Oral History Transcript — Dr. Theodore Hänsch
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Theodore Hänsch; January 19, 1984
ABSTRACT: Laser research at the University of Heidelberg, 1965-1970. Thesis research. Collaboration on a commercial laser. Hänsch's laboratory style. Frustrations of doing spectroscopy with the early, nontunable lasers. Laser research at Stanford University, 1970-ca. Comparison of resources at Heidelberg and Stanford. The high-resolution, tunable laser of 1971 and the research program it engendered.
Bromberg:I'm in the office of Professor Hänsch and we are going to talk first about his experiences at Heidelberg.
Hänsch:I have in front of me some questions, and am trying to remember. In ‘65 I was a graduate student at Heidelberg University, actually starting out in nuclear physics, with a betatron accelerator at the institute of Prof. Kopfermann and then I learned of some work on lasers that was going on at the Institute of Applied Physics, of Professor Schmelzer. Professor Schmelzer was planning to build a linear accelerator for heavy ions, an accelerator that actually was built and now is at GSI in Darmstadt. He felt that maybe these newly discovered lasers would be useful tools in synchronizing the phases of the resonators in this accelerator.
So he had got a young assistant from Bonn University, Peter Toschek, and had asked him to see if he could not set up a laser group to develop some expertise in this fascinating new area.When I learned about that group, some students had actually succeeded in building working helium-neon lasers at that group. That's when I came in, And at that time, since there was hardly any commercial suppliers, we had to do everything ourselves, so we were working with glass blowers, We had our own coating facility, a Balzers machine to make mirrors. We had to fill in the gas, we had to build the power supplies, and it we really a lot of work to actually get one of these lasers going. That's pretty much how it started. Let's see what other questions there are,
Bromberg:Well, you don't have to follow that literally, it's just to give you an idea.
Hänsch:Well, I was so fascinated by these marvelous light sources that I decided I would like to do my thesis work exploring what one can do with lasers. In Germany, before one could start with one's doctoral thesis, one had to do a diploma thesis, and it wasn't really quite well defined what I was going to do in that thesis. Everything just was so new and there was so much unknown, and there were so many different directions one could charge into that it was a period that was initially maybe quite confused.
But in the end it turned out that there were actually some new forms of spectroscopy that were made possible by the high intensity of laser light, new forms of spectroscopy based on saturation effects that really could reveal a lot about the processes going on in gas discharges, about collisions, relaxation rates, and—
Bromberg:I'd be interested to know some of the things that you decided not to work on, some of the other possibilities. Your feeling for what the problematic of laser work were at that point. If you remember what people were talking and thinking about.
Hänsch:Well, it was in some ways rather frustrating. We had these marvelous ideal light sources, much more monochromatic than anything that people had known before, and it was also clear that there were new ways of studying spectra of atoms with much better resolution, but you couldn't tune these lasers to wavelengths that were interesting. I remember that much of my time I felt a sense of frustration. I wished there was a way that I could tune laser wavelengths, say to the sodium lines. All we could do was look at neon itself, with helium neon lasers, because neon naturally had resonances, where the laser would emit light.
So that's one thing we would have liked to do in the early days but couldn't.
Bromberg:The neon was presenting interesting problems, or just?
Hänsch:Well, neon was interesting because it was the material used in laser, and as it turned out at the time, very little was known about the actual transitions that lased, in particular the transition probabilities, the Einstein coefficients. There was no tabulation, as far as I knew no measurement even, and it turned out that by studying the interaction of the light from the helium-neon laser with the neon discharge, I was able to actually find out what these Einstein coefficients were, and so, in that sense it was interesting, but certainly not anything fundamental or of earth shaking importance. It just gave a little bit of a flavor of what might be possible if one could make these lasers widely tunable.
Bromberg:One of the things I'm asking there was whether you had any connection or relationship with people who were trying to commercially make lasers?
Hänsch:Well, actually in Germany there was really no laser industry to speak of. We had found a way to make helium-neon lasers rather simply and inexpensively, by glueing mirrors directly onto plasma tubes, and I wasn't aware that others had done that at all, so we got a patent for this configuration and we actually had an agreement with a German company, Leyboldt, to try to manufacture and sell these lasers. Later I learned—
Bromberg:How did that turn out?
Hänsch:It turned out that Leyboldt wasn't able to make lasers that would last long enough. The problem was that the mirrors had been glued and the quality of the epoxy apparently wasn't good enough to give a long lifetime, so the project was not commercially successful. And of course we were just students, we learned that there was a difference between making a working laser in the laboratory and making one that you can send out to customers.
Bromberg:Was that an extensive interaction? You went over there and worked with them to—?
Hänsch:Yes, we tried to teach them to make it, and they learned to make these lasers. The laser just wouldn't last very long. Of course, in the early days that was a common problem. There was a company in Palo Alto, Optics Technology, which independently had also found the same method of making lasers. I learned about that when I came to this country. They eventually went bankrupt with similar problems, they couldn't make lasers that lasted.
Bromberg:So then just how extensive should we think about this connection with Leyboldt? Was it a couple of months?
Hänsch:Yes, it was a relationship of a couple of months.
Bromberg:Then you just went away from that and went back to academic work?
Hänsch:Yes. Well, I never left the University, that was something we did on the side.
Bromberg:...Who else was involved?
Hänsch:A colleague of mine. He was a student at the same institute at that time and is now lay brother-in-law. Dieter Weiss is his name. Peter Toschek, my thesis advisor, was also part of the enterprise.
Bromberg:I asked you here about the scientific style of your co-workers. That's one way to get some feeling for investigators, to ask people who work with them how they would characterize their styles—people who do a lot of theory before they sit down to experiment, or whether they would rather go into the laboratory and just try quickly. Then we ask people themselves how they would describe their own styles.
Hänsch:Well, I don't think there was time enough for things to settle down into a particular style. Everything was just so new that we just tried to do the darnedest we could under the circumstances, without following any set style. Much of it was really just labor. We had to clean pumping stations, check for leaks, worry about how to keep mirror coatings from peeling off, lots of technical problems that really kept us from thinking too deeply about the theory.
Later on, when I went on to do my PED thesis, I found the time to really do calculations for a couple of months. That's when I started to understand some of the things that previously had just seemed like miracles.But that was done afterwards, after we had experimental results.
Bromberg:So then I asked next, —some of these questions may be good or bad—about the method of laser differential spectroscopy, which I took, when I looked up the paper,to be an innovation that you had made. Is that correct?
Hänsch:Yes. We were not the only people working on that. There was in particular the group of Ali Javan of MIT, who. were interested in similar phenomena. I believe that I was able to show some new approaches to this technique, that then eventually led to Doppler free separation spectroscopy, techniques of very high resolution spectroscopy, that are now in quite general use.
But it was certainly an area that was recognized as being interesting by people other than us.
Bromberg:There are a lot of interviews which deal in great detail with how an idea evolved. A lot of these conceptual stories in our other interviews—you don't have a great deal of how instrumentation evolved, how new experimental techniques evolved. That's why I have allot of questions like this. I think it would really enrich people's understanding of how science is done, to see how a new experimental method—
Hänsch:— I think different people have different styles. My own style is maybe somewhat chaotic. I find it very difficult to make a detailed plan. I hate to go shopping with a shopping list, I'd much rather just go to the supermarket and see what I like by looking at the shelves, and I think it's similar in doing experimental work. I like to decide each day, based on what I did the last day and what I want to do next, and I find it rather boring to follow a set path for a long period of time.
Bromberg:I certainly had some problems with my thesis advisor because of this, because we had discussed an idea and he thought that that was my thesis project, and then the next day I'd thought of something more interesting and I would go off in a different direction. He found it difficult to adapt to that. But in the end I guess he was quite happy with what came out of it.
Hänsch:If I remember correctly what Peter Toschek had suggested to me is to look at the interaction of atomic transitions with modulated laser light, and essentially to repeat some experiments that Russian physicists had carried out before with an incoherent modulated light source, to see if with laser light it would work better; by modulating the laser light you can essentially find level splittings. There are resonant effects that occur if the modulation frequency agrees with the splitting between closely spaced levels.
He felt that it would be worthwhile to see if lasers wouldn't work better than incoherent light sources. That's what I set out to do, but I didn't pursue this very long because it had already been shown that it can work with incoherent light sources, so I thought that surely it would have to work with the laser better, and I just didn't think it was something very adventuresome and exciting to do.
At the same time, I had thought of a way, how with two independent helium-neon lasers one should be able to study in detail what happens, if, a Doppler-broadened transition, being saturated by a monochromatic traveling wave, a second probe laser then would be able: to examine what happens to the velocity distribution, how collisions affect this type of hole burning.I remember, I had that thought one morning and I immediately went to the glass blower and asked him to make a second laser. We had a very skilled glass blower who would be able to do things on the spot, so within an hour he made laser tube for me, and I walked with that laser tube back to the Institute and met Peter Poschek, and he was wondering, what was that?
When I explained it to him, he was rather upset that I was abandoning this project I was supposed to be working on.
Bromberg:You said Javan also was working on that, but that must have been something you found out later on?
Hänsch:Javan didn't work on exactly the same approach. Javan was mostly concerned with what happens if a standing wave field, there is no good analytical way of describing what happens. People have to resort to rather awkward numerical models. There are many processes that can occur when you have a strong standing wave field, as you have it inside the laser cavity, interacting with a Doppler broadened transition, and there are theorists like Stenholm and Lamb and others who have written long papers elucidating what can happen there.
But it's very difficult to understand in detail, whereas the techniques that I was able to first demonstrate at Heidelberg really had simple theoretical interpretations, so simple that even I, being a student, and certainly no theorist, was able to make some calculations on the interaction of three-level systems with two traveling laser waves, which still get a log of citations even after almost 20 years. If I look at the citation index, every year there may be more than a handful of paper, with the calculations I did as a student.
Bromberg:Tell me, how long did it take between conceiving this idea and coming with this laser tube in your hand, and getting the results proves out?...
Hänsch:Well, different people have different approaches. I tend to be very impatient. So I tried to do it as quickly as I could, taking short cuts wherever possible. And so I guess it was a couple of days until we had some first results. But if you looked at the apparatus, my high voltage supplies and Servo-electronics were built of cardboard boxes *with loose wires running around and things like that. So it certainly didn't look like a proper experiment, but it worked.
Later on I had helpers and the electronic shop and everything was properly put together.
Bromberg:That must have been pretty exciting.
Bromberg:How did Toschek react when you got it to work a few days later?
Hänsch:Oh, then he joined in, and we actually did a lot of measurements together.
Bromberg:Now, this question is just, you got technique and I wanted to know how you decided what to apply it to.
Hänsch:— Well, there really wasn't much you could apply it to at that time, simply because these lasers operated only at fixed frequencies, and if you wanted to look at spectral lines, the only thing to look at really were either the laser transitions themselves, or some molecular transitions-in accidental coincidences, and since we already had the lasers, it was natural to look at the laser transitions themselves, and of course they're interesting because they were the basis on which the laser was founded.
Bromberg:The collision depolarization seems to me very cute. Was that just an obvious problem?
Hänsch:It was a byproduct of this whole new approach to spectroscopy—that you could use laser light to manipulate atoms. You could use it to align atoms or to polarize them, to create atomic ensembles with preferred directions, and then you could see what happens as they collide.
Bromberg:Was that very original to you, or was everybody realizing you could do that?
Hänsch:No, I don't think everybody was realizing it, but a number of people were realizing it. Again, I think we perhaps were able to come up with some new twists, some ways that hadn't been tried before. But there were many other workers also interested in studying collisions. It's not as simply as it might appear, because there are many different collision processes going on at the same time. One has to make rather simplifying assumptions to really interpret what one gets, and there are more direct ways of studying collision processes involving atomic beams. So I don't know how much it has really meant, in hindsight, the investigation of these collisions.
Bromberg:I see. The other question I wanted to ask you about Heidelberg is this. Would you have been fairly isolated, or would you have been seeing a lot of the Americans and Russians?
Hänsch:No, we were rather isolated. We had the papers, Physical Review Letters, and Journal of Quantum Electronics was appearing, but these papers came late. We couldn't afford airmail so we typically would get them with quite some delay..
And that was our main contact with the outside world, till about ‘68 or'69. In ‘68 I went to my first International conference and finally met some other people working in the same area.
Bromberg:Which one was that?
Hänsch:I'm trying to remember. There was a conference in Southhampton in England. I've forgotten the name of it. I presented a little paper reporting on some of these results, and in the audience was the Russian Chebotaev who disagreed with my interpretation, so I remember there was some rather heated argument. The end result of that was that I found myself with an invitation to come to Novosibirsk and attend a conference in the Soviet Union. And at that time I met some Americans, in particular Bill Bennett, who together with Ali Javan, I think, worked on the first helium-neon laser. I met Michael Feld from MIT, who had been working on similar problems. Chebotaev is really one of the pioneers in this area.
So this was the first time that I could actually discuss new work with some of the other experts.
Bromberg:Did you go to the Soviet Union?
Bromberg:Within the next year or so?
Bromberg:What was it like? What was going on there? What did you learn or what did you teach?
Hänsch:Well, I essentially attended the conference. I also visited the laboratory of Dr. Chebotaev, one of the pioneers in this whole area, and I was impressed. It was a fascinating experience, in Siberia, where at that time Western visitors were not yet commonplace, so just walking the streets—I was constantly surrounded by students, who were curious and fascinated to meet a real “Capitalist”.
Bromberg:What's at Novosibirsk, that they would have a—?
Hänsch:They have a science center called “Akadem-Gorodd” where they bring the brightest young people from all over the country to train them in sciences.
Bromberg:Here's some Stanford questions....
Hänsch:OX, how I decided to come to Stanford? Well, after I had gotten my PHD degree, that was in January, ‘69, I believe, I had decided that I would like to spend a year in the United States. At that time that was still quite common for a German physicist. It's what you had to do in order to finish your education.
Bromberg:Completely the reverse of what it was about 50 years earlier. When you had to go to Germany in order to finish your education.
Hänsch:True. And of course I knew very little about the United States. There were few people I could ask. I had met Art Schawlow in Scotland at a summer school, and I was fascinated by him. He seemed to have the same approach, the same maybe somewhat chaotic style of thinking that appealed to me, and I felt that that might not be a bad place to go to, in particular since it was on the West Coast and if I could get a fellowship that would pay for my flight. To the West Coast, I could stop at the East Coast without extra expenses and visit some other places there. So these are the kinds of thoughts that went on in my mind, and I applied for a NATO fellowship at the time and got it, and went off.
Bromberg:You had to apply, but you write to Schawlow and say “would you sponsor me?” I see. What it was like here in comparison with Heidelberg?
Hänsch:Well, first, I was in a somewhat different position. At Heidelberg I was a student. Here I was a visiting scholar, a post-doc. I felt that things were a lot easier here than in Heidelberg. There German university system really seemed bogged down by a lot of bureaucracy. Most anything you wanted to do, involved a lot of paper work, a lot of delays. Here, things seems to go so smoothly. Everybody seemed eager to make it simple for you :to accomplish your task. So it really was very refreshing to me to be at Stanford.
Bromberg:What about technicians? You said you had this wonderful glass blower.
Hänsch:OK, that was something which was better in Germany, I have to admit. We had a wonderful glass blower. We had a good electronics shop, mechanical shop, staffed with master technicians. Here of course all that wasn't there. You had a shop but it was very expensive to use it And despite all that, I still felt that it was very much easier to work here.
Bromberg:What about money? Was it easy to get money?
Bromberg:You had to make out the forms, but was money lacking also?
Hänsch:— we certainly were lacking in money in those days. I remember there were extended periods when we had absolutely no money left, so we couldn't buy necessary supplies like stationery.
Bromberg:I see. I notice you had something like a Federal Research Council Grant or something like—Federal Ministry for Scientific Research, I noticed on one paper.
Hänsch:Yes. Of course, as a student I wasn't all that involved in getting money. I just was told from time to time that we are out of money. I have since been had in Germany many times, and I believe that the family situations there improved quite dramatically.
Bromberg:That must be experienced as a real drawback, because you're trying to get a piece of equipment or something that you need, and you don't have any money.
Hänsch:Yes. Well, of course we relied very heavily on building things ourselves, and improvising. And in hindsight maybe that was a good training. But it certainly wasn't an efficient way of doing research.
Bromberg:It sounds as if the Americans would just naturally be able to do things more quickly.
Hänsch:If you have money, you can go out and buy it—if you are in Silicon Valley, with all the companies around.
Bromberg:There were these kinds of Silicon Valley firms that you could just order things from?
Hänsch:Just driving down the freeway, I was amazed by all the company signs. Being in Germany, of course, we knew about these companies, looking through the catalogues. It was dream equipment we would have liked to order. And here they were all together, everything, and the laser companies, Some we knew, Spectra-Physics, we had actually bought some equipment from Spectra Physics in Germany. And Coherent Radiation. In Germany, when a laser tube broke, it was a lengthy process of negotiations, dealing with the sales man to get it fixed or replaced, and here we could just put it in the car and drive by the company.
Bromberg:I See. I was thinking of Spectra-Physics earlier on when you were talking, because they already were commercially manufacturing helium-neon lasers in'62, I guess.
Hänsch:They didn't come to Germany right away. Later on when they were available they were mostly still out of reach. We didn't have enough money to be able to afford more than one or two of these commercial lasers.
Bromberg:OK. They were pretty expensive. I guess the earlier ones were as much as $10,000. That's high. OK, so things were easy to get here and there was less red tape. Was it a bigger group, a little group, or?
Hänsch:When I first came here, let me try to remember, I think the groups were probably comparable in size, Schawlow had quite a few students. At one time I think he had as many as 16 students. That was somewhat reduced when I arrived. I don't remember exactly how many there were.
Bromberg:That's a lot. And with the group under Schmelzer, you had—Toschek was in charge, and—
Hänsch:This laser group was really just a little appendix. The main group was the accelerator design group. That was fairly large sized. A typical German institute at the time, 60, 70 people. But the laser, it was just one little enterprise on the side.
Bromberg:You and Toschek and anyone else?
Hänsch:And a few others. There may have been five, six students in all.
Bromberg:Who were you interacting with mostly when you got here? Were there any among the group of a dozen or something that you really talked to?
Hänsch:Well, when I came I found that there were other faculty members that I had known from papers, in particular Steve Barns. I was fascinated to see that he was just across the street, and I talked to him. We actually collaborated on some experiments. Bob Byer was here.
Of the students who were here, I didn't really interact with them too much initially because I had these plans of trying to make a dye-laser monochromatic enough that one could use it for the same type of Doppler-free spectroscopy that I had started to explore in Heidelberg using helium-neon lasers, and none of the students were working on anything like that. The main area of research was in studies of ions and crystals, using techniques that hid little to do with high resolution. So I was working at the beginning pretty much by myself..Later on, I was joined by an Egyptian student, Isa Shahin, who now is chairman of the physics department in Amman, Jordan. So we two worked together on experiments. Then there was a visitor, Dr. Frank Varsanyi, who actually has founded “High-tech” company called Isoray, but had run into—economic difficulties, so he spent some time in the group just trying to heal his wounds, and we worked together on some experiment involving image amplifications with dye lasers.
Bromberg:So this was all the first year, or where are we?
Hänsch:Approximately the first year. Art Schawlow himself had gone off on a sabbatical. He was in England much of the fall. But before he left, he had agreed that I could buy an Avco nitrogen laser as a pump source to play with, which led to the nitrogen pumped dye laser, and that really was the key to actually making tunable lasers that are highly monochromatic, and suddenly there seemed to be so much one could do. Any visible transition, any atom, any molecule was now accessible to laser spectroscopy.' It was difficult to decide what one should do first, and once the word got out that we had such a laser, our laboratory really was like a bee hive. Every day we had droves of visitors, coming to see how it was done. That was in ‘71 then.
Bromberg:When I looked at that paper, it was the high intensity, and the short pulse of the nitrogen lasers.
Hänsch:That made it feasible, yes.
Bromberg:That's why you went to get it, because you—?
Hänsch:Yes, Actually on my way to Stanford, I had stopped in two other laboratories where people were working with nitrogen pumped dye lasers. There was one laboratory at the. East Coast, Bell Laboratories, that I visited. Chuck Shank and Herb Kogelnik and I guess Andrew Dienes was there at the time, and they were experimenting with nitrogen pumped dye lasers, and they also had the dream that maybe one could make them highly monochromatic, but they hadn't quite figured out how to do it yet.
Then I stopped in Boulder, JILA, and visited John Hall, and he showed me some experiments which were not his own. But he showed me an Avco nitrogen laser which they had, and he showed me how easy it was to pump dyes and make them lase with that, and so when I came to Stanford, I sort of decided that that was the most exciting thing I had seen, and I had thoughts I would like to get a nitrogen laser and see what one can do with it.
Bromberg:Were there any other parts of the puzzle, the solution to the puzzle,that you were picking up as you moved across the country? How did you put together all the components?
Hänsch:Well, I had only some vague thoughts, how it might be accomplished. There were some other experiments which I had learned about by word of mouth. Herbert Walther had been a visitor in Boulder and he had made that flash-lamp pumped laser, using an electro-optic Lyot filter, that seemed to produce fairly narrow lines, and so my thought was that maybe, if I make a nitrogen pumped dye laser with a Lyot filter, and maybe that might be a way to make a workable laser.
So that was my first thought, and I spent the summer trying to make holographic gratings, with moderate success, but unfortunately that wasn't the answer to the problem.
I had actually announced a paper at the meeting of the American Physical Society in December of 1970, before I had results — I-tad promised I was going to report on a single-mode dye laser pumped by a nitrogen laser, and then as the time of that meeting approached, and I could see my original scheme wasn't working well, I was getting rather panicky.
I tried all kinds of different schemes. I don't remember any more how exactly that occurred to me, but it just seemed that all people who had worked with nitrogen pumped dye lasers at the time had tried to use a diffraction grating to tune the output frequency of the laser, and in the grating only a tiny spot was being illuminated by light, because these lasers had only a very small active volume, and it just seemed to me that that's not a good way of taking advantage of the grating, and I was wondering if one shouldn't simply use a telescope to expand the beam on the grating, John Holzrichter was here, he was a student of Art Schawlow's at the time, and he had built a flash lamp-pumped dye laser. I mentioned to him, and he said, oh, he had thought about such a scheme but it wouldn't work, it would have the opposite effect and make the line width worse.
Bromberg:He'd thought about it but he hadn't tried it?
Hänsch:No, he hadn't tried it, but he had come to the conclusion that it wouldn't work.
I thought some more about it and I said,. well, I have to try it out myself, and. I happened to have a little pocket telescope, a little Zeiss monocular, and I put this into the laser, and it had a beautiful effect. I could make an extremely narrow band width; much narrower than anybody had seen before, and it was very easy. Unfortunately the cemented eyepiece in my telescope was burned out after a little while.
Bromberg:And that was already before the Physical Society meeting?
Hänsch:Yes, it was before, so in time for the Physical Society meeting I could actually report on a laser that was even better than what I had daringly promised.
Bromberg:What was your work life like in those days? Was it one of these seven day a week things?
Hänsch:I think once the results started coming in, yes, it was maybe seven days a week, 16 hours a day or so. Before, I was feeling more a little bit like a tourist. There was California to explore, and I liked to go to San Francisco and visit Death Valley and things like that. But once I realized that I had a way to take a marvelous tool which opened infinitely many new possibilities, then anything else just seemed unimportant.
Bromberg:So it was putting this together, and then it was this whole research program that —
Hänsch:— right. And then of course I couldn't leave. I couldn't go back to Germany, because there was so much that could be done, so I asked for an extension of my fellowship for another year.
Bromberg:And got it?
Hänsch:And go it, yes, and then we applied these nitrogen pumped dye lasers to Doppler-free spectroscopy of atomic resonance lines — we first looked at sodium, sodium D lines. One might, I left on Art Schawlow's desk a picture that I made up when I first succeeded in getting a Doppler-free spectrum of the D lines. It showed the hyperfine components of the D line and a little hitchhiker, trying to get a ride to the next D line, source 300 feet away, when plotted on the same scale.
Bromberg:I did have a question on here about how you chose what you were going to do with this new method based on rhodomine 6. You did the Sodium lines and then hydrogen...
Hänsch:Yes, see, early dye lasers were mostly in the yellow, and any. atomic physicist knows that sodium atoms have a yellow resonance, the famous sodium D lines, so it seemed natural to look at sodium just because of that. I didn't know too much of atomic spectroscopy, but I knew that sodium had yellow lines. (laughter) So that was the first thing we tried.
But then of course it became clear that there were other more interesting things, and in particular atomic hydrogen. I believe Art Schawlow suggested that first. At pretty much the same time, we had a visit by Dick Deslattes from NBS who also was suggesting that we should use our marvelous new tool to look at hydrogen.
Bromberg:That wasn't something that you had in mind from the very beginning?
Hänsch:No. But if you can do anything, then hydrogen seemed to be maybe the most interesting, because it's the simplest atom, and anything you can measure precisely in hydrogen has immediate implications. You can measure fundamental constants. You can test quantum electrodynamics. So clearly there were infinitely many choices, but hydrogen was appealing for that reason, and also because its theory is so. manageable, so I actually would understand.
Bromberg:Now, you worked with Schawlow a lot at this point, is that right?
Hänsch:Schawlow had come back from his sabbatical. Yes, he certainly had maintained a very stimulating atmosphere. He did not actually work in the laboratory. He doesn't like to work himself with his hands. But he certainly created a sense of fun and he provided a lot of advice and moral support.
Bromberg:For example, when you see a paper which is authored by two or three people, you never really know how they work together.
Bromberg:That's one of the reasons I wonder about the sodium paper, how you worked with your collaborators.
Hänsch:Well, on the sodium paper, I think Art would agree, essentially I felt that his name should be on this because he was my host and the director of the laboratory here. I felt that his name should be on it. That was the German tradition. And at times he said he felt embarrassed that his name was on all! these papers.
But with hydrogen I think it was really Art who first suggested that we should look at hydrogen, so he in a way got that started. He was not really involved in actually setting it up, but he was interested in it. Every day he would ask how we were coming along.
Bromberg:That's the one with Shahin and Schawlow, and you.
Bromberg:Then what happened next? There are a lot of papers, all sorts of things going on?
Hänsch:Oh yes. Well, before we had there dye lasers, when you asked me who I worked with, I certainly should hive mentioned Mark Levenson. Mark Levenson was a student here at the time, and he seemed to be very very smart. After I got to know him I talked with him a lot.
Iii '70 there was the International Quantum Electronics conference in Japan, in Kyoto. During my first year here, actually, and I found a way to attend that conference, with German support and my thesis advisor, Peter Toschek, happened to be at the same conference, and he on his way back stopped in California and actually spent a couple of weeks at Stanford. At that conference, I had the idea that maybe we could apply our technique of saturation spectroscopy, actually a new improved version of that, that had evolved in some other collaborations that I may have already mentioned with Peter Smith, who happened to be on sabbatical at Berkeley, that we could apply this technique to molecular iodine. It was known that molecular iodine had lines that coincided with emission lines of argon and krypton ion lasers. Mark Levenson had succeeded in making such a laser go in a single mode, and we thought that maybe rather than studying just the laser transitions, we can study these molecular transitions and look at their hyperfine structure by the new technique of saturation spectroscopy.
So while Peter Toschek was here, with the help of Mark Levenson, we tried such an experiment. Unfortunately it didn't work. Peter went home without any result, but then Mark Levenson had found out that our iodine cell was faulty, it had sprung a leak and there was air in it. Mark and I then tried it again with a new good cell and we got beautiful necely resolved hyperfine spectra of iodine, and one of these found its way on the cover of a Spectroscopy Conference in Prance.
Bromberg:Oh, so this is your spectrum. (refers to picture)
Hänsch:This is the spectrum that we took with Mark Levenson here.
Bromberg:Isn't that nice. Very nice.
Hänsch:So that was before the narrows and dye lasers became available, Our monochromatic dye laser war invented at the end of that year, and then of course the same technique could be applied, not just to these few lines but to any atomic lines and to hydrogen.
Bromberg:Let's say a little bit about whom you were seeing outside the university, if anybody, to get this picture of the early years finished up. Whether you were in close touch with Bell Labs or Berkeley or—
Hänsch:I was in very close touch with Peter Smith from Bell Laboratories who happened to be at Berkeley at the time, He spent a sabbatical there, We collaborated on an experiment, that in some ways is perhaps the first demonstration of this very simple method of Doppler-free saturation spectroscopy that was later used to get these iodine spectra and the spectra of sociurn and hydrogen.
Bromberg:Then there was Cal Tech, there was Hughes, a number of companies.
Bromberg:Were they in the picture at all?
Hänsch:I'm trying to remember, I don't think I had much interaction with Cal Tech at the time. Yariv was there, but we never had much interactions with them. There were some people at Berkeley where Peter Smith was staying.
Bromberg:Townes was already there but he was not doing lasers.
Hänsch:No, he was interested in astronomy more than lasers at the time. Steve Harris I mentioned, in the electrical engineering department here,
Bromberg:Did you see each other informally, at lunch time, or were there colloquia which you'd jointly hold, or? How did you interact here?
Hänsch:Oh, there were many different ways. I think with Steve Harris, I knew him from the literature. I just went to see him. Peter Smith, I also knew from his papers, and I had gone over to Berkeley to see what was going on there, and I realized that this was the Peter Smith that I knew from his papers, and he knew me from my papers, and we got talking and decided maybe it would be fun to do something together.
Bromberg:And did you do that in the physically here?
Hänsch:Some of the work was done at Berkeley, and over here, I think I used the campus computer to fit some of the data.
Bromberg:I see, the equipment was at Berkeley.
The equipment was at Berkeley, yes
Hansch and P. Toschek, Phys. Letters 20 (1966) 273, and 22 (1966), 150.
The Repetitively Pulsed Tunable Dye Laser For High Resolution Spectroscopy," App. Optics 11 (1972), 895.
T.W. Hansch, I.S. Shahin, and A.L. Schawlow, "High Resolution Saturation Spectroscopy of the Sodium D lines with a Pulsed Tunable Dye Laser," Phip. Rev. Letters, 27 (1971), 707.
T.W. Hansch, I.S. Shahin, and A.L. Schawlow, "Optical Resolution of the Lamb Shift in Atomic Hydrogen by Laser Saturation Spectrocopy," Nature, Physical Science 235, 63.
Addendum to interview by Peter E. ToschekEarly Years of Laser Spectroscopy in Germany A Contribution to the Oral History Project of AlP For an account of the seminal work on laser spectroscopy at the University of Heidelberg, which includes the remarkable activities of young Theodor Hansch, it seems appropriate to begin in the early sixties.
In February 1961 I passed my PhD graduation at the University of Bonn, finalizing more than three years of construction and experiment, in company with my colleague Klaus Berkling. Under the advice of Wolfgang Paul — who much later won a Nobel prize for his electrodynamic ion trap — and his associate Christoph Schlier, we had embarked on an attempt to discover the anisotropy of interatomic potentials by the scattering of a beam of aligned gallium atoms off a target of thermal noble gas atoms. The directional variation of the cross section seemed of substantial importance in view of Norman Ramsey’s proposal of “collisional alignment” for making collision chambers replace the polarizer (A) and analyzer (B) fields in a Rabi-type resonance experiment[1a]. Since the quantity of interest was a possibly small difference of two cross sections, the latter ones had to be measured with a hundredfold better precision than so far possible[2a]. The outcome of our labors was disappointing: The difference of the cross sections for orthogonal orientations turned out to be on the oder of one part in thousand, far too small to be of any practical use[3a]. The evaluation of the cross sections, however, provided me with some gusto for the kinematics and intricate physics of collisional processes. As Paul offered me a two years position as a post doc in his Institute, I had full leeway to continue playing with our complex apparatus. To my great amazement I found that the gallium atoms inside the molecular beam oven were already naturally aligned, in any particular flight direction, from the collisions among each other[4a], and that the net effect was on the order of 10%! Thus, Ramsey’s “collisional alignment” became vindicated. Since then, the effect has been re-discovered several times in Russia, California, and Italy.
At that time, Christoph Schmelzer had been bestowed with a chair of applied physics at the University of Heidelberg. He had successfully designed the electronic control of the first proton synchrotron at CERN, Geneva, and was now planning a linear accelerator for heavy ions. For this device he considered, as a means of synchronization, a maser signal or, even better, the light beam of a cw laser then being brand new. Thus, following a recommendation by Paul, Schmelzer hired me as his associate in the position of a “Universitäts-Assistent”. This position was by and large comparable with that of an American assistant professor. Schmelzer, as a kind person and perfect gentleman, conceded ample freedom for my activities.
I set about with the job on May 15, 1963. In the new lab, I found two students that I was supposed to take care of, who had set up a vertically mounted rf excited HeNe laser equipped with internal mirrors. The laser blinked occasionally in a useless high transversal mode, as long as no debris had fallen to the surface of the lower mirror. In short: a more useful and reliable laser was in desperate need. I improvised a vacuum system with a gas filling station and an optical bench. Next, I designed horizontal, narrow-bore cw discharges equipped with internal electrodes from commercial neon signs, and with Brewster windows cemented on glass flanges attached by vacuum grease to the corresponding flanges at the ends of the discharge tube. This type of laser turned out to work at 1.15-um wavelength, and a little later at the red 0.63-um line. However, after a few days, gas refill was required since vaporized impurities from the electrodes and the grease spoilt the spectrally pure HeNe mixture. I also tried sealed discharge tubes with the Brewster windows directly cemented on the tube ends which extended the lifetime to a few weeks. With this rudimentary equipment at hand, I hired, in 1964, two students who were supposed to do experimental work for their diploma (masters) graduation. One of them built and characterized a confocal resonator for the analysis of the laser output; the other one measured a magneto-optical effect by help of the laser light.
Now, early in 1965, Ted Hansch entered the scene. He showed up in my office in search of a subject for his diploma thesis. Even in the first few minutes of our interview I was fully aware of this guy to be an extraordinarily keen student, and felt happy to see him interested in my modest activities. Certainly, I was determined to attract him and to make him join, as the third diploma student, my emergent research team.
Before continuing let me put a glimpse on a selection of queries a young and curious physicist determined to solve all puzzles of atoms by way of the just invented laser faced in the mid-sixties. Having been reading quite a bit on rf optical double resonance, I had been confounded by the fact that with rf transitions there is experimental access to the atomic population in an excited level, as demonstrated by many observations in double resonance experiments[5a], e.g., in Rabi nutation, whereas on optical lines this population seemed inaccessible. In spite of the much higher decay rate on an optical line one should expect, under cw excitation in the corresponding flow equilibrium, a detectable atomic occupation of the excited state, which I called “differential ensemble”. Identifying and manipulating this elusive bunch of excited atoms seemed a challenge; but — given my background of colliding aligned atoms — it seemed even more a chance for revealing atomic features and dynamics.
Another item related to rf double resonance had been brought up by the substitution of the rf action on the atomic sample by modulation of the (thermal) light excitation as demonstrated in experiments by Alexandrov et al. in Leningrad (now St. Petersburg)[6a]. I felt that a laser source not only might operate far more efficiently, but the achievable high resolution might provide access to atomic features so far buried in noise. This speculative foresight was justified many years later by the performance of sideband spectroscopy on individual trapped ions[7a][8a][9a].
A third line of speculation concerned experiments on two masers acting in series on the same atomic beam[10a]. They motivated me to initiate a study on Self-Induced Transparency, and later an associate, Wolfgang Krieger, did his PhD work on this subject[11a].
So, what item did we talk about when pondering a topic for Ted’s diploma work? Small wonder that I suggested my pet idea, the differential ensemble, and doing spectroscopy on it. I proposed the construction of a kind of multi-reflexion cell with an internal neon discharge which should be irradiated by the red output of one of my HeNe lasers. A side window of the cell should allow the observation of spontaneous radiation from the discharge. A few days later, Ted came up with a much simpler arrangement, where the observation of the elusive differential ensemble was to be done in the sidelight of the laser discharge tube itself. And thus we started to study the variation of spontaneous emission by the laser action — a phenomenon soon after being considered a not so glamorous saturation effect[12a][13a][14a]. A separate probe discharge in the laser resonator, in series with the gain tube, left some free choice of the discharge parameters. We could distinguish different kinds of collisions of the excited atoms which seemed impossible by conventional methods. We submitted some of the results to the International Quantum Electronics Conference 1968 in Miami, Fla[15a]. The presentation gave me the additional chance of my first tour of US labs, and the acquaintance of outstanding physicists in the field. I saw, among others, William Bridges at Hughes Labs in Malibu Valley, Art Schawlow and Bob Byer at Stanford University, Peter Smith and Herwig Kognelik at Bell Labs, Bill Bennett at Yale, and Au Javan and Michael Feld at MIT. All these renowned researchers which I had come to know from their papers seemed sociable and accessible to a so far unknown foreman of a research team in statu nascendi.
In Europe, in these days, we had some contact with a team at the Ecole Normale Superieure in Paris (Decomps, Dumont, Ducloy), a laser-group in Berne, Switzerland, and the famous theoretician Hermann Haken in Stuttgart. The funding, although still scanty by American standards, started to turn more steady, thanks to a contract with the Deutsche Forschungsgemeinschaft, an agency like NSF.
Sometime in 1968, we got aware of a paper by Helen Holt[16a] who had predicted a conspicuous asymmetry of the laser-induced saturation when recording the fluorescence in the direction of the tube axis. (She later detected it.) This way of observation allows to reveal, via the Doppler effect, the velocity distribution of the atoms in the differential ensemble. This laser-excited sub-ensemble is supposed to correspond to the natural line width of the line, far below the Doppler width determined by the Maxwellian distribution of the thermal atoms in the discharge. In an attempt of seeing such saturation signals in axial direction we soon noticed that narrow-band detection was indispensable for this purpose, which required the use of a high-finesse Fabry-Perot filter. The unsatisfactory result of such an attempt inspired us to shift the strategy: Using absorption or emission stimulated by the spectrally narrow light of a probe laser for the detection. However, this plan left a problem over: How to discriminate the saturating light in the probe-light detection? The solution was probing the atomic differential ensemble on a level shared by a neighboring line, which required the use of a laser on such a “probe” line. Although we had at hand lasers at 1.15 um and therefore on such a line, these sources emitted in many spectral modes. One day, I ran into Ted who came from the glassblower’s shop carrying in his hands a brand new, very short laser tube. Certainly, I was not amused since a rule of the lab had it that, in a permanent situation of limited funds, an order to the shop required the ok of the responsible group leader! But the assembled laser turned out a success: it indeed emitted weak, but single-mode light fit for the job of probing the population of the common level. Thus, a technique of “saturation spectroscopy” was established[17a], that later, in a simplified form, became widely applied for high-resolution spectroscopy.
To our surprise, the recorded spectra indeed showed structures that depended on the relative direction of propagation of saturating light and probe light[18a]. This mysterious and thrilling observation seemed a fitting highlight for Ted’s PhD work. We labored to compose a comprehensive paper, based on calculations for Ted’s thesis that explained the findings in terms of two effects of coherent interaction , which became later interpreted as a two-photon contribution to the non-linear susceptibility, and an interference term between the one- and two- photon contributions. In this theoretical survey, also the feasibility of Doppler-free two-photon spectroscopy is demonstrated, which was discovered much later by Cagnac et al. in Paris. Moreover, the recorded data[18a] and the theoretical model[19a] represent the observation and explanation of what is now known as “electromagnetically induced transparency” (EIT)[20a], and even “gain without inversion.”
Although Ted and I discussed the issue of modulated double resonance, this subject was never considered a topic for Ted’s diploma or PhD work.
For the March of 1969, a “Conference on lasers and opto-electronics” had been announced to take place in Southampton, England. We submitted two papers, and I lobbied for travel funds from the Deutsche Forschungsgemeinschaft for both of us. Our presentations faced skeptical interest in particular from Vesna Chebotaev from Novosibirsk, Russia, who invited us to attend the upcoming conference on gas lasers in his home town. Thus, in July of same year, again luckily sponsored by DFG, we ventured to travel to central Siberia, which, at that time, was indeed kind of an adventurous expedition. In spite of some language problems, the physics conveyed in our two talks[21a][22a] was fully acknowledged, not the least by distinguished figures of the atomic physics establishment, as S.G. Rautian, I.I. Sobelman, A.M. Prokhorov, and V. Letokhov.
Back in Heidelberg, Koichi Shimoda from Japan showed particular interest in our work when he dropped in at his regular visits to Europe[23a]. In the lab, we started to build various versions of the recently invented flash lamp-pumped dye laser. Having passed his PhD graduation, Ted tried out various ways of narrowing the emission band width of these capricious devices, in order to get at hand the universal light source for spectroscopy. Soon, rigged out with a NATO fellowship, he left for Stanford, where he set off on his prodigious career. The International Quantum Electronics Conference in Kyoto, Japan, that took place in September 1970 and that I had the chance to attend, allowed me deep impressions of this buzzing physics community as well as of a fascinating society. Also, this trip offered a welcome occasion of meeting Ted, who had made it from California. On my way back, I had a stopover of a few weeks in Stanford. We used this time to try out, in Art Schawlow’s lab, the somewhat simplified technique of high-resolution saturation spectroscopy with an available expensive argon ion laser, whose blue emission band overlapped with numerous lines of the hyperfine-split spectrum of molecular iodine. A smart student, Mark Levenson, joint in the party, and we had much fun, but for some time no data. The trivial defect was later identified, and beautiful spectra confirmed the importance and power of the novel method[24a].
Two years later, in early 1972 and upon invitation by Tony Siegman, I got the chance of spending almost a year in Stanford. Officially, that is, during the daytime, I worked with a Chinese postdoc in Tony’s lab in the Ginzton Laboratory of the Department of Applied Physics on a then fashionable hollow-cathode laser. At night, and during weekends, Ted and I enjoyed playing with the wonderful pieces of equipment in Schawlow’s lab in the Varian building. In order to enhance the sensitivity for the detection of light absorption, we placed a line absorber — a glass cell that contained a trace of iodine vapor — inside the resonator of a blue multi-mode argon laser. The absorption lines showed up as dark lines, nicely impressed upon the output spectrum of the laser![25a]. Now, many years later, this approach has matured to become the most sensitive technique of absorption spectroscopy available. It is capable of detecting, even on a background of gas, an absorber as weak as a single atom.
In Heidelberg, we had played quite awhile with home-made, pulsed ion lasers driven by a HV transformer for commercial discharge tubes. Now that I was back home, our anxiously awaited first commercial cw argon ion laser was applied as a pump source for various, mostly home-made jet-stream dye lasers.
In the summer of 1974, Hans Dehmelt of the University of Washington, Seattle, WA, spent three months in Heidelberg as a Humboldt Fellow. Notwithstanding the barely adequate technical means available, we conceived a project for the isolation and visual observation of an individual ion[26a].
Hans having left, my associate Werner Neuhauser, my PhD student Martin Hohenstatt, and myself set out to hopefully see, in the light of its laser-excited fluorescence, such an atom, localized in a tiny ion trap having been specially conceived by Hans. But for many months, the scene supposed to unveil the wonders of the micro-cosmos through a microscope remained dark as night, straining the observers’ patience to the extreme. Eventually, in despair, we replaced the trap design by a standard, but miniaturized Paul trap — and the other day we noticed a shiny cloud of barium ions wobbling inside the trap. It was a quick job to show that this swirling bunch of ions was indeed cooled by the irradiating laser light[27a], an effect that had been predicted before by Ted Hansch and Art Schawlow[28a]. Although our stout competitors at the National Bureau of Standards came up with an equivalent observation of laser cooling, we felt happy to have made it by a photo finish....
The number of ions caught inside the trap in a particular attempt was reduced day by day, and finally we faced that evasive unit of matter whose factual existence had been doubted by giants as Erwin Schrodinger and Ernst Mach not so long ago[29a]. Mach once had brushed away the points made by an avowed atomist by commenting: “Droplets emerging from a flask are no proof for the liquid inside being in the form of drops!”
The manipulation of an individual ion was the starting point for novel experimentation in the quantum world, so far unfeasible with atomic ensembles. One of these adventures was the observation of the proverbial “quantum jumps” in the mid-eighties. But this is quite another story.
Peter E. Toschek, Hamburg, January 15, 2010 * * *
[1a]N.F. Ramsey, “Collision Alignment of Molecules, Atoms, and Nuclei”, Phys. Rev. 98, 1853 (1955).
[2a]H.S.W. Massey, Electronic and Ionic Impact Phenomena, Vol. 3, Oxford, Clarendon Press, 1971, p 1436 ff.
[3a]K. Berkling, Ch. Schlier, P.E.T., “Messung der Anisotropie der Van der Waals-Kraft”, Z. Physik 168, 81(1962).
[1a]P.E.T., “StoBausrichtung in Molekularstrahlen”, Z. Physik 187, 56—66 (1965).
[5a]A. Abragam, The Principles of Nuclear Magnetism, Oxford, Clarendon Press, 1961.
[6a]E.B. Alexandrov, Optika i Spektrosk. 14,436 (1963).
[7a]J. Bialas, R. Blatt, W. Neuhauser, P.E.T. “Ultrasensitive Detection of Light Absorption by Few Ions”, Opt.Communic. 59, 27—30 (1986).
[8a]J. Eschner, B. Appasamy, P.E.T., “Vibrational-Sideband Excitation Spectra of a Single Trapped Ion”, Opt.Communic. 118, 123 (1995).
[9a]B. Appasamy, Y. Stalgies, P.E.T., “Measurement-Induced Vibrational Dynamics of a Trapped Ion”, Phys.Rev.Lett. 80, 2805—2808 (1998).
[10a]A.N.Oraevskij, Radiot. Electr. 4, 718 (1959). — Molecular Beam Masers, Nauka, Moscow 1964.
[11a]W. Krieger, P.E.T., “Self-Induced Transparency on the 1.15-um Line of Neon”, Phys. Rev A 11, 276—279 (1975).
[12a]Th. Hänsch, P.E.T., “Measurement of Neon Atomic Level Parameters by Laser Differential Spectrometry”, Phys.Lett. 20, 273—275 (1966).
[13a]Th. Hänsch, P.E.T., “Laser Differential Spectrometry Measurement on Neon Depolarization”, Phys. Lett. 22, 150—151 (1966).
[14a]R. Odenwald, Th. Hänsch, P.E.T., “StoB-Depolarisation angeregter Neon-Atome”, Z.Physik 209, 478—496 (1968).
[15a]Th. Hansch, P.E.T., “Experimental Evidence for Correlation of Depolarizing and Velocity Changing Collisions in He-Ne Discharges”, International Quantum Electronics Conference, Miami, Florida, USA, May 1968, Digest of Technical Papers, p. 73.
[16a]H. Holt, “Frequency-Correlation Effects in Cascade Transitions Involving Stimulated Emission”, Phys. Rev. Lett. 19, 1275 (1967).
[17a]Th. Hänsch, P.E.T., “Observation of Saturation Peaks in a He-Ne Laser by Tuned Laser Differential Spectrometry”, IEEE Journal of Quantum Electronics QE-4, No. 7, 467 (1968).
[18a]Th. Hänsch, R. Keil, A. Schabert, Ch. Schmelzer, P.E.T., “Interaction of Laser Light Waves by Dynamic Stark Splitting”, Z.Physik 226, 293—296 (1969).
[19a]Th. Hansch, P.E.T., “Theory of a Three-Level Gas Laser Amplifier”, Z. Physik 236, 2 13—244 (1970).
[20a]See, e.g., “The Emergence of Electromagnetically Induced Transparency”, Conference Slow and Fast Light, Topical Meeting. Hilton Salt Lake City Center, Salt Lake City, Utah, USA, 8—11 July, 2007, Conference Program p. 15.
[21a]Th. Hansch, P.E.T., “Collisions of Excited Atoms in He-Ne Laser Discharges”, Allunion-Symposium on the Physics of Gas-Lasers, Novosibirsk, USSR, 29. June-04. July 1969; Tezisy Dokladov..., Akademija Nauk SSSR, p. 44.
[22a]Th. Hänsch, P.E.T., “Phase Perturbation of a Three-Level Laser System by Collisions”, Allunion-Symposium on the Physics of Gas-Lasers, Novosibirsk, USSR, 29. June-04. July 1969; Tezisy Dokladov Akademija Nauk SSSR, p. 46.
[23a]K. Shimoda, “Atomic and Molecular Coherence in Quantum Electronics”, VI. Internat. Quantum Electronics Conf, Kyoto 1970, p. 132.
[24a]Th. Hansch, M.D. Levenson, A.L. Schawlow, P.E.T., “Hyperfine Structure in the Visible Absorption Spectrum of Molecular Iodine”, Spring Meeting of the American Physical Society, Cleveland, Ohio, USA, March/April 1971; Bull. Am. Phys. Soc., II, 16, 310 (1971).
[25a]Th. Hansch, A.L. Schawlow, P.E.T., “Ultra-sensitive Response of a CW Dye Laser to Selective Extinction”, IEEE Journal of Quantum Electronics QE-8, No. 10, 802 (1972).
[26a]H. Dehmelt, P.E.T., “Proposed Visual Detection Laser Spectroscopy on Single Ba+ Ion”, Anaheim Meeting of the APS, 29. January — 02. February 1975; BuIl.Am.Phys.Soc. 20, 61(1975).
[27a]W. Neuhauser, M. Hohenstatt, P.E.T., H. Dehmelt. “Optical Sideband Cooling of Visible Atom Cloud Confined in Parabolic Well”, Phys.Rev.Lett. 41, 233 (1978).
[28a]T.W. Hänsch, A.L. Schawlow, “Cooling of Gases by Laser Radiation”, Opt. Communic. 13, 68 (1975).
[29a]W. Neuhauser, M. Hohenstatt, P.E.T., H. Dehmelt, “Localized Visible Ba+ Mono-Ion Oscillator”, Phys.Rev. A 22, 1137 (1980).