Vapor-compression refrigeration and
thermoelectric cooling are
macroscopic cooling methods, as these methods can effectively cool a
large amount of a substance or some other relatively large object.
However, there are
methods of cooling that do not rely on moving
parts and do not require large fans and radiators to dissipate the heat
from a heat sink. These methods are based on reducing the
of the individual particles of a substance (usually a low density gas),
so they are only practical
on a microscopic scale.
One microscopic-scale cooling technique is Laser
Laser cooling is an entirely optical method of reducing the
motion of small particles (such as atoms or ions) in a low-density gas,
and thus reducing their temperature. Laser cooling methods have
been used to cool particles into the millikelvin, microkelvin, and nanokelvin ranges (depending on the exact procedure).
The collection of
low-temperature particles that resulted was nicknamed Optical
Molasses by Steven Chu in 1985.
of Laser Cooling
The first (and simplest) method of laser
cooling is known as Doppler
Cooling. This method exploits the coherent nature of laser
(specifically, how the photons are traveling in the same direction) as
well as the principles behind Doppler-shifts (hence the name of this
How It Works:
If a laser is tuned to the right
particles in the gas will absorb photons and become excited.
However, each atom will emit a photon (with the same
in an arbitrary direction shortly afterwards, returning to a
less-excited state. Since photons have momentum, the atoms
experience a recoil when they absorb and emit photons. The
point is that the photons that are absorbed by the atoms (and emitted
by the laser) all travel in the same direction, while the photons that
the atoms emit travel in any arbitrary direction. So, on
the effect is that the atoms feel a force in the direction away from
Just bombarding the particles with laser
light is not the way to efficiently cool the particles. If the
particles are already moving slowly, then the average force from photon
collision/expulsions would cause them to move more again, raising the
temperature of those particles. This is a step backwards as far as
cooling the gas particles.
So, the laser is tuned to a frequency
slightly lower than the frequency which allows the particles to easily
absorb photons. This is done because the particles that are
moving slowly (or away from the laser) will not absorb photons.
At the same time, the particles that are moving towards the
laser will observe a Doppler shift in the frequency of the laser light,
meaning that those particles could easily absorb photons.
These particles then experience a force backwards away from
If lasers are set up in pairs along an
imaginary x, y, and
z axes, the
particles will experience a net force away from each laser.
This limitation of three degrees of freedom reduces the
particles' kinetic energy and thus cools the gas. To contain
the gas (and not in some sort of physical box which could transfer heat
energy to the particles when they collide with the sides), non-uniform
magnetic fields are used to supply the required trapping force.
As the particles cool (carefully trapped
within their magnetic fields and under the assault of six lasers), the
frequency of the laser light needs to be periodically reduced, since as
the particles cool they will reach the point where not even those that
are travelling towards a laser can readily absorb a photon.
As the frequency is reduced, the process continues until the
limit (the "Doppler limit") is reached, where the random collisions and
interactions of the particles prevents any further cooling. This limit
is typically in the millikelvin range.
Other Methods of Laser Cooling
- Sisyphus cooling:
- cools particles down
to the "recoil limit," which is colder than is possible
using Doppler cooling.
- Velocity-selective coherent population trapping:
- cools the particles even
farther, down to the nanokelvin range.