Ground-based lasers vie with satellites to map Earth's magnetic field

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University of California, Berkeley, physicists have now come up with a much cheaper way to measure the Earth's magnetic field using only a ground-basedlaser.

The method involves exciting sodium atoms in a layer 90 kilometers above the surface and measuring the light they give off.

"Normally, the laser makes the sodium atom fluoresce,"said Dmitry Budker, UC Berkeley professor of physics."But if you modulate the laser light, when the modulation frequency matches the spin precession of the sodium atoms, the brightness of the spot changes."

Because the local magnetic field determines the frequency at which the atoms precess, this allows someone with a ground-based laser to map the magnetic field anywhere onEarth.

Budker and three current and former members of his laboratory, as well as colleagues with the European Southern Observatory (ESO), lay out their technique in a paper appearing online this week in the journalProceedings of the National Academy of Sciences.

Various satellites, ranging from the Geostationary Operational Environmental Satellites, or GOES, to an upcoming European mission called SWARM, carry instruments to measure the Earth's magnetic field, providing data to companies searching for oil or minerals, climatologists tracking currents in the atmosphere and oceans, geophysicists studying the planet's interior and scientists tracking space weather.

Ground-based measurements, however, can avoid several problems associated with satellites, Budker said. Because these spacecraft are moving at high speed, it's not always possible to tell whether a fluctuation in the magnetic field strength is real or a result of the spacecraft having moved to a new location. Also, metal and electronic instruments aboard the craft can affect magnetic field measurements.

"A ground-based remote sensing system allows you to measure when and where you want and avoids problems of spatial and temporal dependence caused by satellite movement,"he said."Initially, this is going to be competitive with the best satellite measurements, but it could be improved drastically."

Laser guide stars

The idea was sparked by a discussion Budker had with a colleague about of the lasers used by many modern telescopes to remove the twinkle from stars caused by atmospheric disturbance. That technique, called laser guide star adaptive optics, employs lasers to excite sodium atoms deposited in the upper atmosphere by meteorites. Once excited, the atoms fluoresce, emitting light that mimics a real star. Telescopes with such a laser guide star, including the Very Large Telescope in Chile and the Keck telescopes in Hawaii, adjust their"rubber mirrors"to cancel the laser guide star's jiggle, and thus remove the jiggle for all nearby stars.

It is well known that these sodium atoms are affected by the Earth's magnetic field. Budker, who specializes in extremely precise magnetic-field measurements, realized that you could easily determine the local magnetic field by exciting the atoms with a pulsed or modulated laser of the type used in guide stars. The method is based on the fact that the electron spin of each sodium atom precesses like a top in the presence of a magnetic field. Hitting the atom with light pulses at just the right frequency will cause the electrons to flip, affecting the way the atoms interact with light.

"It suddenly struck me that what we do in my lab with atomic magnetometers we can do with atoms freely floating in the sky,"he said.

Budker's former post-doctoral fellow James Higbie now an assistant professor of physics and astronomy at Bucknell University– conducted laboratory measurements and computer simulations confirming that the effects of a modulated laser could be detected from the ground by a small telescope. He was assisted by Simon M. Rochester, who received his Ph.D. in physics from UC Berkeley last year, and current post-doctoral fellow Brian Patton.

Portable laser magnetometers

In practice, a 20- to 50-watt laser small enough to load on a truck or boat tuned to the orange sodium line (589 nanometer wavelength) would shine polarized light into the 10 kilometer-thick sodium layer in the mesosphere, which is about 90 kilometers overhead. The frequency with which the laser light is modulated or pulsed would be shifted slightly around this wavelength to stimulate a spin flip.

The decrease or increase in brightness when the modulation is tuned to a"sweet spot"determined by the magnitude of themagnetic fieldcould be as much as 10 percent of the typical fluorescence, Budker said. The spot itself would be too faint to see with the naked eye, but the brightness change could easily be measured by a small telescope.

"This is such a simple idea, I thought somebody must have thought of it before,"Budker said.

He was right. William Happer, a physicist who pioneered spin-polarized spectroscopy and the sodium laser guide stars, had thought of the idea, but had never published it.

"I was very, very happy to hear that, because I felt there may be a flaw in the idea, or that it had already been published,"Budker said.

While Budker's lab continues its studies of how spin-polarized sodium atoms emit and absorb light, Budker's co-authors Ronald Holzlöhner and Domenico Bonaccini Calia of the ESO in Garching, Germany, are building a 20-watt modulated laser for the Very Large Array in Chile that can be used to test the theory.

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Laser welding in the right light

It's a quick process, generates almost no waste and is extremely precise: within a few seconds, alaser beamhas welded the casing and speedometer cover together– without any screws, clamps or glues whatsoever. The result is a perfect weld seam scarcely visible to the naked eye. There are no sparks or particles flying through the air during welding. What's more: the resulting heat is confined to a minimal area. This protects the material. Many industries have now turned to welding plastics with a laser.

Still, the technology has its limits; when it comes to fusing twoplastic componentstogether, for instance, there is little freedom of choice. Up until now, the upper joining part had to be transparent to permit the laser to shine through unimpeded while the lower joining part absorbed the radiation. This usually meant soot particles had to be blended into the plastic. These particles absorb the energy of the laser beam and transmit the fusion heat generated to the upper joining part."Up until now, you usually had to choose a single plastic combination: transparent and black. There are lots of applications– in medical technology, for instance– where what's needed is a combination of two transparent plastics,"explains Dr.-Ing. Alexander Olowinsky, project manager at the Fraunhofer Institute for Laser Technology ILT in Aachen, Germany. The researcher and his team have now managed to erase the previous boundaries of laser welding.

"The industry now also makes infrared absorbers that are nearly transparent, but these are not only very expensive but also have a green, yellowish tint to them,"Olowinsky elaborates."So our goal was to find a way to get the job done completely free of absorber materials."To accomplish this, researchers studied the absorption spectra of a range of transparent polymers in search of wavelength ranges within which plastic absorbs laser radiation. Then the scientists tested and perfected the laser systems to match: systems that emit light of the right wavelengths."Before, you didn't have the right light source,"Olowinsky adds."It was only during the past few years that laser sources have been developed that emit light in these wavelength ranges."To deliver the light energy to the joining level– to the seam along the border between the two transparent plastics– the experts at ILT came up with special lens systems. These systems focus the beam so that the highest energy density occurs at the beam waist– where the beam diameter is the smallest– so that the highest temperature is delivered precisely to the joining level.

The researchers' most promising results were achieved at awavelengthof around 1700 nanometers."This is the peak welding-efficiency range,"Olowinsky summarizes. Nevertheless, the researchers are also continuing work on the EU Commission-sponsored"PolyBright"project in search of the combination of the right absorption bands with the matching light sources."The result has to be the most cost-effective laser system possible that can execute high-precision welding tasks at the highest possible speed."

Medical technology and bioanalytics in particular are among the main beneficiaries of the new welding process: The magic word is"lab on a chip."This refers to automatic, miniature-sized laboratory analysis on the surface of a chip. Whether fluids, protein or DNA analyses– the spectrum of applications is a broad one.

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Cracking the children's fingerprint disappearing act

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Forensic scientists often use techniques like magnetic filings dusting, iodine, and cyanoacrylate fuming to see otherwise invisible, or latent,fingerprints. Although efficient, inexpensive, and relatively fast, these methods make it difficult to preserve trace evidence found in a fingerprint. This is particularly difficult with aged fingerprints left by children, which have been shown to fade faster than those of adults, making them almost impossible to capture.

"Previous research has linked the difference in the longevity of fingerprints to the type of oil found in a person's skin,"said NSLS biophysicist Lisa Miller, one of the authors of the study."This oil, known as sebum, is just one of the components of a fingerprint, which also can contain small pieces of skin and sweat residue. We wanted to determine how these fingerprint components change over time in adults and children, and how these changes alter our ability to predict someone's age based on their fingerprint."

Using a collection of latent fingerprints given by six father (ages 35-45) and son (ages 7-10) pairs, the researchers watched for chemical changes over the course of four weeks. Twice a week, one fingerprint from each participant was dusted, lifted, and analyzed based on the number of features, or minutiae, visible. At the same time, a non-invasive synchrotron technique called Fourier transform infrared microspectroscopy (FTIRM) mapped the location and makeup of the skin and sebum in the prints.

"FTIRM is a very useful tool in this case because it allows us to examine individual fingerprint components— namely, skin and sebum— separately,"Miller said.

At all points in time, the fathers' prints dusted darker than those from their sons. In fact, the fathers' prints remained virtually unchanged during the four-week study, while the fine minutiae of their children became increasingly more difficult to see.

The NSLS study, which was carried out at beamline U10B, helps researchers understand this disappearing act. As predicted by previous studies, FTIRM showed that adults produce more sebum than children, which leads to darker prints."The more oily and moist your skin is, the better print you leave behind to be dusted and lifted,"Miller said.

Researchers also found that the composition of the lipids, or fats, in sebum differ significantly between adults and children.

Adult sebum has higher concentrations of stable lipids such as squalene and wax esters, which are less likely to vaporize over time. Conversely, the sebum of children contains higher levels of cholesterol and branched chain free fatty acids— unstable lipids that break down more quickly.

The results, which appeared in the March 2010 edition of the Journal of Forensic Sciences, indicate that children's prints may disappear faster because they contain different oils. This could pave the path toward more advanced fingerprint detection techniques.

"Based on the differences in chemical composition, children's prints can still be distinguished from adults' prints with FTIRM even when the dusted copies are barely identifiable,"Miller said."To accurately determine the age of the print's individual, FTIRM should be used as a complementary tool to conventional forensic methods."

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Read-write device offers new architecture for information processing

"Right now, information in computers has to be transferred between logic and memory,"Molenkamp continues."But with memory becoming so big, the process is becoming cumbersome."In order to remedy this problem, Molenkamp and his colleagues in Würzburg have developed a device that allows for memory storage and logic processing in the same structure. A description of their device can be found inPhysical Review Letters:"Fully Electrical Read-Write Device Out of a Ferromagnetic Semiconductor."

Traditionally, information processing is based on different components. Metallic ferromagnets can be used to store information in a remanent manner, such as in a hard disk. Semiconductors are used for logic functions and for volatile memory (RAM). There must communication between memory and logic in order to get the type of computing we are used to. However, there are limitations to this.“Heat dissipationis a problem,” Molenkamp points out.“Additionally, the communication takes time and an enormous amount of interconnects, and there is only so much that can be done when logic and memory are separated in information processing architecture.”

The solution, then, is to create a new information processing architecture that puts logic and memory in the same device.“We have a sample device that we have shown works as a read-write device, putting logic and memory together to create the basis for a new information processing architecture,” Molenkamp says.

In order to create the device, Molenkamp and his fellows used the ferromagnetic semiconductor (Ga, Mn)As.“Our device allows you to perform logic operation with the same circuits where you store info,” he explains.“You can do away with the transfer between logic and memory parts.” This would cut down on heat dissipation, as well as making information processing much faster.

So far, the team at Würzburg has created a one bit device.“There is a little disc in the middle of the device which is the logical bit,” Molenkamp says.“However, in order for our design to be a full logic device, to actually make it programmable, we need two discs touching on each other.” This is what the group is working on now.

In order to take the device further, Molenkamp says that a different set up might be needed.“We were able to show that we could use this device. It is more of a principle of operation,” he points out.“Next, we will have to transfer to a different material that is magnetic at room temperature. We think that our new information processing architecture can carry over to metals.” In order to accomplish this, Molenkamp continues,“one needs to grow crystalline metal layers to use as starting material.” Once that is done, it is possible to begin developing devices that can operate at room temperature, as well as more advanced circuits.

“The adoption of our device could lead to much smaller computers,” Molenkamp says.“Because the type of memory we describe stays encoded, you wouldn’t need RAM, and that would help with heat dissipation and size. We hope that, now that we have shown that you can integratememoryand logic in this new information processing architecture, that there will be interest in creating devices that use this technology.”

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Microwave photons can nullify the conductivity of electrons confined to the surface of liquid helium

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Two-dimensional electron gases form naturally at the surface of helium because an intrinsic energy barrier preventselectronsfrom penetrating any deeper into the liquid. These gases vary markedly from their three-dimensional counterparts because the electron motion in one direction becomes quantized—that is, their velocity in this direction is governed by quantum mechanics and is restricted to a range of discrete values.

Konstantinov and Kono cooled liquid helium-3 to 0.3 kelvin. They supplied electrons from a nearby hot filament, and applied voltage to a plate below the helium to control the number of electrons per unit area. Then, they fired microwave radiation at the 2DEG (Fig. 1) and measured the longitudinal conductivity— the current induced by an electric field applied along one direction—as a function of external magnetic field. They saw that the conductivity periodically fell to zero as they increased the magnetic field. When they switched off the source of microwave photons, however, this effect ceased.

This previously unidentified nullifying effect of microwave photons onconductivityis a consequence of energy-conserved scattering of theliquid helium’s electrons between different energy states—specifically, the first excited and ground sub-bands.“When the electrons stay in the ground sub-band, the effects are rather dull,” says Kono.“In our experiment, absorption of microwave photons transfers electrons to a higher energy sub-band,” Konstantinov adds.“As we change the magnetic field, the energies of states in two subbands cross, and scattering redistributes electrons between the sub-bands.”

Kono and Konstantinov believe that the result will lead to the observation of more novel phenomena in these two-dimensional systems when they are shifted out of their equilibrium state.“The study of nonequilibrium transport in the extremely clean helium system will complement studies of electron transport in semiconductors,” explains Konstantinov.

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The 'new' kilogram is approaching: Avogadro constant determined with enriched silicon-28

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The crucial phase of the long-term Avogadro project - which is coordinated by PTB - started in 2003: In that year, several national metrology institutes launched - together with the Bureau International des Poids et Mesures (BIPM) and in cooperation with Russian research institutes - the ambitious project of having approximately 5 kg of highly enriched²⁸Si (99.99 %) be manufactured as a single crystal, of measuring the Avogadro constant with it and of achieving - by the year 2010 - a measurement uncertainty of approx. 2• 10^-8. Meanwhile, the first measurements have been completed on the two 1 kg spheres of²⁸Si - which had been polished in Australia - and their density, lattice parameter and surface quality have been determined.

The single steps: After an extensive check of the crystal perfection, the influence of the crystal lattice defects was assessed. Then, the lattice parameter was determined at the Italian metrology institute (INRIM) by means of an X-ray interferometer, and confirmed by comparison measurements with a natural Si crystal at the American NIST. At BIPM, NMIJ (Japan) and PTB, the masses of the two silicon spheres were linked up in vacuum to the international mass standards. In the respective Working Groups of NMIJ, NMI-A (Australia) and PTB, the sphere volume was measured optically - with excellent agreement - by means of interferometers with different beam geometries. The surface layer (basically composed of silicon dioxide) was spectroscopied with electron radiation, X-ray radiation and synchrotron radiation in accordance with different procedures, analyzed and taken into account for the determination of thesilicondensity. The unexpectedly high metallic contamination of the sphere surfaces with copper and nickel silicides which occurred during the polishing process was measured, and its influence on the results of the sphere volume and of the sphere mass was assessed. This resulted in a higher measurement uncertainty.

What was decisive for the success achieved - i.e. a relative overall measurement uncertainty of 3• 10^-8- was the development of a new mass-spectrometric method for the determination of the molar mass at PTB.

The result is a milestone on the way towards a successful realization of the new kilogram definition on the basis of fundamental constants whose values have been fixed. At present, the agreement of this value with other realizations of the kilogram is not good enough to change the existing definition of the mass unit. The present state of the Avogadro project is, however, so promising that - on the basis of new measurements with improved sphere interferometers - the measurement uncertainty of 2• 10^-8demanded by the Consultative Committee for the Mass (CCM) will in the near future be achieved on contamination-free spheres and will probably even be undercut.

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End of an era: NIST to cease calibrating mercury thermometers

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Mercury is a potent neurotoxin. Elemental mercury is found in thermometers and used in a number of industrial processes such as gold mining. Once released into the environment, mercury makes its way into streams, rivers, and finally the ocean. The mercury is absorbed by sea life and accumulates in the larger fish that humans like to eat. This is the main source of mercury poisoning in humans today.

While many industries follow ASTM standards that stipulate the use of mercury thermometers, these standards have fallen behind the states, many of which have outlawed the sale and transport of mercury thermometers. Presently about 300 of the approximately 700 standards have been amended to allow for the use of both mercury-free liquid-in-glass and digital thermometers.

According to NIST researcher Dawn Cross, each of these ASTM standards is reviewed on a rolling basis. She estimates that all the standards will have been amended to include detailed procedures for making the switch to mercury thermometer alternatives within three years.

"One of our major activities is fielding calls from industry and explaining the science of how they can make the switchover,"says Cross."Change always brings confusion and apprehension, but in every case there is an alternative thermometer to suit the measurement need. It's like learning to use a new cell phone or drive a car with a different kind of transmission; we're simply substituting one technology for another, but they both work equally well."

NIST itself had a stockpile of more than 8,000 industrial-use mercury thermometers hidden away in drawers.

The mercury from these has been sent to specialized recycling centers, which repurpose the mercury to produce compact fluorescent light bulbs. Mercury thermometers contain about 500 milligrams of mercury—an amount equal to the mercury in over 125 compact fluorescent bulbs.

According to Greg Strouse, leader of NIST's Temperature and Humidity Group, that recycling doubly reduces mercury emissions.

"The amount of mercury in a compact fluorescent light bulb is about one to four milligrams,"says Strouse."Most of that mercury is bound to the inside of the glass during the life cycle of the bulb, a process that makes it much less environmentally harmful. Burning of coal is a major source of vaporous mercury released into the atmosphere. Compact fluorescents use less electricity, which reduces the amount of coal burned, which reduces the amount of mercury released by a factor of four."

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