Albert Einstein is often associated with the invention of the laser. There is an element of truth in this - but it's not quite the whole truth.

Albert Einstein is certainly credited with many things. His famous E = mc² equation is occasionally described as the key to building the atom bomb - and some also consider Einstein to be the inventor of the laser. So how true is this?

In one of his most important publications ("The Quantum Theory of Light"), Einstein already described the process of stimulated emission as early as 1917. According to this theory a photon that hits an atom (or an ion or molecule), can not only absorb energy in an absorption process, but rather in reverse an already excited atom can be put into a state of heightened energy. In the latter case the energy taken from the atom produces an additional photon. A critical aspect here is Einstein's finding that this photon must move in the same direction as the original photon. This consequently gives us an amplification process: one photon becomes two and a weak beam of light becomes a stronger beam of light. Furthermore Einstein also identified that in an ensemble of atoms a net amplification can only occur when a "population inversion" has been achieved. The upper energy state must be more intensely populated than the lower one, so that the stimulated emission and not the absorption is predominant. This state is often achieved by "optically pumping" a laser crystal, for example - a technique that is attributed to Alfred Kastler.

Of course a Laser requires far more than this: a "resonator" in which a light beam circulates, whereby an amplification medium can at least balance out the energy loss per circuit. The principle was first demonstrated with microwaves (→ Maser), and it was also applied to light with the ground-breaking work of Schawlow, Townes and Maiman (ca. 1960).

After all the work between 1917 and 1960 has certainly cleared up more than just some small details, it would clearly be an huge exaggeration to describe Einstein as the inventor of the laser. He did, however, actually identify the most important physical principle of the laser - the process of stimulated emission; he did this, incidentally, through theoretical considerations, and not through the observation of physical processes. Thereafter there was still a long way to go before the laser was produced.

The situation was actually quite similar with the atom bomb as well. According to the E = mc² equation an enormous amount of energy is released when even just a couple of grams are converted into energy. This is, without a doubt, an extremely important discovery. However, finding a way to enable this conversion is a different thing altogether. This method was revealed with the discovery of nuclear fission by the Lise Meitner, Otto Hahn and Fritz Strassmann team in 1938, and the ability to cause a nuclear chain reaction that was identified soon after. Einstein therefore has far more to do with the atom bomb because of his famous letter to Roosevelt than because of his equation. And Max Planck can also be attributed an equation (E = hν), but no letter, and doesn't apply as the father of the laser either, but rather at best together with Einstein as the father of the photon (as he was letter named by Gilbert N. Lewis).


Author
Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH
Internet: http://www.rp-photonics.com/
Dr. Paschotta is an expert in lasers and amplifiers, non-linear optics, fiber optics and ultra-short laser pulses. With his company, RP Photonics Consulting GmbH, he supports companies in Laser technology and photonics with product designs, feasibility studies, independent analyses and customized training courses.