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!