LASER

1. LASER

3. LASER A laser is an amplifier of light. When the laser is suitably excited by optical or electrical energy, the light of the proper frequency entering the laser cavity is amplified in such a manner that laser output wave is in phase with input. Practical utility of a laser is as an OSCILLATOR –-- a generator of light. Thus laser is also known as GENERATOR of light.

4. LASER ACTION Laser action is based on amplification of EM waves by means of forced or induced atoms or molecules. A laser radiation uses three fundamental phenomena when EM waves interacts with the matter namely

5. Excited atoms emit photons spontaneously. When an atom in an excited state falls to a lower energy level, it emits a photon of light. Excited level Energy Ground level Molecules typically remain excited for no longer than a few nanoseconds. This is often also called fluorescence or, when it takes longer, phosphorescence.

6. Atoms and molecules can also absorb photons, making a transition from a lower level to a more excited one. Excited level This is, of course, absorption. Energy Ground level Absorption lines in an otherwise continuous light spectrum due to a cold atomic gas in front of a hot source.

7. Spontaneous absorption Let us consider two energy level having energy E1 & E2 resp. The atom will remain in ground state unless some external stimulant is applied to it. When an EM wave i.e photon of particular freq fall on it , there is finite probability that atom will jump form energy state E1 to E2. E2 photon E1

8. Spontaneous emission Consider an atom in higher state (E2). It can decay to lower energy level by emitting photon. Emitted photon have energy hv=E2-E1. Life time of excited state is 10-9sec. E2 Photon hv=E2-E1 E1

9. Stimulated emission There are metastable state i.e. transition from this state is not allowed acc to selection rule. There life time is 10-3 sec. Atom in this state can’t jump to lower state at there own. When an photon of suitable freq arrive it make the atom in metastable unstable. The emitted photon is in coherence with incident photon. Metastable state(10-3sec) Incident photon Emitted Photon coherent

10. Stimulated Emission • The stimulated photons have unique properties: • In phase with the incident photon • Same wavelength as the incident photon • Travel in same direction as incident photon

11. Stimulated vs Spontaneous Emission Stimulated emission requires the presence of a photon. An “incoming” photon stimulates a molecule in an excited state to decay to the ground state by emitting a photon. The stimulated photons travel in the same direction as the incoming photon. Spontaneous emission does not require the presence of a photon. Instead a molecule in the excited state can relax to the ground state by spontaneously emitting a photon. Spontaneously emitted photons are emitted in all directions.

12. In 1916, Einstein showed that another process, stimulated emission, can occur. Before After Spontaneous emission Absorption Stimulated emission

13. The processes that account for absorption and emission of radiation and the attainment of thermal equilibrium. The excited state can return to the lower state spontaneously as well as by a process stimulated by radiation already present at the transition frequency.

14. In 1916, Einstein showed that another process, stimulated emission, can occur. Before After Spontaneous emission Absorption Stimulated emission

15. EINSTEIN’S THEORY OF RADIATIONS E2 Spontaneous emission Incident photon Stimulatedemission E1

16. EINSTEIN’S THEORY OF RADIATIONS Ra=rate of absorption per unit volume It depends upon: 1.N1: no. of atom in ground state. 2.ρ(v): energy density per unit freq of incident wave.

17. EINSTEIN’S THEORY OF RADIATIONS Rsp=rate of emission per unit volume. It depends upon: 1.N2: no. of atom in exicited state.

18. EINSTEIN’S THEORY OF RADIATIONS Rst= rate of stimulated emission per unit volume It depends upon: 1.N2: no. of atom in exicited state. 2.ρ(v): energy density per unit freq of incident wave.

19. Properties of Laser • Monochromatic • The light emitted from a laser is monochromatic, that is, it is of one wavelength (color).  In contrast, ordinary white light is a combination of many different wavelengths (colors).

20. Properties of Laser Directional: Lasers emit light that is highly directional.  Laser light is emitted as a relatively narrow beam in a specific direction.  Ordinary light, such as coming from the sun, a light bulb, or a candle, is emitted in many directions away from the source.

21. Properties of Laser Coherent The light from a laser is said to be coherent, which means the wavelengths of the laser light are in phase in space and time.

22. Population Inversion A state in which a substance has been energized, or excited to specific energy levels. More atoms or molecules are in a higher excited state.

23. Population Inversion • The process of producing a population inversion is called pumping. • Examples: →by lamps of appropriate intensity →by electrical discharge

24. Achieving inversion: Pumping the laser medium I I0 I1 I3 I2 Laser medium R = 100% R < 100% Now let I be the intensity of (flash lamp) light used to pump energy into the laser medium: Will this intensity be sufficient to achieve inversion, N2 > N1? It’ll depend on the laser medium’s energy level system.

25. In what energy levels do molecules reside? Boltzmann population factors Ni is the number density of molecules in state i (i.e., the number of molecules per cm3). T is the temperature, and kB is Boltzmann’s constant. N3 E3 N2 E2 Energy N1 E1 Population density

26. Boltzmann Population Factors In equilibrium, the ratio of the populations of two states is: N2 / N1 = exp(–DE/kBT ), where DE = E2 – E1 = hn As a result, higher-energy states are always less populated than the ground state, and absorption is stronger than stimulated emission. In the absence of collisions, molecules tend to remain in the lowest energy state available. Collisions can knock a mole- cule into a higher-energy state. The higher the temperature, the more this happens. Low T High T 3 3 Energy Energy 2 2 1 1 Molecules Molecules

35. Components of LASER

36. Pump Source A pump is basic energy source for a laser. It gives energy to various atoms of laser medium & excites them . So that population inversion can take place & it is maintained with time. The excitation of atom occur directly or through atom or atom collision. There is various type of pump depending upon nature of medium .Examples: electric discharges, flashlamps, arc lamps and chemical reactions. The type of pump source used depends on the gain medium. →A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon gas mixture. →Excimer lasers use a chemical reaction.

37. Gain Medium When energy is given to laser medium a small fraction of medium shows lasing action. This part of laser medium is called Active centers. For examples in ruby laser Cr+++is active center, in He-Ne laser Ne are active centers. It is the Major determining factor of the wavelength of operation of the laser. Excited by the pump source to produce a population inversion. Where spontaneous and stimulated emission of photons takes place. Example: solid, liquid, gas and semiconductor.

38. Optical Resonator • It is an set up used to obtain amplification of stimulated photons, by oscillating them back & forth between two extreme limits. Consist of: • Two plane or concave mirrors placed co-axially. • One mirror is reflecting & other is partially reflecting.

39. Optical Resonator Two parallel mirrors placed around the gain medium. Light is reflected by the mirrors back into the medium and is amplified . The design and alignment of the mirrors with respect to the medium is crucial. Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output.

40. Stimulated emission can lead to a chain reaction and laser emission. If a medium has many excited molecules, one photon can become many. Excited medium This is the essence of the laser. The factor by which an input beam is amplified by a medium is called the gain and is represented by G.

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

43. Four-level Laser System Laser transition takes place between the third and second excited states. Rapid depopulation of the lower laser level.

44. FOUR LEVEL LASER: STEP 1- PUMPING: atoms are excited to higher energy level by providing energy from ext. source. STEP 2- POPULATION INVERSION: atom via radiation less decay, decays to metastable state and hence population inversion take place.

45. FOUR LEVEL LASER: STEP 3- LASER ACTION: atom from metastable state decays to lower state by stimulated emission and hence laser action take place. STEP 4- BACK TO GROUND STATE: atom from excited state decays to lower state by spontaneous emission.

46. FOUR LEVEL LASER:

47. Three-level Laser System Initially excited to a short-lived high-energy state . Then quickly decay to the intermediate metastable level. Population inversion is created between lower ground state and a higher-energy metastable state.

48. Three-level Laser System

49. Two-level Laser System Unimaginable as absorption and stimulated processes neutralize one another. The material becomes transparent.

50. Em, Nm Em, Nm En, Nn En, Nn Two-Level System Even with very a intense pump source, the best one can achieve with a two-level system is excited state population = ground state population