QUANTUM DOT LASER

1
QUANTUM DOT LASER

• A Novel Approach

AAKASH GUPTA UE5501 B.E. (E.C.E.) 8TH SEMESTER
2
Overview

• 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.

3
Contents

• 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

4
Quantum Structures

• 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.

5
Bulk Crystal (3D) ? 3 Degrees of Freedom (x-,
y-, and z-axis)
Quantum Well (2D) ? 2 Degrees of Freedom (x-,
and y-axis)
Quantum Dot (0D) ? 0 Degrees of Freedom
(electron is confined in all directions)
Quantum Wire (1D) ? 1 Degree of Freedom (x-axis)
6
Quantum Dots

• 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

7

• 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

8

• Quantum dot layer

9
How Quantum Dots Work

• 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
10
Properties of Quantum Dots

• 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

11

• 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

12
LASER

• 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.
• Einsteins 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.

13

• Lasing Process

14
(No Transcript)
15
Types of LASERS

• 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 neodymiumyttrium-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.

16

• 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.

17

• Materials for semiconductor lasers

18
QD Lasers Historical Evolution
19
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
20
Quantum Dot LASER

• 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.

21

• 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)

22
QDL Application Requirements

• 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
  

23
Bottlenecks

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

24
QD Laser 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
• Low threshold at room temperature
• High output power
• Large modulation bandwidth
• Superior temperature stability
• Suppressed diffusion of non-equilibrium carriers
  ? Reduced leakage

25
Market demand of QD lasers
Microwave/Millimeter wave transmission with
optical fibers
QD Lasers
Datacom network
Telecom network
Optics
26
APPLICATIONS

• 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.

27
References

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