* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Tunable External-Cavity High
X-ray fluorescence wikipedia , lookup
Thomas Young (scientist) wikipedia , lookup
Silicon photonics wikipedia , lookup
Diffraction grating wikipedia , lookup
Optical coherence tomography wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Optical tweezers wikipedia , lookup
Birefringence wikipedia , lookup
Super-resolution microscopy wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Confocal microscopy wikipedia , lookup
Ellipsometry wikipedia , lookup
Optical amplifier wikipedia , lookup
Retroreflector wikipedia , lookup
Harold Hopkins (physicist) wikipedia , lookup
3D optical data storage wikipedia , lookup
Nonlinear optics wikipedia , lookup
Photonic laser thruster wikipedia , lookup
Ultrafast laser spectroscopy wikipedia , lookup
Mode-locking wikipedia , lookup
Spin-Polarizing 3He at 8atm with a frequency narrowed diode laser C.W. Arnold, T.V. Daniels, A.H. Couture, T.B. Clegg UNC / TUNL Outline • • • • General Overview Goals Our System Results General Overview • For experiments in which spin polarized 3He is needed, lasers tuned to circularly polarized 795 nm light are used to optically pump Rb atoms into states that exchange spin with 3He nuclei through collisions. Polarization • Definition for spin ½ systems: Polarization 100.00% N N NP N P N N N N 90.00% 80.00% Polarization 70.00% 60.00% 50.00% 40.00% 30.00% 20.00% 10.00% 0.00% 50% 55% 60% 65% 70% 75% Nup/Ntot 80% 85% 90% 95% 100% Optical Pumping Optical Pumping RCP light Source: http://physics.nist.gov/Divisions/Div846/Gp3/Helium/production/SpinEx.html Spin Exchange The build up of nuclear spin Rb He polarization in the gas ensemble is simply described by 3 laser light P(t ) Psat (1 e I·S t / su ) Fermi-contact hyperfine interaction The saturation polarization will be proportional to the amount of laser power available in the region of D1 Rb absorption of Rb. Therefore one desires a laser with high power and a very narrow linewidth in the region of absorption. S • Works best for I = ½ noble gases (3He and 129Xe). • Takes hours for 3He. I 3He Goals • To increase polarization of 3He target nuclei • To develop a versatile and easily transportable system Our System Top view Laser 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. 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 Diode Lasers • “Impurity Recombinations” from Conduction Band to Valence Band • Large Linewidths ~3nm c c • 1nmfcorresponds to 475GHz for our f 2 setup; • 3nm ~1400 GHz pressures f 475GHz • At modest the acceptance linewidth is ~40GHz • A lot of power wasted...or worse. *from Elementary Solid State Physics The Laser • What is “smile”? Laser – 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. Lenses “4x afocal Telescope” Laser Cylindrical lenses f1 f1 + f2 f2 Grating So for a grating with 2400 lines/mm and a λ=795nm we find that our θi= 72.5 External Optical Cavity Laser φ1 Θi Θi The Grating Littrow Mounting: Equation: The a(sin mGrating Helps us tune our i1 sin m ) i laser to the desired output frequency and provides the Diffracted angle Wavelength iof m Order of th order desired narrowness of the Diffraction output light. 2a sin Groove spacing Incident Angle 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 Lens-Grating System 0 2 2 *From B. Chann, I. Nelson & T.G. Walker Laser φ1 The Lenses with the grating Help to reduce the effects of SMILE Θi Θi 0 M Laser lenses & Grating 795.0 794.9 794.8 794.7 794.6 794.5 Thus, we stimulate the emission of the desired wavelength! Wave Plates 2 To reduce excessive feedback! Laser Our mirrored grating preferentially Adiffracts wave plates performance light with E field in depends one on the angle between the E fieldlight of orientation ando simply reflects the lighttoand axis of withpolarized E field 90 the the firstfast orientation. the wave It effects rotate a 2θ the Thus we plate. can essentially rotation the E field where θ is the plane ofofpolarization of our laser to angle between the E field and the control the amount of feedback we fast axis. need. 4 To change linearly polarized light to circularly polarized light •Img from Optics, Eugene Hecht & Alfred Zajac 1976 Wave Plates Wave Plates 1.60E-03 1.40E-03 1.20E-03 1.00E-03 8.00E-04 Series1 6.00E-04 4.00E-04 2.00E-04 0.00E+00 791 792 793 794 795 796 797 797 798 Wave Plates Mirrors Laser Mirrors to steer the light to where we need it to go. Our System Results Polarization Measurement 3 3 He 3 He 3 He 3 He 3 He 3 He 3 He 3 He He 3 He Polarization Measurement 3 NMR Coil He Results •We measure polarization with an NMR coil. •3He 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 Results ~0.3 nm linewidth ~2nm linewidth 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 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 • • • • • Acknowledgements • Thanks to Tom Clegg, Tim Daniels, Alex Couture, Bastian Driehuys, Stephen Daigle, UNC Professors, UNC & TUNL machine shops Thank You Wave Plates Narrowed Output *Applied Physics Letters Vol. 76,No. 11 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 Goals • To frequency narrow our laser output *Applied Physics Letters Vol. 76,No. 11 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 Summary • We will have a small, relatively lightweight, portable laser system • We will be able to achieve higher polarizations of 3He than we can with the laser we have been using (4050% up from ~25%) • We spent a relatively small amount of money to achieve this Applications • Spin Exchange Optical Pumping – Tim’s Experiment – n+3He Experiment – Photodissociation of 3He at HIGS – Any experiment where you want Highly spin polarized 3He The Laser Laser “Smile” 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 Lasers Overview Wave Plates 2 To reduce excessive feedback! Laser 4 To change linearly polarized light to circularly polarized light Lenses