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.
[Illustration: Figure 14 _An atom can release absorbed energy
spontaneously or it can be stimulated to do so._]