Materialwissenschaft und Nanotechnologie. Introduction to Lasers

Transcription

1 Materialwissenschaft und Nanotechnologie Introduction to Lasers Dr. Andrés Lasagni Lehrstuhl für Funktionswerkstoffe Sommersemester Introduction to LASER Contents: Light sources LASER definition Properties of LASER light History of the LASER The wave-particle-duality Atomic transitions Inversion of energy levels Stimulated emission Types of Lasers Market for Laser Applications

2 Light sources Sun Flashlight L.A.S.E.R. Light source Sun 100 W Filament-lamp He-Ne- Laser CO Laser Pulsed Laser Light Power 10 6 Watt 3 Watt 1 mwatt 60 Watt 1 GWatt Power density 5 x 10 W/cm² 10 - W/cm² 4 x 10 4 W/cm² 5 x 10 6 W/cm² W/cm² LASER definition LIGHT AMPLIFICATION by STIMULATED EMISSION of RADIATION Licht Verstärkung durch stimulierte Emission von Strahlung

3 Monochromaticity: Properties of LASER light Nearly monochromatic light Example: He-Ne Laser λ 0 = 63.5 nm λ = 0. nm Diode Laser λ 0 = 900 nm λ = 10 nm Comparison of the wavelengths of red and blue light Directionality: Properties of LASER light Divergence angle (θ d ) ) Conventional light source Beam divergence: θ d = β λ /D β ~ 1 = f(type of light amplitude distribution, definition of beam diameter) λ = wavelenght D = beam diameter

4 Properties of LASER light Coherence: Incoherent light waves Coherent light waves Laser light cannot: be perfectly monochromatic be perfectly directional have perfect coherence Properties of laser light However Laser light is far more coherent than light from any other source.

5 History of the LASER Invented in 1958 by Charles Townes (Nobel prize in Physics 1964) and Arthur Schawlow of Bell Laboratories Was based on Einstein s idea of the particlewave duality of light, more than 30 years earlier Originally called MASER (m = microwave ) History of the LASER The first patent (1958) MASER = Microwave Amplification by Stimulated Emission of Radiation The MASER is similar to the LASER but produced only microwaves

6 Outline history of the development of the LASER Particles have a momentum The momentum can be also classified by the wavelength The wave-particle-duality Louis de Broglie(193) : λ = h/ m v = h/p

7 Energy is quantized To raise an electron from one energy level to another, input energy is required When falling from one energy level to another, there will be an energy output given by the Planck s law Atomic transitions Almost all electronic transitions that occur in atoms that involve photons fall into one of three categories: Stimulated absorption

8 Atomic transitions Spontaneous emission Energy of the emitted photon = Stimulated emission 1 Photon with produces two photons with the same energy Atomic transitions The frequency of the emitted photon going from Level to 1 is given by: E E1 ν = h Defining Ni as the electron population of level i and considering the Boltzman equation which describes the relation between the electrons in level 1 and at thermal equilibrium: E E N = N1 exp kbt 1 Level E E = ν h Level 1 (k B = Boltzman constant) 1 giving that E > E 1 and T>0 => N 1 >N!

9 Atomic transitions To amplify light, the stimulated emission must by stronger than the absorption (N >N 1 ) but how is this possible??? An electromagnetic wave with frequency ν traveling in z-direction through the media with atom levels is normally exponentially absorbed: ( z) I = I exp µ µ > 0 is the lineal Absorption coefficient. It can be demonstrated that µ N 1 -N > 0 The amplification of light is only possible if N 1 -N < 0 (=> µ < 0) 0 That means THE MEDIA IS ACTIVE! Inversion of energy levels E Population density (N) E E N = N1 exp kbt N N 0 1 < 1? E 3 N = N N1 > Population density (N) E = hγ = hc / λ

10 Inversion of energy levels One solution to this problem is to use three energy levels (example He-Ne laser): E 3 τ 3 Level 3 relaxation τ >τ τ τ 1 E ( N > ) 1 N τ 3 long short Level N > N 1!!! meta-stable level Pumping electrons to E 3 Laser light τ 0 τ 1 Level 0 E ( N 1 1) Helium Level 1 Neon Stimulated emission Now N > N 1, however we must amplify the intensity of our beam! We must build a LASER resonator! Mirror 1 Mirror However, with this set-up the intensity will grow up to infinite! What can we do to obtain the LASER beam?

11 Elements of a laser Totally reflecting mirror R = 100 % Excitation mechanism Partially reflecting mirror R < 100 % Active medium Laser beam Excitation mechanism Laser optical cavity Excitation mechanisms Optical pumping (flash lamps or other lasers) Generally used with dye-lasers and solid-state lasers Cylindrical quartz tubes with metal electrodes mounted on the ends, filled with a gaseous species A voltage is applied across the electrodes of the flashlamp current flows through the gas produced populating excited levels of the atoms intense light emission. The process is similar to that of electron excitation of lasers except that a population inversion is not produced and the radiating material of the lamp radiates via spontaneous emission

12 Active medium Atoms: helium-neon (HeNe) laser; heliumcadmium (HeCd) laser, copper vapor lasers (CVL) Molecules: carbon dioxide (CO) laser, ArF and KrF excimer lasers, N laser Liquids: organic dye molecules dilutely dissolved in various solvent solutions Dielectric solids: neodymium atoms doped in YAG or glass to make the crystalline Nd:YAG or Nd:glass lasers Semiconductor materials: gallium arsenide, indium phosphide crystals. Energy balance Energy balance in an optically pumped solidstate laser system. The percentages are fractions of the electrical energy supplied to the lamp

13 Types of Lasers Gas Lasers Solid State Lasers Semiconductor Lasers Excimer Lasers etc etc Solid State Lasers Nd:YAG Laser: λ = µm YAG = Yttrium- Aluminium-Garnet (Y 3 Al 5 O 1 ), it is transparent and colourless. Nd:YAG Laser is doped with about 1% Nd3+ ions into the YAG crystal. The crystal color then changed to a light blue color.

14 Solid State Lasers Ruby Laser: λ = nm First laser invented (1960). Ruby: Al O 3 in which some of the Al atoms have been replaced with Cr. Cr gives its characteristic red color and is responsible for the lasing behavior of the crystal. Cr atoms absorb green and blue light and emit or reflect only red light. Laser technological processes Micromachining Laser power flux (W/cm ) Liquid Shock processing Vapor Drilling Melting Vaporization Cutting Welding Vaporization and melting Melting Heating Interaction time (s) Stereolithography 10-1

15 Worldwide nondiode-laser sales by application Methods of cutting The gas is needed both to aid the cutting operation and to protect the optic from spatter