Einstein and the Laser

The word laser is actually an acronym (The first letter of each descriptive word in the phrase spells a new word). It stands for: "Light Amplification by the Stimulated Emission of Radiation."

You all know what light is. And amplification is to increase something, in this case, light. But what does "Stimulated Emission of Radiation" mean? Lets find out.

Electrons in atoms can occupy many different energy levels. The further away an energy level is from the nucleus, the higher its energy. So an electron occupying energy level E4 in this picture, would have a higher energy than an electron occupying any of the other three energy levels.

When an electron moves from a higher energy level to a lower energy level, a photon (particle of light) is emitted. Light emitted this way (from movement of charged particles) is called radiation. A photon of light is depicted as a squiggly line with an arrow on one end, as in this picture.

Einstein came up with the idea of the photon in 1905 with his paper describing the photoelectric effect. Without photons, laser light would not be possible as we will soon see. Thus, Einstein's work made possible all devices today we use which use laser light. Next time you watch a DVD or listen to a CD, think of Einstein.

Stimulated Emission is what makes a laser different from ordinary light from a normal light bulb. In an ordinary light bulb, light is emitted from atoms in the filament. In these atoms, electrons move from high energy orbits to low energy orbits at random, giving off light. This process is called spontaneous emission, and since it is random, the photons produced in this way have a very low level of coherence. This results in less intense light than what we get with a laser.

Lasers emit light by stimulated emission. In stimulated emission, an electron in a higher orbit is brought to a lower orbit by the presence of a photon of exactly the same energy as the energy difference between the two levels.

When this happens, another photon is produced which is identical to the first photon. Since photons are also waves, they can add like waves. Since our two photons are identical, they add together to make the most intense wave possible. Amplifying the radiation in this way creates what we call coherent radiation, which is the most intense radiation possible.

By putting many of the same atoms together, as in a gas, the previously mentioned photons created by stimulated emission can further stimulate the emisssion of photons in neighboring atoms. This is one way of building a laser.

Here is a schematic of a laser. Stimulated emission occurs in the gas in the discharge tube. The photons bounce back and forth between the two mirrors(M1 and M2), stimulating emission of more photons along the way. One of the mirrors(M2) is slightly "leaky" and some of the photons escape through this mirror, creating the laser beam.