heterostructure semiconductor laser
QD lasers are established by spontaneous formation of QDs at growth temperatures between
460 and 550°C. Fabrication method for such QDs is Stranski-Krastanovgrowth. Thelow growth
temperature and the low dot density can causeseveralproblems concerning threshold and in.
The cladding layer and the GaAs QD barrier typically grown at these lower temperatures area
possiblesourcefor current leakage and non-radiativerecombination. On the other hand, the
QDs exhibit someintermixing with the surrounding barrier material if temperatures of about
700°C are used to grow high quality cladding layers.
Figure 8 - Self-organized QDs
The self-organization of nanoscale three-dimensional coherentstrained islands following
Stranski-Krastanov growth mechanismis considered as the most promising way of in-situ QDs
fabrication. The ordered arrays so formed may result in distributed feedback and in
stabilization of single-modelasing. In addition, intrinsically buried QDs spatially localize
12. 12
carriers and prevent them from recombining non-radiatively at resonator facets. Overheating
of facets at high power operation may thus be avoided. A real challenge lies in the
optimization of growth parameters to achieve a dense and uniformarray of QDs, identical in
sizeand shape.
Figure 9 - Schematicrepresentationof differentapproachestofabricationof nanostructures:Selforganizedgrowthof
nanostructures
Required Characteristics for Quantum Dot Laser Applications
Quantumdot lasers utilize an oscillator strength that is condensed into a narrow energy width.
Because of that reason, the absolute energy level of the QDs should be the same. In other
words, thesize, shape and alloy composition of QDs should be close to identical. Therefore,
the inhomogeneous broadening of QD luminescence is eliminated, and real concentration of
the electron energy states can be obtained. If a macroscopic physicalparameter is desired,
such as light output in laser devices, the density (the number of interacting QDs) should beas
high as possible. The reduction of nonradiative centers in QDs is important for QDL
applications. Nanostructures madeby high-energy beam patterning cannot be used damage is
incurred fromthe beam around the nanostructures. Sincethe surface-to-volumeratio of QDs
is drastically increased compared to QWs, this type of damage around the surfaceof self-
assembled QDs is critical for the development of the QDL applications. QDs are put into
layered structures to create lasers. At this point, electrical controlis very important becausean
electric field applied to the structurecan changecertain physicalproperties of QDs in a
desirable way and carriers can be injected into the structureto create light emission.
13. 13
Figure 10 - comparison of efficiency between a QWL and a QDL
In order for QD lasers compete with QW lasers, two major issues have to be addressed:· A
large array of QDs has to be used because their active volumeis very small. An array of QDs
with a narrow sizedistribution has to be produced to reducein homogeneous broadening.
Furthermore, that array has to be without defects that degrade the optical emission by
providing alternate nonradiativedefect channels. · The phonon bottleneck created by
confinement limits the number of states that are efficiently coupled by phonons due to energy
conversation. Therefore, it also limits the relaxation of excited carriers into lasing states. This
bottleneck causes degradation of stimulated emission (Benisty et al., 1991). However, other
mechanisms can be used to suppress thatbottleneck effect. (e.g. Auger interactions)
Different Types of Quantum Dot Lasers
High speed quantum dot lasers
There are severalepitaxials were proposed to get the predicted advantages of QD lasers,
among them are: overgrowth of QDs with quantum well layers, stacking of quantumdots,
close stacking of quantumdots leading to the vertical coupling of quantumdot layers, p-
doping of the GaAs barrier layers, etc.
14. 14
Directly modulated quantum dot lasers:
Being the key point of the fiber-based datacom application, directly modulated
quantum dot lasers could convert electrical signals into digital optical signals at the rate
of around 10Gb/s. Themodulation speed needs to be further improved, the power
consumption should be reduced and the temperature performanceneeds to be better.
Figure 11 - BER measurement of QD laser module at 8 Gb/s and 10Gb/s (a) and at 10 Gb/s for different
temperatures (b), inset shows the corresponding eye patterns.
Mode-Locked quantum dot lasers:
With the applications of Mode-Locked quantum dot lasers, severaladvantages could be
received: shortoptical pulses, narrow spectralwidth with a small footprintdevice.
Besides, Mode-Locked quantum dot lasers are able to providea much broader gain
spectrum(>50nm), longer cavities (approximately 1cm) , sub-ps width and a very low
factor which leads to low chirp.
15. 15
Figure 12 - Autocorrelation trace of a passively mode-locked quantum dot laser at 1.3 μm and 80 GHz
repetition rate. The side peaks correspond to the cross-correlation of two successive pulses, while the
middle peak presents the autocorrelation of a pulse (a). Field scan of autocorrelation traces with
colorcoded FWHM pulse widths of a 80 GHz passively mode-locked QD laser. Three regimes of
operation can be distinguished (b).
InP based quantum dot lasers:
Compared with QW lasers, the emission wavelength of the InP based quantumdot lasers
is much lower (0.2nm/K compared to 0.55nm/K). This property could allow this kind of
quantumdot lasers operate within a much wider temperature range. Although there still
exist somelimitations in speed dueto the inhomogeneouslinewidth broadening,thedata
transmission could still be ‘’possibly over 10Gb/s for InP based quantum dot lasers
High power quantum dot lasers
With severalpromising properties of the quantumdot materials, it is widely realized
that quantu dot lasers are able to get a good power performance. The advantages of
quantum dot materials to be suitable applied to high power application fields are: zero
linewidth enhancement factor, the free geometric parameters of the quantum dots, e.
g. quantumdots size, dots density and size
distribution could allow to get the gain without considering the material composition .
Extra expensive cooling by Peltier elements is then not needed.
16. 16
QD lasers for coolerless pump sources.
The devices with these properties are recently developed based on the GaInAs/Ga(Al)As
QD layers emitting at 920nm. In this research, the sizeof the quantum dot is reduced by
modifying the growth parameter and In composition with a constant emission
wavelength of the transition. A power splitting of 65ev could be received at room
temperature wavelength of 920nmwith the size reduced quantum dot structures.
Single mode tapered lasers
New device geometry is used to get a similar performanceas multi-mode emitting
devices. Such kind of lasers enables the amplification of the single mode during the
propagation. Compared with quantum well lasers, the wavelength shiftis smaller due to
the better temperature performance(temperature sensitivity).
Market demand:
Because of the approved advantages of Quantum Dots Lasers, such as low threshold
current, enhanced differential gain, lower chirp/high spectralpurity, independent of the
threshold current on temperature and a decreased a factor, QDs Lasers wereintensively
researched all through the previous decade. They are suitable to be used in optical
applications, microwaveor millimeter wavetransmission with optical fibers and other
telecom and datacom networks. However, QD lasers werecommonly regarded as only a
theoretical topic which is almost impossible to be broughtto the market. The early
models werebased on the assumptions:
Only one confined electron level and hole level
Infinitebarriers
Equilibrium carrier distribution
Lattice matched heterostructures
The emerge of self-assembling growth technology which forms today the very basis of
optoelectronic devices such as edge emitting lasers, which has great potential for the
future applications, pushes quantumdot lasers to the boundary between theoretical
field and commercial applications. Thoseupdated QD based lasers employ
fundamentally differentmodels compared to the original models:
17. 17
Lots of electron levels and hole levels
Finite barriers
Non-equilibrium carrier distribution
Strained heterostructures
The predictions of decreased factor and wavelength chirp have already been proved on
real devices. In the lightwave applications, lasing in the 1.3umspectral range, using
GaAs substrate, both surfaceand edge emitters havebeen commercially produced at 6
inch diameter.
Nevertheless, as can be expected, due to the challenges listed below, the way of fulfill
the QD based lasers into commercial markets is not smooth.
· First, the lack of uniformity. · Second, QuantumDots density is insufficient.
· Third, the lack of good coupling between QD and QD. Recently, a Tunnel Coupling
Layer for Efficient QuantumDot Lasers technology has been published as a Commercial
Opportunity Announcement. In order to enhance transportation of electron-hole pairs
among quantumdots, get more efficient quantum dot lasers and break the limitation of
the older QD technologies, a solution of coupling the sheet of uniformand dense layer
of quantumdots, via a thin barrier, to a quantum well (QW) layer. This technology has
been proved in the visible red wavelength. InAlGaP was used as the coupling barrier
layers and InGaP was used in quantumwell layers. has stated that GaAs-based QD lasers
will be a good choice for light wavecommunication networks in terms of performance
and expense.
Although difficulties weremet on the way of realizing QD lasers, with those attractive
properties, QuantumDot laser is still predicted to maintain a hot research field in a few
years. Someresearchers areseeking someother ways to push their research toward.
FUTURE:
The advantages of quantum dot based lasers compared to other conventional
technologies have been realized for several years. Especially the free geometric
parameters of quantumdot layers give probabilities to tailor the spectralgain profile
applied to different types of QD lasers applications. Nevertheless, due to the intrinsic
limitation of technologies, to realize quantum dot lasers with predicted properties met
severaldifficulties. The requirement of further widening the parameters rangein order
to reducing the inhomogeneous linewidth broadening (weneed homogeneous
linewidth) is one of the aspects of developing quantum dot lasers. Using surface
preparation
18. 18
technologies, lots of groups are working on the issue of further controlling the position
and dot sizefor the self-organized technology. Oncethe developed methods can be
implemented in the high density systems, the new technology will become the
breakthrough in the history of quantum dot lasers development. Since the speed of
carrier capture extremely increasethe transporttime and affects themodulation
bandwidth, it is required to decouple the carrier capture fromthe escape procedure.
Employing tunnel injections to quantumdots is a choice. Allowing the injection of
cooled carriers, this method is able to achieve good performancewithoutloosing the
extra carriers which often happens before due to the thermal relaxation. With the
experiment done by comparing the QW lasers and QD lasers in term of raised gain at
the fundamentaltransition energy with the constantbroad band characteristics of
quantum dot lasers, it is concluded the combination use of quantum dot and quantum
well would tailor the material properties in a much wider range than using quantum
dots or quantum wells alone.
With the employment of further controlof parameters and better coupling technology
and the breakthroughs which arealready done, realizing quantum dot lasers as well as
other quantum dot optoelectronic devices in commercial market is not so far away.
