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Einstein Predicts Stimulated Emission

The laser’s invention launched a multi-billion dollar industry. Lasers are used to remove unwanted tattoos; to correct vision defects in laser eye surgery; to cut through steel and other materials in industrial assembly lines; to scan prices in supermarkets and department stores; for optical communications and optical data storage; and in electronic devices like CD and DVD players. The root of all this technological innovation lies in fundamental physics research, specifically, a 1917 paper by Albert Einstein on the quantum theory of radiation.

“Laser” is an acronym for Light Amplification by Stimulated Emission of Radiation. It describes any device that creates and amplifies a narrow, focused beam of light whose photons are coherent. In a laser, the atoms or molecules of the lasing medium–either a crystal like ruby or garnet, or a gas or liquid–are “pumped,” so that more of them are at higher energy levels than at the ground state.

Einstein with his second wife, Elsa.
Einstein with his second wife, Elsa.

The end result is a sudden burst of coherent light as the atoms discharge in a rapid chain reaction. This process is called “stimulated emission.” Albert Einstein first broached the possibility of stimulated emission in a 1917 paper, having turned his attention the year before from general relativity to the interplay of matter and radiation, and how the two could achieve thermal equilibrium. Einstein devised an improved fundamental statistical theory of heat, embracing the quantum of energy.

First, Einstein proposed that an excited atom in isolation can return to a lower energy state by emitting photons, a process he dubbed spontaneous emission. Spontaneous emission sets the scale for all radiative interactions, such as absorption and stimulated emission. Atoms will only absorb photons of the correct wavelength: the photon disappears and the atom goes to a higher energy state, setting the stage for spontaneous emission. Second, his theory predicted that as light passes through a substance, it could stimulate the emission of more light.

Einstein postulated that photons prefer to travel together in the same state. If one has a large collection of atoms containing a great deal of excess energy, they will be ready to emit a photon randomly. However, if a stray photon of the correct wavelength passes by (or, in the case of a laser, is fired at an atom already in an excited state), its presence will stimulate the atoms to release their photons early–and those photons will travel in the same direction with the identical frequency and phase as the original stray photon. A cascading effect ensues: as the crowd of identical photons moves through the rest of the atoms, ever more photons will be emitted from their atoms to join them.

It wasn’t until the 1940s and 1950s that physicists found a use for the concept, even though all that was required to invent a laser was finding the right kind of atom, and adding reflecting mirrors to fortify the stimulated emission process by producing a chain reaction. Charles Townes had worked on radar systems during World War II. After the war ended, he turned his attention to molecular spectroscopy, a technique that studies the absorption of light by molecules. Just like radar, molecular spectroscopy bombards the surface of molecules with light and analyzes the scattered radiation to determine the molecule’s structure.

But the technique was limited by the wavelength of the light produced: in this case, the microwave regime of the electromagnetic spectrum. Townes noticed that as the wavelength of the microwaves shortened, the more strongly the light interacted with the molecules, and the more one could learn about them. He thought it might be possible to develop a device that produced light at much shorter wavelengths. The best way to do this, he thought, would be to use molecules to generate the desired frequencies through stimulated emission.

Townes mentioned the idea to a colleague (later his brother-in-law), Arthur Schawlow, who proposed that the prototype laser be fitted with a pair of mirrors, one at each end of the lasing cavity. Photons of specific wavelengths would then reflect off the mirrors and travel back and forth through the lasing medium. By doing so, they would in turn cause other electrons to relax back into their ground states, emitting even more photons in the same wavelength. So only photons in the selected wavelength and frequency range would be amplified.

The two men wrote a paper detailing their concept, published in the December 1958 issue of the Physical Review, although they had yet to build a working prototype. They received a patent for their design two years later–the same year that the first working laser was built by Theodore Maiman at Hughes Aircraft Company.

Further Reading:
Pais, Abraham. Subtle is the Lord: The Science and the Life of Albert Einstein.
New York: Oxford University Press, 1982.

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