Rydberg

1
Rydberg plasma physics using ultra-cold
strontium
James Millen
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
2
Outline

• Motivation

• Spectroscopy of strontium Rydberg states using
  electromagnetically induced transparencyMauger,
  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 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 1854-1919
Motivation
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
4
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 316 705-708
Motivation
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
5
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 316 705-708
Motivation
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
6
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
  I0), 87Sr has I9/2 (Fermionic)
• Negligible vapour pressure at room temperature

Motivation
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
7
88Sr energy level diagram
5sns 1S0
5snd 1D2
1S
1P
1D
3S
3P
Motivation
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
8
Why strontium?

• Singlet-triplet mixing leads to narrow
  intercombination lines, allowing cooling to ltµK
• 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
The experiment

• 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
The experiment

• 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. 98
113003
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

• 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

• Saturated absorption spectroscopy was used to
  resolve the 5s1S0? 5p1P1 lines for 88Sr and 86Sr

• A fit based on the sum of six Lorentzians was
  used. Scaling parameter was used to calibrate the
  frequency axis

32 MHz
Eliel et. al. Z. Phys. A 311 1, Kluge Sauter Z.
Phys. 270 295
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 ?(v)

• ?3 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.
74 666
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
Signal / V
88Sr
88Sr
Time / s
Time / s
86Sr
4)
3)
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 isnt 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

Spectroscopy of strontium Rydberg states using EIT
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
18
Further study
Coupling
1
2
Atomic beam
Oven Nozzle
Probe
Spectroscopy of strontium Rydberg states using EIT
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
19
Further study
Coupling
1
2
Atomic beam
Oven Nozzle
Probe
Spectroscopy of strontium Rydberg states using EIT
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
20
Strontium energy level diagram
5sns 1S0
5snd 1D2
1S
1P
1D
3S
3P
Motivation
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
21
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 30-180µW
• Results were inconclusive
• Could be Rydberg autoionization

Coupling
2
1
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
5s 1S0
5s2 1S0
5s5p 1P1
5sns 1S0
5pns 1P1
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
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
The ultra-cold strontium experiment
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
25
Requirements

• Three orthogonal axis for a blue (460.7nm) MOT
• Potential for a red (689nm) MOT (sub µK cooling)
• Axis for a dipole trap

• 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
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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
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
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 ultra-cold strontium experiment
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
31
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 calculations

• Field changes by lt1 in central 4mm cube

The ultra-cold strontium experiment
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
33
Online resources

• See website http//massey.dur.ac.uk/resources/lab
  _resources.html

The ultra-cold strontium experiment
Rydberg plasma physics using ultra-cold
strontium Seminar 28/05/08
34
Current progress - Apparatus

• Pumped down to 10-10 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
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