This write up is about the physics behind laser operation.

The word LASER is a rare gem: an acronym that not only sounds way cool, but also describes the device nicely. It stands for Light Amplification by the Stimulated Emission of Radiation. I'll discuss how this thing works based on this acronym. Disclaimer: This is a slightly dumbed down version of the truth. Some lasers work a bit differently than described here, and a few thorny details are left out for the convenience of the reader.

Light: Lasers used to work with light. Nowadays, however, we can make lasers that work with electromagnetic radiation other than light, such as ultra-violet and infrared. Brave souls are also researching the possibility of X-ray lasers. The laser was preceded by the MASER, Microwave Emission by the Stimulated Emission of Radiation, which works with microwaves, another form of electromagnetic radiation. While similar in principle, it's technically a very different machine.

Now, we skip the amplification to go to the crux of the matter: Stimulated Emission. We'll start with a bit of basic atomic physics. An atom exists of a positive nucleus with negative electrons spinning around it. Normally, this atom is settled down with all its electrons in the lowest possible energy state. It is, however, possible that an atom is excited by an interaction with another particle-maybe a photon, phonon, ion, or an electron, to a higher energy state. This is called absorption. An atom does not like being in an excited state, and after some time, will drop down to a lower energy state, emitting a photon. This is called emission. The key to laser working is a process described by Einstein in 1917 called stimulated emission. In this process, we start out with an excited atom. Now, a photon, with an energy exactly corresponding to the energy the atom gained in being excited, interacts with the excited atom. Through the wonders of quantum mechanics, this photon stimulates the atom to emit. But this emission is very special: the emitted photon is a clone of the original photon. Same energy, same momentum, same direction, coherent, everything. So, now we have a mechanism of breeding similar photons.

The technical challenge lies in the amplification part. We need to have some way of making sure that we get more and more of these identical photons. This is achieved by placing the lasing medium between two highly reflective mirrors, say 99.99%. This device is known as a Fabry-Perot Interferometer. The photons will start to bounce between the mirrors, creating friends by stimulated emission, and becoming stronger and stronger, forming a standing electromagnetic wave between the mirrors. The 0.01% that leaks out becomes a laser beam.

If only it were this simple. Remember I mentioned absoption? Well, it's linked to stimulated emission via the Einstein relations. If you don't do anything special, the absorbtion will kill so many photons that you will never get significant stimulated emission. The only way to make sure stimulated emission can become dominant is making sure there are more particles excited state than in the lower state. Unfortunateley, statistical physics teaches us that it's impossible to have more particles in the excited state than in the lower state if we are in thermal equilibrium. (For the pedants: this is not strictly true if we also consider degeneracy; however, because it also effects the rates of emission and absorption, the effect cancels). This situation is called population inversion

So, we are now left with making sure we somehow break thermal equilibrium between the lower and higher state. There is a wide variety of tricks to do this, like:

  • In a helium neon laser2, helium is excited. This excited helium transfers its energy to neon, populating an excited state of neon. Of this effect is strong enough, you can get laser action.
  • In an excimer laser3, the excited state consists of molecules of excited atoms, called excimer. However, in the ground state, these molecules cannot exist because the atoms don't bond if they are not excited. So, no ground state population.
  • In many pulsed lasers, flashing lights are used to pump the higher level
I think the Radiation in the word laser was mainly added for coolness.

Because the photons emitted by the lasers are pretty much clones, a laser beam is highly monochromatic, very coherent, and well collimated5. These properties make the laser suitable for a very wide variety of applications.

The first laser was a solid-state ruby laser demonstrated by Maimain in 19606, altough there is some controversy whether Gould was earlier. Gould was awarded a patent for lasers in 19771. Interesting bit of trivia: There is a well-known picture of Maiman posing with a large flash lamp and ruby rod. This is not the first laser; the reporter taking the picture suggested using it because it looked better than the smaller tube and rod Maiman was actually using. More than a few physicists tried to replicate Maiman's laser using such a big flashtube7.

In summary, a laser is a device that creates a collimated, coherent, monochromatic bundle of light using stimulated emission of radiation. A good way of causing population inversion is essential for laser operation.