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Quantum Dots (QD) Confinement Effect
Quantum Dot Lasers (QDL) Historical Evolution Predicted Advantages Basic Characteristics Application Requirements
Q. Dot Lasers vs. Q. Well Lasers 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
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(qqnπ/dn)2 / 2mc
Quantization of density of states: (a) bulk (b) quantum well (c) quantum wire (d) QD
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 obtainedHigh density of interacting QDs
Macroscopic physical parameter light outputReduction 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
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
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.
The early models were based on the assumptions:
Only one confined electron level and hole level Infinite barriers Equilibrium carrier distribution Lattice matched heterostructures
Breakthroughs
Temperature dependence of light-current characteristics
Modulation waveform at 10Gbps at 20°C and 70 °C with no current adjustment
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!