Optics & Photonics News

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.

30

M

3 2 1 0

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 labeled M.

(J, K) = ( 3, 3)

kV/cm 20 40 GHz
0
1
2
3
0

– 30

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.