Physics Tipsheet #85, September 24, 2008
American Physical Society
Highlights in this Issue:
Tsunami Invisibility Cloak
MFarhat, SEnoch, SGuenneau and A.BMovchan
Rather than building stronger ocean-based structures to withstand tsunamis, it might be easier to simply make the structures disappear. A collaboration of physicists from the Centre National de la Recherche Scientifique (CNRS) and Aix-Marseille Universite in France and the University of Liverpool in England have conducted laboratory experiments showing that it's possible to make type of dike that acts as an invisibility cloak that hides off-shore platforms from water waves. The principle is analogous to the optical invisibility cloaks that are currently a hot area of physics research
Tsunami invisibility cloaks wouldn't make structures disappear from sight, but they could manipulate ocean waves in ways that makes off-shore platforms, and possibly even coastlines and small islands, effectively invisible to tsunamis. If the scheme works as well in the real world as the lab-scale experiments suggest, a tsunami should be able to pass right by with little or no effect on anything hidden behind the cloak- JR
Dark Energy v. the Void: What if Copernicus was Wrong?
Timothy Clifton, Pedro GFerreira, and Kate Land
Dark energy is at the heart of one of the greatest mysteries of modern physics, but it may be nothing more than an illusion, according physicists at Oxford University. The problem facing astrophysicists is that they have to explain why the universe appears to be expanding at an ever increasing rate The most popular explanation is that some sort of force is pushing the accelerating the universe's expansion. That force is generally attributed to a mysterious dark energy.
Although dark energy may seem a bit contrived to some, the Oxford theorists are proposing an even more outrageous alternative. They point out that it's possible that we simply live in a very special place in the universe - specifically, we're in a huge void where the density of matter is particularly low. The suggestion flies in the face of the Copernican Principle, which is one of the most useful and widely held tenants in physics.
Copernicus was among the first scientists to argue that we're not in a special place in the universe, and that any theory that suggests that we're special is most likely wrong. The principle led directly to the replacement of the Earth-centered concept of the solar system with the more elegant sun-centered model.
Dark energy may seem like a stretch, but it's consistent with the venerable Copernican Principle. The proposal that we live in a special place in the universe, on the other hand, is likely to shock many scientists. The maverick physicists at Oxford conclude their paper by pointing out that forthcoming tests of the Copernican principle should help us sort out the mystery in the next few years-JR
Meta-Screens: Squeezing Light into Sub-Wavelength Spots
Loic Markley, Alex M.HWong, Yan Wong, and George VEleftheriades
In a new study, physicists at the University of Toronto have invented a simple structure called a meta-screen, designed to focus light into tiny spots smaller than the wavelength of the photons in use. These sub-wavelength spots overcome the diffraction limit, thus allowing even the smallest details of an object to be visualized.
Researchers developed a meta-screen composed of slots cut into a metallic screen, each narrowly spaced (less than half a wavelength apart) and of a precise length. They found that screen was able to effectively increase the range of sub-wavelength spots, enabling greater image resolution.
The meta-screen is the first sub-wavelength focusing technique capable of being scaled to any arbitrary wavelength, thereby offering unprecedented levels of resolution and flexibility for imaging and sensing apparatuses involving electromagnetic waves, such as radio waves for medical diagnostics or light for optical microscopy.-NR
Tweezers Trap Nanotubes by Color
TRodgers, SShoji, ZSekkat and SSatoshi Kawata
Singled-walled carbon nanotubes are graphene sheets wrapped into tubes, and are typically made up of various sizes and with different amounts of twist (also known as chiralities)Each type of nanotube has its own electronic and optical properties. Physicists at Osaka University in Japan used colored light to selectively manipulate different types of carbon nanotubes. They found that some of nanotubes displayed a tendency to cluster at the focal area of a focused laser beam.
Nanotubes are known for their strong color-dependant interactions with light By using an optical tweezer, a device that traps microscopic or nanoscopic objects in laser beams, researchers were able to selectively pull only specific colors of nanotube into focus.
Their results are the first experimental evidence demonstrating that colored light drives the clustering of nanotubes in a laser tweezer. Moreover, this color dependence can be exploited to select one type of nanotube over another. The study is a significant step towards developing optical methods for sorting and purification of nanotubes, a process that remains a major challenge for the application of nanotubes to engineering.-NR
50 Years of PRL
Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of visit Marty's Milestone PRL Project.
This week, Marty is taking a look at a milestone paper from 1988 that led to Nobel Prizes for laser cooling (1997) and giant magnetoresistance (2007).
Nobel Prize in Physics - 1997
Observation of Atoms Laser Cooled below the Doppler Limit
Laser Cooling below the One-Photon Recoil Energy by Velocity-Selective Coherent Population Trapping
After the demonstration of "optical molasses" by Steven Chu et al(see Milestone for 1985), several groups made attempts to overcome the limits that were apparently set by those authors on the lowest temperatures reachable by their techniques. The Letters above reported that laser cooling could in fact lower the temperature of atoms below those limits.
In the first Letter, sodium atoms were measured to have reached a temperature of around 40 μK, a factor of 6 below the "Doppler limit", which had been achieved by Chu et al.; this result was a great surpriseIt was soon realized that while the theory leading to the Doppler limit assumed a two-level atom, real atoms have several Zeeman sublevels. These levels are differently excited depending on the polarization of the laser field, which varies rapidly with space in the optical molasses created by multiple opposing laser beams; a full analysis of this situation led to an understanding of the ability to achieve lower temperatures.
The second Letter reports achieving a temperature below the "recoil limit" This limit arose from the fact that (in prior laser-cooling experiments) all the atoms were continuously absorbing and emitting photons; these processes inevitably gave the atom a small recoil energy. For the helium atoms involved in this experiment, the recoil limit was 4 μK. The, but the atoms reached a temperature of 2 μK. The experimenters accomplished this by pumping those atoms which were already moving slowly into a nonabsorbing coherent superposition of states; since they were no longer absorbing they did not recoil, and the recoil limit did not apply. Later experiments eventually reached a temperature which was a factor of 22 below the recoil limit. The net result of both these experiments was that it is possible to make a "thicker optical molasses" than was initially accomplished.
As stated in the Chu et. al. Milestone, the 1997 Nobel Prize in physics was awarded jointly to Steven Chu, William DPhillips, and Claude Cohen-Tannoudji "for development of methods to cool and trap atoms with laser light." For further information, see the links in the 1985 Milestone, the Nobel lectures of Phillips [RevModPhys 70, 721 (1998)] and Cohen-Tannoudji [RevMod Phys 70, 707 (1998)], and PhysRevFocus 21, story 11.
Nobel Prize in Physics - 2007
Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices
Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange
Magnetoresistance is the change in resistance of a conductor in the presence of a magnetic field. In the mid 19th century Lord Kelvin observed that in iron there is a difference in the magnetoresistance when the electric current is along or perpendicular to the direction of the spontaneous magnetization This anisotropy, which is a relatively weak effect, has been used to read the direction of magnetic fields in the magnetic memories of computers.
The work highlighted here involves the discovery of a much more pronounced effect - dubbed giant magnetoresistance- in artificially produced nanoscale materialsIn the Letter of Baibich et al., the group of Albert Fert described their production of a stack of alternating thin layers of iron and chromium, which displayed giant magnetoresistanceWe have included also the work of the group of Peter Grünberg which, while published later in Physical Review B, was actually submitted before the Letter of the Fert group. They reported on the large magnetoresistance observed, but with a single layer of chromium.The magnetoresistance that they observed was smaller than that seen by the Fert group, for two reasons. First, the Fert observations were on multiple layers, so that the effect was considerably enhanced, and second, they carried out their observations at liquid helium temperatures. Grünberg's group worked initially at room temperature (of great importance for applications), except for a single measurement at low temperature. Credit clearly belongs to both groups.
This discovery has had a significant practical impact in greatly shrinking the size and increasing the reliability of computer memories and of other magnetic measuring devices. It is a prime example of the fast pace of transforming a fundamental discovery into a large-scale technological application. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg "for the discovery of Giant Magnetoresistance." For further information see the 1997 Nobel Physics Press Release and other links on that site, and PhysRevFocus 20, story 13.
Nadia Ramlagan and James Riordon contributed to this Tip Sheet.
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