Laser Cooling Laser Cooling 1. Doppler Cooling – optical molasses. 2. Magneto-optical trap. 3. Doppler temperature.
Laser Cooling 1. Doppler Cooling – optical molasses. 2. Magneto-optical trap. 3. Doppler temperature.
Dec 31, 2015
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Laser CoolingLaser Cooling
1. Doppler Cooling – optical molasses.
2. Magneto-optical trap.
3. Doppler temperature.
Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?Lab frame
vground
excited
Lab frame
vground
excited
Atom’s frame
' '
Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?
Lab frame
vground
excited
Atom’s frame
' '
Lab frame, after absorption
v-vrecoil
Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?
Lab frame
vground
excited
Atom’s frame
' '
Lab frame, after absorption
v-vrecoil
Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?
Lab frame
vground
excited
Atom’s frame
' '
Absorb a photon atom gets momentum kick.
Repeat process at 107 kicks/s large deceleration.
Emitted photons are radiated symmetrically do not affect motion on average
Absorb a photon atom gets momentum kick.
Repeat process at 107 kicks/s large deceleration.
Emitted photons are radiated symmetrically do not affect motion on average
k
Lab frame, after absorption
v-vrecoil
Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?
Lab frame
vground
excited
Atom’s frame
' '
Absorb a photon atom gets momentum kick.
Repeat process at 107 kicks/s large deceleration.
Emitted photons are radiated symmetrically do not affect motion on average
Absorb a photon atom gets momentum kick.
Repeat process at 107 kicks/s large deceleration.
Emitted photons are radiated symmetrically do not affect motion on average
k
Lab frame, after absorption
v-vrecoil
m/s
m/s2 87Rb: = - I = Isat
Vdoppler ~ 10 cm/s
Vrecoil = 6 mm/s
Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
Problem:
Doppler cooling reduces momentum spread of atoms only.
Similar to a damping or friction force (optical molasses).
Does not reduce spatial spread.
Does not confine the atoms.
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
Solution:
Spatially tune the laser-atom detuning with the Zeeman shift from a spatially varying magnetic field.
zB,
~10 G/cm~14 MHz/cm
Problem:
Doppler cooling reduces momentum spread of atoms only.
Similar to a damping or friction force (optical molasses).
Does not reduce spatial spread.
Does not confine the atoms.
Magneto-Optical TrapMagneto-Optical Trap
E
e
g
2-level atom2-level atom
E
e
g
2-level atom2-level atom
magnetic gradientmagnetic gradient
+
zBB
Magneto-Optical TrapMagneto-Optical Trap
E
e
g
4-level atom
|g “F=0”, |e “F=1”
4-level atom
|g “F=0”, |e “F=1”
magnetic gradientmagnetic gradient
+
z
mF=0
mF=0
mF=-1
mF=+1
BB
Magneto-Optical TrapMagneto-Optical Trap
E
e
g
4-level atom
|g “F=0”, |e “F=1”
4-level atom
|g “F=0”, |e “F=1”
magnetic gradientmagnetic gradient
+
mF=0
mF=0
mF=-1
mF=+1
zBB
Magneto-Optical TrapMagneto-Optical Trap
E
e
g
4-level atom
|g “F=0”, |e “F=1”
4-level atom
|g “F=0”, |e “F=1”
magnetic gradientmagnetic gradient
+
mF=0
mF=0
mF=-1
mF=+1
zBB
Magneto-Optical TrapMagneto-Optical Trap
E
e
g
4-level atom
|g “F=0”, |e “F=1”
4-level atom
|g “F=0”, |e “F=1”
magnetic gradientmagnetic gradient
+
mF=0
mF=0
mF=-1
mF=+1
zBB
-
+
Magneto-Optical TrapMagneto-Optical Trap
E
e
g
4-level atom
|g “F=0”, |e “F=1”
4-level atom
|g “F=0”, |e “F=1”
magnetic gradientmagnetic gradient
+
mF=0
mF=0
mF=-1
mF=+1
zBB
-
+
Magneto-Optical TrapMagneto-Optical Trap
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
~ 100 K
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)
109 87Rb atoms
Francium MOTFrancium MOT
PROBLEM: Accelerator produces only 106 Fr atoms/s.
Very difficult to work with.
SOLUTION:SOLUTION: Attach a Francium Magneto-Optical Trap to the accelerator.
Cold Francium is concentrated in ~1 mm3 volume.
With T < 100 K, Doppler broadening is negligible.
Long integration times.
Minimally perturbative environment (substrate free).
Francium MOTFrancium MOT
PROBLEM: Accelerator produces only 106 Fr atoms/s.
Very difficult to work with.
SOLUTION:SOLUTION: Attach a Francium Magneto-Optical Trap to the accelerator.
Cold Francium is concentrated in ~1 mm3 volume.
With T < 100 K, Doppler broadening is negligible.
Long integration times.
Minimally perturbative environment (substrate free).
MOT with ~105 210Fr atoms
MOT collection efficiency ~ 1 %
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