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Laying foundations for spectroscopy's future

17 Jun 2002

Ted Hänsch has made a career out of pushing the limits of spectroscopy to ever more precise levels. Now he plans to measure the fundamental physical constants to see just how constant they really are. Michael Hatcher reports.

From Opto & Laser Europe April 2002

Physical constants are not supposed to change - that is why they are called constants, after all. But last year, an international group of astronomers published some research that may yet cause the physics textbooks to be revised. Having analysed the light emitted by distant quasars at the edge of the universe, they think that the fine structure constant alpha - which is a combination of the speed of light, Planck's constant and the electronic charge - might have actually changed slightly over the years. The group measured a fractional change of just 10-5 over about 7 billion years.

The problem is, how exactly do you measure such minute changes objectively? This is where Ted Hänsch comes in. Internationally renowned for his work in precision spectroscopy, Hänsch thinks that he might have the technique to help cosmologists find some answers.Since gaining his doctorate in 1969 at the University of Heidelberg, Hänsch has come up with a steady stream of innovative designs and ideas that have had an impact on spectroscopy laboratories everywhere. One of the most celebrated, practical and promising of these is the optical frequency comb. This is the device that Hänsch thinks could help the astronomers.

The frequency comb works by firing an ultrashort laser pulse down a photonic crystal or "holey" fibre. The result is a series of regularly spaced, discrete emission lines extending from the optical frequencies into the near-infrared. Frequency differences at the photodetector output produce lines all the way to the microwave region.

The development of optical frequency combs has given spectroscopists a tool with which to measure the frequency of light anywhere in the region with unprecedented precision. The device has already spawned a new commercial venture, Menlo Systems, as a direct spin-off of Hänsch's lab.

But how exactly will these combs aid the cosmologists? "If you accept that this [alpha] constant can slowly change with time, one possibility is that this would induce a change in the mass or magnetic moment of nucleons, as some theorists argue," Hänsch explains. "The caesium clock as we know it depends on the magnetic moment of caesium nucleus and electron processes around that. So if alpha changes at 10-15 per year, as the astronomers speculate, then the caesium clock would slow down by 10-13 per year. This of course means that you wouldn't be able to measure the effect with two caesium clocks. However, you should be able to tell by comparing a caesium clock with an optical resonance. Nearly three years ago, we made the first such comparison - to two parts in 10-14. So we should be able to see these changes if they are really there."Hänsch's innovation habit (he says that the "eureka" moment has an addictive quality) has certainly stood the test of time - his 1970s design for high-intensity dye lasers can still be found in many laser laboratories. His pet subject in those days at Stanford University with Art Schawlow was the hydrogen atom - a system so simple that "we can really confront experiment with theory like nowhere else in atomic physics", as Hänsch puts it.

In 1972 the Stanford group was the first to resolve the red Balmer lines of the hydrogen spectrum, which led directly to a new precision measurement of the Rydberg constant (the number that defines the separation between energy levels in hydrogen atoms). Hänsch and Schawlow later came up with the idea for laser cooling, which has reshaped the world of atomic physics through the ever-growing field of Bose-Einstein condensates (BECs).

More recently, Hänsch and his teams at the Max-Planck Institute for Quantum Optics in Garching, Germany, and the nearby Ludwig-Maximilians-University of Munich have also turned their attention to BECs. A recent highlight was the Garching team's generation of a new state of matter, which earned them a place on the cover of Nature in January.

"We loaded a BEC into an array of optical dipole traps - an optical lattice - and by ramping up the potential depth, we saw a phase transition to a state in which the atoms no longer behave in a wave-like fashion and materialize as particles. By lowering the potential depth again, we can return to the BEC state. It's an intriguing experiment that we think opens up many new avenues of research," says Hänsch.

Having also managed to produce a BEC on a chip and create tools for manipulating coherent beams of atoms - which are analogous to lasers - Hänsch is now looking forward to the development of some real-world applications for BECs.

"In the near term, sensing is the most likely application. One can imagine atom interferometers for gyroscopes, perhaps for use in space navigation, and also gravimeters, which could be useful for geological investigations," he predicts.

Hänsch's own future is now something to ponder on. He turned 60 last year, and in Germany professors tend to retire at 65. "I've been thinking of either continuing my academic work somewhere else, if it's not possible in Germany, or - more seriously - going into some commercial ventures."At the moment, Hänsch's involvement in the commercial world is limited to being one of the five founding partners in Menlo, and acting as scientific advisor to it and other companies. "The interface between new technologies and basic science has always been the most fascinating for me, and I'm looking for something strongly based on research and innovation, where I can continue to contribute to new ideas, and where I know the scene and potential customers. It might not make billions, but it could still be satisfying."

Technology areas that Hänsch finds intriguing include the possibility of making diamond lasers that emit in the ultraviolet, as well as ultrafast sources, photonic bandgap materials and blue laser diodes. "I think that there's still a lot to achieve with ultrashort pulses, such as greater control over the light. If you can focus a pulse of some tens of Joules to a diffraction-limited spot, and shorten the pulse length to just a few cycles, some unexpected things are bound to happen," he says.

Hänsch also has some ideas that he thinks might improve the way in which optics research is directed: "One problem is that government and funding agencies do not have many people who are scientifically trained - people who can judge what is important. They often fall victim to good salespeople who might not be offering something that relevant."

"The distinction between applied and fundamental research is often unclear in the minds of the people at the agencies - one doesn't have to spend a lot on fundamental research, but a little should be spent on risky ideas," he adds.

So, what does that "eureka" moment feel like? "It's a moment of intense pleasure. Well, not just a moment but a period," Hänsch explains. And when does inspiration strike? "I think one needs to really struggle with a problem, to get the unconscious mind programmed to be aware of the details. Then, in a relaxed moment over breakfast or on an aeroplane, the solution might come. But not without work. At that moment, it might appear to be easy, but you have to lay the groundwork."

Hänsch group www.mpq.mpg.de/~haensch

Menlo Systems www.menlosystems.com

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