Saturation

1. Saturation Roi Levy

2. Motivation • To show the deference between linear and non linear spectroscopy • To understand how saturation spectroscopy is been applied

3. Motivation

4. Outline • Widths and Profiles of Spectral Lines theory • Nonlinear Spectroscopy theory • Experimental Schemes • Some papers

5. Widths and Profiles of Spectral Lines Natural Line width Doppler width Collisional Broadening Saturation Broadening

6. Natural Line width • An excited atom can emit its excitation energy as spontaneous radiation • Describing it as dumped harmonic oscillator with dumping constant • As a result the amplitude decreases (in time) and the emitted radiation is no longer monochromatic

7. Natural Line width The square of amplitude A(w) as function of w we can see it has the shape of a Lorentzian. The normalized intensityis Where the full halfwidth at half-maximum is

8. Natural Line width • Using the uncertainty principle we can relate the mean life time of an exited level i to it’s energy, • Where Aikis Einstein coefficientfor spontaneous emission.

9. Doppler Width • One of the major contributions to the spectral line width in gases. • Due to the thermal motion of the absorbing or emitting molecules. • Because Doppler Width is of our main interest we shall present the outline of the derivation.

10. Doppler Width Consider an excited molecule with a velocity v relative to the rest fame of the observer. The central frequency of the emission is w0 At the rest frame the frequency is The same relation holds for the absorption frequency

11. Doppler Width At thermal equilibrium the molecule of a gas follow a Maxwellian velocity distribution ni - density of molecules in level Ei - the most probable velocity

12. Doppler Width Using the relation and that the absorbed radiant power is proportional to the Density we get the intensity of the Doppler- Broadened spectral line Gaussian profile with full halfwidth

13. Doppler Width More detailed consideration will have to include the natural Line width for every molecule.

14. Doppler Width The spectral intensity distribution of the total Absorption or emission is This intensity profile is called Voigt profile.

15. Saturation Broadening • At sufficiently large laser intensities the optical pumping rate on an absorbing transition become larger than the relaxation rates. • This saturation causes additional line broadening

16. Saturation of Level Population by Optical pumping For two-level system with population N1 and N2 The rate equations are, With the rate for stimulated emission (absorption), and Rithe relaxation Probability for level i.

17. Saturation of Level Population by Optical pumping Solving for the steady state we get for N1 And for the difference between the population of the levels is Where S the saturation parameter is the ratio of the Pumping rate to the average relaxation rate.

18. Saturation of Level Population by Optical pumping The pump rate due to monochromatic wave with intensity is so that And the saturated absorption coefficient is

19. Saturation Broadening of Homogeneous Line Profiles Since the absorption profile of a homogeneously broadened line is Lorentzian, the induced absorption probability is And the saturation parameter is

20. Saturation Broadening of Homogeneous Line Profiles The absorption coefficient will be With

21. Nonlinear Spectroscopy Linear and Nonlinear Absorption Saturation of Inhomogeneous Line Profiles Saturation Spectroscopy

22. Linear and Nonlinear Absorption Assume that a monochromatic plane lightwave With the mean intensity passes through a sample of molecules. The Absorption in volume dV is

23. Linear and Nonlinear Absorption In case the incident wave with spectral energy density And spectral width Which is large compared to the halfwidth of the Absorption profile The total intensity becomes

24. Linear and Nonlinear Absorption The absorbed power is then Remembering that the absorbed power is proportional to the number of absorbed photons We can obtain

25. Linear and Nonlinear Absorption Let us discuss what happens in open systems Many relaxation channels Also Molecules can diffuse in and out of the excitationvolume

26. Linear and Nonlinear Absorption The rate equations Where Is the contribution of other levels to the population of level i , Diis the diffusion rate of the molecules in level i into the excitation volume

27. Linear and Nonlinear Absorption Solving these equations under stationary Conditions (dN/dt=0) we get for And for

28. Linear and Nonlinear Absorption The saturation parameter , The power decrease of the incident light wave from absorption along the length dz

29. Linear and Nonlinear Absorption In case of incoherent light sources S<<1 And P is This is the Lambert-Beer law of linear absorption

30. Linear and Nonlinear Absorption In open systems the saturated population density N1 Can be very small. Where C1 andC2are small compared to R1 and R2 For close systems

31. Saturation of Inhomogeneousbroadened Line Profiles Inhomogeneous broadened line profiles such as Doppler-broadening we treat two cases • Hole Burning • Lamb Dip

32. Hole Burning When a monochromatic light wave passes through a gas with Maxwell-Boltzmann velocity distribution the laser frequency in the frame of the molecule is With fall within the linewidth

33. Hole Burning The absorption cross section for the molecule Due to the saturation the population Decreases within the velocity range and the population increases. Let us right the equations for N1 and N2

34. Hole Burning where is the homogeneous width of the transition. And

35. Hole Burning Bennet hole (peak) For the depth of the hole in N1is different from the height of the peak in N2.

36. Hole Burning The saturated population difference The absorption coefficient

37. Hole Burning Solving the integral we get • We can not detect Bennet hole by tuning the laser through the absorption profile. • Something missing ?

38. Hole Burning The Bennet hole can be detected if two lasers are used • The saturating pump laser with the wave vector k1which is kept at the frequency and which burn the hole • A weak probe laser with the wave vector k2and frequency tunable across the Voigt profile

39. Hole Burning The absorption coefficient for the probe laser

40. Hole Burning The absorption coefficient for the probe laser

41. Lamb Dip • Pump and probe waves can be generated by a single laser when the incident beam is reflected back into the absorption cell • The absorption profile will be Doppler-broadened profile with a dip at the center at • This dip is called Lamb dip after W.E.Lamb who first explained it theoretically

42. Lamb Dip The saturated absorption coefficient in case of equal intensities (weak filed )

43. Lamb Dip For strong laser fields

44. Lamb Dip In case the intensity of the reflected wave is very small

45. Saturation Spectroscopy • Experimental Schemes

46. Saturation Spectroscopy • Experimental Schemes

47. Saturation Spectroscopy • Laser-induced fluorescence Instead of measuring the attenuation of the probe beamthe absorption can be monitored by the laser-induced fluorescence. • Advantageous when the density of the absorbing molecule is low.

48. Saturation Spectroscopy Intermodulated fluorescence the pump beam and the probe beam are chopped At different frequencies The fluorescence intensity is

49. Saturation Spectroscopy At the center of an absorption line Only the term with both intensities contribute to the saturation effect

50. Saturation Spectroscopy