Laser Cooling0 pages

Lab Application Notes
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Laser Cooling

Tools to help you lead, not follow

Introduction

L

aser cooling is the starting point for many experiments in
atomic physics. By laser cooling, one can prepare a sample of
atoms with a temperature on the order of a few 100 μK and an
RMS velocity < 1 ms-1. Having a sample of slow atoms eliminates
systematic effects due to the Doppler effect and improves
resolution by allowing for long interaction times. It also enables
the atoms to be loaded into shallower magnetic and dipole-force
traps.
On absorbing and emitting light, an atom recoils; the photon
imparts some momentum, albeit a tiny amount, to the atom. This
causes the atom to be both accelerated and decelerated depending
on whether it is moving towards or away from the light source. In a
gas, atoms collide with each other and move in all directions so, on
average, one would not expect the light to have any effect.
However, laser cooling uses a neat trick which makes use of the
Doppler effect1. An atom absorbs light of a specific frequency, a
condition referred to as resonance. The trick is to red-detune the
frequency of the light to just below the resonance frequency of
the atoms. Now, atoms moving towards the light are blue-shifted
into resonance and are more likely to absorb the light whilst
those moving away from the light are red-shifted further away
from resonance and are less likely to absorb the light. This leads
to preferential absorption of the light for atoms moving towards
the light source. As this is the direction in which they receive a
momentum kick opposite to their direction of motion, the atoms
are decelerated or slowed. Following absorption, atoms re-emit
spontaneously and the recoil is in a random direction, however,
they re-absorb and, over many absorptions and emissions, the
momentum exchange accumulates with the average effect that the
atoms are slowed down or cooled.

Optical Molasses

T

he typical configuration for laser cooling atoms uses three
orthogonal pairs of oppositely-directed beams as shown in
Figure 2. In the region where the beams intersect an atom will
always be moving towards at least one of the beams and will feel
a force in the opposite direction to its motion. This type of force
is analogous to that experienced by particles in a viscous medium
and, for this reason, the arrangement of beams is referred to as
optical molasses. The molasses is indeed very sticky: on entering
the molasses region, atoms moving at 300 ms-1 are rapidly slowed
down to < 1 ms-1.

Figure 1. On absorbing a laser photon an atom recoils
and is slowed down. The atom re-emits spontaneously
and recoils in a random direction. Over many
absorptions and emissions, the atom is slowed down or
cooled.

The Doppler effect is where the apparent frequency of the
light source is shifted up/down in frequency when the atom is
moving towards/away from the light source. One can think of
frequency in terms of colour and refer to the light as being blue/
red-shifted.
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