Laser Cooling Of Atoms

                            Laser Cooling Of Atoms

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 fields.

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

How it works

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 = p_photon/mv = Δv/v

Δv = p_photon/m

Momentum of the photon is: p = E/c = h/λ
Suppose you are floating on a hovercraft, moving with a significant velocity in one direction (due north, for example). Heavy metallic balls are being thrown at you from all four directions (front, back, left, and right), but you can only catch the balls that are coming from directly in front of you. If you were to catch one of these balls, you would slow down due to the conservation of momentum. Eventually, however, you must throw the ball away, but the direction in which you throw the ball away is completely random. Due to conservation of momentum, throwing the ball away will increase your velocity in the direction opposite the ball's. However, since the "throw-away" direction is random, this contribution to your velocity will vanish on average. Therefore your forward velocity will decrease (due to preferentially catching the balls in front) and eventually your movements will entirely be dictated by the recoil effect of catching and throwing the balls.η_cooling = P_cooling/P_electric
η_cooling = cooling efficiency
P_cooling = cooling power in the active material
P_electric = 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 = λ_photon/x
x = number of photons needed to stop the momentum of an atom with mass m and at velocity v
Na atom
m_Na = 3.818∙〖10〗^(-26) kg/atom
v_Na ≈ 300meters/second
λ_photon = 600 nm
λ_photon/x = h/(m_Na v_Na ) ⟹ x = 10372
Conclusion: A total of 10372 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 1 millisecond.
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