2. Spin-Polarizing 3 He at 8atm with a frequency narrowed diode laser C.W. Arnold, T.V. Daniels, A.H. Couture, T.B. Clegg UNC / TUNL

3. Outline General Overview Goals Our System Results

4. General Overview For experiments in which spin polarized 3 He is needed, lasers tuned to circularly polarized 795 nm light are used to optically pump Rb atoms into states that exchange spin with 3 He nuclei through collisions.

5. Polarization Definition for spin ½ systems:

6. Optical Pumping

7. RCP light Source: http://physics.nist.gov/Divisions/Div846/Gp3/Helium/production/SpinEx.html

8. Spin Exchange 3 He Rb 3 He Fermi-contact hyperfine interaction Rb Works best for I = ½ noble gases ( 3 He and 129 Xe). Takes hours for 3 He.  I · S I S laser light The build up of nuclear spin polarization in the gas ensemble is simply described by The saturation polarization will be proportional to the amount of laser power available in the region of D1 absorption of Rb. Therefore one desires a laser with high power and a very narrow linewidth in the region of absorption.

9. Goals To increase polarization of 3 He target nuclei To develop a versatile and easily transportable system

10. Our System Laser Top view

11. The Laser Laser These diodes put out 50 watts of laser power at the source, and we get about 30 – 36 watts of laser power into our system after losses.

12. Diode Lasers In semiconductor crystals the atomic spacing is very low. –Wave functions of electrons start to overlap –Energy levels split satisfying the Pauli exclusion principle –Energy level spacing ~10 -18 eV The nearly continuous levels form “bands” * from Fundamentals of Semiconductors

13. Diode Lasers “Impurity Recombinations” from Conduction Band to Valence Band Large Linewidths ~3nm 1nm corresponds to 475GHz for our setup; 3nm  ~1400 GHz At modest pressures the acceptance linewidth is ~40GHz A lot of power wasted...or worse. *from Elementary Solid State Physics

14. The Laser What is “smile”? –Displacement of a particular diode from the mean position of the array of diodes. –Causes linewidth broadening due to how the way light is fedback into the diode. –We want “smile” to be as little as possible. Laser

15. Lenses Laser “4x afocal Telescope” Cylindrical lenses f1f1 f 1 + f 2 f2f2

16. Grating Laser The Grating Equation: Groove spacing Incident Angle Diffracted angle of m th order Order of Diffraction Wavelength Littrow Mounting: So for a grating with 2400 lines/mm and a λ=795nm we find that our θ i = 72.5 ΘiΘi φ1φ1 ΘiΘi The Grating Helps us tune our laser to the desired output frequency and provides the desired narrowness of the output light. External Optical Cavity

17. Lasers Overview Stimulated Emission –Excited atoms are triggered into emission by the presence of photons of the proper frequency –Stimulated Emission Photons have the same phase, direction and polarization of the stimulating photon

18. Lens-Grating System Laser ΘiΘi φ1φ1 ΘiΘi The Lenses with the grating Help to reduce the effects of SMILE *From B. Chann, I. Nelson & T.G. Walker

19. Laser lenses & Grating 794.5794.6794.7794.8794.9795.0 Thus, we stimulate the emission of the desired wavelength!

20. Wave Plates Laser To reduce excessive feedback! To change linearly polarized light to circularly polarized light A wave plates performance depends on the angle between the E field of the polarized light and the fast axis of the wave plate. It effects a 2θ rotation of the E field where θ is the angle between the E field and the fast axis. Our mirrored grating preferentially diffracts light with E field in one orientation and simply reflects light with E field 90 o to the first orientation. Thus we can essentially rotate the plane of polarization of our laser to control the amount of feedback we need. Img from Optics, Eugene Hecht & Alfred Zajac 1976

21. Wave Plates

24. Mirrors Laser Mirrors to steer the light to where we need it to go.

25. Our System

26. Results

27. 3 He Polarization Measurement

28. 3 He NMR Coil Polarization Measurement

29. Results We measure polarization with an NMR coil. 3 He Polarization  NMR signal strength measured in mV After the cells and the NMR coil cooled The new laser polarization read 3600mV and the old laser read 3000mV. This represents a 20% increase in polarization from the old laser ~30 W narrowed laser vs. ~60 watt non-narrowed laser

30. Results ~0.3 nm linewidth ~2nm linewidth

31. Summary System is versatile and portable –Has been coupled to two different setups Laser linewidth narrowed by ~ order of magnitude Observed 20% increase in polarization

32. Sources http://science.howstuffworks.com/laser.htm http://hyperphysics.phy-astr.gsu.edu/HBASE/hph.htm http://physics.nist.gov/Divisions/Div846/Gp3/Helium/production/SpinEx.html Tunable Lasers Handbook, F.J. Duarte Ch. 8, 1995 Polarized Light Production and Use, William A. Schurcliff Optics, Eugene Hecht & Alfred Zajac 1976 Fundamentals of Semiconductor Lasers, Takahiro Numai, 2004 Elementary Solid State Physics, M. Ali Omar, 1993 High power diode lasers: Fundamentals Technology and applications, R. Diehl,2000 Using Diode Lasers for Atomic Physics, Carl E. Weiman & Leo Hollberg, Rev. Sci. Instrum.Vol 62, No.1 1991 Narrowing the Laser Diode Array, Xing Zong, Duke Physics Frequency-Narrowed External Cavity Diode Laser Array Bar, B. Chann, I. Nelson, & T.G. Walker (April 4, 1999) Spin Exchange optical pumping of nobel-gas nuclei, Thad G. Walker& William, Happer, Reviews of Modern Physics, Vol 69, No.2, April 1997 Spin-Exchange optical pumping using a frequency narrowed high power diode laser, I.A. Nelson, B. Chann, T.G. Walker, Applied Physics Letters, Vol 76, No.11, March 13, 2000. Private Communications with Alex Couture, Tom Clegg, Brian Collins, Bastian Driehuys

33. Acknowledgements Thanks to Tom Clegg, Tim Daniels, Alex Couture, Bastian Driehuys, Stephen Daigle, UNC Professors, UNC & TUNL machine shops

34. Thank You

35. Wave Plates

36. Narrowed Output *Applied Physics Letters Vol. 76,No. 11

37. Lasers Overview Population Inversion Stimulated Emission –Excited atoms are triggered into emission by the presence of photons of the proper frequency –Stimulated Emission Photons have the same phase, direction and polarization of the stimulating photon

38. Goals To frequency narrow our laser output *Applied Physics Letters Vol. 76,No. 11

39. Lasers Overview Laser: Light Amplification by Stimulated Emission of Radiation. Atoms –Absorb energy – electrons transition to an excited state –Electrons return to lower state – Can release energy in the form of a photon

40. Summary We will have a small, relatively light- weight, portable laser system We will be able to achieve higher polarizations of 3 He than we can with the laser we have been using (40- 50% up from ~25%) We spent a relatively small amount of money to achieve this

41. Applications Spin Exchange Optical Pumping –Tim’s Experiment –n+ 3 He Experiment –Photodissociation of 3 He at HIGS –Any experiment where you want Highly spin polarized 3 He

42. The Laser Laser “Smile”

43. Concerns Losses –Try to minimize the number of things the laser light has to interact with –Anti-reflection coatings on lenses –Compensate for SMILE Safety –Blindness –Fire

44. Lasers Overview

53. Wave Plates Laser To reduce excessive feedback! To change linearly polarized light to circularly polarized light

54. Lenses