The crucial difference between the sun’s light or any ordinary kind of light and laser light lies in the extent to which the emission of energy can be controlled. In the production of ordinary light the atoms, as we know, emit spontaneously, or in an uncontrolled fashion. But if the atoms could be forced to take in the proper amount of energy, store it, and release it when we wanted them to, we would have stimulated, rather than spontaneous, emission.
This, however, is practically the same as the amplification principle we discussed earlier. In that case, a small radio signal is jacked up into a large one by stimulating an available power source to release its energy at the same wavelength and in step with the smaller signal.
The question is, how can we do this with light?
CONTROLLED EMISSION
The laser and its parent, the maser, can be traced back half a century to its theoretical beginnings. The great physicist Albert Einstein is most widely known for his work in relativity. But he did early and important work on that other gigantic 20th century scientific achievement, the quantum theory.[10] In one of his papers, published first in Zurich, Switzerland, in 1916, Einstein showed that controlled emission of light energy could be obtained from an atom under certain conditions. When an atom or molecule has somehow had its energy level raised, the release of this stored energy could be stimulated by subjecting the atom or molecule to a small “shot” of electromagnetic radiation of the proper frequency.
Einstein wrote that when such a photon of energy caused an electron to drop from a higher to a lower orbit, the electron would emit another photon of the same frequency and in the same direction as the one that hit it.[11] In other words, the energy of the emitted photon would be added to that of the photon that stimulated the emission in the first place. Here, potentially, was light amplification. The three major factors, absorption of energy, spontaneous emission, and stimulated emission are diagrammed in [Figure 14].
There the matter lay for more than 30 years.
In 1951 Charles H. Townes, then on the Columbia University faculty, was interested in ways of extending to still higher frequencies the range of microwaves available for use in communications and in other scientific applications. Townes and other scientists who were interested in the problem were to meet in Washington, D. C., on the 26th of April. The night before the meeting he slept in a small Washington hotel; but he awoke at 5:30—pondering, pondering the high frequency problem.
He dressed and took a walk, then sat on a park bench and savored the beauty of azaleas in bloom. But all the while his mind was running over the various aspects of the problem.
