In the paper that Arthur L. Schawlow and Charles H. Townes wrote in
1958 in whcih they showed on,theoritical grounds that laser action should
be possible, they described a system in which the active material was potassium vapor enclosed in a tube. It was something of surprise when the first laser ever made, two years later, turned out to contain a solid active material "ruby" rather than a gaseous one. Schawlow and Townes chose a gas quite deliberately. The molecules in gas are, for the most part very well
spaced out and far away from each other. Although each one moves about and frequently collides with other molecules, it spends most of its time on its
own;or, to put it another way, by far the greatest part of the volume of a
gas consists of empty space. This makes it much easier to calculate how the
gas will behave when one tries to stimulate the emission of radiation in it,
because each molecule can be considered on its own, and there is no need to
worry about the influence of other molecules on it. In potassium vapor,each
molecule is in fact a single atom, and this makes the calculation of the gas's
behaviour even simpler, because molecules that are made up of several atoms combined together are more complicated than individual atoms.
For these reasons most of the scientists taking part in the race to
make the first laser were trying with monoatomic gases as the active materials, and Ali Javan of the Bell Telephone laboratories, together with two of his colleages, made the first laser of this type towards the end of 1960,
only a few months after Maiman's ruby laser appeared. Since 1960, many different kinds of laser have been built, and active materials now include
liquids, polyatomic gases, and solids other than ruby.
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