1. Rydberg & plasma physics using
ultra-cold strontium
James Millen
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

2. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Outline
Motivation
Spectroscopy of strontium Rydberg states using electromagnetically induced transparency Mauger, Millen, Jones J. Phys. B: At. Mol. Opt. Phys. 40 (2007) F319-F325
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

3. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Rydberg physics
A Rydberg state is one of high principle quantum number n
Rydberg atoms can be very large (orbital radius scales as n2)
Very strong Rydberg-Rydberg interactions (van-der-Waals interaction scales as n11)
This can lead to “frozen” Rydberg gases, where the interaction energy is much greater than the thermal energy.
Johannes Rydberg
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

4. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Ultra-cold plasma physics
Most plasmas are hot, dense and dominated by their kinetic energy
The behaviour of ultra-cold neutral plasmas is governed by Coulomb interactions
Other “strongly coupled” plasmas are not accessible in the lab
Killian, Science
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

5. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Ultra-cold plasma physics
Plasmas can be formed from cold atoms by optically exciting above the ionisation threshold
Some electrons leave, leading to the system being bound
The initial electron energy can be set
Killian, Science
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

6. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Introduction to Strontium
Atomic Number: 38
An alkaline earth metal (Group II)
Four naturally occurring isotopes: 88Sr (82.6%), 87Sr (7.0%), 86Sr (9.9%) & 84Sr (0.6%)
88,86,84Sr have no hyperfine structure (Bosonic I=0), 87Sr has I=9/2 (Fermionic)
Negligible vapour pressure at room temperature
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

7. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
88Sr energy level diagram
5sns 1S0
5snd 1D2
412.7nm
460.7nm
32MHz
689nm
7.5kHz
698nm
1mHz (87Sr) 1S 1P 1D 3S 3P
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

8. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Why strontium?
Singlet-triplet mixing leads to narrow intercombination lines, allowing cooling to <μK
This also allows high spectroscopic resolution
1S0 ground state can make spectroscopy more simple (no optical pumping required)
Singly charged ion Sr+ has many transitions in the visible, allowing spatially resolved diagnostics (5s 1S0 → 5p 1P1 transition is at 420nm)
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

9. Spectroscopy of strontium Rydberg states using electromagnetically induced transparency
Mauger, Millen, Jones: J. Phys. B: At. Mol. Opt. Phys. 40 (2007) F319-F325
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

10. 420nm frequency doubled diode laser (max. output ~15mW)
The experiment
420.47nm
420.40nm
460.7nm
Probe
Coupling
5s2 1S0
5s5p 1P1
5s19s 1S0
5s18d 1D2
461nm frequency doubled diode laser with tapered amplifier (max. output ~350mW)
420nm frequency doubled diode laser (max. output ~15mW)
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

11. Strontium is heated in an oven and collimated with a nozzle
The experiment
Probe
Coupling
Atomic beam 1 2
Oven + Nozzle
Strontium is heated in an oven and collimated with a nozzle
The transmission of the probe beam is measured as it is scanned across the transition
When the coupling beam is turned on there is an increase in the transmission of the probe beam on resonance
Mohapatra, Jackson, Adams Phys. Rev. Lett
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

12. Electromagnetically induced transparency
When the probe laser is scanned across the transition at 460.7nm you see a Doppler broadened absorption profile
When the coupling laser is resonant with the transition under investigation there is an increase in transmission on the probe beam
~5MHz
~150MHz
~5MHz
~150MHz
By subtracting the Doppler broadened background this peak can be studied. It can have a width as small as 5MHz.
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

13. Frequency axis calibration
A fit based on the sum of six Lorentzians was used. Scaling parameter was used to calibrate the frequency axis
Saturated absorption spectroscopy was used to resolve the 5s1S0→ 5p1P1 lines for 88Sr and 86Sr
124.5 MHz
Isotope
Abundance % I F
Shift (MHz)
Rel. Strength
84Sr
0.56 -
-270.8 1
86Sr
9.86
-124.5
7.00
9/2
7/2
-9.7
4/15
87Sr
-68.9
1/3
11/2
-51.9
2/5
88Sr
82.58
32 MHz
Eliel et. al. Z. Phys. A 311 1, Kluge & Sauter Z. Phys
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

14. Fitting – EIT peaks
In order to fit to our EIT lineshapes we use the following expression for the susceptibility χ(v)†
γ3 is the decay rate of the Rydberg state, and includes all line broadening mechanisms as well as the natural lifetime
The absorption is given by the imaginary part of the susceptibility
We sum over all four isotopes, and integrate the absorption over the transverse velocity distribution
†Xiao, Li, Jin, Gea-Banacloche Phys. Rev. Lett
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

15. Isotope shift of EIT peaks
Coupling laser tuned to the 5s5p1P1→5s18d1D2 transition 1) 2)
Signal / V
88Sr
Signal / V
88Sr
Time / s
Time / s
86Sr 3) 4)
86Sr
88Sr
Signal / V
Signal / V
88Sr
Time / s
Time / s
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

16. Isotope shift of EIT peaks - Results
Coupling tuned near 5s18d1D2 transition
Singlet-triplet mixing with the 5s18d3D3 state cause massive (~GHz) hyperfine splitting in 87Sr, so the peak isn’t visible†
Coupling tuned near 5s19s1S0 transition
The transition to the 5s19s1S0 is much weaker than to the D state, so a lock-in amplifier was used
†Beigang et. al. J. Phys. B: At. Mol. Phys. 15 L201-L206
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

17. Doppler mismatch
Due to the difference in wavevectors between the probe and coupling beams you cannot read the shift straight from the frequency axis
Δωp = -{ (1 - λc/λp )Δω2 + (λc/λp)Δω3 }
(~0.1)
(~0.9)
Transition
88Sr→86Sr
(MHz)
88Sr→87Sr
5s2 1S0→5s18d1D2
226±7 -
5s2 1S0→5s19s1S0
213±7
62±8
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

18. Further study 1 2 Atomic beam Oven + Nozzle Probe Coupling
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

19. Further study 1 2 Atomic beam Oven + Nozzle Probe Coupling
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

20. Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Strontium energy level diagram
5sns 1S0
5snd 1D2
420nm
460.7nm
32MHz
689nm
7.5kHz
698nm
1mHz (87Sr) 1S 1P 1D 3S 3P
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

21. The original beam separation was set by the beamsplitter to 4mm
Beam translation
The original beam separation was set by the beamsplitter to 4mm
A translatable mirror enabled separations of 3-13mm
Varied probe power from μW
Results were inconclusive
Could be Rydberg autoionization
Coupling 1 2
Atomic beam
Oven + Nozzle
Translatable
mirror
Probe
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

22. Rydberg Autoionization
5s2 1S0
5s5p 1P1
5sns 1S0
5pns 1P1
5s 1S0 e- e- e- e-
Sr+
Sr2+ Sr
Sr+ e-
460nm
420nm
420nm
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

23. EIT could be used for laser stabilization
Conclusion
Electromagnetically induced transparency provides a useful, non-destructive spectroscopic tool
The population dynamics of our system are not well understood, further modelling is required
EIT could be used for laser stabilization
Need to move towards cold strontium to fulfil our aims of studying “frozen” Rydberg gases and plasmas
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

24. The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

25. Three orthogonal axis for a blue (460.7nm) MOT
Requirements
Three orthogonal axis for a blue (460.7nm) MOT
Potential for a red (689nm) MOT (sub μK cooling)
Axis for a dipole trap
MOT from Tino group: LENS, Florence
Axis for excitation of atoms and imaging
Detection via a micro channel plate (MCP)
Electrodes for charged particle control / state-selective field ionisation
MOT coils inside chamber
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

26. The vacuum system The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

27. The chamber 30cm flange to flange 12 DN40 flanges (separated by 30°)
2 DN200 flanges, one with 8’’ viewport, the other with 1.5’’ viewport and feed-throughs
Beam height is 190mm above optical bench
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

28. Coils wound from 1mm Kapton insulated copper wire
Internals – MOT coils
Coils wound from 1mm Kapton insulated copper wire
Can produce a field gradient of 30Gcm-1 at 2.5A
Mounted directly on top flange so can directly “plug” into the chamber
No electrical connections in any optical path
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

29. Split ring geometry mounted onto MOT coil formers
The electrodes
Split ring geometry mounted onto MOT coil formers
Blocks no optical access
8 independently controllable electrodes
Can produce reasonably flat fields and also gradients
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

30. Calculating the electric field
The electric potential (in 2D) Φ(x,y) is the solution to Laplace’s equation
Φ(x,y)xx + Φ(x,y)yy = 0
Map Φ(x,y) onto an array of points with spacing h
Taylor expand
[Φ(x±h,y) + Φ(x,y±h) + Φ(x ±h,y±h)] =
8Φ(x,y) + 3h2(Φ(x,y)xx + Φ(x,y)yy) + O(h4)
→ Φ(x,y) ≈ 1/8[Φ(x±h,y) + Φ(x,y±h) + Φ(x ±h,y±h)]
The average of all neighbouring points
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

31. Set an initial electrode configuration
Realization in MatLab
Create a 40x40x40 array
Set an initial electrode configuration
Use the “circshift” command to take average of neighbouring points
Image across various slices
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

32. Field changes by <1% in central 4mm cube
Field calculations
Field changes by <1% in central 4mm cube
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

33. See website: http://massey.dur.ac.uk/resources/lab_resources.html
Online resources
See website:
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

34. Current progress - Apparatus
Pumped down to ~ Torr
New oven currently being built
Waiting to move into new lab
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

35. Our apparatus for cooling and trapping strontium is almost complete
Conclusion
We have shown that EIT can be used as a spectroscopic tool for strontium
Our apparatus for cooling and trapping strontium is almost complete
Once we have achieved a MOT we can move towards creating an ultra-cold Rydberg gas or neutral plasma
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08

36. Team Strontium would like to thank you for your attention
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08