Laser Cooling

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Laser Cooling
The First Steps
Colin Fletcher Union College Physics
Dept. Physics 190 fall 02
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What Is Laser Cooling?

• Proposed only 25 years ago

Laser Cooling ? The use of light to exert forces
on neutral atoms that will slow them down, thus
cooling them.
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Why Do We Care?

• Atoms are difficult to study
• Transit time
• Blurred spectroscopy

Cool them, slow them down, and better
measurements can be made.
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Scattering Force

• Photons carry momentum
• Upon absorption, atoms receive a Kick
• Spontaneous emissions all cancel out
• Allows lasers to apply force

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Doppler Cooling

• Need velocity sensitivity
• Utilizes Doppler Effect
• Use red-detuned laser
• Velocity always reduced

• Provides a tool for deceleration, but one big
  problem,
• Atom slows, no longer absorbs
• No more Doppler shift

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Increasing Effectivness

• Chip the laser
• or
• 2) Apply magnetic field to utilizing Zeeman
  Effect

• Zeeman Effect
• Atoms behave like small magnets
• Applying spatially dependent magnetic field
  changes transition size!

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Optical Molasses

• Add counter propagating laser
• Each laser is red-detuned
• Forces only oppose motion
• Results in viscous-like damping

Can easily be made 3-D, just add pair for each
Cartesean axis.
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MOT (Magneto-Optical Trap)

• Use lasers to cool,
• Use magnetic field to trap.
• Spatially varying
• Utilizes Zeeman Effect
• Utilizes polarization of beam

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First Step In Construction

• Need a laser for cooling
• Must have specific frequency.
• Must be extremely constant.

• For this we will use
• Stable He-Ne laser as a base
• Fabry-Perot interferometer
• Digital feedback control

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Fabry-Perot Interferometer