Laser Cooling and Optical Molasses

    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 motion 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 cooling.  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.

        Methods of Laser Cooling

                Doppler Cooling

    The first (and simplest) method of laser cooling is known as Doppler Cooling.  This method exploits the coherent nature of laser light (specifically, how the photons are traveling in the same direction) as well as the principles behind Doppler-shifts (hence the name of this method).

            How It Works:

    If a laser is tuned to the right frequency, the particles in the gas will absorb photons and become excited.  However, each atom will emit a photon (with the same frequency) 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 key 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 average, the effect is that the atoms feel a force in the direction away from the laser.

    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 the laser.

Visualization of why the net force on a particle is away from the laser

    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.

An array of 6 lasers along the x, y, and z axes

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.

Home Vapor-Compression Refrigeration The Peltier Effect and Thermoelectric Cooling Laser Cooling and Optical Molasses Magnetic Cooling References
S H Price      26 March 2007     Physics 212 Web Project