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American Institute of Physics



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Eric Lerner

Subfemtosecond control
Femtosecond (10–15 s) lasers, which emit only a few cycles of a light wave, have become standard benchtop instruments used to study ultrafast atomic processes or deliver ultrahigh-power radiation. However, users have had no way to control the exact phase of the light wave produced—where the wave lies within the envelope of the pulse. This has prevented researchers from taking the next step and tailoring pulses that users could control at subfemtosecond durations. For some applications, such as the development of X-ray lasers and compact electron accelerators, breaking the femtosecond barrier is essential.

X-ray pulses with durations of a few hundred attoseconds combined with a few-cycle laser pulse allow time-resolved observation of the Auger decay of core-excited krypton.
( Technische Universität Wien)

Now, a research group at the Institut für Photonik, Technische Universität Wien (Vienna, Austria) and the Max- Planck-Institut für Quantenoptik (Garching, Germany) has achieved this level of control (Nature 2003, 421, 611). The team was able to accomplish this feat by using soft X-ray emissions to measure the phase of the wave inside the pulse envelope.

“Two years ago, our group and others developed a method to stabilize and change the phase inside the envelope,” explains Wien researcher Ferenc Krausz, one of the team leaders. In this technique, the frequency of the pulse is doubled, and the doubled-frequency pulse is allowed to interfere with the initial pulse. The phase of the wave in the envelope is thereby shifted forward, backward, or kept the same, but the absolute phase is unknown.

To find the absolute phase, the team took advantage of the extremely high electric fields created by the light pulse. These fields were intense enough to cancel out the electric field holding electrons to atoms, allowing one outer electron to be stripped way from the atom. When the direction of the field reversed as the wave passed, the electron was then thrown back into the atom. That collision caused the emission of a soft X-ray.

“We set the amplitude of the pulse so that only when the field was most intense would the ionization occur,” says Krausz. If the phase of the pulse was such that the peak of the wave corresponded to the peak of the pulse envelope, that condition would occur only once for each pulse, and only one X-ray photon would be emitted in a pulse lasting only 0.25 fs. The short pulse meant that the frequency of the X-ray would be smeared out evenly over a range of energies.

However, if the phase of the light pulse was such that the wave was zero at the peak of the pulse envelope, there would be two distinct peak fields and thus two X-ray pulses emitted for each light pulse. The interference of these two X-ray pulses would produce a spectrum with clear peaks and valleys. By observing the spectrum of the emitted X-ray photons, the team could measure the phase of the light pulse to a small fraction of a femtosecond. Knowing the absolute phase, the team could then use the frequency-doubling interference technique to change the phase at will.

Such precise control, and the production of X-ray pulses that last less than a femtosecond, can provide tools for probing processes that occur when an atom is excited. This will be useful in the development of compact X-ray lasers. In addition, attosecond (10–18 s) pulses can take snapshots of chemical bonds in formation, as electrons move into new positions during the formation of a molecule. Finally, attosecond control of electric fields can be used to accelerate bunches of electrons to relativistic velocities as the first stage of a compact electron gun. These applications will require 10 times the power that the team has achieved, above 1 TW, and the researchers are preparing an attempt to achieve that level.

Turbulent secrets
Turbulence is a central concern in several fields, including aerodynamics, meteorology, hydrology, and chemical processing, yet it remains little understood. In particular, turbulence is highly intermittent, exhibiting periods of relatively smooth flow disrupted by sharp bursts of rapidly rotating vortices. How these bursts of turbulence develop is not fully clear, in part because there has been no good way to study turbulence in three dimensions over time.

Now, a collaboration between researchers at the University of Maryland (College Park) and Arizona State University (Tempe) has succeeded in obtaining three-dimensional, time-resolved videos of turbulent motion and started to illuminate the process of intermittent intense turbulence (Nature 2003, 421, 146). The team focused 1-mm-wide sheets of laser light on three sides of a 1-mm cube placed in a turbulent flow of water. Each of three high-speed video cameras focused on a single sheet and recorded the motion of 1-µm fluorescent polystyrene particles. Although each camera tracked different particles, the flow was smooth enough on the 1-µm scale to measure all 3 components of flow velocity and all 9 gradients of velocity change every 8 ms. Based on measurements of particle velocities, the researchers derived time sequences of two quantities. One was dissipation—the rate at which kinetic motion is degraded to heat (which is also a measure of the strain on the fluid)—and the other vorticity, a measure of rotational flow.

As expected, the videos showed smooth flow interrupted periodically by “hyperbolic” flow patterns, in which the flow changed course in a small volume, and by intense vortices. “What we found was that the turbulence begins with an increase in dissipation,” explains Daniel P. Lathrop of the University of Maryland’s department of physics, a leader of the team. “Strains suddenly build up and start to amplify themselves.” This, in turn, starts to stretch out small vortices, which intensifies them and causes a sudden rise in vorticity. Because a vortex is an efficient way for the fluid to flow, strains are released, and the dissipation—the release of heat—also falls. As the strains are eliminated, the vortex gradually dies away.

Turbulent flow in water seeded with fluorescent particles is recorded by three high-speed video cameras focused on three sides of a 1-mm cube illuminated by 1-mm-wide sheets of laser light.
(University of Maryland, College Park)

“The way the vorticity rises had already been predicted by earlier work, but this was the first time we could see how the vorticity in turn reduced the dissipation, something that had never been proven before,” says Lathrop. Being able to track the strains and energy exchange in the origin and decay of vortices will have a major impact on several applications. Vorticity may be desirable in increasing the mixing of chemicals, but it needs to be minimized in aerodynamic design and in the design of structures to minimize wind damage. Vorticity is also important in the study of violent weather, such as tornadoes or, on a smaller scale, dust devils.

The researchers are now improving their instrument to simultaneously track flow in many different cubes in the moving water. These measurements enable them to relate a local change to global change in flow that affects large areas. In addition, they plan to study the effects of adding polymers, which are often used in industrial applications to reduce turbulence and drag on liquids.

Micromachines in vacuum
Conventional processes can make micrometer-scale machines, but their size has a major impact on how well they run. The National Aeronautics and Space Administration, in particular, has been concerned that trying to use micromachines in the vacuum of space will lead to major problems, such as tiny parts welding together, a process called “stiction” (from “sticky” and “friction”). However, new research at the Southwest Research Institute (SwRI) in San Antonio, Texas, shows that not only can micromachines work in a vacuum, they can work much better than in air. The research will be published in Review of Scientific Instruments.

Test facility at the Southwest Research Institute is equipped with probes and longfocal-length microscopes to study and test micromachines in vacuum.

David J. McComas and his colleges at SwRI developed a vacuum-test facility equipped with long-focal-length microscopes to test and study micromachines in vacuum. They looked at a simple oscillator consisting of a beam with a small mass at its end, powered by a comb electric drive. Many researchers had theorized that air molecules acted as lubricants to prevent the sticking of microscopic moving parts, a lubrication that would be missing in a vacuum. However, the researchers found that the disappearance of air led to a need for far less power to drive the oscillator. Drive voltage dropped from 70 V at atmospheric pressure to 5 V at 0.04 Torr. Because generators of higher voltages weigh much more than those for lower voltages, this finding is good news for many space applications.

In addition, the oscillator produced a sharper resonance in the vacuum. The Q value (the ratio of the frequency of oscillation to the bandwidth of the frequency) increased from 30 at atmospheric pressure to more than 10,000 at 0.1 Torr, a surprisingly large increase. A higher Q value means more accurate functioning.

“We are not sure why there was such a large improvement in Q values,” says McComas. “If it was just simple air resistance, one would think that once 10 Torr was reached, there would be very little increase in Q, but in fact, the improvement became more pronounced down to 0.1 Torr.” More work will be needed to find the physical mechanisms involved.

“We found that, with careful design, stiction just was not a problem,” says McComas. The researchers tested a 40-µm-wide sliding door, running it back and forth 10 billion times without any failures.

Because such a significant improvement in performance occurred in a vacuum, vacuum packaging may become worthwhile for micromachine applications on Earth. The research team is now looking to develop some of these applications, but starting with those aimed at space. Some of the first such applications might be for variable apertures of instruments such as mass spectrometers and neutral atom imagers.

Fluid nanostructuring
Building nanoscale circuits and machines may require different approaches than the photolithographic methods long used for microcircuits. One promising idea is self-assembly, inducing nanoparticles to assemble themselves into the desired arrays through their own interactions (see “Hybrid Semiconductor–Molecular Nanoelectronics,” this issue). To control such processes, researchers need to understand how nature produces order out of chaos, one of the basic tendencies of evolution in the universe.

Tiny bronze spheres selfassemble into a honeycomb pattern (a) and pulsating rings (b) under the influence of an electric field, gravity, currents in the poorly conducting fluid, and each other.
( Argonne National Laboratory, Materials Science Division)

A research group at Argonne National Laboratory (Argonne, IL) recently succeeded in producing ordered patterns using tiny bronze spheres in a poorly conducting fluid under the influence of a static electric field (Phys. Rev. Lett. 2003, 90, 114301). Not only did they produce orderly, static patterns such as honeycomb arrays, but to the researchers’ surprise, the spheres formed two highly organized dynamic entities as well-toroidal vortices and pulsating rings. Such dynamic entities may serve as parts of nanomachines or have applications far afield from nanostructures, such as in space engineering.

The electrocell in which the team performed its experiments consisted of two enclosed charged plates 1.5 mm apart, across which is an electric field that is varied up to 3 kV. Dispersed 120-µm bronze spheres moved about in a mixture of toluene and ethanol. By increasing the percentage of ethanol in the mix, the researchers could increase the conductivity of the solution from 5 × 10–11 to 5 × 10–9/W•m. The bronze spheres acquired a charge from contact with the bottom conducting plate, and the spheres then moved under the influence of the electric field, gravity, the field generated by other spheres, and the currents moving though the fluid.

As the experimenters increased the ethanol in the solution and, thus, the conductivity, the behavior of the fluid changed from that of a randomly moving gas to the creation of static structures. Depending on the direction of the field, either up or down, the particles generated a regular hexagonal honeycomb or a semiregular pattern termed a Wigner crystal.

With still higher conductivity, the fluid began to behave like a plasma, which is governed by similar forces. “We were very surprised to see the particles organize themselves into rotating toroidal vortices,” comments I. S. Aranson, one of the Argonne researchers. The toroidal vortices, in turn, attracted each other and merged into large vortices limited only by the spacing between the plates. Sometimes, dynamic structures that looked like pulsating rings formed.

The toroidal vortices are fascinating in themselves and could be used as models to study natural phenomena, such as ball lightning. They could also be applied to keeping particles in suspension, efficient mixing of chemicals, and conducting heat in space, where ordinary convection does not function. The team also hopes that the static structures could be used for such applications as forming nanoscale versions of quantum-dot arrays. Work is ongoing to scale down the size of the conducting spheres, initially to 2 µm and later to 100 nm.