ECE 580 – Term Project Betul Arda Huizi Diwu Department of Electrical and Computer Engineering University of Rochester

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

2. 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

3. 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

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 • 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

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

15. Comparison

16. Comparison

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. -----www.fibers.org

21. 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 Future Directions to 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