A Novel Approach AAKASH GUPTA UE5501 B.E. (E.C.E.) 8 TH 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).light  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.

    IEEE spectrum Jan 2009 Issue