1. Quantum Dot Lasers Betul Arda Huizi Diwu ECE 580 – Term Project
Department of Electrical
and Computer Engineering
University of Rochester

2. Outline Quantum Dots (QD) Quantum Dot Lasers (QDL)
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

3. Quantum Dots (QD) Semiconductor nanostructures Unique tunability
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

4. 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

5. 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

6. QD Lasers – Historical Evolution

7. 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

8. QDL – Basic characteristics
Components of a laser
An energy pump source
electric power supply
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

9. 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)

10. 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

11. 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)

12. Q. Dot Laser vs. Q. Well Laser
Comparison of efficiency: QWL vs. QDL

13. Market demand of QD lasers
Microwave/Millimeter wave transmission with optical fibers
QD Lasers
Datacom network
Telecom network
Optics

14. Market demand of QD lasers
Earlier QD Laser Models
Updated QD Laser Models
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
Before and after self-assembling technology

15. 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

16. Comparison QD lasers for Coolerless Pump Sources
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

17. Bottlenecks First, the lack of uniformity.
Second, Quantum Dots density is insufficient.
Third, the lack of good coupling between QD and QD.

18. 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

19. 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,
France
2006

20. 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

21. Future Directions Widening parameters rangeto
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
using by
In term of

22. 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.

23. Thank you!
Q & A