2. Atomic model

3. MIT 2.71/2.710 Optics 10/20/04 wk7-b-21 Semi-classical view of atom excitations Energy Atom in ground state Atom in excited state

4. Absorption

6. Spontaneous emission

7. Stimulated emission

9. E1E1 E2E2 n 1 - the number of electrons of energy E 1 n 2 - the number of electrons of energy E 2 Boltzmann’s equation example: T=3000 K E 2 -E 1 =2.0 eV

10. Einstein’s theory of spontaneous and stimulated emission

11. Einstein’s coefficients Probability of stimulated absorption R 1-2 R 1-2 =  ( ) B 1-2 Probability of stimulated and spontaneous emission : R 2-1 =  ( ) B 2-1 + A 2-1 assumption: n 1 atoms of energy  1 and n 2 atoms of energy  2 are in thermal equilibrium at temperature T with the radiation of spectral density  ( ): n 1 R 1-2 = n 2 R 2-1 n 1  ( ) B 1-2 = n 2 (  ( ) B 2-1 + A 2-1 ) E1E1 E2E2

12. B 1-2 /B 2-1 = 1 According to Boltzman statistics:   ( ) = = Planck’s law

13. The probability of spontaneous emission A 2-1 /the probability of stimulated emission B 2-1  (  : 1.Visible photons, energy: 1.6eV – 3.1eV. 2.kT at 300K ~ 0.025eV. 3.stimulated emission dominates solely when h  /kT <<1! (for microwaves: h <0.0015eV) The frequency of emission acts to the absorption: if h  /kT <<1. x~ n 2 /n 1

14. Population inversion

16. For lasing action Active medium Pumping mechanism – Optical – Electrical discharge – Chemical pumping Optical resonator Resonator

17. Laser characteristics

18. Principle Carbon Di Oxide LASER Principle The transition between the rotational and vibrational energy levels lends to the construction of a molecular gas laser. Nitrogen atoms are raised to the excited state which in turn deliver energy to the CO2 atoms whose energy levels are close to it. Transition takes place between the energy levels of CO2 atoms and the laser beam is emitted. Type:Molecular gas laser Active Medium:Mixture of CO2, N2, He or H2O vapour Active Centre:CO2 Pumping Method:Electric Discharge Method Optical Resonator:Gold mirror or Si mirror coated with Al Power Output:10 kW Nature of Output:Continuous or pulsed Wavelength Emitted:9.6 μm or 10.6 μm

19. Symmetric100C - stationary O - vibrates simultaneously along molecular axis Bending010, 020 C & O vibrate perpendicular to molecular axis Asymmetric Stretching 001, 002 C & O atoms vibrate in opposite directions along molecular axis

21. Applications Bloodless surgery Open air communication Military field

23. Principle Characteristics Nd (Neodymium) – YAG (Yttrium Aluminium Garnet) LASER Principle Characteristics Doped Insulator laser refers to yttrium aluminium garnet doped with neodymium. The Nd ion has many energy levels and due to optical pumping these ions are raised to excited levels. During the transition from the metastable state to E1, the laser beam of wavelength 1.064μm is emitted Type:Doped Insulator Laser Active Medium:Yttrium Aluminium Garnet Active Centre:Neodymium Pumping Method :Optical Pumping Pumping Source :Xenon Flash Pump Optical Resonator :Ends of rods silver coated Two mirrors partially and totally reflecting Power Output:20 kWatts Nature of Output :Pulsed Wavelength Emitted :1.064 μm

24. Nd (Neodymium) – YAG (Yttrium Aluminium Garnet) LASER Power Supply Capacitor Resistor Laser Rod Flash Tube M1– 100% reflector mirror M2 – partial reflector mirror

25. E1, E2, E3 – Energy levels of Nd E4 – Meta Stable State E0 – ground State Energy Level Applications Transmission of signals over large distances Long haul communication system Endoscopic applications Remaote sensing Energy Level Diagram of Nd– YAG LASER Non radiative decay Laser 1.064μm Non radiative decay E3 E2 E0 E1 E4 Nd

26. Principle The electron in the conduction band combines with a hole in the valence band and the recombination produces radiant energy. This photon induces another electron in the CB to combine with a hole in the VB and thereby stimulate the emission of another photon. Type:Homojunction Semiconductor laser Active Medium:P – N junction Active Centre:Recombination of electrons and holes Pumping Method :Direct Pumping Optical Resonator :Polished junction of diode Power Output:1 mW Nature of Output :Continuous or pulsed Wavelength Emitted :8400 – 8600 Angstrom Units HOMOJUNCTION SEMICONDUCTOR LASER (Ga-As Laser)

27. P- and N-type Semiconductors In the compound GaAs, each gallium atom has three electrons in its outermost shell of electrons and each arsenic atom has five. When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal. The result is the shortage of one electron from one of the pairs, causing an imbalance in which there is a “hole” for an electron but there is no electron available. This forms a p-type semiconductor. When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. This type is called an n- type semiconductor.

28. Reverse-biased pn Junction Optical Fiber communications, 3 rd ed.,G.Keiser,McGrawHill, 2000 A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field.

29. Forward-biased pn Junction Optical Fiber communications, 3 rd ed.,G.Keiser,McGrawHill, 2000 Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction.

32. Applications Compact & used in fibre optic communications CD writer Relieves pain Laser printers

33. Excimer LASER Excited dimer – Short lived molecule formed from one or two species, at least one of which is in an electronically excited state – May not be stable in ground state Excimer LASER: – Electron pumped LASER – Dimer (excimer)/complex (exciplex) formation – LASER radiation: relaxation from excited state dimer to ground state Excimer Function Chemicals Characteristic applications Organic Dye Chemicals Function Characteristic applications

34. Excimer e - + A → A* A* + B → AB* → AB + hν Immediately AB → A + B Two important facts: 1.The lower state does not exist! 2.No rotational/vibrational bands Excimer Function Chemicals Characteristic applications Organic Dye Chemicals Function Characteristic applications

35. Excimer LASER Excimer Function Chemicals Characteristic applications Organic Dye Chemicals Function Characteristic applications Energy states of an excimer

36. Excimer Excited Dimers – F 2, Xe 2 ect. Excited Complexes (Exciplex) – Combination of rare gas atoms and halogen atoms – Ar, Kr, Xe – F, Cl, Br Excimer Function Chemicals Characteristic applications Organic Dye Chemicals Function Characteristic applications

37. Excimer LASER Excimer Function Chemicals Characteristic applications Organic Dye Chemicals Function Characteristic applications ExcimerWavelength Ar 2 126 nm Kr 2 146 nm F2F2 157 nm Xe 2 172 and 175 ArF193 nm CaF 2 193 nm KrCl222 nm KrF248 nm Cl 2 259 nm XeBr282 nm XeCl309 nm N2N2 337 nm XeF351 nm Many wavelength possibilities Depends upon the excited dimer Repetition rate from 0.05 Hz to 20 kHz High power: several 10-200 W

38. Excimer LASER Micromaching – Ink jet cartidges (drilling the nozzles) Radiation for changing the structure and properties of materials – Active matrix LCD monitors – Fiber bragg gratings – High temperature superconducting films “Short wavelength light bulb” in optical litography – Computer chips Excimer Function Chemicals Characteristic applications Organic Dye Chemicals Function Characteristic applications

39. 1. Introduction Magnetostatic “wiggler” field Relativistic electron beam EM radiation N SN SN SN SN S The Free Electron Laser (FEL) consists of a relativistic beam of electrons (v≈c) moving through a spatially periodic magnetic field (wiggler). Principal attraction of the FEL is tunability : - FELs currently produce coherent light from microwaves through visible to UV - X-ray production via Self- Amplified Spontaneous Emission (SASE) (LCLS – 1.5Å) (wavelength w )  w /   << w

41. Principle Two beams (object beam and reference beam) are superimposed on a holographic plate to form an image called a hologram.

42. Principle A beam of light (reading beam) having the same wavelength as that of the reference beam used for constructing the hologram, is made to fall over the hologram, which in turn gives rise to a 3-D image in the field of view.

43. Extra slides

44. Review of Semiconductor Physics a) Energy level diagrams showing the excitation of an electron from the valence band to the conduction band. The resultant free electron can freely move under the application of electric field. b) Equal electron & hole concentrations in an intrinsic semiconductor created by the thermal excitation of electrons across the band gap Optical Fiber communications, 3 rd ed.,G.Keiser,McGrawHill, 2000

45. n-Type Semiconductor a)Donor level in an n-type semiconductor. b) The ionization of donor impurities creates an increased electron concentration distribution. Optical Fiber communications, 3 rd ed.,G.Keiser,McGrawHill, 2000

46. p-Type Semiconductor a)Acceptor level in an p-type semiconductor. b) The ionization of acceptor impurities creates an increased hole concentration distribution Optical Fiber communications, 3 rd ed.,G.Keiser,McGrawHill, 2000

47. The pn Junction Optical Fiber communications, 3 rd ed.,G.Keiser,McGrawHill, 2000 Electron diffusion across a pn junction creates a barrier potential (electric field) in the depletion region.