LASER. Light Amplification by Stimulated Emission of Radiation

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2 LASER Light Amplification by Stimulated Emission of Radiation

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5 Energy Level, Definitions The valence band is the highest filled band The conduction band is the next higher empty band The energy gap has an energy, E g, equal to the difference in energy between the top of the valence band and the bottom of the conduction band

6 Conductors When a voltage is applied to a conductor, the electrons accelerate and gain energy In quantum terms, electron energies increase if there are a high number of unoccupied energy levels for the electron to jump to It takes very little energy for electrons to jump from the partially filled to one of the nearby empty states

7 Insulators The valence band is completely full of electrons A large band gap separates the valence and conduction bands A large amount of energy is needed for an electron to be able to jump from the valence to the conduction band The minimum required energy is E g

8 Semiconductors Electrons A semiconductor has a small energy gap Thermally excited electrons have enough energy to cross the band gap The resistivity of semiconductors decreases with increases in temperature The white area in the valence band represents holes Holes

9 Atomic Transitions Stimulated The blue dots represent electrons When a photon with energy ΔE is absorbed, one electron jumps to a higher energy level These higher levels are called excited states ΔE = hƒ = E 2 E 1 In general, ΔE can be the difference between any two energy levels Absorption

10 Atomic Transitions Spontaneous Emission Once an atom is in an excited state, there is a constant probability that it will jump back to a lower state by emitting a photon This process is called spontaneous emission

11 Atomic Transitions Stimulated Emission An atom is in an excited state and a photon is incident on it The incoming photon increases the probability that the excited atom will return to the ground state There are two identical emitted photons, the incident one and the emitted one The emitted photon is exactly in phase with the incident photon

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13 Absorption and Spontaneous Emission Consider a two-level system E 2, N 2 E 2, N 2 absorption emission E 1, N 1 E 1, N 1 Light from bulbs are due to spontaneous emission

14 Absorption and Stimulated Emission E 2, N 2 E 2, N 2 absorption stimulated emission E 1, N 1 E 1, N 1 Laser light results from stimulated emission

15 Two-Level System E 2, N 2 E 2, N 2 E 1, N 1 E 1, N 1 Even with very a intense pump source, the best one can achieve with a two-level system is excited state population = ground state population

16 Three-Level System The first laser, the ruby laser, was a three-level system upper lasing state lower lasing state Laser light due to transition from 2 E state to 4 A 2 state

17 He-Ne laser Four-Level System

18 Four-Level System Nd:YAG laser upper laser state lower laser state Laser light due to transition from 4 F to 4 I

19 Dye Lasers: Four-level systems

20 efficient pumping slow relaxation Requirements for Laser Action fast Metastable state slow Population inversion Fast relaxation

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22 Laser Conditions and Requirements A) Stimulated emission B) Population inversion (more stimulated emission than absorption) Excited state: full Ground state: empty Stimulated emission C) Cavity (to gain the stimulated emission)

23 Light Amplification Create a laser cavity, which consists of the lasing medium and two highly reflective mirrors.

24 How to build a Laser? Pumping Active medium Resonator

25 How to build a Laser? (a) Pumping (=energy input) (b) Cavity to make the pumping effective enough (c) Laser emission due to population inversion

26 Laser Beam Ruby Example (a) (b)

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29 Al2O3 (Cr2O3) Xe.1.2 Neodymium YAG (Y3AL5O12)

30 ليسر هليوم-نئون He - Ne ليسر گبز كربنيك Co2 N2 He ليسر اگسايمر Eximer

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32 Transverse laser modes

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35 Dye Tuning Curves Dye lasers - tunable laser radiation

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37 1. Photochemical interaction 2. Thermal interaction : Hyperthermia, Coagulation, Vaporization, Melting 3. Photo ablation 4. Photo disruption

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