lECTURE 5 Semiconductor,Excimer

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The various types of laser.

Transcript of lECTURE 5 Semiconductor,Excimer


    PH 0101 UNIT-3 LECTURE 5

    PH 0101 UNIT 3 Lecture 5

  • Semiconductor(Ga-As) lasersIntroduction

    The semiconductor laser is today one of the most important types of lasers with its very important application in fiber optic communication.

    These lasers use semiconductors as the lasing medium and are characterized by specific advantages such as the capability of direct modulation in the gigahertz region, small size and low cost.

    PH 0101 UNIT 3 Lecture 5

  • Basic Mechanism

    The basic mechanism responsible for light emission from a semiconductor is the recombination of electrons and holes at a p-n junction when a current is passed through a diode. There can be three interaction processes

    1)An electron in the valence band can absorb the incident radiation and be excited to the conduction band leading to the generation of eletron-hole pair.

    PH 0101 UNIT 3 Lecture 5

  • 2) An electron can make a spontaneous transition in which it combines with a hole and in the process it emits radiation3) A stimulated emission may occur in which the incident radiation stimulates an electron in the conduction band to make a transition to the valence band and in the process emit radiation. To convert the amplifying medium into a laserOptical feedback should be providedDone by cleaving or polishing the ends of the p-n junction diode at right angles to the junction.

    PH 0101 UNIT 3 Lecture 5

  • When a current is passed through a p-n junction under forward bias, the injected electrons and holes will increase the density of electrons in the conduction band.

    The stimulated emission rate will exceed the absorption rate and amplification will occur at some value of current due to holes in valence band.

    As the current is further increased, at threshold value of the current, the amplification will overcome the losses in the cavity and the laser will begin to emit coherent radiation.

    PH 0101 UNIT 3 Lecture 5

  • Simple structure (Homojunction) The basic semiconductor laser structure in which the photons generated by the injection current travel to the edge mirrors and are reflected back into the active area. Photoelectron collisions take place and produce more photons, which continue to bounce back and forth between the two edge mirrors. This process eventually increases the number of generated photons until lasing takes place. The lasing will take place at particular wavelengths that are related to the length of the cavity.

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 5

  • Laser heterostructure (a) Schematic projection (b) Refractive index profile

    PH 0101 UNIT 3 Lecture 5

  • HeterostructuresThe heterostructure laser is a laser diode with more than single P and N layers.

    GaAs/AlGaAs is a heterojunction laser.

    The notations P+ and N+ and P- and N- indicate heavy doping and light doping respectively. The P-N structure consists of the two double layers, P+ - P- and N+ - N- . A thin layer of GaAs is placed at the junction-the active region.

    PH 0101 UNIT 3 Lecture 5

  • The P and N regions are lightly doped regions that have an index of refraction n2 less than n1 of the active region. These three layers, n2-n1-n2, form a light waveguide much like the optical fiber, so that the light generated is confined to the active region.Applications:


    CD players

    Pumping solid state lasers.

    PH 0101 UNIT 3 Lecture 5

  • Excimer laserSome diatomic molecules are unstable in the ground stateThey form excited Dimers which are stable.Eg. Xe2*,Kr2*,Ar2*Excited Dimers-ExcimerExcited Complex- Exciplex

    PH 0101 UNIT 3 Lecture 5

  • The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) disassociates back into two unbound atoms.

    This forms a population inversion between the two states.

    Most "excimer" lasers are of the noble gas halide type The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet region

    PH 0101 UNIT 3 Lecture 5

  • Excimer Wavelength Excimer lasers are usually operated with a pulse rate of around 100 Hz -8 kHz and a pulse duration of ~10-30 nsEnergy from 0.1-1J

    ExcimerWavelengthArF193 nmKrF248 nmXeBr282 nmXeCl308 nmXeF351 nmCaF2193 nmKrCl222 nmCl2259 nmN2337 nm

    PH 0101 UNIT 3 Lecture 5

  • Construction:SS structure-Halogens are corrosive

    Discharge tube to excite gas to form excited dimer

    Highly reflecting mirror and quartz output coupler

    PH 0101 UNIT 3 Lecture 5

  • Typical excimer laser configuration

    PH 0101 UNIT 3 Lecture 5

  • Applications Material processing

    Medical applications Laser surgery Corneal sculpting Dermatology

    Lithography- Microchip as small as 0.18-0.25 mm

    Pumping other lasers

    PH 0101 UNIT 3 Lecture 5

  • Free electron laserIntroductionInvented by J.M.J. Madey in 1971.

    Combination of particle-accelerator and laser physics .

    Electrons moving at relativistic velocity acts as a laser source

    Provide wide tunability - microwave,UV,X-ray

    High output power

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 5

  • Free electron laser

    Electrons accelerated to relativistic speeds.

    Passes through undulator/Wiggler which produces ocillating field

    e assume a sinusoidal path

    Accelerated electron emits radiation

    The radiation emitted is in phase

    Mirrors used for feedback

    Frequency of laser depends on the energy of e and period of alternating magnetic field

    PH 0101 UNIT 3 Lecture 5

  • Applications Medical



    Material processing

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 6*

    Applications of Laser


    Applications of holography

    PH 0101 UNIT-3 LECTURE 6

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 6*Industrial applicationsDrilling

    Use single or multiple pulses of the laser-percussion drilling

    Laser focussed to a small spot

    Holes of sized 10mm to 1mm can be made

    Inert gas is blown to remove vaporized material

    Used to make holes in plastic, diamonds and aircraft engines

    High power pulsed or CW lasers are used.CO2, Nd:YAG or Ruby lasers

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 6*Industrial applications2. Cutting

    Uses CW or pulsed CO2 laser

    A gas jet used to assist in cutting

    SS of 5mm thick with 500 W lasers

    Advantages- Precision,speed,special materials, lack of tool contact

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 6*Industrial applications3. Welding

    Uses CW CO2 or pulsed YAG laser

    High speed and low distortion

    Seam weld using CO2 and spot weld using Nd:YAG

    Inert gas used for cooling and prevent oxidation

    4. Heat treatment: Surface hardnening of steel

    5. Cladding Coating layers of different material on the surface

    PH 0101 UNIT 3 Lecture 5

  • PH 0101 UNIT 3 Lecture 6*Medical applications1. Photodynamic therapy

    Destroys malignant tumors

    Chemical agents accumulate at the tumor cells

    Laser triggers photochemical reaction destroying the cells2. Dermatology (low power IR)3. Dentistry- root canal treatment, remove cavities4. Gastric tumors- Nd:YAG

    PH 0101 UNIT 3 Lecture 5

  • *Medical applications5. Ophthamology

    Reattach tissues in the eye-Ar ion laser

    Treatment of cataract

    Excimer laser used for cutting the cornea6. Surgery-cardiovascular, remove lesions or clots

    CommunicationsOptical frequency= 1015 Hz

    Larger bandwidth

    Used in space communication

    PH 0101 UNIT 3 Lecture 5