Transcript Slide 1

Rydberg & plasma physics using
ultra-cold strontium
James Millen
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
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)
Johannes Rydberg
1854-1919
• This can lead to “frozen” Rydberg gases, where the
interaction energy is much greater than the thermal
energy.
Motivation
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 316 705-708
Motivation
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 316 705-708
Motivation
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
88Sr
energy level diagram
5sns 1S0
5snd 1D2
412.7nm
460.7nm
689nm
32MHz
7.5kHz
698nm
1mHz (87Sr)
1S
1P
1D
3S
3P
Motivation
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
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
The experiment
5s19s 1S0
5s18d 1D2
Coupling
420.47nm
420.40nm
5s5p 1P1
460.7nm
5s2 1S0
• 461nm frequency doubled
diode laser with tapered
amplifier (max. output
~350mW)
Probe
• 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
The experiment
• Strontium is heated in an
oven and collimated with a
nozzle
Coupling
1
• The transmission of the
probe beam is measured
as it is scanned across the
transition
2
Atomic beam
Oven + Nozzle
• When the coupling beam
is turned on there is an
increase in the
transmission of the probe
beam on resonance
Probe
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
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.
~5MHz
~150MHz
~150MHz
~5MHz
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Frequency axis calibration
• Saturated
A fit basedabsorption
on the sumspectroscopy
of six Lorentzians
was
1P lines
was
Scaling
to
usedused.
to resolve
theparameter
5s1S0→ 5pwas
1 usedfor
88
calibrate
Sr and 86the
Sr frequency axis
124.5 MHz
Isotope
32 MHz
Abundance
%
I
F
Shift
(MHz)
Rel.
Strength
84Sr
0.56
0
-
-270.8
1
86Sr
9.86
0
-
-124.5
1
7/2
-9.7
4/15
9/2
-68.9
1/3
11/2
-51.9
2/5
-
0
1
87Sr
88Sr
7.00
82.58
9/2
0
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
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. 74 666
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Isotope shift of EIT peaks
Coupling laser tuned to the 5s5p1P1→5s18d1D2 transition
1)
Signal / V
Signal / V
2)
88Sr
88Sr
Time / s
86Sr
4)
88Sr
Time / s
Signal / V
Signal / V
3)
Time / s
86Sr
88Sr
Time / s
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Isotope shift of EIT peaks - Results
• Singlet-triplet mixing with the
5s18d3D3 state cause massive
(~GHz) hyperfine splitting in
87Sr, so the peak isn’t visible†
Coupling tuned near 5s18d1D2 transition
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
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)
88Sr→86Sr
88Sr→87Sr
(MHz)
(MHz)
5s2 1S0→5s18d1D2
226±7
-
5s2 1S0→5s19s1S0
213±7
62±8
Transition
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
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
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
Strontium energy level diagram
5sns 1S0
5snd 1D2
420nm
460.7nm
689nm
32MHz
7.5kHz
698nm
1mHz (87Sr)
1S
1P
1D
3S
3P
Motivation
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Beam translation
• The original beam
separation was set by the
beamsplitter to 4mm
Coupling
1
2
Atomic beam
Oven + Nozzle
• A translatable mirror
enabled separations of 313mm
• Varied probe power from
30-180μW
Translatable
Probe
• Results were inconclusive
mirror
• Could be Rydberg
autoionization
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
Rydberg Autoionization
5s2 1S0
5s5p 1P1
5sns 1S0
e-
e-
5s 1S0
e-
eSr+
Sr
5pns 1P1
Sr+
Sr2+
e460nm
420nm
420nm
Spectroscopy of strontium Rydberg states using EIT
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
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
The ultra-cold strontium experiment
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
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 / stateselective field ionisation
• MOT coils inside chamber
MOT from Tino group: LENS, Florence
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
The vacuum system
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
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 feedthroughs
• Beam height is 190mm above optical bench
The ultra-cold strontium experiment
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08
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
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
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)
0
→ 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
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
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
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
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
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
Team Strontium would like to thank you for your attention
Rydberg & plasma physics using ultra-cold strontium– Seminar 28/05/08