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ScienceWeek
EXPERIMENTAL PHYSICS: ON ULTRAFAST QUANTUM CONTROL
The following points are made by Yaron Silberberg (Nature 2004 430:624):
1) When a light pulse is short enough, it interacts with atoms and molecules before they can be affected by their environment. The interaction is then described by simple quantum mechanical rules. By carefully tailoring the shape of the optical pulse, it is possible to manoeuvre the system into desirable final states, particularly those that are hard to reach through simple thermodynamic processes. For example, it might be possible to break a certain bond in a molecule while leaving other, perhaps weaker, bonds intact. The general approach is known as "quantum coherent control", a field that developed as a theoretical exercise in the mid-1980s, but which has seen intense experimental effort in recent years[1-3].
2) Coherent-control experiments start with light pulses that last typically a few tens of femtoseconds (one femtosecond is 10^(-15) seconds). Such pulses are now routinely produced by commercial lasers. The pulses are sent through an optical set-up known as a pulse shaper, which can be programmed to generate complex temporal shapes. The shaper acts as a frequency-domain synthesizer, separating a short pulse into many frequency components. The phase, and possibly the amplitude, of each component can be tweaked individually. The result is a longer pulse with an internal structure that can be defined with great precision.
3) It used to be the case that all quantum control experiments with shaped pulses used linearly polarized light -- light whose electric-field vector is confined to a single direction. The optical field of a linearly polarized pulse puts a force on the charges in the system -- be they electrons or ions -- along only one direction. It is quite easy to convert this linearly polarized light into other polarization states, say circular or elliptical ones, by placing simple polarization converters in the beam. But this modifies the polarization of the entire pulse uniformly.
4) What is needed is the ability to change the polarization direction continuously -- that is, to modify the polarization direction within the optical field. This was recently realized by Brixner and Gerber[4]: in their polarization pulse shaper, not only the amplitude and phase but also the polarization state of the different frequency components can be changed, creating pulses with complex twisted polarization structures. In such a pulse, the polarization direction may change on the scale of a few femtoseconds.[5]
References (abridged):
1. Suzuki, T., Minemoto, S., Kanai, T. & Sakai, H. Phys. Rev. Lett. 92, 133005 (2004)
2. Brixner, T. et al. Phys. Rev. Lett. 92, 208301 (2004)
3. Shapiro, M. & Brumer, P. Principles of the Quantum Control of Molecular Processes (Wiley, New York, 2003)
4. Brixner, T. & Gerber, G. Opt. Lett. 26, 557-559 (2001)
5. Oron, D., Dudovich, N. & Silberberg, Y. Phys. Rev. Lett. 90, 213902 (2003)
Nature http://www.nature.com/nature
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CHEMISTRY: ON ULTRAFAST PROBES OF WATER STRUCTURE
The following points are made by Y. Zubavicus and M. Grunze (Science 2004 304:974):
1) Despite the apparent simplicity of the water molecule, liquid water is one of the most mysterious substances in our world. Because of its vital importance for virtually any area of human activity, ranging from geophysics through cell biology, there has been a continuous attempt to explain the behavior of liquid water by referring to the properties of the water molecule. Bulk water is truly multifaceted (1). This pertains to a common beverage and solvent as well as to one of the 13 known crystalline modifications of ice or one of the three, or probably infinite number, of its amorphous modifications (2,3). Not to mention overcooled, supercritical, interfacial, low-dimensional cluster, confined, and many other forms of water.
2) The interaction of a water molecule with its neighbors -- the so-called water "structure" -- is of key importance for understanding the unique properties of aqueous systems. Since the first two-state-mixture model (capable of explaining the anomalous dependencies of some macroscopic properties of liquid water on temperature and external pressure) was suggested by Roentgen (4), tremendous progress has been made with sophisticated models augmented with extended theoretical formalisms (such as flickering cluster, percolation, fluctuating charges, random network, continuum models, and so forth) (5).
3) Nevertheless, many structural aspects of liquid water remain a subject of debate. The traditional techniques for structural studies of ice and liquid water, such as neutron and x-ray diffraction, are yielding diminishing returns, owing to the collective efforts of many generations of researchers. New results should instead be expected with new techniques unrelated to these mainstream research efforts. For instance, studies of water-based systems under confinement or extreme pressure and temperature conditions could extend the existing knowledge base. Alternatively, one could focus on the structural flexibility of water as reflected in the structural relaxation following an external perturbation or the fluctuations within a network of hydrogen bonds.
4) The question arises whether recent results with ultrafast probes can be explained within the commonly envisioned "static picture" of water, taking into account the dynamical behavior of H2O molecules in the liquid phase, together with the near-edge x-ray absorption fine structure, which probes instantaneous positions of the oxygen and hydrogen atoms in an ensemble of water molecules, integrated over time. Whether -- or to what extent -- the time-averaged instantaneous positions reflect the equilibrium position of the oxygen atoms in the hydrogen bond network of water will be a subject for debate; that is, if the results obtained with the attosecond "snapshot" approach are comparable to the time- and ensemble-averaged results obtained by x-ray and neutron diffraction. What is disturbing, and probably of major consequence for all molecular dynamics simulations of water performed with established and widely used computer program packages, is their disagreement with the water structure "snapshots" measured experimentally by Wernet et al (Science 2004 304:995).
References (abridged):
1. P. Ball, H2O: A Biography of Water (Weidenfeld & Nicolson, London, 1999)
2. C. A. Tulk et al., Science 297, 1320 (2002)
3. M. Guthrie, J. Urquidi, C. A. Tulk, D. KLug, J. Neuefeind, Phys. Rev. B 68, 184110 (2003)
4. W. C. Roentgen, Ann. Phys. Chem. 45, 91 (1892)
5. I. Ohmine, H. Tanaka, Chem. Rev. 93, 2545 (1993)
Science http://www.sciencemag.org
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ON ULTRAFAST GENERATION OF LOCAL MAGNETIC FIELDS
The following points are made by Y. Acremann et al (Nature 2001 414:51):
1) For the development of future magnetic data storage technologies, the ultrafast generation of local magnetic fields is essential. Subnanosecond excitation of the magnetic state has so far been achieved by launching electric current pulses into micro-coils and micro-striplines, and by using high-energy electron beams. Local injection of a spin-polarized current through an all-metal junction has been proposed as an efficient method of switching magnetic elements, and experiments apparently confirm this. Spin injection has also been observed in hybrid ferromagnetic semiconductor structures.
2) The authors report a different scheme for the ultrafast generation of local magnetic fields in such a hybrid structure. The basis of the approach of the authors is to optically pump a Schottky diode with a focused approximately 150 femtosecond laser pulse. The laser pulse generates a current across the semiconductor-metal junction, which in turn gives rise to an in-plane magnetic field. This scheme combines the localization of current injection techniques with the speed of current generation at a Schottky barrier. The authors suggest specific advantages include the ability to rapidly create local fields along any in-plane direction anywhere on the sample, the ability to scan the field over many magnetic elements, and the ability to tune the magnitude of the field with the diode bias voltage.
Nature http://www.nature.com/nature
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