Our calculations had shown a bare chance that we would achieve an
oscillator, but there was no safety factor. If it didn’t work, then what?
Laying the groundwork for the maser
The maser was not created from scratch; there were antecedents to our work. The first was Einstein
(who seems to be everywhere in fundamental physics);
in 1917, he studied the conditions for the equilibrium
of energy and momentum transfers between radiation
and atomic or molecular systems. He didn’t much like
quantum mechanics; nevertheless, he identified the
processes of absorption, stimulated emission and spontaneous emission and the relationship among them. He
also found that the emission of energy quanta (photons)
needed to be completely directional— apparently quite
unlike the classical picture.
In 1924, Richard Tolman discussed the possibility of
negative absorption (amplification) by molecules. Then,
in 1939, the Russian V. A. Fabricant conceived of eliciting
amplification from an excited gas. His experiments were
not successful, however, and no one followed up on
them. Willis Lamb and Robert Retherford later resurrected the idea of negative absorption in their 1950 paper
on the Lamb shift. Also in 1950, Edward Purcell and
Robert Pound invented the term “negative temperature”
to describe quantum systems with inverted populations
with transitions within a finite frequency range.
In 1953, Joseph Weber at the University of Maryland
discussed a scheme for obtaining coherent microwave
amplification from ammonia gas—but it was clearly
impractical. In addition, his work was not exactly an
antecedent, since our research on what would become
the maser started in 1951.
The rotational angular momentum of the molecule is denoted
by the quantum number J. The projection of J on the molecular axis is labeled K, and the projection of J on some laboratory axis, as provided, for example, by an electric field, is
(J, K) = ( 3, 3)
kV/cm 20 40 GHz
accepted there—fortunately, as it turned out). The Columbia physics department at that time comprised an excellent
group of people.
In my first two years, I had an initial course in atomic
physics from Isidor Rabi; a class on quantum mechanics from
Willis Lamb; and a seminar in advanced quantum mechanics
from Hideki Yukawa, who had come to Columbia in 1949.
Among my memories was the first session with Lamb, where
he wrote the wave function symbol ψ on the blackboard along
with the comment: “Don’t worry about what this means,
you’ll get used to it!” Another is of Yukawa, whose English
was poor; he mumbled towards the blackboard as he wrote
in tiny symbols. He was not the best teacher, but of course he
was a great theorist. Townes and Polykarp Kusch were also
members of the group.
In Townes’s office, I met with Herbert Zeiger and George
Dousmanis, who had already been thinking about the project.
Herb was a post-doc, having earned his Ph.D. at Columbia
working with molecular beams, and George was a student
from Greece who was then doing calculations of beam tra-jectories in the electrostatic focuser. This device was called
a focuser because at that time we thought that its focusing
properties would be important to the design of the apparatus.
If I decided to join Townes’s project, it would have to turn
out something new to provide me with a Ph.D. thesis. I recall
having an important meeting in Townes’s office during which
we decided to move forward. Our calculations had shown a
bare chance that we would achieve an oscillator, but there was
no safety factor. If it didn’t work, then what?
As I remember, I pointed out that the relatively long cavity
resonator that we had designed to increase the interaction of
the molecules with the field would also cause a tenfold increase
in the resolution of the ammonia spectrum. Townes immediately pointed out that there was as-yet-unseen hyperfine
structure in the spectrum to be found that had been previously
hidden by Doppler broadening. Thus, happily, we went ahead.
The first time the project was written up in some detail
was in the December 1951 quarterly report of the Columbia Radiation Lab—a report that was required by the Joint
Services Command in return for its financial support. As
Townes recounts in his 1999 book How the Laser Happened,
these reports were not official publications; rather, they were
generously distributed to whoever asked for them. Whether
they influenced what came later will never be known, but for
sure most initial reactions were ho-hum. For example, Townes
had arranged for Herb and me to visit Prof. Malcom Strand-berg at MIT, who had had some acquaintance with ammonia
beams. He listened to what we were trying to do, gave us some
advice which I don’t remember, and wished us well.
36 | OPN Optics & Photonics News