The intensity gradients of inhomogeneous laser-light fields impose ponderomotive forces on charged particles. Such forces have been used to trap and manipulate ions, diffract electrons, and generate charge waves in plasmas. But they were thought to act only very weakly on neutral atoms—having to rely on the polarizability of an atom’s charge distribution. Now, however, a group at the Max Born Institute in Berlin has reported the use of intense ultrashort laser pulses to accelerate neutral helium atoms for about 100 femtoseconds at 1015 m/s2. That’s eight orders of magnitude greater than the acceleration (or deceleration) one can get with the continuous-wave techniques used in laser cooling of neutral atoms. The ponderomotive force of an inhomogeneous light field pushes a charged particle toward lower light intensity with a force proportional to the square of its charge—irrespective of sign—and inversely proportional to its mass. The Berlin group argues that the strong laser pulse excites an electron to the outer reaches of the helium atom where it “quivers” in the oscillating light field and experiences the ponderomotive force almost as a free electron would. But still bound to the atom’s ionic core, it tugs the much heavier core with it away from the laser beam’s focus. The figure shows how the maximum velocity thus acquired by neutral atoms in the Berlin experiment increases with pulse duration. The dashed curves show the theoretical expectation for the group’s model of electron excitation and the consequent ponderomotive force. Such “ultrastrong” acceleration of neutral atoms, they suggest, could be exploited for atomic-beam optics, atom deposition, and controlled chemical reactions. (U. Eichmann et al., Nature 461, 1261, 2009.) —Bertram Schwarzschild
October 2009 Archives
Photonic devices that can detect and control the polarization of light across a range of wavelengths are rare. More common are materials such as quartz that can be made into monochromatic optical retarders, which through their intrinsic birefringence convert a specific wavelength of linearly polarized light into circularly polarized light, or vice versa. Some multilayered thin films exhibit achromatic retardation through fabricated periodic nanoscale structures that effectively combine the dispersive properties of each layer to achieve wavelength-independent birefringence. But engineering nanoscale structures is tricky, and even the best synthetic achromatic retarders perform poorly across the full visible range, varying by as much as 9.1°. But Nature has already solved the puzzle in animals that have evolved biophotonic structures for signaling, vision, and coloration (see Physics Today, January 2004, page 18). Now, an international team of researchers from the UK, Australia, and the US has discovered a near-ideal achromatic retarder in the eyes of the colorful peacock mantis shrimp, Odontodactylus scyllarus, shown in the image. This mantis shrimp’s biophotonic retarder is the R8 photoreceptor cell—a UV-photopigment-filled lipid bundle with critical radii of 26 nm and 40 nm, which are subwavelength for visible light. When subjected to linearly polarized light, the R8 cell acted as a quarter-wave retarder, converting the incident light to circularly polarized light, as confirmed by close experimental agreement with theoretically determined Stokes parameter values. Moreover, the extent of retardation varied by only 2.7° across the visible spectrum. (N. W. Roberts et al., Nat. Photonics, in press, doi:10.1038/nphoton.2009.189. Image courtesy of Roy Caldwell, University of California Berkeley.)—Jermey N. A. Matthews
With political sentiment growing in favor of greenhouse gas (GHG) restrictions, biofuels from plant cellulose are being considered among the alternatives to fossil fuels: Plants are renewable and biodegradable, and they sequester carbon. Yet a new report validates concerns that a global biofuels program could put intense pressure on land supply and distribution. To predict the impact of a biofuels-based economy on climate change, an international team of researchers from the US, Brazil, and China linked an economic model of land use with a terrestrial biogeochemical model of global GHG levels. The team considered two cases for cellulosic biofuel crop growth: The primary focus of case1 is on converting unfarmed areas such as forests, as shown in the image; of case 2, on exploiting existing farmland to the extent possible. In both cases, biofuel feedstock becomes a dominant global crop by year 2100, but in the process, total forest area is cut—by 56% in case 1 and by 24% in case 2. The loss of carbon-sequestering trees in case 1 results in a net release of carbon. In case 2, the gains from biofuel production ultimately lead to increased carbon sequestration in the farmed soil from the addition of nitrogen fertilizer, which paradoxically releases N2O, another potent GHG. The research suggests that stabilizing GHG levels will require a limited and more efficient use of forests and fertilizers for biofuel crop production. (J. M. Melillo et al., Science Express, 22 October 2009, doi:10.1126/science.1180251. Image courtesy of Chris Neill, Marine Biological Laboratory, Woods Hole, MA.)—Jermey N. A. Matthews
A common step in industrial cooling processes is the liquefaction of a vapor on a condenser. If, however, a liquid film forms on the condenser, the cooling may be compromised. The problem can be addressed by coating the condenser with a hydrophobic material conducive to drop formation and then letting the drops slide off due to gravity. Now Chuan-Hua Chen of Duke University and his student Jonathan Boreyko report a different approach. By depositing carbon nanotubes on silicon micropillars and coating both with hexadecanethiol (C16H34S), they engineered a rough “superhydrophobic” surface. The water drops that condensed on it were about a hundred times smaller than those on a conventional hydrophobic surface that the Duke team considered as a standard; the surface roughening offers the promise for more efficient cooling. Furthermore, as the figure and video show, when two sufficiently large drops coalesce into a single drop, that drop literally springs off the condenser—no external prompting needed.
The post-combination drop has less surface energy than do the two drops from which it forms. Most of the released surface energy is dissipated, but Chen and Boreyko observed that the vertical component of the drop’s velocity can be as much as one-sixth of the theoretical maximum. Nature has her own version of the jumping trick. Coalescence of a wet portion of a spore with a dew drop provides the energy for spore ejection in certain mushrooms. (J. B. Boreyko, C.-H. Chen, Phys. Rev. Lett., in press.) —Steven K. Blau
In recent years, photon pulses in the attosecond (10-18 s) regime have been precisely engineered and are being increasingly put to work—for example, in experimental quantum control and chemical dynamics (see Physics Today, March 2005, page 39). But can much shorter pulses be generated and put to use? Three physicists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have proposed some answers. They modeled the photon emission in the early expansion of a quark–gluon plasma, a hot dense stew of fundamental particles created when heavy nuclei smash into each other at relativistic speeds. Prompt gamma rays in the GeV range, produced primarily by quark–gluon Compton scattering and quark–antiquark annihilation, would exit the expanding QGP in at most a few yoctoseconds (10-24 s). With certain collision parameters and detectors nearly aligned with the collision axis, the model predicts a double-peaked pulse before the QGP disappears. One peak is blueshifted, arising from the approaching side of the QGP, the other is redshifted from the receding side, and the peaks are separated roughly by the light-travel time across the hot soup. The dip between the peaks occurs during an intermediate time at which the stew acquires an anisotropy and emits nothing in that direction. If the model proves correct, such a double pulse could enable pump–probe experiments at the nuclear scale, though new detection schemes would first need to be invented. (A. Ipp, C. H. Keitel, J. Evers, Phys. Rev. Lett. 103, 152301, 2009.) —Stephen G. Benka
Colossal magnetoresistance is aptly named. By subjecting a piece of appropriately doped manganite to a strong magnetic field and a moderately low temperature, one can raise its electrical conductivity by 10 000%. Despite its prodigious magnitude, CMR has not led to any commercial devices since its discovery 15 years ago. Still, it continues to fascinate physicists. At its most basic level, CMR arises as a paramagnetic insulating phase yields to a ferromagnetic conducting phase. But evidence suggests that a third, charge-ordered antiferromagnetic phase could play a role too. To elucidate the issue, Jing Tao of Brookhaven National Laboratory and her collaborators developed a new experimental technique. Called scanning electron nanoscale diffraction (SEND), their technique combines electron diffraction's ability to reveal the presence of ordered structures with scanning microscopy's ability to reveal those structures' real-space distribution. The patches in the figure correspond to charge-ordered regions. As the temperature approaches the 253-K CMR transition, the volume occupied by the charge-ordered phase increases. Simulations by Elbio Dagotto of Oak Ridge National Laboratory and his collaborators suggest an explanation: The charge-ordered phase vies with the ferromagnetic phase to become the predominant phase below the transition temperature. Although it loses the battle, the charge-ordered phase nevertheless delays and thereby intensifies the onset of CMR. (J. Tao et al., Phys. Rev. Lett. 103, 097202, 2009.)—Charles Day
Ferromagnetism usually arises from transition metals rich in 3d and 4f electrons. The occurrence of ferromagnetism in pure carbon, which contains only s and p electrons, is thus surprising—even controversial, given the weakness of the magnetic signal and a Curie temperature well above room temperature. Using magnetic force microscopy and a superconducting quantum interference device to probe the surface and bulk magnetization of graphite, Netherlands researchers Jiri Červenka and Kees Flipse (Eindhoven University of Technology) and Mikhail Katsnelson (Radboud University) offer evidence that the ferromagnetism arises from a two-dimensional network of point defects at grain boundaries. The breaking of the lattice’s translational symmetry by the defects leads to localized electron states at the Fermi level. Because of electron–electron interactions, those states become polarized, which, in turn, leads to the formation of local magnetic moments. Grain-boundary defects are more complicated than single vacancies: The figure here shows a 2D plane of periodic defects, each an extended zigzag discontinuity that propagates through individual graphene sheets of the bulk crystal. A magnetic moment can be associated with each defect; and the step edge at the surface is a manifestation of the grain boundary buried underneath it. The Curie temperature deduced from experiment is, reassuringly, comparable to the theoretical value based on weak interlayer coupling. (J. Červenka, M. I. Katsnelson, C. F. J. Flipse, Nat. Phys., in press, doi:10.1038/nphys1399.)—R. Mark Wilson
The winners of this year's Nobel Prize in Physics are Charles K. Kao for what the Royal Swedish Academy of Sciences cites as "groundbreaking achievements concerning the transmission of light in fibers for optical communication" and Willard S. Boyle and George E. Smith for "the invention of an imaging semiconductor circuit—the CCD sensor."
Two-dimensional spectroscopy of materials, in its simplest form, measures the change in a sample’s interaction with radiation at one frequency caused by excitation at another. First used in nuclear magnetic resonance and later applied at IR and optical frequencies, the technique can separate overlapping peaks in a complicated spectrum or probe the flow of energy through systems such as the light-harvesting molecules involved in photosynthesis. (See Physics Today, July 2005, page 23.) In 2D Fourier-transform optical (FTOPT) spectroscopy, the sample is excited not with monochromatic light but with a pair of phase-related femtosecond pulses separated by a variable delay t1; the time-domain data are transformed into a spectrum as a function of frequency ω1. The second field is likewise replaced by a pair of phase-related pulses, one of which is a so-called local oscillator that allows the radiated field to be recorded as a function of time t3 or frequency ω3. Now, Keith Nelson and colleagues at MIT have produced 3D FTOPT spectra, in which the signal is a function of ω1, ω3, and also ω2, corresponding to the time t2 between the two pulse pairs. Applying their technique to semiconductor quantum wells—a system in which electrons are excited from nondegenerate valence-band states into the same conduction-band states—they found that important pathways whose signals were indistinguishable in a 2D spectrum could often be separated in a 3D spectrum. (D. B. Turner et al., J. Chem. Phys., in press.) —Johanna Miller
In hierarchical models of galaxy formation, large galaxies grow by ingesting dwarf galaxies and the spherical minihalos of dark matter that envelop them. Some of those dwarfs would be tidally shredded when they venture too close to the center of the “host” galaxy’s enormous halo, leaving streams of stars as long-lasting relics of the carnage. Searching within the halo of our neighboring Andromeda galaxy for such relics and for dwarf galaxies surviving intact, the Pan-Andromeda Archaeological Survey has been systematically imaging the galaxy’s environs out to a radius of half a million light-years. The newly published PANDAS stellar population-density map of the halos of Andromeda and the adjacent Triangulum galaxy shows seven relic tidal streams (circled numbers in the figure) stretching over enormous distances. One of them, emanating from Triangulum, gives evidence of a cataclysmic encounter between the two galaxies two or three billion years ago, as shown in the linked animation. The survey results thus far already put an upper limit of about 90 on the total number of surviving dwarf galaxies brighter than the detection limit (Roman numerals in the figure) within Andromeda’s halo. That’s an order of magnitude less than the number of minihalos expected from dark-matter simulations. There has long been evidence of such shortfalls around large galaxies, but PANDAS is the most systematic census of satellite dwarf galaxies to date. Perhaps, theorists suggest, dark-matter minihalos are as abundant as the simulations predict but the shortfall appears because actual star formation in most minihalos is aborted by some threshold-mass effect. (A. McConnachie et al., Nature 461, 66, 2009.)—Bertram Schwarzschild
