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Interview of Joseph Anthony Giordmaine by Joan Bromberg on 1984 May 31,
Niels Bohr Library & Archives, American Institute of Physics,
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Graduate and postgraduate work at Columbia University under Charles H. Townes, 1955-1961; the maser receiver for the Naval Research Laboratory. The early quantum electronics conferences at Schwanga Lodge (1959), Berkeley (1961) and Puerto Rico (1965). Nonlinear optics researches at Bell Laboratories and Munich's Technische Hochschule, 1961 to about 1968; optical parametric oscillators. Picosecond pulse measurement techniques.
OK, I’ve divided it into Columbia and Bell Labs, and I want to begin by getting you into Townes’s group, so to get some feeling for when you first began to know about this general subject matter, and how you became a member of Professor Townes’ group, and we’ll talk a little bit about [???] then we’ll talk a little bit about what that group was like.
I first became familiar with quantum electronics approximately in 1954. At that time I was a junior undergraduate at the University of Toronto. At that time, I was a major in physics and chemistry, and as I tried to choose a subject for graduate work. I became interested in the field of microwave spectroscopy, because the spectroscopy element was familiar because of my chemistry undergraduate interests, and I had a hobby of electronics and knew something about experimental electronics in the field of microwaves, which was fashionable and very interesting and productive at that time, was of some interest to me, and so the combination of microwave spectroscopy was of a lot of interest, and at that point, I was doing some literature searching. I had identified Duke University as a center for research in microwave spectroscopy, particularly Professor Walter Gordy’s laboratory, and in fact had applied to and been accepted at Duke. Now, in the course of my — it was either late junior year or early senior year, I think early senior year, I guess it was late ‘54, Professor Townes paid a visit to Toronto at the invitation of Professor Welch, a prominent optical spectrometrist who knew Townes professionally. Townes gave a seminar, a colloquium at Toronto, on the subject of the maser. The maser had just been constructed. The first paper had just been published or the first talks given, in any case, it was very shortly after the first operation of the maser, and it was clear from Townes’ talk that, clear to me, that there was something completely new, something completely novel and something completely out of the realm of anything that was being talked about in the area of microwave spectroscopy. It was something completely different and new, and it was, what was new about it was the quantum electronics issue, the topic of the, the idea of coherent radiation, and this was my first exposure to it, and I can’t claim to have understood the nuances of the talk. It was something new and very exciting. So after big talk, I went up to him and told him about my interest, and this was succeeded by some correspondence, and I got accepted into the g, as a graduate student at Columbia, and I made a request before accepting and changing my plans that I be a member of his laboratory group, rather than a teaching assistant which was more normal. So at that point I became accepted and I came to New York in September of ‘55 and joined the Columbia Radiation Laboratory.
I’d like to get some feeling next about what the group was like, both its social arrangements, who was in it, how did you communicate with each other, and the intellectual situation, what problems were people most interested in, and maybe you want to say a little bit about who was interested in what — to got some feeling for bow the group looked, not immediately as you came in, but maybe for the first six, nine months.
Well, when I came in September, ‘55, I was a beginning graduate student, and the normal situation is, you spend most of your time on your courses, but I was a laboratory assistant or lab research assistant at that time, in the Columbia Radiation Laboratory, one of my assignments was to accumulate and maintain the collection of references in microwave spectroscopy that Professor Townes maintained, so I had a chance, an unusual chance to get familiar with the field earlier than would be normal and to meet a lot of members of the group. And it was an unusually exciting and very vibrant time in the field, because either in the Radiation Laboratory or in other groups on the same floor of Pupil Physics Building at that time, there was Professor Rabi, Professor Lamb, Professor Kusch, Townes, and others, so that there was a real ferment going, not only in microwave spectroscopy but in physics and electronics generally which made it an extremely exciting and intellectually fast moving kind of climate. The channels of communication, the main one, the principal formal one was the seminar, microwave seminar, which people spoke on topics related to microwave spectroscopy for the most part, but frequency stabilization, to molecular and atomic lines, which was an important thrust in research related to microwave spectroscopy at the time.
One that was being actively carried on at Columbia?
Yes, that was the important theme. In fact, the maser had been invented, as you know, by Townes with a great deal of focus on its initial application, the need for a highly stabilized frequency source, and that was the theme of Townes’ research for years before I came there, and the initial idea of the application of the maser was, it would be a highly stable source of radio frequency radiation suitable for physics experiments of various kinds. And in fact, it was so used in tasks of special relativity and other applications during that next five or ten years. Now, the other channel of communication I guess in addition to the seminar was a set of informal meetings with Townes. I became more exposed to this when I became a more active graduate student of his during a year later, but let me jump ahead to that for a moment. Townes’ style at that point was to hold regular meetings with his graduate students, both for the students working in subjects that he was especially interested in — these should be about once a day every morning — now, this didn’t begin in my case for about a year, year and a half after I came there, but at that point, this was every morning, and the agenda would consist of a report by a graduate student on some recently published paper. The graduate student would have to go through this paper well enough to be able to talk about it, and this was an extremely important source of information, very effective source of communication. It was Townes’ way of keeping up with the literature to some extent, and it was surely the graduate student’s way of learning a lot of physics, mostly because in preparing these papers, you really had to deal with a lot of questions that you hadn’t been exposed to before.
Was this a one to one meeting?
No, this was a meeting of Townes with a number of, not all of his students, he had about anywhere from 10 to 15 students at a time there, but this would be typically a group of half a dozen students, so these would be lots of opportunity for give and take and there was a lot of encouragement. If you didn’t understand something, it was quite normal to say that. If you differed with an opinion — so it was a chance for very open and candid discussion, and this really contributed to the intellectual level and the excitement of the physics climate there. So there were both formal processes, such as the microwave seminars, set of colloquia and other related seminars in the physics department, but I think this almost one on one kind of interaction with Townes, through a large part of that period, was especially important.
Much of this is going to be more interesting in terms of starting the next year, which would be, let’s see, you came?
I came in September of ‘55, and I began.
— then Townes was already going on a sabbatical at that point.
Yes, he was away for part of the time, part of my first year, but I didn’t begin my own research until approximately a year later, I forget the exact date but roughly a year later.
Many of these questions are going to be referring to that period. As you started in, especially questions like the facilities that you had for your research and the [???], your initial research project. I should say that we’re not going to —
— well, I began research about a year after coming there to Columbia. I came to Columbia, or at least formed during the first year that I was at Columbia an intention to work on a particular area of microwave spectroscopy having to do with free radical spectroscopy, microwave studies of materials like the O OH radical, which seemed like an exciting chemical material, but Townes discouraged me from that and interested me in the topic that he wanted to study, namely, a maser for radio astronomy. Now, at that time, to get a good picture of the vision and the daring involved there, you have to get a feeling for where things were. At that point, the only maser that had operated continuously was the ammonia maser, and this was a maser that operated over, that produced an extremely, that worked with an extremely narrow sharp line, narrow in frequency. This meant that as an amplifier of radiation, for example, microwaves radiation, that you study in radio astronomy, negligible usefulness just because its line width was so small and the amount of radiation that it could amplify, that is the width of radiation, was sufficiently small that it wasn’t useful. Now, that was one maser that had operated, the maser that had operated continuously. A second maser experiment that had been down by Townes during his sabbatical in France, working with [???] and [???] in France, was an experiment that demonstrated a population inversion in a Para-magnetic material, this was a transient experiment in which the population was transiently inverted and there was evidence of negative temperature, and perhaps an amplification, I’ve forgotten that, but it was only on a transient basis. It was completely useless as a practical tool, but Townes interested me in developing a maser that could actually be used in a practical way on a radio telescope to make observations in the microwave frequency range, the purpose being to make these observations with a lower noise figure, greater sensitivity, than had been achieved in any microwave amplifier to date. This reflected Townes’s continuing interest in astronomy. Townes became interested in astronomy when he was at Bell Labs, well, I guess, further back than I know about, but at Bell Labs he’d been interested in astronomy and he’d always had that interest, so this was his way of pushing that at the same time that he was developing a new application for the maser. But he had the vision of a maser that could be used for a practical tool, at a time when there was practically, when this was before the Bloembergen three level maser idea, so there was essentially no way of doing this, in the technology that existed at that time, but he had a vision, on the scale of a graduate student’s tenure, a few years, that it would be practical as a graduate student thesis, and this is without any existing technology that was relevant. So I began to study this, study the maser, and do some preliminary experiments with the people in his laboratory that were working on masers. They included P.C. Wong and Au Javan particularly. I had worked as an assistant with Ali Javan during the first year that I’d been there at Columbia.
I do want to ask, Townes told you at one point that he had also alerted the people at Bell Laboratories to the importance of this subject, he said he did that sometime during his sabbatical year, and I wondered if you were working in any relation with them, perhaps just talking to them or what have you, people like Scoville or Thayer or Gordon here?
No, I had no contact with them, except when the three level maser came out from Bloembergen, it was immediately evident that that would be a way, you know, well, when that idea came out and was quickly followed up by Scoville, Thayer, work at Bell Labs demonstrating, it was clear that that was the way to make an amplifier for radio astronomy. It was very significant. Townes sort of understood that there was a possibility of something of this general sort, although perhaps that wasn’t directly of course him contribution, but he anticipated that in a very timely way, so when that came up, that was immediately the way to approach this topic of amplification of microwaves in astronomy.
So, you were here working away at this and then Bloembergen’s paper comes up within the context that you’re already very interested in this?
That’s right. That’s right. So the Bloembergen type of maser provided a practical, for the first time a practical means of doing what I had been charged by Townes to do, for my thesis, so we immediately began to repeat the Bloembergen and Thayer Scoville experiment, using the material potassium chromi cyanide. Now, potassium chromi cyanide is a paramagnetic material. I believe that was, I don’t think that was one of the ones that Scoville and Thayer worked with, I’ve forgotten, but that was the material that we picked out, and did the preliminary paramagnetic resonance apparatus, and I point out, at that time, there was no paramagnetic resonance in Townes’ laboratory at all. In fact, there was no solid state electronics in Townes’ laboratory. Townes was a completely gas molecular atomics spectroscopist at that point.
Does that mean that your previous efforts, previous to seeing what Bloembergen and [???] was theoretical, or you were not doing anything yet on a solid state maser?
Yes, I was working in collaboration with T.C. Wong, with Au Javan, and really just studying the subject at that time.
I didn’t know any of this that this was going on at Columbia.
Yes, so this began before the Thayer, Scoville, Bloembergen work, but that was absolutely essential, to have that result, before my progress could be made. I began starting with building a solid state paramagnetic resonance paramagnetic resonance phenomenon in potassium chromi cyanide, and that required learning the paramagnetic resonance. An important visit was the visit of Townes, who brought me along at that time, to Rutgers, where there was a very active group working in paramagnetic resonance, and the host there — the host at Rutgers gave a colloquium, and we saw their paramagnetic resonance laboratory, and reproduced a great deal of that at Columbia very quickly, because we saw the techniques. Let me get the name of the Rutgers — one of our Rutgers hosts was Ellhu Abrahams, who was a very active, was and is active researcher, at that time in the field of paramagnetic resonance relaxation phenomena and the visit to his lab expedited our entry into the field of paramagnetic resonance. We were fairly quickly able to achieve paramagnetic resonance in materials like potassium chromi cyanide which was known from the work of others, and demonstrated the three level maser operations in potassium chromi cyanide. Again, however, it soon became clear that the spin density and the absorption strength in potassium chromi cyanide was sufficiently weak that it would be possible only to make an amplifier of limited strength, and that would offer a limited improvement in noise figure to the microwave radio telescope, using potassium chromi cyanide, and again, just at that time, work of Professor [???] at University of Michigan demonstrated paramagnetic resonance in ruby, and the spin density in ruby was such that it was a, this is ruby doped with chromium, it’s the chromium doping in ruby that gives it its red color, was the key element here, and that proved to be a much more adequate material for a microwave maser. We demonstrated microwave maser action and that proved to be ideal material for a microwave maser for a radio telescope. So around I guess in ‘58, we brought, we being myself and another graduate student, Leonard Allsop, — both working —
— was he working from the very beginning?
Yes. Well, Allsop joined the group a little later, a little after I did, but we did our thesis research in parallel, and in fact we collaborated on the maser for radio astronomy, so we got this maser to the Naval Research Laboratory, and mounted it on their radio telescope. This is the large radio telescope that you see on the banks of the Potomac as you land at Washington Airport.
This was an already extant piece of equipment, not redesigned or changed.
Yes — no, this was a very active piece of equipment under Cornell Mayer, who was the department head responsible for radio astronomy research at the Naval Research Laboratory.
Well, I don’t really want us to get there quite yet, because I’d like to know, in the course of working these things out, is there anything that we should record in the way of obstacles, breakthroughs, false starts, triumphs, you know, in the things that were novel that were coming out of it. GIORDMAJNE: I think, I guess, again, I’m repeating myself perhaps, but the anticipation that Townes had of, first of all, a technique for using masers in a useful way, before any technology existed, ties followed by the appearance of potassium chromleyanide as a, at an appropriate time, and the very fortunate appearance of ruby at an appropriate time, and again —
That was just something you read in the literature?
Yes. Yes. No, it was done; the work was just done at that time. These materials and the understanding of their potential for this application only appeared at that time. So a lot of things happened in a timely way, but for me of course it was an expression of Townes’ vision really in anticipating that this, you know, the needed tools would appear in a timely kind of way. OK, so, while you’re working with them, why did you decide to make a 3 centimeter detector, amplifier? You must have somewhere decided on the region that you wanted to work in.
Well, there were two issues there. One was the fact that in paramagnetic resonance, you need a magnet to provide the magnetic field. Now, these experiments were done
I see. So were you already in interaction with NRL, even before the maser amplifier was prepared?
Oh yes. Very early in the project, Townes established a relationship between Columbia and NRL, anticipating this; and throughout the design of our amplifier at Columbia, there was very close collaboration with them, so that we ended up with an apparatus that could be brought down to Washington and simply attached by bolts to their antenna, and operated in that way.
You did a lot of that yourself, didn’t you?
Yes. Lee Allsop and I did all of the experimentation work ourselves. And this was a major undertaking in a lot of ways, because simply, even the logistics of the patching and operating liquid helium doer at the focus of an antenna, you know, which is being pointed everywhere, and loading it you had to climb up a ladder about 20 feet in the air to get to the focus, and pull up a, you know, a doer full of liquid helium to the focus of the antenna and install that there. The doer contained enough liquid helium for a run of about eight or ten hours. But the logistics here were really novel. But we had a lot of collaboration from the Naval Research Laboratory.
Hopping around a bit... I want to know as much as you think is important here about how you actually worked, how you decided on the, you were working for sub planetary [???] temperatures, isn’t that right?
That’s right, yes.
How you decided what you wanted to measure, and any novelties that emerged in the course of getting this going. Any surprises that might have happened also.
Well, the purpose of the maser was to increase the sensitivity of the receiver, and it did that by in fact, very successfully, by reducing the effective noise temperature of the receiver by [???], and improving the noise figure of the receiver, by about a factor of 10. And one way of saying that is, the effective noise temperature of the receiver was 85 degrees Kelvin, as distinct from about 800 or a thousand degrees Kelvin before the apparatus was put in, so an improvement of a factor of ten in the sensitivity was obtained.
Why was that? You put it on and immediately — or was it something that would work after —
Well, with the amplifier operating, with the maser amplifier operating, one way of seeing this is that there’s a theoretical quantum noise which ultimately limits the effective noise coming in which is a lower limit to the effective noise than the maser amplifier, that’s H nu over K, and that’s a 3 centimeter wavelength, that’s less than 5 degrees Kelvin. Now, we produce a receiver with a temperature of 80 degrees Kelvin, the difference being due to the stray radiation coming in from various other sources. Now, the applications that were chosen included planetary astronomy, which was my, which I emphasized. Lee Allsop emphasized radiation from ionized hydrogen regions, which also radiated in the microwave region. This was a rather arbitrary split of the kinds of observations that were being made, and these kinds of choices reflected the ongoing research program of the Naval Research radio astronomy people, in part, and one of the focuses was on Venus, and at that time, the Naval Research people had just recently discovered the extremely high surface temperature of Venus, which had not been known before, because it had previously been accessible only at infrared, only with infrared radiation which saw only the top of the cloud. The microwave radiation that was detected came from the surface of the planet, and that had only recently been discovered by the Naval Research Laboratory, people working in 3 centimeters. Then our contribution was to make that observation much more accurate than it had been before, which we could do because the sensitivity of the receiver was greater. We also made observations of Jupiter and Lee Allsop made a number of observations of planetary nebulae, ionized hydrogen regions, and one other planet.
In all this, by the way, were you as a graduate student being funded by NRL contracts at this point?
No, this was funded by Columbia’s funding source, a joint services contract. And that’s worth pointing out. The sense that the graduate students had of funding, funding from the graduate student’s point of view really wasn’t an issue. You took for granted, and I think with the exception that once a year there was a visit by the representative of the joint services, and one was aware there of people that one had a great deal of respect for. They had a lot of knowledge of what was going on, a lot of interest, and particularly noteworthy were the people from the Office of Naval Research. They seemed to have a great deal of vision and seeing where all this might be going, and one had a sense there of real understanding and-
— was that [???] or?
You know, I think —
Johnson and [???] and, those are names that occur to
Going back a little bit to the climate in the Columbia Radiation Lab, there was a real climate of innovation there. Some of the experiments that involved masers, because of the unfamiliarity of the quantum electronics kinds of concepts, some of these experiments were extremely exciting in principle. One of the early experiments that was done there by Javen and T.C. Wong had to do with beams of ammonia molecules producing the ammonia maser, and they were able to achieve a beam of molecules which was excited in a first cavity, by passing through a first microwave cavity, and then this beam of molecules then would be detected in a second microwave cavity, which had no radio frequency radiation in it, but because of the coherent excitation of these molecules, they would radiate, when they entered the second cavity, in a coherent kind of way, not simply because they had an inverted population or a negative temperature, but because they actually had a rotating dipole moment that was generated in the first cavity, so that they would radiate in the second cavity independently of whether there was a negative temperature or not. They would radiate simply because they had a rotating dipole moment, and it was graphic experiments of that kind that really tended to, a multitude of those kinds of experiments that particularly delineated a completely new field, so because of these kinds of experiments, it was a very stimulating kind of atmosphere, and a kind of atmosphere which, people tend to come up with their own ideas, which are often kind of crazy, and they were listened to, and you listened to other people’s ideas, but it was a very unusual time.
It sounds as if you actually felt, you were conscious of setting the basis for this whole new kind of technology. Is that a correct interpretation, that you had the feeling there that you were laying or helping to lay the basis?
Well, you felt that the whole group — the whole group was doing that, yes. Yes, oh, absolutely. Absolutely.
I also asked this about the telescope, but maybe we should look at it more widely, whether you were in touch with other groups in the country that were working an similar things? I asked specifically whether you were in touch with the Michigan group or the Harvard group, which was working on radio telescopes with masers, and I don’t know whether as a graduate student you would have yourself felt this relationship to other groups in the nation.
Well, yes. I tended to meet them primarily at meetings, where we reported the results. That tended to be more in the ‘58, ‘59 kind of time frame, and yes, we did meet others working in the field. Carl Sagan at that time was working in planetary astronomy, and we had a number of conversations with others working Tithe field. I didn’t meet him but Townes reported extensively on meeting that he had with Professor Kuiper, who was a very prominent planetary astronomer, and I think these observations were an important stimulus. These and other observations, early observations, were an important stimulus to radio frequency spectroscopy, radio frequency observations of planets.
In addition you were doing some other work on masers. You did some molecular beam work, and you did the paramagnetic relaxation work. I’m just wondering —
The paramagnetic relaxation work was done just as a sideline during the development of our paramagnetic resonance technology for masers, and in the course of these experiments on potassium chromi cyanide, this was a slightly earlier stage now, ‘57 or ‘58, in the course of these experiments with potassium chromi cyanide, which were focused entirely on getting a practical maser amplifier, it became clear that there were some very unusual features in the relaxation of these paramagnetic lines. Now, if you look at the spectrum of the absorption, microwave absorption in a material like potassium chromi cyanide, there were several lines that were very close together, and I discovered, completely by accident, that if you saturated one of these lines by observing the spectrum and at the same time, taking a second microwave source and setting it to the frequency of one of these lines, and saturating it, putting so much power in that the line absorption decreased, that that saturation could spread to another line, in ways that were, at least at that time, sort of unexpected, and this prompted a pretty serious study on the part of our group in paramagnetic relaxation, and this , I guess at that time, the initial understanding that we had was provided by Townes, who introduced the idea of a spin flip, that is, that the relaxation, proposed that the saturation was being spread from one line to an adjoining line by a kind of spin flip process. We published a paper on that which stimulated a lot of discussion, and there was follow-up word by Bloemberger, Pershan and others, which I think really fully clarified that situation and provided a full understanding of that process, which I think we understood only in part, but that stimulated a lot of work, a lot of additional work on paramagnetic relaxation. At that time, we had a visit from, at the Columbia Radiation Lab, from Professor van Vleck of Harvard, and that was a great stimulus to our work.
OK, well, the other thing I wanted to know and this — there may be other things you want to go back to [???] is about the [???] Lodge —
May I make one other remark here? You mentioned the molecular beam work. I guess, shortly after I arrived at Columbia, I was assigned as an assistant to T.C. Wong, who was doing work related to the ammonia maser, and some of the novelties in the experimental design of the ammonia maser were Townes’ and Gordon’s use of what was called a, what was called a crinkly [???] of sorts, this was a, you took a strip of metal foil, and you put it through a serrating tool which gave you a sort of serration along the foil, and you wound the foil up into a tight bundle, and this left you with a set of tubes effectively that could flow through, and the ammonia gas was injected into this ammonia beam maser apparatus by flowing through this set of tubes, the so called crinkly [???] of sorts, and the question was always how to optimize the flow of gas through this source so that it was most focused in the direction of the ammonia maser cavity that was sitting out there in the vacuum, so you bad to have as collimated a beam as possible. I was assigned that project, as an assistant to T.C. Wong, and in the course of that, I gave a number of calculations about how gas flowed through tubes, and was able to delineate the properties of gas flow through tubes in a particular regime that hadn’t been investigated before, namely, when the mean free path of the pas is comparable with the diameter of the tubes in a source of that kind, and that result obtained widely used in recent years. But that was a real introduction to my own personal research at that time. Now, on the [???] Lodge Conference —
I understand that you were very heavily involved in getting the manuscript ready for —
— Yes, Townes enlisted the help of all of his graduate students in editing work and various other assistance in getting that ready for publication. I remember the conference very well, because I had just completed this work on beam focusing, and at that point, I didn’t think, although I was very excited about it personally, I didn’t feel that it was really very closely related to quantum electronics. I thought it was nice physics but it wasn’t quantum electronics, but Townes nevertheless encouraged me to present a paper at that conference, which I did, and which was a very valuable thing for me to do. So I remember the conference very well for that reason. For personal reasons... At the conference also, there was a very active discussion on the subject of paramagnetic relaxation, and there was a first discussions of the potential for maser action at higher frequencies. And lasers. Now, you can read the account, you can read in the book itself who spoke on those topics. Schawllow was one of the people who spoke on that topic, and so it was in the air at that time, the possibility, extending the maser to maybe even higher frequencies.
I’m particularly interested in whether you recall anything that one would not pick up by just reading the Proceedings. Sometimes there are attitudes that participants pick up that you can’t really read out of the Proceedings, or things that are surprising, you know, or things that people that are, there are papers that are given that people just don’t believe, that kind of thing. So if you have any memories of [???] Proceedings, or certain things that would be very influential that you really don’t know just by reading the Proceedings, which papers are going to be — carry more weight than others — I’m fishing for any memories you might have.
Perhaps we could come back to that in a second discussion. I could stimulate myself here by really looking through the proceedings of the conference. But I think, for the first time it became absolutely obvious that this was a field that had ramifications that would go far beyond anything that had been done so far, and there were the suggestions and, one didn’t know how to do it, that is, to carry out the experiments at higher frequencies, perhaps into the far Infrared, although one didn’t know how to do it, one sensed a thrust being to really extend the frequency range in a systematic kind of way.
That’s interesting, because there are a number of researchers who came home from that conference and started for the first time to work in this area.
Now, a few other comments about that era — shortly after the maser amplifier was working at the Naval Research Laboratory, we had a visit from Rudy Kompfner at Bell Laboratories. and around that time I had a chance to visit Bell Laboratories end see some of the microwave work going on there, and I remember, this was an awesome experience, because the microwave work at Bell Labs had to do with very sophisticated microwave structures, such as slow wave structures, which were specifically designed, which were influenced by the whole tradition of traveling wave tubes that had emerged at Bell Laboratories, and reflected the full understanding of traveling wave phenomena, and I remember at the time being very impressed because our technology, at the level of microwave technology and the maser, was relatively primitive. It was a simple cavity kind of structure, just a resonant cavity, a rectangular box basically, the prototype microwave cavity, and reflected from a microwave technology point of view, a relatively primitive level of technology, and the sophistication was in the maser and Quantum electronics concepts rather than in the microwave. One recognized, coming to Bell Labs, that the potential of the microwave technology was not even being sampled in our work.
When Kumpfner came to Columbia was that to speak about that kind of thing?
Actually he visited the Naval Research Laboratory, and be came with the idea of evaluating masers for microwave transmission applications, and — let’s see, at that time I guess the Echo satellite , the Echo balloon satellite , I’ve forgotten whether it had already gone up at that time, anyway the concert was there, I’ve forgotten whether it was flown in ‘58, I believe it had, but the idea was, a maser for radio astronomy was something, that really interested Bell Labs immediately, because of future satellite applications, and indeed the first Telstar receivers that were built by the Bell system had maser receivers. I’m not sure if it was the very first, but the early receivers, Telstar receivers, had maser amplifiers.
It sounds as if Kumpfner was coming directly to you and Allsop to talk about that, right?
Well, yes. He came to visit Townes and we all spoke to him at that time, but he saw the apparatus, and —
Was that an important point for you to get to know a little more about Bell Labs? I mean, you were very soon to go.
No, that really wasn’t — it was really his information gathering at that time, so I didn’t learn very much about Bell Labs at that point. Now, it was later, around 1960 that I had the first chance to, ‘59 or ‘60 that I had the first real exposure to Bell Labs. At that time, Ali Javan , who had been a kind of mentor for me when I first arrived at Columbia, had moved to Bell Labs, and at that time my career directions were focused entirely toward academic kinds of directions, but I went to visit him because at that time be had just — I visited him, I guess, in 1960, at the time of the first laser had been constructed. I also visited him a year, six months or so before that. In any case, I saw his laboratory. I saw the way he was being funded. I saw the level of equipment that be had. I guess the one that I remember most was a very — a large research kind of magnet which was sitting beside the wall, and it had its original plastic wraps around it, and I remarked on this, and I he made a remark, “Well, I thought I might need this but I really didn’t.” I said, “Well, If this is the way that research is funded here, this is certainly an attractive kind of climate. I hasten to assure you, this is not the way Bell Labs normally operates, now or then, but I have to say it was extremely impressive, in his case, to see that he had been funded very adequately. And again, my decision to come to Bell Labs was affected by the attitude of Bell Labs toward research funding and toward the freedom that they offered to many in the research laboratories.
Now, you surprise me, when you get there, by going into a whole new field, which is nonlinear optics. Were you already thinking of doing that at Columbia?
No, when I left Columbia, I really felt that I had a, I was at a crossroads, because I had been exposed to radio astronomy, and I’d also been exposed to quantum electronics, and I realized that I had to make a choice between radio astronomy and quantum electronics, and I chose to remain in quantum electronics, but my colleague at that time, Penzias, decided to stay in radio astronomy. I think he had, I guess I under-estimated the potential of radio astronomy, you know, what there was really to be seen, what the future of radio astronomy was. I saw more future in quantum electronics than in radio astronomy, so I opted for quantum electronics, stimulated of course by the discovery of the laser just at that time.
When you say your colleagues, did you enter into Bell with Penzias?
He came about a year later, a year or two years later, but we did a lot of work in parallel. He was about a year or two behind me. See, he built a receiver at another frequency. I believe he built his at 10 centimeters, and used another Naval Research Laboratory antenna. (crosstalk) But we —
— He was at Columbia?
Oh yes. He was a student of Townes. And he came through the same sort of environment that I did, and again, he was open to the same influences, on radio astronomy particularly.
Well, before I left Columbia, I really decided to get into optics, although I was not doing any optics at Columbia at the time. And at that time, in the ‘59, ‘60, time, in the microwave seminar, we had many talks about ideas, and of course by that time, Townes’ idea of the maser was well known, and it was very much in the air. People at Columbia knew what Au Javan was doing at Bell Labs, and it was just a question of when it was going to happen.
And you had your own group, you had Cummings and Bella.
That’s right, yes. Now, at that time, I was not doing optics, but it became evident that that was the direction of the future, so I decided that when I came to Bell Labs, I would just drop everything and get into optics, so as soon as I came to Bell Labs, I did nothing more in microwaves at all, and just picked up optics and devoted myself to the work that was going on at Bell Labs at that time, interacting with people like Schawllow and Kaiser and Nelson Kleinmann, [???],he was also a student of Townes, and particularly Bob Collins, that group, Collins, Nelson, Kaiser, Schawllow , had produced the first paper on lasers, on solid state lasers, at Bell Labs, immediately following Maiman’s discovery; after Maiman’s discovery this group immediately reproduced it and I think probably felt a little disappointed that they had not done the experiment with lasers because there was a great deal of work on fluoride crystals at Bell Labs which was pointed toward lasers; but of course Maiman was first with the ruby.
Now, who was your department head when you came in?
And what was be working on
No, he was not working on masers or lasers at all, although he had had some early ideas on the subject of lasers and masers, in fact , had filed an early patent with a filing date in the late fifties, now , you’ll have to check the timing and chronology here, but there was a patent that he filed in the late fifties with some other Bell Labs person proposing a population [???] system, and I’ve just forgotten, I’d have to recall the particular system that it was, see if I can recall that.... interested in lasers. I’d had a chance to work with others as well as Boyle, but the intellectual climate in his group seemed to be very exciting and so I joined him.
You had a chance to select whom you would join?
That’s right. Yes. Yes.
Is it also the case that you just selected the particular kind of work you wanted to do with the nonlinear optics and?
No, not that.
How did that go?
At that point, when I came in September of ‘61, there was no nonlinear optics, September of’60? I’m sorry; it was June of’61.
The [???] Franken experiment was probably just going on about then.
Yes, but I didn’t become aware of it until it was actually done, and that must have been the autumn, late autumn of ‘61, so when I came there, the first thing I set about to do was to find out how to use the ruby laser, ruby maser, and set out to reproduce my own ruby maser. The ruby maser at that time was built inside of a professional photographer’s flash tube, you know, it flashed, probably some of them are still around, but it was a coiled lamp of the sort that Maiman used, was the type used by photographers for flash exposures, and we mounted the ruby rod inside of this flash tube, so that we had several apparatuses that were quickly set up following Maiman’s experiment and I acquired one of these and simply started to do experiments with masers. I mean, it was such a remarkable thing, seeing this beam of light coming out, and I started taking pictures of diffraction patterns and just exploring what happened when you sent light through transparent materials, the damage, and nobody had ever seen optical damage before. Nobody had ever seen any of these diffraction phenomena. So I really started out in a completely open ended sort of way, not really knowing what I was — you know, really just trying to acquire familiarity with the laser and studying the diffraction phenomena, but not really, at that point, without a very specific goal in mind, other than to really get a feel for what the potential of this device was. The direction that I had set out for myself was to do work in Brouillon scattering.
Oh really, that early you had set on that?
When I came to Bell Labs it was my idea that, you know, the laser was here, the Javan laser had been operated by that time, and I set out with the idea of doing Brouillon scattering. This had come about also from discussions at Columbia. At Columbia, Townes was very interested in the topic of Brouillon scattering, and part of this had come from discussions with [???] at conferences, and [???] had visited at Columbia, and he had done beautiful experiments on Brouillon spectroscopy with conventional optical sources at the National Research Council in Canada, and so there was exposure to that field as well at Columbia, and it became clear that to do these kinds of high resolution experiments which required high resolution spectroscopy, that they would also benefit from high resolution sources, so I came up with the idea of getting into that kind of spectroscopy, but this was, at the time I came, the ruby laser came along, so I started to just familiarize myself with that, with the general idea of moving toward this kind of spectroscopy, but it was so exciting, just playing with this new device, that I was spending several months really just learning to use it and sort of exploring the primitive diffraction, damage effects, behavior in passing through [???] fringent? Crystals of various kinds. But again, I did not have any clear concept at that point of the nonlinear optical phenomena. In fact, it was not until the Franken experiment — well, the Franken experiment, as soon as I heard about it, I mean, we immediately put a piece of potassium dehydrogenate phosphate in front of the laser — I think It was a day or two after we heard about it — and sure enough, you know, there was the ultraviolet light at twice the frequency coming out. And at that point I began; at that point, all that was known was that if you put laser light into a crystal, you got some ultraviolet light out at twice the frequency. That was all that was known. So I began to investigate that, at first in a rather open, a rather informal kind of way. I put a crystal in there which had been carefully polished, and started to study the dependence of the output light on the angle of the crystal, and it was beginning to be clear that there were some systematic fringe effects, — as you turned the crystal, the output power moved up and down in systematic kinds of ways and of course, investigating that, it was clear that moving in certain directions increased the intensity, and, I investigated this effect further and soon found that you could increase the intensity by thousands and thousands, literally thousands and thousands of times, by choosing a particular direction in the crystal, which I discovered experimentally. But as I was approaching, you know, realizing what enormous increase in intensity was possible in this process, I studied the mathematics a little bit more carefully and recognized that there was a resonance condition, in which the velocity of the fundamental light, the laser light, became, could become equal to the velocity of the second harmonic light; that the normal dispersion difference in the velocities could be offset by the difference velocities due to the fact was a [???] fringing crystal. You played them off against each other, and that you got an exact match, the increase in intensity could be literally hundreds of thousands of times, and this is a very, was for me a very very exciting discovery. You know, it really represented the first case that I’m aware of phase matching in nonlinear optic process.
So you really got this experimental result and then you began to realize that it was a phase matching phenomenon?
Yes. Oh, I didn’t understand it at first. No, I just saw the very systematic angular dependence of the light, and in exploring that systematically, recognized that it was a very fundamental effect, and then went back and analyzed the light propagation of the crystal more carefully, and discovered what it was.
The thing that occurred to me was that you might know, or someone in your position might know that to be able to get a second harmonic generation would be important for applications as well as for science, and you might have your eye on increased intensity — now, is there anything whatsoever — this would be... an historian’s hypothesis?
No, I think the motivation here, I mean, in general, one appreciated that because one now has an ultraviolet source, that one didn’t have before, that there was some potential usefulness here, but the real motivation was that it was such an exciting new principle, you know, that one didn’t have before, it was really just the excitement of doing something that really hadn’t been, you know exploring such a novel feature which, you know, when you understood seemed so very simple, of matching velocities, and yet, you — it didn’t occur to you at first, that — the whole issue —
Let me say parenthetically — you’ve got a situation here with lasers and masers that’s just on the borderline between science and technology, and I’m a little bit searching to see how the motivation combined, in this particular concrete instance, or how they didn’t, but —
No, for me at least the technology had no relevance at all. I was simply interested in investigating the new physical phenomena, you know, of really great interest and excitement, and from my point of view, that was the deal driving, I mean, the mechanisms by which the technology — in a laboratory like Bell Labs, where this is a whole different areas, but you know, the technology is served here by the laboratory sort of instituting and encouraging research in certain fields, and from the laboratory standpoint, anything that was novel in the area of optics was potentially relevant for communications, so it was all in the context of the search for technology, but that wasn’t the driving force. And I think in the case of most researchers, they may choose certain areas because they have a technological, an area of technological impact, but once that’s chosen, you know, at least I tended to be driven almost completely by the interest of the physics, and the novelty and the sense of innovation.
Well, that’s really bringing us to question 4, where I ask about the interaction Kleinmann, whether, how important or unimportant we should understand that to have been.
Well, now, the interaction with Kleinmann began, I guess, I had sort of delineated the gross features of the phenomena, I mean the dependence on the refractive index, on the birafringents, on the dispersion of the material, the dependence of diffraction on wavelength, the gross features of it, but it remained for Kleinmann to really show in real detail the details of the wave optics interaction between the fundamental, and the harmonic. It was Kleinmann for the first time that really worked out in detail the really quantitative mechanism of the process, and really developed a full understanding of the process. Later also Gary Boyd in collaboration with Kleinmann pursued this further in the context of Gaussian beams, that were introduced by. But that was I guess one or two years later.
Were you working intimately with these people at this point, or was that a close association, he just took the results that you had and did the theoretical niceties? I’m really seeking for who it was that you were primarily in touch with, primarily.
I had many discussions with Kleinmann in the course of this [???], in part because he had a very thorough understanding of phonons in quartz, and a real understanding of crystals, that was his connection, but it was the context, at a certain point, there were others here who, you know, one consulted with, and I had no real background in optics, except the courses, you know, it was a theoretical knowledge, but there were people at Bell Labs such as Walter Bond who had worked with piezoelectric crystals for many years, and was extremely influential. His writings taught about, from a practical and accessible point of view, the understanding of crystal symmetry and its relation to optical properties. This came from the long gradation of piezoelectric crystal research at Bell Labs. There were many contacts here. Another one is the concept of modes of radiation. I remember coming here in my interview visit, having lunch with Art Schawllow, and we had discussion of, I remember, matchbox antennas, the possibility of antennas that were smaller than a wavelength of producing directed radiation. In the course of that discussion, it became clear, this was in the early sixties, early ‘61, this was before the full understanding of modes of a resonator, you know, were really understood; I mean in the Townes-Schawlow paper, and the [???] paper and so on, there was an understanding of what a cavity should look like, but the fundamental understanding of what a mode was, quantitively, you know, in more than a qualitative kind of way, wasn’t there, and it wasn’t until Boyd and one of his colleagues at Bell Labs, now retired, did a calculation which paved the way, a computer calculation, the way the waves moved back and forth between mirrors, actually settled into a steady state kind of long lived excitation of the cavity, that one understood what modes were. So at that point, the concept of modes was completely new. A second concept that was completely new for me was the importance in nonlinear optics of the crystal symmetry. Now, it’s obvious from the work of Franken, after the work of Franken, but you know, I had done some early experiments to explore what the effect of putting this intense light through transparent materials would be, but I did not understand at the very beginning the importance of the lack of symmetry, till it was pointed out. Now it seems so obvious, but you know, at the time it was very very far from obvious. And In fact, —
You did some work on modes yourself, with Collins.
Yes, that was very shortly after I came. That was probably in early ‘62. This was in the context of this followed up on the original work of Collins and Kaiser and Nelson and others demonstrating the coherence of light from a ruby rod, and the work that we did was to show that — well, you know, the result that Collins had was that there was some evidence that the light reflected from the sides of the laser rod were playing some role in the output, because the rods that were polished on the sides behaved differently than rods that were ground to a rough finish on the sides, and so we explored that some more and we did that by fabricating special rods that were in the form of rectangular parallel pipeds? each side being polished, and we did various experiments with some of the sides having these evaporated layers, some of the sides being, — having not evaporated layers — and systematically explored modes in which the light actually uses the bouncing off the walls in order to form the resonance structure, and you could form cavities in which you had beams of light coming out in very symmetric directions, each beam was very coherent, like the regular beam of your laser, but they would come out in specific directions, all very symmetrically arranged, add these directions correspond to the directions that a ray? running around inside the box would be re-entering upon itself, and so we explored that in a very systematic way. Really a very beautiful radiation pattern from the sides of lasers, and from that experiment emerged some understanding of the fact that a laser could operate indeed in structures that looked very different from a Fabry-Pleot [???] resonator, and in fact, that you know, laser action could occur in a box that was the shape of a rectangular parallel piped? Although that’s not a preferred arrangement, but it showed it was possible.
There were a number of groups at Bell Labs that did various work on modes.
When you worked with Collins, you went into his lab and worked with his equipment, or you?
I came into his lab. We shared a lot of equipment. As soon as I arrived at Bell Labs, he was very helpful in letting me use his equipment and loaning me some equipment, lending me the designs of his equipment, and really helped me get started.
OK, now, question 5, you partly answered that already, that part of your background was already there and useful in nonlinear optics, and I also wanted to know whether you already knew and were familiar with radio frequency and microwave nonlinearities?
No. No, I was not familiar with their nonlinearities, no. My background really was in microwave and in electromagnetic theory. I had taught a course to graduate students in electromagnetic theory at Columbia, so I came to Bell really fluent in how to think about electromagnetic wave propagation at that time, not in a quantum electronic sense but in a conventional wave kind of theory, so that I could, I felt at home with these kinds of concepts, but I had no previous background in nonlinear effects per se.
And then you already said that you were boning up on stuff like crystal properties and —
— Yes, and crystal symmetry and the relation of piezoelectric properties to nonlinear optical properties, and realizing that the mathematics, the mathematical symmetry properties of those was very very closely related.
And what about Bloembergen’s group? Did they impinge on your studies at this early period, or does that happen only later?
No, not at this point. Now, of course, we’re now out of the three level masers. That had enormous impact.
Yes. I mean his new work.
No, that didn’t come up for a couple of year more — maybe ‘63?
I remember that coming out in ‘62.
Was it ‘62? OK.
But if it were late ‘62, then that might shove it all till — I notice I have a habit of looking at submission dates —
Yes. Well, I think in a general way that it conveyed the breadth of the nonlinear optical effects, and you know, it systematized one’s understanding of these effects and the relation between them. One of the things that Bloembergen, [???] condition, showed symmetries between the various waves that are present in nonlinear optical experiment, and so that provided a [???] first time, a real solid foundation for the understanding of these effects. So in that way, it was certainly influential. And then another very influential experiment for me was the stimulated Raman experiments that were first reported from the people at Hughes, and that was a direction that I pursued at some length over a number of years.
I’m going to turn this off for just a moment...
In my own work. from ‘61 Into ‘62, I guess my emphasis in early ‘62 and through mid-’62 was on modes of radiation of cavities including these studies of bouncing ball modes in rectangular and other cylindrical cavities, with polished walls. That was in collaboration with Bob Collins. Now, late in’62, the stimulated Raman experiments became available, and so work started in stimulated Raman activity, are at the same time, I became interested in the relationship between optical rotation and nonlinear optics. I was fascinated with the idea that liquids lacked a center of symmetry, liquids that have crystal molecules, liquids that have optical rotation lack a center of symmetry, and I became interested in observing second harmonic activity in liquids, and somewhat later, well, actually it wasn’t until later in the sixties in collaboration with Peter [???] that we were successful in seeing these interactions in liquids. The reason these initial experiments failed in 1962 was that the peculiar symmetry of liquids, namely a very high symmetry, is such that in order to do a nonlinear experiment, a harmonic experiment, the beams have to be coming into the liquid in different directions. A single beam will not generate harmonic generation, so the initial experiments that I did at that time were not successful because I did not understand the full implications of the symmetry. A little later in 1964, ‘65, I studied the symmetry of liquids more carefully, and understood that, and then went back to it later in collaboration with [???].
This work, I have down here in ‘64 with Kaiser and Myer, this really went back several years in your own history, to some preliminary work you were doing. There’s an article —
The stimulated Raman effect was with Kaiser represented a combination of interests. On the one hand, the stimulated Raman effect, and on the other hand, in phase matching and second harmonic generation experiments. Now, the first stimulated Raman effects were such that you observed light coming out at a new frequency, shifted in frequency from the laser frequency by a difference frequency which represented the frequency of the phonons, or some other molecular or lattice excitation in the material, and there was no evident phase matching issue there. Experiments that I did with Kaiser were an attempt to demonstrate the interaction of pairs of frequencies that were differing by the frequency of the lattice excitation, so these experiments used multiple frequencies and explored the phase relationships that were part of the stimulated Raman effects. Those experiments were the first experiments in fact in which the coherent scattering from the lattice excitation was observed. These experiments with Kaiser, we were to show that in a stimulated Raman experiment, the lattice phonons are being driven in a coherent way. The phase of the lattice excitations is related to the difference in phase of the two light waves that are present, the laser and the shifted frequency. We were able to do this experiment, at the same time, scattered light, the second laser beam, from the excitation from the excited phonons in the solid, and look at the scattering from those excited phonons, and study the phase of those, study the coherence of those phonon excitations in a direct kind of way. So those experiments were a kind of combination of earlier work of others on stimulated Raman effects, and work that I had been involved in on the phase matching and nonlinear second harmonic generation.
I was actually thinking of this paper. I think we were just discussing this one.
And I had been thinking of this. When I said that you were working with Kaiser and Myer.
Oh, yes, now, this work with Kaiser and Myer was an attempt to understand the possibility, to evaluate the possibility that the Raman excitation in one liquid, in a liquid solution for example, could influence the vibration in another liquid, and I think there was some evidence that in this paper, but that was not a direction that really opened up new directions, and so that there’s some evidence of an interaction there between the stimulated excitation of the molecules in one liquid and another, but that was not a path that really led anywhere at that time.
How did these paths open up? Did you have — of course, here it’s very difficult to distinguish what seems to happen in retrospect from what was going on at the time, but was there a common sort of research drive out of which these various things were emerging, or were there several parallel research directions that you were simultaneously interested in — with the Raman effects, you had this early interest in liquids, which you’re talking about, and the —
Individual researchers, in large part, tended to go in their own directions that reflected the particular mix of background that each one had. And of course, the people that one collaborated with. Now, in the case of Kaiser, Kaiser, for example, had a very strong background in optical studies of solids and spectroscopy of solids, particularly of materials such as diamonds. He was the one who discovered that enormous quantities of nitrogen can be contained in diamonds, in certain flawed diamond crystals, so that he had a good understanding of crystal optics, and his interest in crystal physics and my interest in nonlinear optics led to this work on coherently driven lattice vibrations in calcites, and that was an example. Another one, in the case of [???] for example, his chemistry background, and mine in nonlinear optics, were the natural confluence, if you like, of interests. People tended to follow their own direction, and sort of mesh with colleagues, you know, where there was a common goal to be served. But it wasn’t as though there was, you know, a whole department or a whole laboratory with a certain set of coherent directions.
Did you have, in your own work, any goal, I mean, so that this research was steps to that goal, or, I want to try to get at the structure of your own research program, whether you were interested in investigating certain lines, or whether you had one coherent plan that you were sort of approaching from various angles. I’m trying to fit together the various problems that you worked on in this period, was it really a question that you were just brainstorming and at some point you said to each other, “Hey, let’s try this,” or whether you had some idea and then you’d go over and get some nice person to tell, think, this might be a nice person to fall in with at this point my general direction. I wonder if I’m making myself clear at all.
The general direction that I had was really to try to open out the understanding of interactions of light beams with each other. That was my interest — that broad, I mean, particularly the interactions of light beams and the differences in frequencies of light beams, coincided with certain molecular and atomic or solid state excitation, so it was very broad, and I just looked for areas that, where there were interesting ramifications and new phenomena. I was particularly excited by the possibility of new phenomena, for example, the optical parametric oscillator, and —
That’s exactly what I was getting at.
At that point, it was known at that point that — well, in fact, the idea of a parametric oscillator in its relation to nonlinear properties of materials, that was known now in the microwave region, in a very clear way, and so it became — a number of people here, myself among them, were recognizing the Principle of parametric oscillators at optical frequencies might be possible. But at one point, I did a kind of back of the envelope calculation, to estimate how much power would be needed, and it looked like a reasonable amount of power, and I remarked about this, to a number of people; particularly I remember an interaction with Morrie Tannenbaum, who is now the president of AT&T and T communications, and at that time he was in the research area here, and I remember being stimulated by a remark he made, “Well, if it’s that good, go ahead and do it.” It was a very brief interaction, but I remember being very moved by it. So in that case, now, there was this kind of confidence of interests. Now, this —
(crosstalk) ... had this crystal.
Yes, now, it depended entirely on the crystal. I mean, a lot of people could have the idea, because the idea of parametric oscillators was not novel. I mean, it hadn’t been done at the optical frequencies but it had been down at lower frequencies and the concept was pretty familiar. But to do it at optical frequencies required excellent crystals, with strong nonlinear properties, and there was such a crystal that had just been introduced at Bell Labs by Garry Boyd, and his colleagues here at Bell Labs, and Carry Boyd and Kurt [???], [???]
Did this happen before or after you had made these back of the envelope calculations?
This was before, yes. The calculations really had to assume a particular nonlinear optical coefficient, and a certain, you know, assumptions about how long a crystal you could get, to do the experiment with, because unlike harmonic generation which has no threshold, in other words, harmonic generation, the light comes out a proportion of the square of the input light, there’s no threshold. If you have a quarter of the light, you get 1/16 of the output light, whereas optical parametric oscillation is quite different. You get nothing out unless, essentially nothing but noise, unless you have exceeded a certain threshold in power, so only with a very perfect crystal, so it all depends on the crystal, and the fact that these crystals were available and understood at Bell Labs was a very crucial part of it. I mean, they were available. So I teamed up with Bob Miller who had done some of the key measurements on these crystals, key measurements of nonlinear optical coefficients, and who had worked with those crystals and understood them, so he — although he of course had worked in nonlinear optics as well, but he had the special crystal expertise and I had the special interest in optical parametric mechanisms. That was something that I thought about. He was, he had focused more on the materials aspect than the nonlinear coefficients and what they meant from a materials point of view. I had concentrated more on the electromagnetic aspects of the mechanisms of the optical parametric interactions. So there was a good teamwork there that represented materials and the electromagnetic aspects of it. So again that’s an example of the same thing, in which two individuals who had particular strengths mesh together to get a useful result.
Was this idea of parametric oscillation something that you had in your mind but it was only after the crystal was developed that you sort of gave it more form? Or was it that you didn’t really think about it but then when you did come to think about it you realized that there was a crystal that would work? Does that question mean much to you?
Yes. I guess at this point I no longer remember when I first thought about applying parametric oscillators, whether it came before the knowledge of the crystal was available, or after the knowledge that the crystal was available, but it was, I think it was about the same time. One wouldn’t have done the calculations unless one knew that there were considerably better crystals available than those that had been used in the earliest experiments, the hydrogen phosphate crystals, for example.
But on the other hand, one might have had in one’s head, wouldn’t it be nice to have an optical parametric oscillator —
— Oh yes —
— and then when the crystal came, there might have been a kind of a putting together of those things. I was trying to fish for whether you had any memories of that kind of thing happening or not happening. Because you mention in that paper that there had been proposals coming out right along on extending these techniques to the optical region, in 1962, you cite —
[???] was the key person there. I’m not sure whether I found out about his proposal before or after my paper. Let me ... Yes, the paper I was referring to, and [???], in ‘63, they had them.... (off tape) So the key thing here was the appearance of the availability of the crystal, which was developed here by Kurt [???]
You brought that new phenomenon to the Puerto Rico Conference. Do you have any memory of what the reaction was here?
Yes, I remember it was a topic of a lot of discussion, and I guess one of the questions that Bob Miller and I had been able to deal with was the further understanding of the role of the multi-mode issue, in other words, there are two frequencies coming out, not just the second harmonic, but two frequencies that add up to the original frequency, and an important issue was the fact that it required a coincidence of cavity modes in order to get that most efficient, so that there are only certain pairs of frequencies where that coincidence between the available cavity modes and the frequencies of the two output frequencies would coincide. And again, we saw that as a kind of periodicity in the efficiency of the oscillation. We didn’t understand it at first. It wasn’t until we analyzed it in detail that I understood that this was the result of the periodic availability of the modes of the two frequencies that were being generated. So that was a topic of considerable discussion.
That was something you already understood by then?
Yes, we had reported on that.
— They didn’t, they needed to be talked to about it in some way?
Yes, well, I think at that time we were the only people with an optical parametric oscillator, so there was, you know, nobody had thought about it very much, in detail.
Were [???] and [???] there at the meeting?
That I don’t remember. No, I met [???] only — he may have been at the meeting. I only met him of course some time later. I found him a very very unusual person, a delightful person in every possible way, gained a lot of respect for him. Yes, he was a great loss.
Was there anyone in particular at the Puerto Rico meeting who hadn’t been there before? That there was now a contact which was of some interest?
Again, the Puerto Rico meeting, like the Paris meeting, somehow didn’t seem to be a turning point for me. You know, one met people in a number of different kinds of, a number of different meetings. There were Physical Society meetings, for example. So that didn’t reflect, represent a real turning point.
Well, then, I would like to make sure that we have said — have spoken sufficiently about the origins of the parametric amplifier, optical things, and are there things we’ve left out, in getting the experiments going, getting the concept, working it out, that we should be putting in there? And then we want to talk a little bit about — I guess you continued to work on it until you went to Munich, Is that right?
That’s right. That’s right.
Do you want to talk a little about the extension —
Just, you know, on the idea of looking for parametric oscillation at optical frequencies, somehow there’s a real hurdle. I mean, it looks so obvious, like anything else, after you do it, but it was a real hurdle to get started, because it’s a question of confidence — you have to invest a certain amount of time here in getting oriented to do all this. You have no background you know, in a completely new experiment, an experiment with a threshold. It wasn’t the case that you could put things together in a readily imperfect and primitive kind of way and see an effect and then gradually optimize it. The — at least as I saw it, for us, the optical parametric oscillator was an effect of a threshold, so that you had to have everything right, and yet you had no guidelines until you actually saw a substantial amount of power coming out of it, unlike a — you know, it was in a small way a little bit like the maser, for example, in which the ammonia beam maser was an experiment with a threshold, something like the nuclear chain reaction. Nothing much there of interest until you reach a certain level of the experiment, so that you have to invest a great deal in the experiment before you see anything, and that issue, I found, a kind of hurdle just in approaching it.
How did you approach it? You calculated first what levels you had to have to have an effect?
Yes, that’s right, and the fact that you calculated the level of the effect, you needed harmonic light to numb the device, which we generated in a harmonic generator crystal, and we calculated the intensity that should be needed, and it was in the, you know, the kind of intensity that was available, so we went ahead and did it, but that was a kind of hurdle, unlike those of other nonlinear optic experiments, where you can look for an effect and you may see some very, or even — or even — you will see an effect before you have everything optimized, but you’re helped by seeing the effect and guided by the way it increases, you know, in intensity as you optimize the experiment, but that advantage wasn’t present here, and that was a kind of hurdle.
That’s a very interesting point, which I had not realized. I really did feel that the nature of the experimental work was insufficiently put down in histories and books and I’m therefore particularly interested to get that kind of detail onto these tapes, so that people get the feeling for what is involved in different kinds of experiments, and the approach people take and why they take it, all of this kind of thing.
There’s a kind of confidence necessary, too. I mean, I came into these experiments from a whole set of experiments involving stimulated Raman effect and harmonic generation, and it was sort of beginning to get a feeling, you know, the kind of gut feeling that you have that these interactions and coherent mixing effects, you begin to get a sufficient feel for it that you have the confidence to go on to a new experiment, where there’s absolutely nothing that’s been seen, but you have to somehow have that confidence to, you know, approach an experiment in which there’s a possibility that absolutely nothing will come out of it. So I find that there’s a kind of psychological hurdle there that has to be overcome.
Did it take long to set that one up?
No, it didn’t take very long, as a matter of fact; if I remember correctly, it took only a few weeks, before we saw an effect.
So it was more a question of a kind of nervousness.
Yes. Yes, that’s right.
Well, then you came back and you started to increase the range of wavelengths for oscillations. You extended the range of oscillations by a factor of five, and you added this tuning by electric fields. I want to make sure that I have here the main ways in which you extended the parametric work.
Yes, I think they were extended by optimizing the cut of the crystal by optimizing the pump intensity, by optimizing a variety of experimental conditions. Now, at the conclusion of that experiment, I became interested at that point in [???], again in liquids, and generations of second harmonic in liquids, and I moved away from the parametric oscillator at that point, and I think in retrospect, probably, I really probably should have stayed with that, to really develop that more into a more useful kind of source before moving on to other things, but at the time, I was so fascinated with the nonlinear optical properties in relation to symmetries and materials and so on that I moved off to these material experiments in liquids, and I think as a result, I probably missed an opportunity here to really develop the optical parametric oscillator into a practical source, because I now realize of course, you know, years later, other sources such as the dye lasers have had enormous impact, you know, on the application of lasers in science, and the dye lasers were not available at this time, and I think if this had been pushed more at that time, into a practical source that others could use — you know, at this point in the experiment, maybe a few months after this work was done, it was something that I could use because I had, with Bob Miller, had built the thing, but you know, it wasn’t an accessible source for scientific application, and —
Now, you at this point we’re not carrying anything into that kind of stage.
No, I was solely interested — no, that just wasn’t what I was interested in at the time. I was really only interested in, you know, exploring new kinds of nonlinear optical interactions, new kinds of nonlinear optical phenomena, and subsequently I did experiments in liquids. I did experiments on picoseconds pulse effects and Raman scattering, new techniques of measuring the duration of picoseconds pulses. My main moving force here in research was to really explore new phenomena and delineate new phenomena, identify new phenomena in nonlinear optics. I was just confident; you know that where they were valuable, the applications would just develop themselves. Now that I’m concerned with the responsibility for development here at Bell Labs, I feel, I realize more in greater detail that that takes a tremendous amount of effort, but at the time, my main interests were in delineating, identifying new phenomena.
Now, there is something, in reading through, in examining some of the Bloembergen group, some of the Townes group and MIT papers, I was struck by the difference in approach that I thought I saw, more phenomenological on Bloembergen’s part and more concerned with molecular and medium motions in terms of Townes’ group, and I was wondering how that looked to workers in the field around this time, and whether there were any other ways of approaching the subject that marked themselves out as maybe semi-distinct schools.
I think that’s a good distinction to make. It just reflects the different styles of working, and these different styles appeal to different people. Of course, a lot depends on your training. I mean, I, having been trained in the Townes’ school, I tended to be attracted to the issues of identifying new phenomena and describing these new phenomena in terms of — well, I guess my own interests were heavily involved in electromagnetic aspects of this, of these phenomena, as distinct from the atomic and molecular aspects. But one couldn’t help but be very conscious of those differences in approach. I think the Bloembergen school was very mathematical, and was able to find, to demonstrate [???] connections between a large part of nonlinear optics. In fact, you know, in a few equations, in a sense, was able to consolidate the whole field of nonlinear optics in a few equations. I think that was a stimulus in developing the field. But I came from a different kind of background, and I guess most Townes’ students tended to think about the field less cosmic, less all-encompassing kinds of ways, and just in terms of more specific, tended to think of the field in more localized kinds of ways and to be attracted to new phenomena in the field, as distinct from the perhaps more formal approach of the Harvard school.
So then to what extent should somebody think of it as being too well defined and somewhat distinct approaches? It sounds like the correct way. That’s the way people in the field at the time were looking at it, that there were two somewhat well defined different approaches, in the literature. Were there others? I mean, are there other schools that I’m just ignorant of now, that — when you look back at that period and you remember who was working in it, how would you identify the big important group and their points of view?
Let’s see, we’re not identifying Townes and Bloembergen specifically, we’re identifying them simply because their work had perhaps the greatest impact of anybody’s in the field, but I think Bloembergen, and the broad all — encompassing formalism be developed, I think that was rather, I think most people in the field would have an approach here a little closer to what you and I are referring to as the Townes approach.
Then I also asked you, a lot was going on in laser physics at this time, and I’m just trying to get some feeling for someone who was working as you were, what the relation was — for example, you’re getting the coherence studies, the semi classical studies of Wolfe and his people and van Dell [???] and so on, and then the quantum electrodynamics studies of [???] was that having any impact? You were getting the controversy with people like Jaynes? as to how much you could just use classical physics. Was that having an impact on your part of the world? This kind of thing.
Well, it had an impact, in the sense that that was really exciting, all of it. [???] to follow that very very closely. That came out, in fact that was one of the highlights, I guess of the Paris conference, the —
He was certainly at the Paris conference.
Yes, I think that’s where it really, really became exposed, the apparent conflict between the quantum view of coherence and the classical view, whether in fact the classical view was really an adequate one for getting a rough picture of what was going on. Oh yes, I think that most people followed that very very closely. But it didn’t affect my work in a very direct way. But we all followed it, and were very stimulated by it, but it didn’t affect us in a very direct way.
Someone said to me once just informally that he didn’t believe that coherence studies really had much of any effect on lasers altogether, and I’m not completely sure what he meant by that remark. But —
— Well, I think it had an effect. It was more of an intellectual effect. You know, a feeling of real understanding of what was going on in the sense of, after you read [???]’s paper, initial paper, you know, you really felt that for the first time you had an understanding of what the characteristics of light were all about, but it is very true, I don’t think that the statistical aspect of light has had a big impact on the — certainly they haven’t had much on the applications of lasers, but I think there’ll be universal agreement that that work, as well as Wolfe’s work and Mandel’s work and a few others, really provide, you know, the framework for understanding and thinking about the statistical properties of light. But I’d have to agree with your statement that I think from an impact point of view on the applications, on the scientific experimental scientific applications, the engineering applications, I think it would have a pretty limited impact.
One of the ways I wanted to get at this is, in talking person by person to see how they, received these —
Oh, I was very excited about it. Some things you can be excited about even though they have no impact.
And I think, you know, it wouldn’t be right or fair to say it had no impact. It had small impact on applications, on lasers; it had a very important intellectual impact. I think it probably impacted a lot of good people.
That’s interesting. What other theory or work should we really be conscious of, and now I’m saying with a cutoff point say of 1966, that’s just a convenient cutoff point for the moment, on things that were coming out in general that were important for your own interest or for your own development of your researches or —
I guess for me, it would be, going back to an early stage, it was the discovery of the stimulated Raman effect and its interpretation. I guess one of the first people to really interpret that in a very physical way was Hellworth, and the discovery of that by the Hughes people and Hellworth’s interpretation, I think — that and the Fran ken experiment of course were two of the key things here.
You were in touch with Hellworth or just read it in the literature?
I met him. We talked to each other at meetings, but I guess the real communication there was through his papers.
This has been May 31, 1984, and this is Joan Bromberg, I’ve been talking with Dr. Joseph Giordmaine at Bell Laboratories.