Laser Cooling:Definition,Working Principle,Techniques,5 Uses

What is laser cooling?

Laser cooling refers to the variety of techniques used for cooling down atomic and molecular samples to a near absolute zero temperature. The techniques of laser cooling are based on the fact that an atom (of any metal sample) changes its momentum (and energy) when it absorbs and then re-emits a photon.

The thermodynamic temperature of an atom or molecule ensemble depends on the variance in their momentum or velocity. When the velocities of the particles are homogeneous, their collective temperature is lower. This thermodynamic principle is combined with atomic spectroscopy for conducting laser cooling techniques on molecular or atomic samples.

What is the principle of laser cooling?

Laser cooling is primarily based on the fact that an atom (of any metal sample) changes its momentum (and energy) when it absorbs and then re-emits a photon. For laser cooling, the frequency of the laser is tuned below the frequency of the wave emitted by the atomic transition.

When the atom approaches the laser beam, as a result of the Doppler effect the frequency of light increases with respect to the atom. Therefore, the atoms that move towards the laser beam have an increased probability of absorbing a photon. The converse happens when the atoms move away from the laser beam.

What is the Doppler effect?

The Doppler effect or the Doppler shift refers to the change in frequency of a wave with respect to the observer moving along the wave source. When the waves from a source approach an observer each wave take a bit less time than the previous wave. So, the time span of successive wave crest approaching the observer is reduced. Hence, the frequency is increased. Conversely, when the wave source moves away from the observer, the timespan is increased and the frequency is reduced.

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Waves emitted by a source moving from the right to the left. Frequency increases from right to left. Image source: !Original:TkarcherVector: TatouteDoppler effect diagrammaticCC BY-SA 3.0

What are the types of laser cooling?

The various techniques of laser cooling are:

Doppler cooling:

Doppler cooling the most commonly used laser cooling technique. Doppler cooling involves tuning the frequency of light a little below the electronic transition in an atom. The atoms absorb more photons when they move towards the source of light because of the Doppler effect as the light is detuned to a lower frequency. Hence, the atoms scatter more photons and lose a momentum equivalent to the momentum of the photon each time. With a decrease in momentum, the kinetic energy of the atoms is reduced thereby lowering the overall temperature of the sample to the Doppler cooling limit (which is around 150 microkelvin)

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Demonstration of Doppler cooling. Image source: Rickky678Rubidium85 laser cooling, marked as public domain, more details on Wikimedia Commons

Sisyphus cooling:

Sisyphus cooling is also known as polarization gradient cooling. It is a variant of laser cooling technique that involves the shining of two counter-propagating laser beams having orthogonal polarization on an atom or molecule sample. A standing wave is generated as a result of the two interfering laser beams. The atoms tend to lose kinetic energy as they move along with the standing wave towards the higher potential. At the maximum potential, optical pumping moves the atoms to a lower-energy state removing the potential energy that it gained. This loss of energy contributes towards the cooling of the atoms below the Doppler cooling limit.

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Demonstration of Sisyphus cooling principle. The atoms move along the standing wave to a higher potential and are then sent back to a lower potential. Image source: http://By Stefan.Original uploader was StefanPohl at German Wikipedia – selbst gemalt, Public Domain,

Resolved sideband cooling:

Resolved sideband cooling is another variant of laser cooling techniques that specializes in cooling down tightly bound ions and atoms below the Doppler cooling limit. Resolved sideband cooling is generally conducted after the application of Doppler cooling for trapping the atoms to their motional ground state. The cooled atom is then considered as a good quantum mechanical harmonic oscillator. In this technique, cooling is achieved by tuning the laser beam frequency to the lower red sideband.

Internal structure of Ca 40 ion with zeeman splitting
Demonstration of resolved sideband cooling. Blue: Doppler cooling, Red: sideband cooling path, yellow: spontaneous decay, Green: spin polarization pulses Image source: BtabakovInternal structure of Ca 40 ion with zeeman splittingCC BY-SA 3.0

Raman Sideband cooling:

Raman sideband cooling refers to a sub-coil cooling technique that cools atoms below the Doppler cooling limit by using optical methods. In Raman cooling, the process begins from atoms present in a magneto-optical trap. The sites having atoms can be converted into a harmonic trap if the lasers of the optical lattice are powerful enough. The atoms are likely to be trapped in one of the levels of the harmonic oscillator. The main target of Raman sideband cooling is to put the atoms of the lattice into the ground state of the harmonic potential. This provides a high density of atoms at a low temperature.

Raman sideband cooling demonstration. Image source: LcormanRamanSidebandCoolingCC BY-SA 3.0

What are the uses of laser cooling?

Laser cooling is mainly used for experimental purposes. Quantum physics experiments require ultracold atoms that are generated by laser cooling. Quantum effects like Bose-Einstein condensation need atoms near absolute zero temperature. Earlier, laser cooling was conducted only on atoms. However, nowadays laser cooling is performed on more complex systems such as a diatomic molecule or a macro-scale object. Laser cooling has contributed a lot to the study of quantum particles.

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