Transcript Quantum Dot Lasers - Department of Electrical and Computer

Quantum Dot Lasers
ECE 580 – Term Project
Betul Arda
Huizi Diwu
Department of Electrical
and Computer Engineering
University of Rochester
Outline
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Quantum Dots (QD)
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Quantum Dot Lasers (QDL)
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Confinement Effect
Fabrication Techniques
Historical Evolution
Predicted Advantages
Basic Characteristics
Application Requirements
Q. Dot Lasers vs. Q. Well Lasers
Market demand of QDLs
Comparison of different types of QDLs
Bottlenecks
Breakthroughs
Future Directions
Conclusion
Quantum Dots (QD)
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Semiconductor nanostructures
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Size: ~2-10 nm or ~10-50 atoms
in diameter
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Unique tunability
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Motion of electrons + holes = excitons
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Confinement of motion can be created by:
 Electrostatic potential
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the presence of an interface between different
semiconductor materials
 e.g. in the case of self-assembled QDs
the presence of the semiconductor surface
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e.g. in e.g. doping, strain, impurities,
external electrodes
e.g. in the case of a semiconductor nanocrystal
or by a combination of these
Quantum Confinement Effect
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E = Eq1 + Eq2 + Eq3, Eqn = h2(q1π/dn)2 / 2mc
Quantization of density of states: (a) bulk (b) quantum well (c) quantum wire (d) QD
QD – Fabrication Techniques
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Core shell quantum
structures
Self-assembled QDs
and StranskiKrastanov growth
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MBE (molecular beam
epitaxy)
MOVPE
(metalorganics vapor
phase epitaxy)
Monolayer fluctuations
representation of different approaches to
Gases in remotely Schematic
fabrication of nanostructures: (a) microcrystallites in
glass, (b) artificial patterning of thin film structures, (c)
doped
heterostructures self-organized growth of nanostructures
QD Lasers – Historical Evolution
QDL – Predicted Advantages
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Wavelength of light determined by the energy levels not by
bandgap energy:
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improved performance & increased flexibility to adjust the
wavelength
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Maximum material gain and differential gain
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Small volume:
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low power high frequency operation
large modulation bandwidth
small dynamic chirp
small linewidth enhancement factor
low threshold current
Superior temperature stability of I
I
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threshold
(T) = I
threshold
threshold
(T ref ).exp ((T-(T ref ))/ (T 0))
High T 0  decoupling electron-phonon interaction by increasing the
intersubband separation.
Undiminished room-temperature performance without external thermal
stabilization
Suppressed diffusion of non-equilibrium carriers  Reduced
leakage
QDL – Basic characteristics
Components of a laser
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An energy pump source
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electric power supply
An active medium to
create population
inversion by pumping
mechanism:
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photons at some site
stimulate emission at
other sites while
traveling
Two reflectors:
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to reflect the light in
phase
multipass amplification
QDL – Basic characteristics
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An ideal QDL consists of a 3D-array of dots with
equal size and shape
Surrounded by a higher band-gap material
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confines the injected carriers.
Embedded in an optical waveguide
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Consists lower and upper cladding layers (n-doped
and p-doped shields)
QDL – Application Requirements
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Same energy level
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High density of interacting QDs
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Macroscopic physical parameter  light output
Reduction of non-radiative centers
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Size, shape and alloy composition of QDs close
to identical
Inhomogeneous broadening eliminated  real
concentration of energy states obtained
Nanostructures made by high-energy beam
patterning cannot be used since damage is
incurred
Electrical control
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Electric field applied can change physical
properties of QDs
Carriers can be injected to create light emission
Q. Dot Laser vs. Q. Well Laser
In order for QD lasers compete with QW lasers:
 A large array of QDs since their active volume is
small
 An array with a narrow size distribution has to be
produced to reduce inhomogeneous broadening
 Array has to be without defects
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may degrade the optical emission by providing
alternate nonradiative defect channels
The phonon bottleneck created by confinement
limits the number of states that are efficiently
coupled by phonons due to energy conservation
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Limits the relaxation of excited carriers into lasing
states
Causes degradation of stimulated emission
Other mechanisms can be used to suppress that
bottleneck effect (e.g. Auger interactions)
Q. Dot Laser vs. Q. Well Laser
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Comparison of efficiency: QWL vs. QDL
Market demand of QD lasers
Microwave/Millimeter wave transmission with optical fibers
Optics
Datacom network
Telecom network
QD Lasers
Market demand of QD lasers
Earlier QD Laser Models
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Only one confined
electron level and
hole level
Infinite barriers
Equilibrium carrier
distribution
Lattice matched
heterostructures
Updated QD Laser Models
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Lots of electron
levels and hole
levels
Finite barriers
Non-equilibrium
carrier distribution
Strained
heterostructures
Before and after self-assembling technology
Comparison
High speed
quantum dot lasers
Advantages
Directly Modulated Quantum
Dot Lasers
•Datacom
Mode-Locked Quantum Dot
Lasers
•Short
InP Based Quantum Dot
Lasers
•Low
application
•Rate of 10Gb/s
optical pulses
•Narrow spectral width
•Broad gain spectrum
•Very low α factor-low chirp
emission wavelength
•Wide temperature range
•Used for data transmission
Comparison
High power
Advantages
Quantum Dot lasers
QD lasers for
Coolerless Pump
Sources
•Size
reduced
quantum dot
Single Mode Tapered
Lasers
•Small
wave length
shift
•Temperature
insensitivity
Bottlenecks
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First, the lack of uniformity.
Second, Quantum Dots density is
insufficient.
Third, the lack of good coupling
between QD and QD.
Breakthroughs
Fujitsu
Temperature Independent QD laser
2004
Temperature dependence of light-current characteristics
Modulation waveform at 10Bbps at 20°C and 70 °C with no current adjustment
Breakthroughs
InP instead of GaAs
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Can operate on ground state for much shorter cavity
length
High T0 is achieved
First buried DFB DWELL operating at 10Gb/s in
1.55um range
Surprising narrow linewidth-brings a good phase
noise and time-jitter when the laser is actively mode
locked
Alcatel Thales III–V Laboratory,
France
2006
Commercialization
Zia Laser's quantum-dot laser structures comprise an active region that looks
like a quantum well, but is actually a layer of pyramid-shaped indium-arsenide
dots. Each pyramid measures 200 Å along its base, and is 70–90 Å high. About
100 billion dots in total would be needed to fill an area of one square
centimeter. -----www.fibers.org
Future Directions
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Widening
parameters range
to
Further controlling
the position and
dot size
using
Decouple the
carrier capture
from the escape
procedure
Combination of QD
lasers and QW
lasers
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Reduce inhomogeneous
linewidth broadening
Surface Preparation
Technology
by
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Allowing the injection of
cooled carriers
In term of
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Raised gain at the
fundamental transition
energy
Conclusion
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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.
Thank you!