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Quantum Dot Laser Seminar Report 2004

Dept of Electronics & Communication GEC Thrissur

TABLE OF CONTENTS

INTRODUCTION 1

LASER 2

• PRICIPLE OF OPERATION 3

• LASER DIODE 4

PRINCIPLE OF OPERATION 5

TYPES OF LASER DIODE 6

SEMICONDUCTOR LASER 7

QUANTUM WELL LASER 7

QUANTUM DOT LASER 8

QUANTUM DOT 9

GROWTH TECHNIQUES 12

CHALLENGES 13

SPECTRAL ANALYSIS OF QUANTUM DOT 14

LASERS

HIGH TEMPERATURE PROPERTIES 21

CONCLUSION 24


Dept of Electronics & Communication GEC Thrissur 1

INTRODUCTION

Since the invention of semiconductor lasers in 1962, significant

progress has been made in terms of high performance in many applications

including telecommunications, optical storage, and instrumentation. Most

modern semiconductor lasers operate based on quantum mechanical effects.

Quantum well lasers have been used with impressive performance, while

novel quantum dot lasers, a subject of intense research, show a great promise

Lasers come in many sizes and can be made from a variety of

resonant cavities and active laser materials. Generally, increasing

confinement enforces an increasing quantization in the energy of electrons.

Therefore quantum dots, essentially zero-dimensional bits of material, will

(once excited) re-emit light at nearly a single wavelength. Quantum dots are

therefore a good starting point for producing laser light Some existing

quantum dot lasers employ dots made epitaxially: the atoms in the dots are

laid down meticulously using beams of atoms or molecules In the MIT laser

the gain medium consists of nm-sized particles of CdSe coated with a layer

of organic molecules and then immersed in a glassy film. The medium sits

in a waveguide atop a grating.

The fabrication advantage in this case derives from the fact that one

uses simple solution processing rather than the more exacting technique of

epitaxy usually needed for semiconductors. Furthermore, the color of the

output laser light can be varied by changing the size of the CdSe particles,

the grating spacing, or the refractive index of the waveguide, giving great

flexibility to the design and application of the laser



LASER

A Laser (light amplification by stimulated emission of radiation) is a

device which uses a quantum mechanical effect, stimulated emission, to

generate a coherent beam of light. Light from a laser is often very collimated

and monochromatic, but this is not true of all laser types.

Common light sources, such as the electric light bulb, emit photons in

all directions, usually over a wide spectrum of wavelengths. Most light

sources are also incoherent; i.e., there is no fixed phase relationship between

the photons emitted by the light source.

By contrast, a laser generally emits photons in a narrow, well-defined

beam of light. The light is often near-monochromatic, consisting of a single

wavelength or color, is highly coherent and is often polarised. Some types of

laser, such as dye lasers and vibronic solid-state lasers can produce light

over a broad range of wavelengths; this property makes them suitable for the

generation of extremely short pulses of light, on the order of a femtosecond

(10-15 seconds).

Laser light can be highly intense — able to cut steel and other metals.

The beam emitted by a laser often has a very small divergence (highly

collimated). A perfectly collimated beam cannot be created, due to the effect

of diffraction, but a laser beam will spread much less than a beam of light

generated by other means. A beam generated by a small laboratory laser

such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6

kilometres) in diameter if shone from the Earth's surface to the Moon. Some

lasers, especially semiconductor lasers due to their small size, produce very



divergent beams. However, such a divergent beam can be transformed into a

collimated beam by means of a lens. In contrast, the light from non-laser

light sources can generally not be collimated.

A laser medium can also function as an optical amplifier when seeded

with light from another source. The amplified signal can be very similar to

the input signal in terms wavelength, phase and polarisation; this is

particularly important in optical communications.

The output of a laser may be a continuous, constant-amplitude output

(known as c.w. or continuous wave), or pulsed, by using the techniques of

Q-switching, modelocking or Gain-switching. In pulsed operation, much

higher peak powers can be achieved.

Principle Of Operation

The basic physics of lasers centres around the idea of producing a

population inversion in a laser medium by 'pumping' the medium; i.e., by

supplying energy in the form of light or electricity, for example. The

medium may then amplify light by the process of stimulated emission. If the

light is circulating through the medium by means of a cavity resonator, and

the gain (amplification) in the medium is stronger than the resonator losses,

the power of the circulating light can rise exponentially. Eventually it will

get so strong that the gain is saturated (reduced). In continuous operation,

the intracavity laser power finds an equilibrium value which is saturating the

gain exactly to the level of the cavity losses. If the pump power is chosen too

small (below the 'laser threshold'), the gain is not sufficient to overcome the

resonator losses, and the laser will emit only very small light powers.



A great deal of quantum mechanics and thermodynamics theory can

be applied to laser action (see laser science), though in fact many laser types

were discovered by trial and error.

Population inversion is also the concept behind the maser, which is

similar in principle to a laser but works with microwaves. The first maser

was built by Charles H. Townes in 1953. Townes later worked with Arthur

L. Schawlow to describe the theory of the laser, or optical maser as it was

then known. The word laser was coined in 1957 by Gordon Gould, who was

also credited with lucrative patent rights in the 1970s, following a protracted

legal battle

Even low-power lasers with only a few milliwatts of output power can

be hazardous to a person's eyesight. The coherence and low divergence of

laser light means that it can be focused by the eye into an extremely small

spot on the retina, resulting in localised burning and permanent damage in

seconds. Lasers are classified by wavelength and maximum output power

into safety classes numbered I (inherently safe) to IV (even scattered light

can cause eye and/or skin damage). Laser products available for consumers,

such as CD players and laser pointers are usually in class I or II

.

LASER DIODE

A laser diode is a laser where the active medium is a semiconductor

p-n junction similar to that found in a light-emitting diode. Laser diodes are

sometimes referred to (somewhat redundantly) as injection laser diodes or

by the acronyms LD or ILD.



Principle of operation

When a diode is forward biased, holes from the p-region are injected

into the n-region, and electrons from the n-region are injected into the p-

region. If electrons and holes are present in the same region, they may

radiatively recombine—that is, the electron "falls into" the hole and emits a

photon with the energy of the bandgap. This is called spontaneous emission,

and is the main source of light in a light-emitting diode.

Under suitable conditions, the electron and the hole may coexist in the

same area for quite some time (on the order of microseconds) before they

recombine. If a photon of exactly the right frequency happens along within

this time period, recombination may be stimulated by the photon. This

causes another photon of the same frequency to be emitted, with exactly the

same direction, polarization and phase as the first photon.

In a laser diode, the semiconductor crystal is fashioned into a shape

somewhat like a piece of paper—very thin in one direction and rectangular

in the other two. The top of the crystal is n-doped, and the bottom is p-

doped, resulting in a large, flat p-n junction. The two ends of the crystal are

cleaved so as to form perfectly smooth, parallel edges; two reflective parallel

edges are called a Fabry-Perot cavity. Photons emitted in precisely the right

direction will be reflected several times from each end face before they are

emitted. Each time they pass through the cavity, the light is amplified by

stimulated emission. Hence, if there is more amplification than loss, the

diode begins to "lase".



Types of laser diode

The type of laser diode just described is called a homojunction laser diode,

for reasons which should soon become clear. Unfortunately, they are

extremely inefficient. They require so much power that they can only be

operated in short "pulses;" otherwise the semiconductor would melt.

Although historically important and easy to explain, such devices are not

practical.

Double heterostructure lasers

In these devices, a layer of low bandgap material is sandwiched between two

high bandgap layers. One commonly-used pair of materials is GaAs with

AlGaAs. Each of the junctions between different bandgap materials is called

a heterostructure, hence the name "double heterostructure laser" or DH

laser. The kind of laser diode described in the first part of the article is

referred to as a "homojunction" laser, for contrast with these more popular

devices.

The advantage of a DH laser is that the region where free electrons and holes

exist simultaneously—the "active" region—is confined to the thin middle

layer. This means that many more of the electron-hole pairs can contribute to

amplification—not so many are left out in the poorly amplifying periphery.

In addition, light is reflected from the heterojunction; hence, the light is

confined to the region where the amplification takes place.



SEMICONDUCTOR LASER

Semiconductor lasers are key components in a host of widely used

technological products, including compact disk players and laser printers,

and they will play critical roles in optical communication schemes. The basis

of laser operation depends on the creation of nonequilibrium populations of

electrons and holes, and coupling of electrons and holes to an optical field,

which will stimulate radiative emission. Calculations carried out in the early

1970s by C. Henry (Dingle and Henry 1976) predicted the advantages of

using quantum wells as the active layer in such lasers: the carrier

confinement and nature of the electronic density of states should result in

more efficient devices operating at lower threshold currents than lasers with

"bulk" active layers.

QUANTUM WELL LASER

In addition, the use of a quantum well, with discrete transition energy

levels dependent on the quantum well dimensions (thickness), provides a

means of "tuning" the resulting wavelength of the material. The critical

feature size-in this case, the thickness of the quantum well-depends on the

desired spacing between energy levels. For energy levels of greater than a

few tens of millielectron volts (meV, to be compared with room temperature

thermal energy of 25 meV), the critical dimension is approximately a few

hundred angstroms. Although the first quantum well laser, demonstrated in

1975, was many times less efficient than a conventional laser (van der Ziel

et al. 1975), the situation was reversed by 1981 through the use of new

materials growth capabilities (molecular beam epitaxy), and optimization of

the heterostructure laser design (Tsang 1982).



If the middle layer is made thin enough, it starts acting like a quantum

well. This means that in the vertical direction, electron energy is quantised.

The difference between quantum well energy levels can be used for the laser

action instead of the bandgap. This is very useful since the wavelength of

light emitted can be tuned simply by altering the thickness of the layer. The

efficiency of a quantum well laser is greater than that of a bulk laser due to a

tailoring of the distrubution of electrons and holes that are involved in the

stimulated emission (light producing) process.

The problem with these devices is that the thin layer is simply too

small to effectively confine the light. To compensate, another two layers are

added on, outside the first three. These layers have a lower refractive index

than the centre layers, and hence confine the light effectively. Such a design

is called a separate confinement heterostructure (SCH) laser diode.

QUANTUM DOT LASER

Even greater benefits have been predicted for lasers with quantum dot

active layers. Arakawa and Sakaki (1982) predicted in the early 1980s that

quantum dot lasers should exhibit performance that is less temperature-

dependent than existing semiconductor lasers, and that will in particular not

degrade at elevated temperatures. Other benefits of quantum dot active

layers include further reduction in threshold currents and an increase in

differential gain-that is, more efficient laser operation (Asada et al. 1986).



Quantum dots.

A quantum dot is a potential well that confines electrons in three

dimensions to a region of the order of the electrons' de Broglie wavelength

in size, a few nanometers in a semiconductor. Compare to quantum wires

and quantum wells.

Because of the confinement, electrons in the quantum dot have

quantized, discrete energy levels, much like an atom. For this reason,

quantum dots are sometimes called "artificial atoms." The energy levels can

be controlled by changing the size and shape of the quantum dot, and the

depth of the potential.

A potential well is the region surrounding a local energy minimum.If

potential energy is imagined as corresponding to the height of the Earth's

surface on a map, so that the resulting landscape of hills and valleys is a

potential energy surface, then a potential well would be the region around a

minimum of potential that could be filled with water without any flowing

away toward another minimum

Quantum dots are so small that quantum mechanical effects come into

play in controlling their behavior. Quantum mechanics apply in the

microscopic realm but its effects are largely unseen and unfelt in our

macroscopic world

Stimulated recombination of electron-hole pairs takes place in the

GaAs quantum well region, where the confinement of carriers and of the

optical mode enhance the interaction between carriers and radiation In

particular, note the change in the electronic density of states, as a function of



the "dimensionality" of the active layerThe population inversion (creation of

electrons and holes) necessary for lasing occurs more efficiently as the

active layer material is scaled down from bulk (3-dimensional) to quantum

dots (0-dimensional). However, the advantages in operation depend not only

on the absolute size of the nanostructures in the active region, but also on the

uniformity of size. A broad distribution of sizes "smears" the density of

states, producing behavior similar to that of bulk material

With the demonstration of the high optical efficiency self-assembled

formation of quantum dots formed without need of external processing and

having the natural overgrowth of cladding material (which addressed issues

of surface recombination), there ensued a marked increase in quantum dot

laser research.

The first demonstration 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

quantum dot structures, stacking successive, strain-aligned rows of quantum

dots and therefore achieving vertical as well as lateral coupling of the

quantum dots. In addition to utilizing their quantum size effects in edge-

emitting lasers, self-assembled quantum dots have also been incorporated

within vertical cavity surface-emitting lasers. Table 5.4 gives a partial

summary of the work and achievements in quantum dot lasers.

As with the demonstration of the advantages of the quantum well laser

that preceded it, the full promise of the quantum dot laser must await

advances in the understanding of the materials growth and optimization of

the laser structure. Although the self-assembled dots have provided an



enormous stimulus to work in this field, there remain a number of critical

issues involving their growth and formation: greater uniformity of size,

controllable achievement of higher quantum dot density, and closer dot-to-

dot interaction range will further improve laser performance.

Better understanding of carrier confinement dynamics and capture

times, and better evaluation of loss mechanisms, will further improve device

characteristics. It should be noted that the spatial localization of carriers

brought about by the quantum dot confinement may play a role in the

"anomalous" optical efficiency of the GaN-based materials, which is

exceptional in light of the high concentration of threading dislocations (~ 108

- 1010 cm-2) that currently plague this material system. The localization

imposed by the perhaps natural nanostructure of the GaN materials may

make the dislocation largely irrelevant to the purely optical (but not to the

electrical) behavior of the material.

Quantum dot lasers work like other semiconductor lasers, such as

those found in home-audio compact disc players. Just as in the

semiconductor laser chip in a CD player, the goal of a quantum dot laser is

to manipulate the material into a high energy state and then properly convert

it to a low energy state. The result is the net release of energy, which

emerges as a photon.

. In quantum dots, the electrons are confined within a very small

volume that forces them to strongly interact with each other. These strong

interactions can lead to deactivation of the dot through the so-called "Auger

process," preventing it from emitting a photon.



Quantum dots offer this performance over a range of temperatures,

making them suitable for a variety of applications, and also can be "tuned"

to emit at different wavelengths, or colors. The emission wavelength of a

quantum dot is a function of its size, so by making dots of different sizes

scientists can create light of different colors

GROWTH TECHNIQUES

Several growth approached have been developed to fabricate

Quantum Dots arrays with high luminescence efficiency and low dislocation

density using Molecular Beam Epitaxy (MBE) and Metal Organic Chemical

Vapor Deposition (MOCVD). The parameters of such QDs arrays (QD

density, size and shape) can be controlled by growth conditions. The

emission range of InAs-GaAs nanostructures is extended up to 1.75 m at

room temperature

Parameters of some QD lasers.

Wavelength Output power, 300K

Growth approach Growth technique

1.3 m m 2.7 W Activated Alloy Phase Separation

MBE

1.1 m m 3.7 W Stacking of QDs MOCVD

0.94 m m 4 W Submonolayer deposition MBE



QD Lasers grown by MBE Energy diagram of the layers in a QD “dots-in-a-well” laser.

CHALLENGES

Thus, the challenge in realizing quantum dot lasers with operation

superior to that shown by quantum well lasers is that of forming high

quality, uniform quantum dots in the active layer. Initially, the most widely

followed approach to forming quantum dots was through electron beam

lithography of suitably small featured patterns (~300 Å) and subsequent dry-

etch transfer of dots into the substrate material. The problem that plagued

these quantum dot arrays was their exceedingly low optical efficiency: high

surface-to-volume ratios of these nanostructures and associated high surface



recombination rates, together with damage introduced during the fabrication

itself, precluded the successful formation of a quantum dot laser.

The challenge, however, is that there are competing mechanisms by

which the energy can be released, such as vibrational energy or electron

kinetic energy

SPECTRAL ANALYSIS OF QUANTUM DOT LASERS

Since the early eighties, predictions have indicated that quantum-dot

lasers should have superior characteristics to other higher dimensional

structures such as quantum well devices and, with the advent of the self-

organized growth technique, progress towards this goal has been made—at

the present time, the best results being for lasers incorporating InGaAs or

InAs dots.

One unexpected feature of InGaAs/GaAs quantum-dot lasers is the

nature of the longitudinal mode distribution. It has been observed that the

laser emission spectra are broad and consist of peaks at regularly spaced

intervals (approx 1–5 nm) superimposed on the normal longitudinal Fabry–

Perot modes. Such behavior has been attributed to the discrete nature of the

dots and the resulting inhomogeneous broadening (lack of a global Fermi

function) leading to either spatial or spectral hole burning.

Further hypotheses have been advanced to account for the periodic

nature of the spectra where different subsets of dot sizes contribute to

different groups of modes, the groups of longitudinal modes do not

necessarily have a regular spacing . The suggested mechanisms include

intracavity photon scattering a nonuniform distribution of dot electronic



states (due perhaps to some preferred dot sizes), a gain that is dot size or

shape dependent (due to size and shape dependence of either the oscillator

strength or the efficiency with which dots capture carriers) and a modulation

of the losses by constructive interference with the reflection of a transverse

leaky mode propagating in the transparent substrate. The effects due to the

leaky mode have previously been reported in quantum well lasers operating

at the same wavelength. They lead to an optical mode loss and an optical

confinement factor that vary as a function of wavelength with a period that is

inversely proportional to the device thickness.

The laser structure we have examined is represented in Fig. 1 and

consists of three layers of InGaAs quantum dots each of which is grown in a

matrix of GaAs (10 nm thick). These are themselves grown in Al0.3Ga0.7As,

and together comprise the waveguide core of the device. Atomic force

microscopy (AFM) studies indicate the dots are lens like in shape, are 2.2

nm high and 36 nm in diameter with a dot density of 4.5x1010 cm -2.



FIG. 3. Spectra of the device of Fig. 2 taken at 1.4 and 1.53Ith and at a temperature of

280 K. The larger wavelength range shows the presence of a second group of lasing

modes at higher energies. The two spectra are offset on the vertical scale.

Typical spectra for 50 mm wide oxide isolated stripe devices

fabricated from the above structure are presented in Fig. 2 (a)The spectra

were measured, using a spectrum analyzer(0.07 nm resolution), as a function

of drive current (I=1.1, 1.2, 1.3, and 1.4x Ith ) at a temperature of 280 K. The

devices being operated pulsed with a pulse length of 300 ns and a duty cycle

of 0.03%. In addition to the normal longitudinal modes (spacing ~0.09 nm

for the device that is 1500 mm long), which we can just resolve with the

spectrum analyzer and just pick out in the spectrum shown magnified in the

inset, there is a more widely spaced periodicity present in the data. The

groups of longitudinal modes or supermodes are much more obvious than

the longitudinal modes themselves and have a spacing of approximately 0.6

nm.



The Fourier transform of each of the four spectra (plotted in terms of

wave number so that the conjugate variable is length) are shown in Fig. 2(b)

and demonstrate the presence of a periodicity in all four spectra even at the

relatively low drive currents used here. At still higher currents, lasing

spreads to a second group of higher lying energy states as shown in Fig. 3

for spectra recorded at currents of 1.4 and 1.5x3Ith. This second group of

modes complicates the Fourier transform, introducing extra detail, but the

Fourier transforms of each of the two groups taken individually indicate a

similar periodicity within each group.

FIG. 2.a Quantum-dot laser spectra taken at drive currents of 1.1, 1.2, 1.3, and 1.43Ith

and a temperature of 280 K for a 50 mm wide, 1500 mm long oxide isolated stripe

device. The spectra have been offset on the vertical scale for ease of comparison ~higher

current have larger offsets!. The spectra exhibit groups of longitudinal modes separated

by approximately 1nm intervals in addition to the normal longitudinal modes shown in

the magnified section of the 1.33Ith spectrum in the inset.



Fig 2.b. Fourier transforms of the data in (a) plotted in terms of wave number. The

spectra are offset on the vertical scale for clarity (increasing offsets for higher currents).

The spectra of the three sets of quantum-dot lasers with different

substrate thickness were measured as a function of drive current and

temperature. Fourier transforms were used to simplify the analysis of the

spectra. As recently shown by plotting the spectra in terms of wave number,

the Fourier transform gives information about the optical path length within

the laser cavity. Furthermore, by using the refractive index and refractive

index energy dependence this information can be converted into the device



length. The length dependence of any other periodicity within the spectrum

also then becomes apparent. In Fig. 4, we have plotted the Fourier transform

of the wave-number spectrum of a 1500 mm long, 260 mm thick device

operated at 23Ith and at a temperature of 150 K.

The low temperature allows us to drive the device well above

threshold without exciting the higher energy states observed in Fig. 3. In the

upper trace, which is the lower trace amplified by a factor of 20, a feature

exists at both the device length (B1) and twice the device length (B2). The

largest feature (A1), which is readily apparent in the trace that has not been

amplified, corresponds to a length of250 mm, with another feature (A2),

apparent in the amplified trace, at 500 mm. Similar measurements taken on

the other devices of different thickness and cavity length are summarized in

Table I. The features apparent in the Fourier transform spectra, which

represent the periodicity present in the measured spectra, show a correlation

with the thickness

These results indicate that the dominant mechanism leading to the

regular modulation of the emission spectra in these quantum-dot lasers is

related to the device thickness, although there are some additional features

present in some of the measured spectra that do not appear to be related to

the cavity length or thickness. It may be that in quantum-dot devices where

substrate effects are suppressed that other mechanisms cause regular or

quasiregular mode distributions.



.

FIG. 3. Spectra of the device of Fig. 2 taken at 1.4 and 1.53Ith and at a temperature of

280 K. The larger wavelength range shows the presence of a second group of lasing

modes at higher energies. The two spectra are offset on the vertical scale.



HIGH TEMPERATURE PROPERTIES

The growth of selforganized InAs quantum dots allows the realization

of lasers emitting at 1,3 µm on GaAs substrates. Beside the principal

advantage that GaAs substrates can be used instead of InP substrates for

large volume production quantum dot related effects like very low

transparency current densities and low internal absorption are also of

importance. Other predictions of quantum dot lasers like low temperature

dependence could not be realized up to now for room temperature and

above. Especially in the case of 1.3 µm emitting quantum dot lasers good

high temperature performance is still a problem.

We have realized 1,3 µm emitting graded index separate confinement

heterostructure lasers with InAs dots embedded in a 10 nm thick

Ga0.85In0.15As quantum well. Our structure additionally uses short period

superlattices (SSLs) in the graded regions to improve the carrier

confinement by electron back reflection [1, 2]. This improvement allow

ground state lasing at temperatures > 80 °C. Multi quantum dot structures

with large dot layer separation of 50 nm were used to avoid any strain

coupling and to minimize strain accumulation. The growth temperature for

the quantum dot layers was 510 °C and for the 1.6 µm thick cladding layers

570 °C, respectively.

The influence of the amount of quantum dots on the laser performance

was investigated by varying the number of dot layers from 3 to 8 layers. The

best results were obtained with 6 uncoupled quantum dot layers with

transparency current densities of less than 40 A/cm2 ( Fig. 1), an internal

quantum efficiency of about 35% and an internal absorption of 1-2 cm-1.

Ridge waveguide lasers with 4 µm ridge width and cavity lengths as short as



800 µm long can be operated at room temperature in cw mode without any

facet coatings. These devices show good temperature characteristics with T0

> 70 K up to about 50 °C and 54 K up to 140 °C, respectively ( Fig. 2). The

maximum operation temperature was above 150 °C which is the highest

value known up to now for 1.3 µm emitting quantum dot lasers.

Fig. 1: Threshold current density of 2 samples with different numbers of quantum dot

layers as function of the inverse cavity length. Values determined in pulsed operation for

100 µm wide broad area lasers at 20 °C.

Due to the improved gain by 6 dot layers with an average dot density

per layer of about 1x1011 cm-2 and the low internal absorption high

performance short cavity devices could be realized using high reflection

facet coatings (83% for front and 95% for backside facets, respectively). 400

µm long devices exhibit threshold currents as low as 6 mA and more than 5

mW output power at 30 mA Emission from the fundamental dot states was



achieved from cw operating unmounted devices up to 70 °C with more than

2 mW output power. The maximum cw operation temperature was 90 °C

Fig. 2: Temperature dependence of the threshold current density of a 2.5 mm long

uncoated ridge waveguide laser with 6 quantum dot layers.



CONCLUSION

Though QD lasers show immense potential for superior device

performances, there are still some significant problems associated with the

control of emission wavelengths reproducibilty of the dots,high temperature

reliability and long term stablity of the dots.The current challenge is to

match and surpass the performance of the quantum well lasers.There is still

need for the development of QD strength of lasing around 1.55 micrometre

,which is a principal wavelength in fibre optic communications.This would

give QD lasers a chance to move into application such as ultrafast optical

data transfer .A key aspect of QD production challenge will be to improve

our control over the dot distribution produced in the self assembly process

.Reliable continuous wave room temperature operation of QD lasers has

already been reported; structure improvements are required to get the

operation characteristics more desirable,especially the elimination of several

mechanisms that have a detrimental effect at room temperature.

From a bird’s eyeview ,the research on QD lasers is still newly

emerging from its beginning stages .Several promient group of researchers

around the world are all going down their own avenues ,grappling with a

portion of the overall problem ,identifying and overcoming obstacles one by

one individually .This is not surprising ,considering the research on QD

lasers , as opposed to somewhat more well established research on basic

QD’s themselves began to hit the stage truly only around 1995-1996.Still

consideing the efforts and the emergence of well defined directions ,there

seems to be hope that the field will settle down and become established .If

the collective effort succeeds in bettering the performance of quantum well

lasers ,which it might ,then QD lasers can finally be up there along with the

MOSFET,quantum well lasers and monolithic integration technology.



REFERENCES

Long wavelength quantum dot lasers in Journal of

materials science: Materials in electronics January 2002

1.3 micro metre QD lasers with improved high temperature

properties by F.Klopfs and R.Krebs

Spectral analysis of InGaAs/GaAs quantum dot lasers by

P.M.Smowton in Journal of Applied physics letters

Volume 75,October 1999

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