Laser cooling
From Wikipedia, the free encyclopedia
Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled
down to near absolute zero through the interaction with one or more laser light fields.
Simplified principle of Doppler laser cooling:
1 A stationary atom sees the laser neither red- nor blue-shifted and does not absorb the photon.
2 An atom moving away from the laser sees it red-shifted and does not absorb the photon.
3.1 An atom moving towards the laser sees it blue-shifted and absorbs the photon, slowing the atom.
3.2 The photon excites the atom, moving an electron to a higher quantum state.
3.3 The atom re-emits a photon. As its direction is random, there is no net change in momentum over
many atoms.
The first example of laser cooling, and also still the most common method (so much so that it is still
often referred to simply as 'laser cooling') is Doppler cooling. Other methods of laser cooling
include:

Sisyphus cooling

Resolved sideband cooling

Velocity selective coherent population trapping (VSCPT)

Anti-Stokes inelastic light scattering (typically in the form of fluorescence or Raman
scattering)

Cavity mediated cooling

Sympathetic cooling

Use of a Zeeman slower
Contents
[hide]

1How it works

2Doppler cooling

3Other methods of laser
cooling

4Uses

5See also

6References

7External links
How it works [edit]
A laser photon hits the atom and causes it to emit photons of a higher average energy than the one
it absorbed from the laser. The energy difference comes from thermal excitations within the atoms,
and this heat from the thermal excitation is converted into light which then leaves the atom as a
photon. This can also be seen from the perspective of the law of conservation of momentum. When
an atom is traveling towards a laser beam and a photon from the laser is absorbed by the atom, the
momentum of the atom is reduced by the amount of momentum of the photon it absorbed. Δp/p =
pphoton/mv = Δv/v
Δv = pphoton/m
Momentum of the photon is: p = E/c = h/λ If you were floating on a hovercraft, moving a significant
velocity in one direction, and randomly threw metallic balls off the hovercraft, eventually your
velocity would slow down and your movements would entirely be dictated by the recoil effect of
throwing the balls. That is how laser cooling works..[1]
ηcooling=Pcooling/Pelectric
ηcooling=cooling efficiency
Pcooling=cooling power in the active material
Pelectric=input electric power to the pump light source
h/λ = p= mv
h=Planck's constant (h=6.626∙〖 10〗
(-34)
J∙s)
λ = de Broglie's wavelength
p = momentum of the atom
m = mass of the atom
v = velocity of the atom
Example: λ = h/mv = x∙λphoton
x = number of photons needed to stop the momentum of an atom with mass m and at velocity v
Na atom
mNa = 3.818∙〖 10〗
(-26)
kg/atom
vNa ≈ 300meters/second
λphoton = 600 nm
x∙λphoton = h/(mNa vNa ) ⟹ x = 1037
Conclusion: A total of 1037 photons are needed to stop the momentum of one sodium atom with a
velocity of about 300 m/s. Experiments in laser cooling have yielded a number of 10^7 photons to
be emitted from a laser per second. This sodium atom could be stopped in space in just a matter of
0.1 milliseconds.
Doppler cooling [edit]
Main article: Doppler cooling
Doppler cooling, which is usually accompanied by a magnetic trapping force to give
a magneto-optical trap, is by far the most common method of laser cooling. It is used to cool low
density gases down to the Doppler cooling limit, which forRubidium 85 is around 150 microkelvin.
As Doppler cooling requires a very particular energy level structure, known as a closed optical loop,
the method is limited to a small handful of elements.
In Doppler cooling, the frequency of light is tuned slightly below an electronic transition in the atom.
Because the light is detuned to the "red" (i.e., at lower frequency) of the transition, the atoms will
absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one
applies light from two opposite directions, the atoms will always scatter more photons from the laser
beam pointing opposite to their direction of motion. In each scattering event the atom loses
a momentum equal to the momentum of the photon. If the atom, which is now in the excited state,
then emits a photon spontaneously, it will be kicked by the same amount of momentum, but in a
random direction. Since the initial momentum loss was opposite to the direction of motion, while the
subsequent momentum gain was in a random direction, the overall result of the absorption and
emission process is to reduce the speed of the atom (provided its initial speed was larger than the
recoil speed from scattering a single photon). If the absorption and emission are repeated many
times, the average speed, and therefore the kinetic energy of the atom will be reduced. Since
the temperature of a group of atoms is a measure of the average random internal kinetic energy,
this is equivalent to cooling the atoms.
Other methods of laser cooling [edit]
Several somewhat similar processes are also referred to as laser cooling, in which photons are
used to pump heat away from a material and thus cool it. The phenomenon has been demonstrated
via anti-Stokes fluorescence, and
both electroluminescentupconversion and photoluminescent upconversion have been studied as
means to achieve the same effects. In many of these, the coherence of the laser light is not
essential to the process, but lasers are typically used to achieve a highirradiance.
Uses [edit]
Laser cooling is primarily used for experiments in Quantum Physics to achieve temperatures of
near absolute zero(−273.15°C, −459.67°F). This is done to observe the unique quantum effects
that can only occur at this heat level. Generally, laser cooling has been only used on the atomic
level to cool down elements. This may soon change, as a new breakthrough in the technology has
successfully cooled a macro-scale object to near absolute zero.[2]
See also [edit]

List of laser articles

Optical tweezers

Mössbauer effect

Mössbauer spectroscopy

Timeline of low-temperature technology

Researchers in laser cooling

Claude Cohen-Tannoudji

Steven Chu
References [edit]
1. ^ Anissimov, Michael, and Bronwyn Harris. What Is Laser Cooling?. WiseGeek.
Retrieved April 11, 2013, fromhttp://www.wisegeek.com/what-is-laser-cooling.htm
2. ^ Massachusetts Institute of Technology (2007, April 8). Laser-cooling Brings
Large Object Near Absolute Zero. ScienceDaily. Retrieved January 14, 2011,
from http://www.sciencedaily.com/releases/2007/04/070406171036.htm

D.J. Wineland, R.E. Drullinger and F.L. Walls (1978). "Radiation-pressure cooling of bound
resonant absorbers". Phys. Rev. Lett. 40 (25):
1639. Bibcode:1978PhRvL..40.1639W. doi:10.1103/PhysRevLett.40.1639.

W. Neuhauser, M. Hohenstatt, P. Toschek and H. Dehmelt (1978). "Optical-sideband
cooling of visible atom cloud confined in parabolic well". Phys. Rev. Lett. 41 (4):
233. Bibcode:1978PhRvL..41..233N. doi:10.1103/PhysRevLett.41.233.

Nobel Lecture by William D. Phillips, Dec 8, 1997.

Foot, C.J. Atomic Physics. Oxford University Press (2005).

Cohen-Tanoudji, Claude (2011). Advances in Atomic Physics. World Scientific.
p. 791. ISBN 978-981-277-496-5.

Laser cooling of a semiconductor by 40 kelvin - Jun Zhang, Dehui Li, Renjie Chen & Qihua
Xiong
External links [edit]

Bowley, Roger; Copeland, Ed (2010). "Laser Cooling". Sixty Symbols. Brady Haran for
the University of Nottingham.