A Novel Approach

AAKASH GUPTAUE5501

B.E. (E.C.E.) 8TH SEMESTER

Quantum-dot laser tightly confines the electrons and holes to produce steady output, regardless of external temperature.

I will discuss quantum structures, laser and lasing action and use of quantum dots in lasers.

Quantum Structures Quantum Dots

How QDs Work Properties of Quantum Dots

LASER Working Principle Types of Lasers

QD Laser Historical Evolution Fabrication Application Requirement Bottlenecks Advantages

Applications References

In nanotechnology, a particle is defined 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.

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)

Non-traditional semiconductor

Crystals composed of periodic groups of II-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

Emission frequency depends on the bandgap, therefore it is possible to control the output wavelength of a dot with extreme 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

Quantum dot layer

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 adjusting the size of the dot

* Motion of electrons + holes = excitons

Tunable Absorption Pattern bulk semiconductors display a uniform absorption

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

the wavelength of the exciton peaks is a function of the composition and size of the quantum dot. Smaller quantum dots result 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 wavelength of the excitation light

Quantum Yield The percentage of absorbed photons that result in an

emitted photon is called Quantum Yield (QY) controlled by the existence of nonradiative transition of

electrons and holes 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

Light Amplification by Stimulated Emission of Radiation.

Laser light is monochromatic, coherent, and moves in the same direction.

A semiconductor laser is a laser in which a semiconductor serves as a photon source.

Einstein’s Photoelectric theory states that light should be understood as discrete lumps of energy (photons) and it takes only a single photon with high enough energy to knock an electron loose from the atom it's bound to.

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

Lasing Process

Lasers are commonly designated by the type of lasing material employed: Solid-state lasers have lasing material distributed in a

solid matrix (such as the ruby or neodymium:yttrium-aluminum garnet "Yag" lasers). The neodymium-Yag laser emits infrared light at 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 for cutting hard materials.

Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton or xenon. When electrically stimulated, a pseudo molecule (dimer) is produced. When lased, the dimer produces light in the ultraviolet range.

Dye lasers use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.

Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic devices are generally very small and use low power. They may be built into larger arrays, such as the writing source in some laser printers or CD players.

Quantum Dot lasers use quantum dots as materials to produce lasing action. These are low power consuming, tunable and have better temperature stability.

Materials for semiconductor lasers

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

A quantum dot laser is a semiconductor laser that uses quantum dots as the active laser medium in its light emitting region. Due to the tight confinement of charge carriers in

quantum dots, they exhibit an electronic structure similar to atoms.

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)

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 parameter light 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

First, the lack of uniformity.

Second, Quantum Dots density is insufficient.

Third, the lack of good coupling between QD and QD.

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

Low threshold at room temperature

High output power

Large modulation bandwidth

Superior temperature stability

Suppressed diffusion of non-equilibrium carriers

Reduced leakage

QD Lasers

Microwave/Millimeter wave transmission with optical fibers

Datacom network

Telecom network

Optics

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.

www.wikipedia.org www.ieee.org www.howstuffworks.com IEEE spectrum Jan 2009 Issue