Progress towards laser cooling
strontium atoms on the
intercombination transition
Danielle Boddy
Durham University – Atomic & Molecular Physics group
The team
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Motivation: Rydberg physics
Ionization
threshold
States of high principal quantum number n.
Energy
Exaggerated size and lifetimes.
Can be prepared through laser excitation.
Greatly enhanced inter-atomic interactions.
Strong, tunable, long-range dipole-dipole
interactions among the atoms.
Applications include quantum
computation.
M. Saffman et. al., Rev. Mod. Phys. 82, 2313 (2010)
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Motivation: Dipole blockade
Y Miroshnychenko et al., Nat. Phys. 5, 115-118 (2009)
E Urban et al., Nat. Phys. 5, 110-114 (2009)
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Motivation: An introduction to strontium
Sr88 is an alkaline earth metal with no hyperfine
structure.
Spectroscopy of strontium Rydberg states using
electromagnetically induced transparency
S. Mauger, J. Millen and M. P. A. Jones
J. Phys. B: At. Mol. Opt Phys. 40, F319 (2007)
Two valence electrons permits two
electron excitation.
Two-electron excitation of an interacting cold Rydberg gas
J. Millen, G. Lochead and M. P. A. Jones
Phys. Rev. Lett. 105, 213004 (2010)
Ground state
Rydberg state
Doubly excited
state
At present, we’re investigating the spatial
excited state distribution.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Motivation: Dipole blockade regime
Blockaded
No blockade
rB
T ~ 5 mK
T ~ 400 nK
Density ~ 1 x 109 cm-3
Density ~ 1 x 1012 cm-3
How do we enter the dipole blockade regime?
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Motivation: Laser cooling of strontium
1S
0
→ 3P1 intercombination transition → TD ≈ 180 nK.
Photon recoil limits TD → Tmin ≈ 460 nK.
1P
1
3P
Introduce two stages of cooling:
First cool on the (5s2) 1S0 → (5s5p) 1P1.
3P
λ = 461 nm
Γ = 2π x 32 MHz
1st stage cooling
3P
2
1
0
λ = 689 nm
Γ = 2π x 7.5 kHz
2nd stage cooling
Second cool on the narrow-line (5s2) 1S0
→ (5s5p) 3P1 .
1S
0
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Outline
Simple laser stabilization set-up
Laser system
Pound-Drever-Hall (PDH)
Locking to an atomic transition
Fluorescence
Electron shelving
Summary
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Simple laser stabilization set-up
Laser
Fabry-Perot
Atomic
system
cavity
signal
Red MOT
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Simple laser stabilization set-up
Laser
Fabry-Perot
Atomic
system
cavity
signal
Red MOT
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Laser system
Compared old and new designs.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Laser system
frequency
time
10 s
OLD
1
2
Wavemeter
NEW
OLD
1
2
Wavemeter
frequency
NEW
10 s
time
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Laser system
Old
New
New design fluctuates more in the short term.
Little difference between the long term stability.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Simple laser stabilization set-up
Laser
Fabry-Perot
Atomic
system
cavity
signal
Red MOT
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Pound-Drever-Hall (PDH) technique
Require the laser linewidth < 7.5 kHz.
Noise broadens the linewidth to the MHz regime.
Uses Fabry-Perot cavity as a frequency reference.
Cavity peaks are spaced
by the free spectral range :
c
 FSR 
2L
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Pound-Drever-Hall (PDH) technique
Phase modulator adds sidebands to the laser.
High-finesse Fabry-Perot cavity measures the time-varying frequency of the
laser input.
An electronic feedback loop works to correct the frequency error and
maintain constant optical power.
Phase

modulator
4
Etalon
Laser
Current
modulation
Lock Box
Piezo
FPD

Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Pound-Drever-Hall (PDH) technique
Atomic signal
Lock Box
Fast
feedback to
diode



2
2
FPD
Feedback to
cavity piezo

PS
4
Slow
feedback to
piezo
Laser
A crystal oscillator phase modulates the 689 nm
beam at a frequency of 10 MHz.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Pound-Drever-Hall (PDH) technique
Laser locks to the central feature of the PDH error signal
(a)
(b)
(c)
(d)
Increasing the gradient of the error signal strengthens the lock and reduces
the linewidth.
Gradient depends on sideband power: carrier
power ratio.
Gradient steepest when Ps = 0.42 Pc
Theory: See E. Black., Am. J. Phys. 69 (1) 79 (2001)
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Pound-Drever-Hall (PDH) summary
Generate PDH signal
Gradient of error signal → strength of lock and laser linewidth
NEXT STEP: Finish high bandwidth servo
IMPROVEMENTS: Build high-finesse cavity
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Simple laser stabilization set-up
Laser
Fabry-Perot
Atomic
system
cavity
signal
Red MOT
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Locking to an atomic transition
CHALLENGE:
Detecting the transition.
Two detection methods:
1.
Electron Shelving
2.
Fluorescence
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving
Excite atoms to the 3P1 and measure the rate at which these atoms decay out of the
state.
Photon scattering rate is proportional to the linewidth of the transition.
1P
1
3P
λ = 461 nm
1
λ = 689 nm
1S
0
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving
photodiode
1P
1
atomic beam
3P
λ = 461 nm
1
The amount of scattered light is proportional to the
number of atoms initially in the 1S0 ground state.
1S
0
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving
photodiode
1P
1
atomic beam
3P
λ = 461 nm
1
λ = 689 nm
1S
0
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Experiment
photodiode
atomic beam
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Experiment
photodiode
atomic beam
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Experiment
photodiode
atomic beam
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Experiment
≈ 32 MHz
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Lifetime measurement
Gradient: (8.9 ± 0.2) x 10-2 mm-1
Using a velocity of 500 ms-1
Lifetime of 3P1 is (23 ± 1) μs
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Crossed beams
photodiode
atomic beam
FWHM crossed beams is ≈ 20 MHz.
Linewidth has reduced by 1/3.
This is not narrow enough for the Fabry-Perot to lock to!
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Electron shelving: Summary
Detected the transition indirectly via electron shelving.
Determined the lifetime of the 3P1 state.
And the lineshape? Work in progress
Tried crossing the beams:

 Did the linewidth reduce?


Is this narrow enough for the laser to lock to?
Try a direct method of detection.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The experiment
Strontium has negligible vapour pressure at
room temperature → heated to 900 K.
CCD camera takes spatially
resolved images of the
fluorescence.
Exposure length set to 65.5 ms.
atomic beam
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The experiment
(a)
(a)
Slice along direction of laser
beam → absorption and decay.
(b)
Slice along direction of atomic
beam → transverse velocity
distribution.
(b)
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The experiment
Gradient: (9.0 ± 0.3) x 10-2 mm-1
Using a velocity of 500 ms-1
Lifetime of 3P1 is (22.2 ± 0.7) μs
Other time resolved fluorescence
detection: τ = (21.3 ± 0.5) μs
See R Drozdowski., Phys. D. 41:125 (1997)
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The experiment
BUT what about the absorption?
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The model
Solves optical bloch equations (OBEs) for a two level atom as a function of
distance.
Velocity distribution of atoms f (v) α v3e

mv 2
2 k BT
Randomly selects a value of f (v) and v ' .
If f (v)  f (v' ) the value of v ' is kept.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The model
Assuming the laser is on resonance the only other unknown in the OBEs is
the Rabi frequency.
Top hat pulse:

d.E

x
Gaussian pulse:

d .E
e

 2 ( x  waist ) 2
waist 2
x
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The model
Velocity of 500 ms-1
Excited population state 22
1.0
0.8
0.6
0.4
0.2
Distance m
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
Top hat pulse
Gaussian pulse
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: The model
Excited population state 22
0.7
0.6
0.5
0.4
Excited population state 22
0.3
2 x waist
0.2
0.6
0.1
Distance m
0.001
0.002
0.003
0.004
0.005
0.006
0.4
0.2
Distance m
0.001
0.002
0.003
0.004
0.005
0.006
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Fluorescence: Summary
Detected the transition directly.
Determined the lifetime of the 3P1 state.
Written code to model absorption and decay.
Data and theory don’t quite agree.
Need to find source of problem.
NEXT: Try locking to this fluorescence signal.
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Summary
689 nm laser built and tested.
Need to finish PDH high bandwidth servo circuit.
Build high-finesse cavity.
Tested an indirect and direct method to detect the transition.
Measured lifetime of 3P1 state from both methods.
Try locking to fluorescence signal.
If this works….GREAT!
If it doesn’t work….try pump-probe spectroscopy
Red MOT → colder atoms
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011
Questions?
Thanks for listening 
Any questions?
Progress towards laser cooling strontium atoms on the intercombination transition - May 2011