quantum dot diwu

8/8/2019 Quantum Dot Diwu

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ECE 580 Term ProjectBetul ArdaHuizi Diwu

Department of Electricaland Computer Engineering

University of Rochester

Quantum Dot Lasers


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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


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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


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Quantum Confinement Effect

E = Eq1 + Eq2 + Eq3, Eqn = h2(q1/dn)2 / 2mc

Quantizationof density of states: (a) bulk (b) quantumwell (c) quantumwire(d) QD


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QD Fabrication Techniques

Core shell quantumstructures

Self-assembled QDs

and Stranski-Krastanov growth MBE (molecular beam

epitaxy) MOVPE

(metalorganics vaporphase 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


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QD Lasers Historical Evolution


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QDL Predicted Advantages

Wavelength of light determined by the energy levels not bybandgap 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 Ithreshold

Ithreshold

(T) = Ithreshold

(Tref).exp ((T-(T

ref))/ (T

0))

High T0 decoupling electron-phonon interaction by increasing the

intersubband separation. Undiminished room-temperature performance without external thermal

stabilization

Suppressed diffusion of non-equilibrium carriers Reducedleakage


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QDL Basic characteristics

An active medium tocreate populationinversion by pumping

mechanism: photons at some sitestimulate emission atother sites whiletraveling

Two reflectors: to reflect the light inphase

multipass amplification

Components of a laser

An energy pump source electric power supply


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QDL Basic characteristics

An ideal QDL consists of a 3D-array of dots withequal 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)


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QDL Application Requirements Same energy level

Size, shape and alloy composition of QDs closeto identical

Inhomogeneous broadening eliminated realconcentration 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 isincurred Electrical control

Electric field applied can change physicalproperties of QDs

Carriers can be injected to create light emission


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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 providingalternate nonradiative defect channels

The phonon bottleneck created by confinement

limits the number of states that are efficientlycoupled 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 thatbottleneck effect (e.g. Auger interactions)


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Q. Dot Laser vs. Q. Well Laser

Comparison of efficiency: QWL vs. QDL


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Market demand of QD lasers

QD Lasers

Microwave/Millimeter wave transmission with optical fibers

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rk

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Optics


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Market demand of QD lasers

Only one confinedelectron level andhole level

Infinite barriers Equilibrium carrier

distribution Lattice matched

heterostructures

Lots of electronlevels and holelevels

Finite barriers Non-equilibrium

carrier distribution Strained

heterostructures

Earlier QD Laser Models Updated QD Laser Models

Before and after self-assembling technology


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Comparison

High speed quantum

dot lasers

Advantages

Directly Modulated QuantumDot Lasers

Datacom application

Rate of 10Gb/sMode-Locked Quantum DotLasers

Short optical pulsesNarrow spectral widthBroad gain spectrumVery low factor-low chirp

InP Based Quantum Dot Lasers Low emission wavelengthWide temperature rangeUsed for data transmission


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Comparison

High power Quantum

Dot lasers

Advantages

QD lasers forCoolerless PumpSources

Size reducedquantum dot

Single Mode Tapered

Lasers

Small wave length

shiftTemperatureinsensitivity


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Bottlenecks

First, the lack of uniformity. Second, Quantum Dots density is

insufficient. Third, the lack of good couplingbetween QD and QD.


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Breakthroughs

FujitsuTemperature Independent QD laser2004

Temperature dependence of light-current characteristics Modulation waveform at 10Bbps at 20C and 70 C with no current adjustment


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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 IIIV Laboratory,France2006


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Commercialization

Zia Laser's quantum-dot laser structures comprise an active region that lookslike a quantum well, but is actually a layer of pyramid-shaped indium-arsenidedots. Each pyramid measures 200 along its base, and is 7090 high. About100 billion dots in total would be needed to fill an area of one square

centimeter. -----www.fibers.org
http://www.fibers.org/http://www.fibers.org/


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Future Directions Wideningparameters range

Further controllingthe position anddot size

Decouple thecarrier capturefrom the escapeprocedure

Combination of QDlasers and QWlasers

Reduce inhomogeneouslinewidth broadening

Surface PreparationTechnology

Allowing the injection ofcooled carriers

Raised gain at thefundamental transitionenergy

using

by

In term of

to


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Conclusion

During the previous decade, there was anintensive interest on the development of quantumdot lasers. The unique properties of quantum dotsallow QD lasers obtain several excellentproperties and performances compared to

traditional lasers and even QW lasers.

Although bottlenecks block the way of realizingquantum dot lasers to commercial markets,breakthroughs in the aspects of material andother properties will still keep the research areaactive in a few years. According to the marketdemand and higher requirements of applications,future research directions are figured out andneeded to be realized soon.


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Thank you!

quantum dot diwu

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