Lasers Can COOL Atoms..... Here Is How It's Done.....
With The Right Type of Light, Atoms Can Be Cooled To Near Absolute Zero
Introduction
A typical view of laser light is that is can heat things with the intense focused light. There are many video examples of lasers popping balloons and igniting matches, and this is true. The high powered laser with their high intensity means that it is delivering a whopping amount of light and energy on a small area, which most of the time will cause an object to heat.
Image credit:Laser Cooling Light
When we arrive to the realm of very small objects like atoms, nano-particles and micro-objects, it is possible with the right frequency or wavelength of light to cool the object. The majority of the article will focus on cooling atoms, but I will mention a bit about larger objects.
Atoms have a property called energy levels, the electrons can only orbit the atoms with specific energies. They can be excited to higher energetic states with excitation from a photon emitted by a laser. When it is excited it can decay back to the lower energy level and emit a photon of the same energy that excited it. By doing so it experiences a force and change in the momentum. This is the mechanism that ultimately allows for cooling of atoms with light.
Image credit: Energy levels Example
Laser Cooling
Image credit:Laser Cooling
So let's get into a bit more detail now. Firstly the physics and formula that is truly involved is not suitable to discuss here as the purpose is to introduce people to Laser cooling, not to teach the subject, so i'll stick to the basics, and i am sure you agree it's a good idea.
Sodium can be heated in a special oven with a hole, it is heated to 900 Kelvin where it has reached the gaseous state, due to the high temperature and pressure in the oven, it is forced through the small hole as a beam of sodium atoms. The beam of sodium atoms at 900 Kelvin has speeds of about 1000 meters per second.
Sodium has one electron in it's outer atomic shell which has a distinct resonance transition between two energy levels of 589 nm (nano meters). So by using a laser that is mono-chromatic (single colour) of 589 nm, the transition between the sodium energy levels can be activated.
Image credit:- Self obtained physics notes
There a laser is used that shines in the opposite direction of the sodium atoms. When the sodium absorbs a photon and increases the energy level, it experiences a change in momentum and force that basically slows the atom down in the direction it is moving. However it then spontaneously decays back to a lower energy level emitting a photon in a random direction, which changes its momentum and speed again. The trick is that over a very large number of photons, the displacement and change in momentum from the random spontaneous emission averages to zero, and the atom has slowed down in the direction of travel.
The atom can be slowed to speed of about 33 meters a second which is a temperature of about 1.5 Kelvin, so its cooled by a factor of 1/600. Its now -272 degree celsius.
It's possible to cool down further, and get colder.
Magneto-Optical Trap M.O.T
The MOT was an invention invisiged by David. E. Pritchard and is a critical device used to cool atoms further.
Image credit:- wikipedia
When the atoms are cool enough so they move less and have lower kinetic energy, then it is possible that a magnetic field can trap the atoms in a negative potential well. The atoms are still quite hot on the quantum scale, the atoms will still be jiggling around with speeds of 10's meters per second. It this point in trap six laser beams are used, 3 orthogonal pairs that propagate in opposite directions as you can see in the image. The cooling technique is called optical molasses, as the atoms almost experience like a syrup effect from the light.
This technique is quite complicated but it effectively reduces the motion in 3-spatial dimensions , using the laser cooling technique described. The point is, the first stage using just the laser is important to cool the atoms enough of that they can be trapped by a magnetic field.
Using the Optical Molasses technique implementing the MOT, the atoms can be cooled to temperatures of about 0.01 Kelvin, which I am sure you agree is a lot colder than before.
Optical Lattice and Evaporative Cooling
Image Credit:- Joint Quantum Institute
By fine tuning the 6 lasers, its possible to create something called an optical lattice but setting up a standing wave of light in the lasers. The image shows a depiction of atoms trapped in a lattice. The optical traps have depths, and the depths are given in terms of energy, an atom with lower energy than the trap depth can be trapped in the well.
Image Credit:- Physics.org
Using this it is possible to cool the atoms further using a technique called evaporative cooling, it's like blowing on a hot cup of coffee. Hotter atoms will move faster and have higher kinetic energies, these atoms if trapped will exist towards the top of the trap. By lowering the height of the trap for a moment the hotter atoms in the lattice can escape, the system is then allowed to reach thermal equilibrium and the process repeated. The process is limited by statistical thermal dynamics by the number of trapped atoms, you still want a system with a high enough number of atoms to make measurements in practice.
Temperatures can be reached of nano Kelvin, that 0.000000001 Kelvin with this technique. At this stage, atoms that are Bosons form something called a Bose-Einstein Condensate, which you can read about here:-
Conclusion
So by using the fundamental properties of atomic energy levels and transitions it's possible to cool atoms to nano Kelvins, using such techniques as Laser Cooling, Optical Molasses with MOT, and Evaporative Cooling.
I hope you enjoyed, to learn more please see the references. If you liked what you read then feel free to upvote, follow, and resteem.
References:-
Laser Cooling
Magneto-Optical Trap
Evapourative Cooling
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