626 ii-vi quantum dot laser

8/12/2019 626 II-VI Quantum Dot Laser

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II-VI Quantum Dot Laser


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Quantum Structures

Quantum Dots

How QDs Work

Properties of Quantum Dots

LASER

Working Principle

Types of Lasers

QD Laser(II-VI)

Historical Evolution

Fabrication

Application Requirement

Bottlenecks Advantages

Applications

References


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In nanotechnology, a particle isdefined as a small object that

behaves as a whole unit in terms of

its transport and properties.

According to size:

fine particles cover a range between

100 and 2500 nm

ultrafine particles are sized between 1

and 100 nm

Nanoparticles may or may not exhibit

size-related intensive properties.


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Bulk Crystal (3D)

3 Degrees of Freedom (x-, y-, and z-axis)

Quantum Well (2D)

2 Degrees of Freedom (x-, and y-axis)

Quantum Wire (1D)

1 Degree of Freedom (x-axis)

Quantum Dot (0D) 0 Degrees of Freedom

(electron is confined in all directions)


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Comparison of the quantization of

density of states:

(a) bulk, (b) quantum well, (c) quantum wire, (d) quantum dot.

NB:- The conduction and valence bands split into overlapping subbands that

get successively narrower as the electron motion is restricted in more

dimensions.


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Non-traditional semiconductor

Crystals composed of periodic groups ofII-VI, III-V, or IV-VI materials

Range from 2-10 nanometres (10-50 atoms) in diameter

An electromagnetic radiation emitter

with an easily tunable band gap

0 degrees of freedom


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Emission frequency depends on the bandgap, therefore it ispossible to control the output wavelength of a dot withextreme precision

Small nanocrystals absorb shorter wavelengths or bluer light

Larger nanocrystals absorb longer wavelengths or redder

light The shape of the dot also changes the band gap energy level


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Quantum dot layer


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Bands and band gaps Electrons and Holes

Range of energies

Quantum confinement Exciton* Bohr Radius

Discrete energy levels

Tunable band gap The size of the band gap is

controlled simply by adjustingthe size of the dot

* Motion of electrons + holes =


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Tunable Absorption Pattern bulk semiconductors display a uniform absorption spectrum,

whereas absorption spectrum for quantum dots appears as aseries of overlapping peaks that get larger at shorter wavelengths

the wavelength of the exciton peaks is a function of thecomposition and size of the quantum dot. Smaller quantum dotsresult in a first exciton peak at shorter wavelengths

Tunable Emission Pattern the peak emission wavelength is bell-shaped (Gaussian)

the peak emission wavelength is independent of the wavelengthof the excitation light


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Quantum Yield

The percentage of absorbed photons that result in an emitted photon iscalled Quantum Yield (QY)

controlled by the existence of nonradiative transition of electrons andholes between energy levels

greatly influenced by the surface chemistry

Adding Shells to Quantum Dots

Shell =several atomic layers of an inorganic wide band semiconductor it should be of a different semiconductor material with a wider bandgap than the

Core

reduces nonradiative recombination and results in brighter emission

also neutralizes the effects of many types of surface defects


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LightAmplificationby StimulatedEmissionofRadiation.

Laser light is monochromatic, coherent, and moves in the samedirection.

A semiconductor laser is a laser in which a semiconductor serves as

a photon source.

Einsteins Photoelectric theory states that light should beunderstood as discrete lumps of energy (photons) and it takes only asingle photon with high enough energy to knock an electron loosefrom the atom it's bound to.

Stimulated, organized photon emission occurs when two electronswith the same energy and phase meet. The two photons leave withthe same frequency and direction.


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Lasing Process


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Lasers are commonly designated by the type of lasingmaterial employed: Solid-state lasershave lasing material distributed in a solid matrix

(such as the ruby or neodymium:yttrium-aluminum garnet "Yag"lasers). The neodymium-Yag laser emits infrared lightat 1,064

nanometers (nm). Gas lasers(helium and helium-neon, HeNe, are the most

common gas lasers) have a primary output of visible red light.CO2 lasers emit energy in the far-infrared, and are used forcutting hard materials.

Excimer lasers(the name is derived from the terms

excitedanddimers) use reactive gases, such as chlorine and fluorine, mixed

with inert gases such as argon, krypton or xenon. Whenelectrically stimulated, a pseudo molecule (dimer) is produced.When lased, the dimer produces light in the ultraviolet range.
http://science.howstuffworks.com/light.htmhttp://science.howstuffworks.com/light.htm


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Dye lasersuse complex organic dyes, such asrhodamine 6G, in liquid solution or suspension aslasing media. They are tunable over a broad range ofwavelengths.

Semiconductor lasers, sometimes called diode lasers,

are not solid-state lasers. These electronic devices aregenerally very small and use low power. They may bebuilt into larger arrays, such as the writing source insome laser printers or CD players.

Quantum Dot lasers use quantum dots as materials to

produce lasing action. These are low powerconsuming, tunable and have better temperaturestability.


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Core shell quantum structures

Self-assembled QDs and Stranski-

Krastanov growth

MBE (molecular beam epitaxy)

MOVPE (metalorganics vapor phaseepitaxy)

Monolayer fluctuations

Gases in remotely doped

heterostructures Schematic representation of different approaches tofabrication of nanostructures: (a) microcrystallites in

glass, (b) artificial patterning of thin film structures, (c)

self-organized growth of nanostructures


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A quantum dot laser is a semiconductor laserthat uses quantum dots as the active laser

medium in its light emitting region.

Due to the tight confinement of charge carriers inquantum dots, they exhibit an electronic

structure similar to atoms.


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Compound Semiconductor

Compound semiconductors are compounds

that show semiconductor behaviour (in

contrast to the insulating compounds

considered earlier).


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Types of Compound Semiconductor


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Why II-VI compound?

IIVI compounds are expected to be one of

the most vital materials for high-performance

optoelectronics devices.

Additionally, the high ionicity of these

compounds makes them good candidates for

high electro-optical and electromechanical

coupling.


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II-VI semiconductor colloidal Quantum Dots (QDs) are highly fluorescent nanocrystals

which are prepared through organometallic synthesis in solution phase.


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Same energy level Size, shape and alloy composition of QDs close to identical

Real concentration of energy states obtained

High density of interacting QDs Macroscopic physical parameterlight output

Reduction of nonradiative 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


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Comparison of efficiency between a QWL and a QDL

Quantum-dot laser tightly confines theelectrons and holes to produce steady

output, regardless of external

temperature


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Atomically ordered II-VI quantum dots possess their own photoluminescence (PL)

spectra.

full line: non-resonant excitation over band gap

CdSe II-VI Quantum dot


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First, the lack of uniformity.

Second, Quantum Dots density is insufficient.

Third, the lack of good coupling between QD

and QD.


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Wavelength of light determined by the energy levels not by bandgapenergy:

improved performance & increased flexibility to adjust the wavelength

Maximum material gain and differential gain

Low threshold at room temperature

High output power

Large modulation bandwidth

Superior temperature stability

Suppressed diffusion of non-equilibrium carriersReduced leakage


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QD Lasers

Microwave/Millimeter wave transmission with optical fibers

Datacom networkTelecom network

Optics


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In telecommunications they send signals for thousands of kilometers along

optical fibers.

In consumer electronics, semiconductor lasers are used to read the data on

compact disks and CD-ROMs.

For detection of gases and vapors in a smokestack.

For fiber data communication in the speed range of 100Mbps to 10Gbps.

Medical lasers are used because of their ability to produce thermal,

physical, mechanical and welding effects when exposed to tissues.

Lasers are also used by law enforcement agencies to determine the speed

and distance of the vehicles.

Lasers are used for guidance purposes in missiles, aircrafts and satellites.


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www.wikipedia.org

www.ieee.org

www.howstuffworks.com IEEE spectrum Jan 2009 Issue
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626 ii-vi quantum dot laser

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