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The Townes - Einstein Connection

Theory Transformed Into Indispensable Technology

How is the work of Charles Townes related to the work of Albert Einstein? Why is the celebration of Townes's achievements and the 50th anniversary of the laser a most appropriate focus for the World Year of Physics? Read on...

Today, Albert Einstein is best known for such things as the theory of relativity and, of course, that single equation which has come to serve as a symbol for the field of physics itself: E=mc2. Just as important, though, is the groundwork Einstein laid that would make the invention of the laser and its predecessor, the maser, possible over 30 years later.

In 1917 Einstein published a paper entitled "On the Quantum Theory of Radiation," which introduced the process of stimulated emission, the theoretical basis upon which both the maser and laser rest. With this concept, Einstein gave a brief, intuitive derivation of Planck's radiation formula.

Einstein's idea was that in addition to the spontaneous, random emission of photons from excited atoms, there is an emission forced by radiation to which the atoms might be exposed. The likelihood of the effect depends on the strength of the component of the radiation whose frequency agrees with that of the photon emitted in the forced transition.

The photons that result from stimulated emission have exactly the same frequency, direction and phase as the photons that shake them out of the excited atoms. This process is repeated as the stimulated photons stimulate other atoms to release photons identical with themselves.

The result is a cascade of photons, all traveling in the same direction and with the same frequency, what physicists refer to as a "coherent amplification of optical waves." These photons, because they share the same frequency and travel in the same direction, essentially mimic each other's behavior precisely, resulting in a powerful beam of light. This beam is monochromatic - constituted by one specific wavelength of light (only one color); coherent - "organized," meaning that each photon moves in step with the others, sharing the same "wave front"; and very directional - all the photons are concentrated into a tight, focused movement. This is the principle of the laser.

Of course, Einstein never actually invented the laser. His discovery of the process of stimulated emission, however, made the laser theoretically possible. It was this process of stimulated emission that Dr. Charles H. Townes harnessed in 1954 to invent the world's first maser, a device to effect Microwave Amplification by Stimulated Emission of Radiation. Townes used ammonia molecules as the medium through which to produce the cascade transition of atoms to lower energy states (and the release of energy as photons). Just four years later, Dr. Townes and his brother-in-law, Dr. A. L. Shawlow, demonstrated that the same process of stimulated emission could be utilized in the optical and infrared regions and therewith proposed the laser.

Since Townes's initial discovery in 1954, lasers have been used in countless applications from military technology and cutting edge scientific research to such mundane purposes as CD players and bar code scanners in supermarkets. This evolution is exemplary of the progress of science itself. Stimulated emission was, for Einstein, a theoretical possibility; now, thanks to Dr. Townes, this process is the practical basis of many of our most fundamental technological luxuries and our most daring scientific projects.

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