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Quantum Dot Lasers. ECE 580 – Term Project Betul Arda Huizi Diwu Department of Electrical and Computer Engineering University of Rochester. Outline. Quantum Dots (QD) Confinement Effect Fabrication Techniques Quantum Dot Lasers (QDL) Historical Evolution Predicted Advantages

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Quantum Dot Lasers

ECE 580 – Term Project

Betul Arda

Huizi Diwu

Department of Electrical

and Computer Engineering

University of Rochester

outline
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
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
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
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

qdl predicted advantages
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
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 characteristics9
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
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
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 laser12
Q. Dot Laser vs. Q. Well Laser
  • Comparison of efficiency: QWL vs. QDL
market demand of qd lasers
Market demand of QD lasers

Microwave/Millimeter wave transmission with optical fibers

QD Lasers

Datacom network

Telecom network

Optics

market demand of qd lasers14
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

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

breakthroughs19
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

commercialization
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

Future Directions

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conclusion
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.