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First Year Seminar: Strontium Project
Graham Lochead 03/06/09
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Introduction and motivation Second generation cell
Outline Introduction and motivation Second generation cell Polarization spectroscopy and sub- Doppler DAVLL Strontium pyramid MOT 689 nm locking progress Graham Lochead 03/06/09
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Motivation: Rydberg physics
Rydberg states are states with large n Rydberg states have large orbital radii We aim to trap ultracold strontium in a 1-D optical lattice and excite to Rydberg states Graham Lochead 03/06/09
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Motivation: Ultracold plasmas
Most plasmas are dominated by their thermal energy Coulomb coupling parameter is ratio of Coulomb energy to thermal energy Strong Coulomb interactions lead to spatial corrrelations Cold plasma in a spatially ordered lattice will be a first T.C. Killian et al., Physics Reports 449, 77 (2007) Graham Lochead 03/06/09
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Alkaline-earth element (Group II) Atomic number 38
Strontium overview Alkaline-earth element (Group II) Atomic number 38 84 Sr % I=0 boson 86 Sr % I=0 boson 87 Sr 7% I=9/2 fermion 88 Sr % I=0 boson Graham Lochead 03/06/09
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1S0 ground state – no optical pumping
Electronic structure 3D 3S1 3 2 1 412 nm 1P1 1D2 3P 2 1 461 nm /2p = 32 MHz 689 nm /2p = 7.5 kHz 698nm /2 = 1 mHz 87Sr 1S0 1S0 ground state – no optical pumping Low decay rate to meta- stable state 3P2 Broad linewidth for 1S0-1P1 transition Intercombination line for further cooling Graham Lochead 03/06/09
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Ion transition can be used for:
Why strontium? Singly ionised strontium has an optical transition at ~ 422 nm for 2S1/2-2P1/2 Ion transition can be used for: imaging observing charge transfer laser cooling Rydberg manipulation T.C. Killian et al., Phys. Rev. Lett., 92:143001, 2004. Graham Lochead 03/06/09
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Laser frequency stabilization “locking”
Laser locking requires an atomic sample to investigate the transition And a detection scheme that gives a slope to lock to Graham Lochead 03/06/09
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Introduction and motivation Second generation cell
Polarization spectroscopy and sub- Doppler DAVLL Strontium pyramid MOT 689 nm locking progress Graham Lochead 03/06/09
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Problems with strontium
Locking to a transition requires an atomic sample Atomic strontium has very low vapour pressure Hot strontium reacts with glass and copper M. Asano, K. Kubo J. Nuclear Sci. & Tech. 15 pp. 765~767 (1978) Graham Lochead 03/06/09
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Sealed in argon with indium plug Directional source of atomic vapour
Dispenser technology Sr Sr Sealed in argon with indium plug Directional source of atomic vapour Flux is dependent on current supplied Graham Lochead 03/06/09
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First generation cell Birefringent sapphire windows not required
No continual pumping No buffer gas Lifetime estimate ~ h Compact size for Sr Strontium acts as a getter Only 15-20% absorbtion stable operation Dispenser A vapor cell based on dispensers for laser spectroscopy E. M. Bridge, J. Millen, C. S. Adams, M. P. A. Jones Rev. Sci. Instr. 80, (2009) Graham Lochead 03/06/09
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Second generation cell
30 cm Dispenser Baffle Designed by Clementine Javaux Graham Lochead 03/06/09
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Second cell absorption
Doppler FWHM of 1.7 GHz at 50% absorption Optically thick for 461 nm transition Wide Doppler profile due to dispenser type Graham Lochead 03/06/09
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Second cell saturated absorption
124.5 MHz Probe = 0.14 mW Pump = 7.3 mW Pump Laser Probe Cell Frequency axis calibrated from 86Sr-88Sr splitting 88Sr transition peak is ~5% of the optical depth Graham Lochead 03/06/09
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Introduction and motivation Second generation cell
Polarization spectroscopy and sub- Doppler DAVLL Strontium pyramid MOT 689 nm locking progress Graham Lochead 03/06/09
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Polarization spectroscopy theory
J = 1 mJ = -1 +1 5s5p 1P1 σ- σ+ Cell 5s2 1S0 J = 0 mJ = 0 M. L. Harris, et al. Phys. Rev. A, 73:062509, 2006. C. P. Pearman et al., J. Phys. B, 35:5141, 2002. Graham Lochead 03/06/09
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Polarization spectroscopy setup
Frequency calibration Laser Metal mirror Cell 2 Cell 1 Differential photodiode Graham Lochead 03/06/09
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Polarization spectroscopy results
Gives a steep gradient – easy to lock to 0.8 MHz rms offset stability over an hour Graham Lochead 03/06/09
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5s2 1S0 5s5p 1P1 5s2 1S0 5s5p 1P1 DAVLL theory
Dichroic Atmoic Vapour Laser Lock (DAVLL) 5s2 1S0 5s5p 1P1 J = 0 mJ = 0 J = 1 mJ = -1 +1 σ+ σ- 5s2 1S0 5s5p 1P1 J = 0 mJ = 0 J = 1 mJ = -1 +1 σ+ σ- Apply a uniform magnetic field to atomic sample with Helmholtz coils Creates a difference in frequency between different transitions Taking the difference of these signals leads to a dispersion signal with zero crossing at the B=0 transition Graham Lochead 03/06/09
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Sub-Doppler DAVLL setup
To frequency calibration Laser Metal mirror Coils Differential photodiode M.L. Harris et al., J. Phys. B. Phys Graham Lochead 03/06/09
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Sub-Doppler DAVLL trace
3 MHz rms offset stability over an hour Graham Lochead 03/06/09
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Sub-Doppler DAVLL characteristics
Graham Lochead 03/06/09
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These two locking schemes have been characterized and written up
Laser locking summary Polarization spectroscopy is used to lock the 461 nm laser with first cell as offset more stable These two locking schemes have been characterized and written up Second cell will be used for thermal Rydberg spectroscopy with pulsed dye laser arXiv: v1 [physics.atom-ph] Graham Lochead 03/06/09
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Introduction and motivation Second generation cell
Polarization spectroscopy and sub- Doppler DAVLL Strontium pyramid MOT 689 nm locking progress Graham Lochead 03/06/09
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Normal (6 beam) MOT Pyramid MOT What is a pyramid MOT?
K. I. Lee et al., Optics Letters, Vol. 21, Issue 15, pp Graham Lochead 03/06/09
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Acts as a cold atom source
Pyramid MOT function Acts as a cold atom source Graham Lochead 03/06/09
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Benefits of a pyramid MOT
Size – much smaller than a Zeeman slower Blackbody radiation effects reduced Graham Lochead 03/06/09
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Design considerations
Chamber design 45 cm Design considerations Trapping gradient of 30 G/cm No water cooling Standard vacuum parts 30 cm Graham Lochead 03/06/09
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Problem Solution Dispensers below mirrors
Mirror mount design Most pyramid MOTs are loaded from background atomic vapour or dispensers above pyramid Problem Low vapour pressure and mirrors get coated Solution Dispensers below mirrors Slits where mirrors meet in corners Graham Lochead 03/06/09
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The mount design 45 mm Mirror size needs to be small to avoid pumping into meta-stable states Graham Lochead 03/06/09
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Atomic beam divergence measurement
Expanding lens Atomic beam Laser Light sheet Collimating lens Cylindrical lenses AOM Light sheet Graham Lochead 03/06/09
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Introduction and motivation Second generation cell
Polarization spectroscopy and sub- Doppler DAVLL Strontium pyramid MOT 689 nm locking progress Graham Lochead 03/06/09
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Motivation for 689 nm laser
We will use a 532 nm optical lattice laser to add periodic spatial confinement to our MOT Doppler limited temperature 1P1 3P 1D2 2 1 461 nm /2p = 32 MHz 689 nm /2p = 7.5 kHz TD ≈ 1 mK TD ≈ 0.2 μK 1S0 Graham Lochead 03/06/09
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Pound-Drever-Hall FPD Oscilloscope PS Laser Graham Lochead 03/06/09
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Pound-Drever-Hall setup
Strontium cell FPD Filter Feedback to cavity piezo Slow feedback to piezo PS Fast feedback to diode Laser Graham Lochead 03/06/09
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Slow lock * Unlocked laser has linewidth of ~ 600 kHz
PDH signal REF02 +15V To piezo AD620 1k 100k Ramp 2k 510 100 10nF * Unlocked laser has linewidth of ~ 600 kHz Locked laser has linewidth of ~ 350 kHz Graham Lochead 03/06/09
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Cavity lock to atomic transition
Going to use frequency modulation spectroscopy as laser already modulated 20 kHz AOM 10 MHz 80 MHz Probe Pump Lock in amplifier Cell Feedback to cavity piezo Graham Lochead 03/06/09
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Summary and future work
Second cell and locking schemes characterized Pyramid MOT design 689 nm locking progress Future work Build the pyramid MOT Achieve a red MOT Load 1-D lattice Graham Lochead 03/06/09
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Saturated absorption spectroscopy fit
Sat. spec. fit is achieved by minimizing sum of six Lorentzians in Matlab 1% scaling accuracy for the frequency Parameters A Amplitude of trace s Scaling factor ω0 Centre frequency Width c Offset m Gradient of background Isotope Abundance (%) I F Shift (MHz) Rel. Strength 84Sr 0.56 - -270.8 1 86Sr 9.86 -124.5 87Sr 7.00 9/2 7/2 -9.7 4/15 -68.9 1/3 11/2 -51.9 2/5 88Sr 82.58 Graham Lochead 03/06/09
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