ECE 580 – Term Project
Betul ArdaHuizi Diwu
Department of Electrical and Computer Engineering
University of Rochester
Quantum Dot Lasers
Outline Quantum Dots (QD)
Confinement Effect Fabrication Techniques
Quantum Dot Lasers (QDL) 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)
Semiconductor nanostructures Size: ~2-10 nm or ~10-50 atoms
in diameter Unique tunability Motion of electrons + holes = excitons Confinement of motion can be created by:
Electrostatic potential e.g. in e.g. doping, strain, impurities,
external electrodes the presence of an interface between different
semiconductor materials e.g. in the case of self-assembled QDs
the presence of the semiconductor surface e.g. in the case of a semiconductor nanocrystal
or by a combination of these
Quantum Confinement Effect
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
Core shell quantum structures
Self-assembled QDs and Stranski-Krastanov growth MBE (molecular beam
epitaxy) MOVPE
(metalorganics vapor phase epitaxy)
Monolayer fluctuations Gases in remotely
doped heterostructures
Schematic representation of different approaches to fabrication of nanostructures: (a) microcrystallites in glass, (b) artificial patterning of thin film structures, (c) self-organized growth of nanostructures
QD Lasers – Historical Evolution
QDL – Predicted Advantages Wavelength of light determined by the energy levels not by
bandgap energy: improved performance & increased flexibility to adjust the
wavelength Maximum material gain and differential gain Small volume:
low power high frequency operation large modulation bandwidth small dynamic chirp small linewidth enhancement factor low threshold current
Superior temperature stability of I threshold
I threshold (T) = I 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
An active medium to create population inversion by pumping mechanism: photons at some site
stimulate emission at other sites while traveling
Two reflectors: to reflect the light in
phase multipass amplification
Components of a laser
An energy pump source electric power supply
QDL – Basic characteristics
An ideal QDL consists of a 3D-array of dots with equal size and shape
Surrounded by a higher band-gap material confines the injected carriers.
Embedded in an optical waveguide Consists lower and upper cladding layers (n-doped
and p-doped shields)
QDL – Application Requirements Same energy level
Size, shape and alloy composition of QDs close to identical
Inhomogeneous broadening eliminated real concentration of energy states obtained
High density of interacting QDs Macroscopic physical parameter light output
Reduction of non-radiative centers Nanostructures made by high-energy beam
patterning cannot be used since damage is incurred
Electrical control 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
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 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
Comparison of efficiency: QWL vs. QDL
Market demand of QD lasers
QD Lasers
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Market demand of QD lasers
Only one confined electron level and hole level
Infinite barriers Equilibrium carrier
distribution Lattice matched
heterostructures
Lots of electron levels and hole levels
Finite barriers Non-equilibrium
carrier distribution Strained
heterostructures
Earlier QD Laser Models Updated QD Laser Models
Before and after self-assembling technology
Comparison
High speed quantum dot lasers
Advantages
Directly Modulated Quantum Dot Lasers
•Datacom application•Rate of 10Gb/s
Mode-Locked Quantum Dot Lasers
•Short optical pulses•Narrow spectral width•Broad gain spectrum•Very low α factor-low chirp
InP Based Quantum Dot Lasers
•Low emission wavelength•Wide temperature range•Used for data transmission
Comparison
High power Quantum Dot lasers
Advantages
QD lasers for Coolerless Pump Sources
•Size reduced quantum dot
Single Mode Tapered Lasers
•Small wave length shift•Temperature insensitivity
Bottlenecks
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 laser2004
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
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, France2006
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 Widening
parameters range
Further controlling the position and dot size
Decouple the carrier capture from the escape procedure
Combination of QD lasers and QW lasers
Reduce inhomogeneous linewidth broadening
Surface Preparation Technology
Allowing the injection of cooled carriers
Raised gain at the fundamental transition energy
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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.
Thank you!