19-07-2014, 10:49 AM
Quantum Dot Lasers
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Abstract
In this paper, we reviewed the recent literature on quantum dot lasers. First of all, we
start with physics of quantum dots. These nanostructures provide limitless opportunities to create
new technologies. To understand the applications of quantum dots, we talked about quantum
confinement effect versus dimensionality and different fabrication techniques of quantum dots.
Secondly, we examined the physical properties of quantum dot lasers along with history and
development of quantum dot laser technology and different kinds of quantum dot lasers
comparing with other types of lasers. Thirdly, since engineering is a practical science, we made a
market search on the practical usage of quantum dot lasers. And lastly, we predicted a future for
quantum dot laser
INTRODUCTION
Quantum Dots
Quantum dots (QD) are semiconductor nanostructures with vast applications across many
industries. Their small size (~2-10 nanometers or ~10-50 atoms in diameter) gives quantum dots
unique tunability. Like that of traditional semiconductors, the importance of QDs is originated
from the fact that their electrical conductivity can be altered by an external stimulus such as
voltage or photon flux. One of the main differences between quantum dots and traditional
semiconductors is that the peak emission frequencies of quantum dots are very sensitive to both
the dot's size and composition. [1], [2]
Quantum Confinement Effect
To understand the QD concept, first of all, we should consider the quantum confinement 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 band and
valence band still apply. However, the electron energy levels can no longer be treated as
continuous - they must be treated as discrete.
ECE 580 – Term Project
Quantum well, or quantum wire confinements 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-dimensional energy 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 are
quantized. We can think this confinement as a box of volume d1d2d3. The energy is therefore
quantized to
QUANTUM DOT LASERS
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.
• Discrete energy levels provide a means of "tuning" the resulting wavelength of the
material. Since the thickness of the quantum well-depends on the desired spacing
between energy levels, tuning can be done by changing the quantum well dimensions or
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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 uniform array of QDs, identical in size and shape. [9], [15], [16]
Required Characteristics for Quantum Dot Laser Applications
Quantum dot 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,
the size, 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 physical parameter is desired, such as
light output in laser devices, the density (the number of interacting QDs) should be as high as
possible. [17]
The reduction of nonradiative centers in QDs is important for QDL applications. Nanostructures
made by high-energy beam patterning cannot be used damage is incurred from the beam around
the nanostructures. Since the surface-to-volume ratio of QDs is drastically increased compared to
MARKET DEMAND AND NEW TECHNOLOGY
Market demand
Because of the approved advantages of Quantum Dots Lasers, such as low threshold current,
enhanced differential gain, lower chirp/high spectral purity, independent of the threshold current
on temperature and a decreased a factor, QDs Lasers were intensively researched all through the
previous decade. They are suitable to be used in optical applications, microwave or millimeter
wave transmission with optical fibers and other telecom and datacom networks. However, QD
lasers were commonly regarded as only a theoretical topic which is almost impossible to be
brought to the market. The early models were based on the assumptions:
• Only one confined electron level and hole level
• Infinite barriers
• 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 quantum dot lasers to the boundary between theoretical field and commercial
Technology Trends
Although Professor Yasuhiko Arakawa of the University of Tokyo predicted that quantum dot
lasers do not rely on temperature, the theory is only valid for very low temperatures as room
temperature for a long time. Since quantum dot lasers are used in high speed applications such as
optical transmitters in metro excess optical systems and optical-LANs, a high speed quantum dot
lasers which can also operate under high temperatures without cooler are needed to be realized
[27]
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 quantum dot layers
give probabilities to tailor the spectral gain profile applied to different types of QD lasers
applications. [23]
Nevertheless, due to the intrinsic limitation of technologies, to realize quantum dot lasers with
predicted properties met several difficulties. The requirement of further widening the parameters
range in order to reducing the inhomogeneous linewidth broadening (we need homogeneous
linewidth) is one of the aspects of developing quantum dot lasers. Using surface preparation
technologies, lots of groups are working on the issue of further controlling the position and dot
size for the self-organized technology. Once the 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 increase the transport time and affects the
modulation bandwidth, it is required to decouple the carrier capture from the escape procedure.
Employing tunnel injections to quantum dots is a choice. Allowing the injection of cooled
carriers, this method is able to achieve good performance without loosing 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 fundamental transition energy with the
constant broad 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 [24], [34].
With the employment of further control of parameters and better coupling technology and the
breakthroughs which are already done, realizing quantum dot lasers as well as other quantum dot
optoelectronic devices in commercial market is not so far away.
CONCLUSION
During the previous decade, there was an intensive interest on the development of quantum dot
lasers. The unique properties of quantum dots allow QD lasers obtain several excellent properties
and performances compared to traditional lasers and even QW lasers. Although bottlenecks block
the way of realizing quantum dot lasers to commercial markets, breakthroughs in the aspects of
material and other properties will still keep the research area active in a few years. According to
the market demand and higher requirements of applications, future research directions are figured
out and needed to be realized soon