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He Wrote the Book on Atom Trapping

Retired Bell Labs scientist Arthur Ashkin discusses his years as a physicist and how he discovered that light could trap atoms -- the discovery that led Steven Chu and two others to the Nobel Prize


Murray Hill (November, 1997) -- When the Swedish Academy of Sciences awarded the 1997 Nobel Prize in Physics to Steven Chu, William Phillips and Claude Cohen-Tannoudji for their development of methods to cool and trap atoms with laser light, they honored a branch of physics -- optical trapping -- that has long been associated with Bell Labs.

Arthur Ashkin invented optical trapping, or the process by which atoms are trapped by laser light, at Bell Labs. He found that radiation pressure -- the ability of light to exert pressure to move small objects -- could be harnessed to constrain atoms. Many consider him the father of the field. In fact, the work that led Chu to the Nobel Prize was started by Ashkin and Chu at the labs in Holmdel. Ashkin retired from Bell Labs in 1992 after a 40-year career during which he contributed to many areas of experimental physics. He authored many research papers over the years and holds 47 patents. Ashkin was elected to the National Academy of Sciences in 1996.

Ashkin recently spoke to the Bell Labs News about his long and distinguished career at Bell Labs and the work that led Steven Chu to the Nobel Prize.

BLN: What led you to become a physicist? How did your interest in physics develop?

Ashkin: As a little kid, I was interested in how things worked. In high school, I was very interested in science, especially in math and physics. I had an older brother who was a physicist and he inspired me.

I went to Columbia as an undergraduate. I got caught in World War II -- I was a sophomore at the time -- and, since I was a good student, I became a technician in the Columbia Radiation Lab. There were three Nobel laureates there at that time. It was tremendously exciting. I worked there for a few years, got drafted, and they assigned me back. That was my introduction to electromagnetic radiation.

After I finished my physics degree at Columbia, I went to Cornell and studied nuclear physics. My brother was in nuclear physics. I met all those famous men there -- Hans Bethe, Richard Feynman, and so on. Feynman was my brother's very good friend. He used to call me "Ashkin's-brother-Ashkin." I thought that amusing.

[ Arthur Ashkin in his lab, 1988 ]

Arthur Ashkin at work in his Holmdel, N.J. lab in 1988

BLN: How did you come to Bell Labs? What did you work on initially?

Ashkin: During the War, I met Sid Millman at Columbia Radiation Labs. Millman came to Bell Labs after the War and ended up executive director of Research. After I got my degree at Cornell, he somehow got wind of it and asked me if I wanted to come to Bell Labs. I accepted.

I got started working on microwaves. I worked on microwaves until about 1960-61, just after the invention of the laser. That was so exciting. Everybody was studying lasers. I started laser work.

In 1963, I got promoted to department head under Chape Cutler. I worked with Gary Boyd. We did a lot of very nice things in nonlinear optics. We were pioneers in parametric amplification and parametric oscillators. One of our papers is going to be in the Classics of Non-Linear Optics, coming out soon.

We also discovered the photo-refractive effect. It has to do with the fact that light changes the index of refraction of piezoelectric crystals. We were trying desperately to make parametric oscillators, and index effects were a problem. We were terribly disappointed. But it turns out this is a very important effect. This was the first discovery.

BLN: Was this all in the late 1960s?

Ashkin: We came to Holmdel from Murray Hill in 1967. This was done before that. It must have been in 1964-65. And then I did a lot of work subsequently.

BLN: Tell me about how you started work on optical trapping, leading up to when you and Steve Chu and colleagues did the now-famous experiments?

Ashkin: I started in 1970 with my discovery that I could optically trap small latex spheres. And I went on to propose that we could do the same thing with atoms.

BLN: How did you start thinking about it in the first place?

Ashkin: Well, I had been interested in radiation pressure for a long time. Even at Columbia, when I used to make megawatt-magnetrons for Millman, I used to think, "What can you do with this power? Maybe you could push small things ..." So I got myself a microphone and I put pulses on it. This was when I was still a sophomore. I heard some noise and I said, "Aaah, I am seeing the effects of radiation pressure." Turns out, you know, I am not sure whether I was seeing the effects or not. Anyway, I was alerted to this problem.

And I was reminded of it when I went to a conference and there was a guy who did an experiment with lasers and little particles in a laser cavity. He saw the particles staying in the cavity and moving back and forth and doing crazy things. He called them runners and bouncers. People were fascinated by this. I heard this talk and he said at the time, "We think it might be radiation pressure."

When I came home, I did a calculation and realized, given the size of the beam and the particles, it couldn't be radiation pressure. More likely, I thought, is that the particles were heated and that led to the crazy behavior. This made me think of radiation pressure again.

I decided to try to see radiation pressure. I made a calculation of how much it would be on a small transparent spheres. That started the whole business for me.

What I did was focus a beam down on little spheres in water and watched as they were pushed along and mysteriously collected at the chamber wall. I tried to understand this and figured it out using simple ray diagrams.

Then, I replaced the glass wall with another opposite beam to hold the particles in place with just light. I tried it and it worked. This was the first optical trap. It turned out to be a pretty important discovery. It led to Steve's Nobel Prize and, I believe, it will lead to two more Nobel Prizes.

BLN: What do you think they will be?

Ashkin: Well, I think, the Bose-Einstein condensation and the atom laser will get a prize. And I hope that the great things that biologists are doing with optical tweezers will be rewarded with another one.

BLN: How did you and Steve Chu get involved in doing the experiments that would ultimately get him the Nobel Prize?

Ashkin: Steve Chu was at Murray Hill, doing his great experiments on positronium. He was promoted and came to Holmdel in 1984 or so as a fellow department head in electronics research. He started talking to me, and to the other guys. We had been working on trapping for almost 15 years. It was a very loose group. It was basically me, working with Joe Dziedzic, playing around with these little spheres, and John Bjorkholm and Jim Gordon helping with atoms. At the same time, I was trying to understand what were the forces were necessary to trap things. And I looked for the same forces with the atoms.

We had hired a new guy, Rick Freeman, in our laboratory and, with Bjorkholm, we did our first experiment with atoms and atomic beams. We showed that we could focus atoms, defocus atoms. We could focus them to small spots and see how the spots were limited by quantum fluctuations. We learned a heck of a lot and got a lot of credit for that.

After that experiment, the field began to heat up. People on the outside began to realize that something important was happening. When Steve Chu came here, he said, "You know, people are very interested in this." He wanted to understand it and we discussed it. He wanted to make an atom trap. We were delighted. And we went and made it.

BLN: So that was the famous experiment with "optical molasses" and traps that you and Chu and colleagues did in 1985?

Ashkin: The initial plan was to combine slowing of atoms, cooling them and trapping them in a single experiment. Steve argued for a simpler first step. Steve was very enthusiastic. He wanted to first study the three-dimensional cooling scheme, using a technique proposed by T.W. Hansch and A. L. Schawlow in 1975. This is now referred to as "optical molasses." This was wise because molasses cooling succeeded so well that it affected our subsequent choice of traps. We used the now-famous optical tweezers trap. Steve was a very hands-on person, a great experimentalist. He did some absolutely brilliant experiments. I always thought in those days that he would one day get the Nobel Prize. I am extremely happy for him.


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