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18. Dezember 2001 © email: Krahmer

website: www.physics.gatech.edu/ultracool

ULTRASOUND SCANS ARE AUDIBLE TO A FETUS,
researchers reported at this week's meeting of the Acoustical Society of America in Fort Lauderdale, Florida. Ultrasound by definition is sound that lies beyond the range of human hearing. So how can a fetus hear an ultrasound scan? As explained by the researchers (Mostafa Fatemi, Mayo Foundation, Minnesota, fatemi.mostafa@mayo.edu), traditional imaging systems produce ultrasound as sequences of short-duration, high-energy bursts, called "pulse trains." When the pulses enter the body, they tap internal organs at a regular rate. When the ultrasound points at the head of the fetus, its sensitive hearing structure gets vibrated at a rate equal to the number of pulses per second. (Typically, several thousand pulses are transmitted per second in a pulse train, a rate equal to several thousand Hertz.) The fetus senses these vibrations as tones, equivalent to the high notes of a piano. The sound can get loud--about the equivalent of 100-120 decibels of airborne sound, or the level of sound of an approaching subway train. Rather than being akin to a sound from the outside world, though, the sensation is more like what you hear when your finger taps a spot close to an ear-which is why it's inaudible to others, including the mother. What's more, the sound is focused on a tiny, square- millimeter spot, and the sound diminishes rapidly from that spot, so that the fetus could quickly adjust its position to avoid the loudness. Fatemi stresses that their findings do not suggest that this sound is harmful to a fetus. These studies can help explain physicians' observations that a fetus moves vigorously when ultrasound is directed at its head. They eliminate the notion that ultrasound is a passive observation technique, but they may also inspire new ultrasound exams for testing normal fetal function. (Paper 1pBB6 at meeting; abstract at http://asa.aip.org/asasearch.html )

TRACKING DNA MOTION WITH PICOMETER ACCURACY.
Scientists don't have to settle for averaged results when studying tiny things with x rays. In x-ray diffraction, for example, a crystallized sample with billions of molecules scatters the x rays into a characteristic pattern of spots on a detector which is then decoded to yield lattice structure information. A team of Japanese scientists have developed a method, which they call diffracted x- ray tracking (DXT), in which the bobbing Brownian motion of single nanocrystallites in water are watched by tracking scattered x rays; with this method one acquires information not about the position but the rotary motion of single nanoparticles (Sasaki et al., Physical Review E, September 2000). Now the process has been extended to single DNA molecules, whose Brownian motion can be tracked, for the first time, with a precision of picometers, or 10^-15 m (see figure at http://www.aip.org/mgr/png ). The researchers will soon broaden their measurements of important biomolecules. For example, they hope to observe the structural changes accompanying the activation of ion channels in living cells. (Sasaki et al., Physical Review Letters, 10 December 2001; contact Yuji Sasaki, Japan Synchrotron Radiation Research Institute, ycsasaki@spring8.or.jp, 81-791-58-0831.)

ATTOSECOND PHYSICS HAS ARRIVED.
An Austria- Canada-Germany collaboration (Ferenc Krausz, Vienna Institute of Technology, 011-43-1-58801-38711, ferenc.krausz@tuwien.ac.at) reports that it has produced and detected, for the first time, isolated x-ray pulses lasting on the scale of attoseconds, where one attosecond is a billionth of a billionth of a second (10^-18 s). The reported pulses, lasting approximately 650 attoseconds (as) and residing in the soft x-ray part of the electromagnetic spectrum, subsequently provided attosecond-scale measurements of a physical phenomenon (specifically, the detachment of an electron from an atom by an x-ray photon). With these observations, and several earlier ones by other groups, attophysics becomes the short-timescale frontier of physics. It replaces femtochemistry, the production of light pulses at the 10^-15 s (femtosecond) scale, in this regard. Just as a strobelight can yield stop-action photographs of a falling water drop, femtosecond pulses can capture the ultrafast steps of a chemical reaction between multiple atoms or molecules. But attosecond pulses are better equipped to capture the even speedier motions of electrons within atoms. If light can be imagined as a wave of peaks and valleys, a one- second visible light pulse is a train of roughly 600 trillion (6*10^14) peaks and valleys in length. The researchers report an attosecond pulse just 200 nanometers long, carrying just over a dozen peaks and valleys. Therefore, the duration of a light pulse can be thought of as the length along its direction of travel. A 1.28-second pulse can stretch from an Earthbound laboratory to the moon; a 650-attosecond pulse would barely span the length of two typical viruses. Previous experiments have reported evidence of trains of attosecond pulses following each other roughly every 1 fs (Papadogiannis et al, Phys. Rev. Lett., 22 November1999; Paul et al., Science, 1 June 2001; Bartels et al., Nature, 13 July 2000), but the new experiment, according to the researchers, represents the first detection and measurement of isolated attosecond pulses. Such isolated pulses, Krausz states, are important for taking attosecond-resolution snapshots of electron motion in atoms. To accomplish their feat, the researchers first prepared an intense fsec pulse and aimed it at neon gas. The interaction between the neon gas and the fsec pulse created an attosecond- scale pulse in the soft x-ray range. According to a helpful theoretical picture (Corkum, Physical Review Letters, 27 September 1993), the fsec pulse ejects electrons from neon atoms, and the resulting oscillations of the electrons in the bath of fsec light produce an even shorter-duration soft-x-ray pulse. Producing attosecond light is only half the battle. The researchers then had to measure its duration. By adjusting the delay between the times at which the x-ray pulse and a fsec visible pulse hit a gas of krypton atoms, the researchers affected the spectrum of energies in the electrons liberated from the atoms. Such modulations in the observed energy spread served as evidence for an x-ray pulse of 650 attoseconds. Henry Kapteyn of JILA/University of Colorado (303-492-819, kapteyn@jila.colorado.edu), a member of a competing group, claims that the evidence is ambiguous as to whether the collaboration detected isolated attosecond pulses or trains of attosecond pulses. Both Kapteyn and Krausz have respected colleagues who back their differing views. However this debate pans out, attosecond metrology has arrived, and it will doubtlessly lead to some staggering physics experiments never before possible. (Hentschel et al., Nature, 29 November 2001; some other associated journal articles can be found at www.aip.org/physnews/select)

INSULATOR TO METAL IN ONLY 100 FEMTOSECONDS.
A new experiment has, for the first time, studied in detail how a crystal undergoes a superfast phase change from the insulating state into a metallic state on a femtosecond time scale. Andrea Cavalleri (now at LBL, 510-495-2536, acavallieri@lbl.gov) and his colleagues at UC-San Diego and the University of Quebec work with a sample consisting of a 200-nm thick film of vanadium oxide (VO2). A 50-fsec laser pulse enters the sample causing what is believed to be not one but two phase transitions: one structural (the unit cell size increases a bit), monitored with short x-ray pulses; and one electrical (insulator-to-metal), monitored by short pulses of visible light. All of this done on an unprecedentedly short timescale. This allowed the researchers to observe that the manifestation of the solid in its new crystalline form did not happen piecemeal but practically all at once; this had never been seen before. For all the speed, though, this experiment still did not settle an old question in condensed matter physics as to which comes, first the structural change in the sample or the electrical change. Because the crystalline reordering is so fast (only hundreds of fsec), and is reversible, and because x rays scatter differently from the two contrasting crystalline forms, it might be possible to use this whole process as a ultrafast "Bragg switch" for sub- picosecond portions of a longer x-ray wavetrain. The transformation from insulator to metal is an important example of the large catalog of solid-to-solid phase transitions in physics which usually occur because of a change in pressure or temperature; the ice-induced failure of the sealing ring on the Challenger mission is one example. (Cavalleri et al., Physical Review Letters, 3 December 2001; text at http://www.aip.org/physnews/select)

HIDDEN OBJECTS REVEALED WITH QUANTUM HOLOGRAPHY.
Second sight and remote viewing are terms used to explain charlatans' supposed psychic ability to see hidden objects in terms of pseudoscientific gibberish. Quantum holography, on the other hand, is a method firmly grounded in modern physics that permits the imaging of hidden objects with entangled photons. Of the quantum entanglement phenomena that Einstein described as "spooky action at a distance," quantum holography may be the spookiest to date. Researchers at Boston University's Quantum Imaging Laboratory (Bahaa Saleh, 617-353-7176, besaleh@bu.edu) propose to create holographic images of objects concealed in a spherical chamber. Ideally, a small opening in the chamber wall permits light to enter, but lets no light out. The photons in a beam of light directed through the hole scatter from the enclosed object, and ultimately strike the inner wall of the chamber (see figure at http://www.aip.org/mgr/png ). According to the scheme, the inside of chamber would be designed to detect the time when a photon hits the wall but not where it hits. Classically, there is no way to generate an image of an object with this sort of configuration. Quantum mechanically, however, it's possible to build a hologram of the hidden object provided that the photons in the illuminating beam are entangled with photons in another beam. Each photon in an entangled pair has properties (such as momentum or polarization) that are unknown until a measurement is performed on one photon or the other. When a property of one of the photons is measured, corresponding information about its entangled mate is instantly determined. That may seem spooky enough, but in quantum holography, things get spookier still. Holograms are typically constructed with interfering beams of light, which provides more information about a subject than simple illumination can. The additional information helps build a three dimensional image of a three dimensional object. In quantum holography, the researchers measure the simultaneous arrivals of an illuminating photon that is sent into the chamber and a companion photon in the other entangled beam. This measurement tells the researchers about the interference of various possible paths that the single photon inside the chamber could travel. And it's the interference of the possible paths that encodes the holographic image of the hidden object. Very spooky indeed. For the moment, quantum holography exists only on paper. But the researchers assert that there are no technological obstacles to the proposal, and they hope to begin building an experimental system soon. (Ayman F. Abouraddy, Bahaa E. A. Saleh, Alexander V. Sergienko, and Malvin C. Teich, Optics Express, 5 November 2001.)

QUANTUM ACOUSTICS.
The flatland world of electrons residing at the two-dimensional interface between semiconductor planes is an integral part of quantum-well lasers found in many popular electro-optical devices such as grocery scanners and CD players. But the physics at work in these two dimensional electron gases (2DEG) is far from exhausted. Two years ago a team of physicists used subtle sound waves (surface acoustic waves, or SAW) rippling through one of the semiconductor planes used to confine the electrons to form up the electrons into orderly lines (in effect "quantum wires") and also to transport controllably these formations (see focus.aps.org/v3/st14.html). Now the team, consisting of physicists at the Institute of Semiconductor Physics in Novosibirsk, Russia (contact Sasha Govorov, temporarily at Ohio University, govorov@helios.ohiou.edu) and the Ludwig Maximilans University in Munich (Achim Wixforth, achim.wixforth@physik.uni-muenchen.de ), propose to use two such surface acoustic waves, oriented at right angles, to confine the electrons to essentially zero-dimensional pockets which can be maneuvered around. Thus initially free electrons are organized into quantum wires and dots by intense sound waves. Furthermore, the train of wires or dots might be able to move through the "quantum film" (the planar region between the semiconductor layers) without resistance; alternatively it can be said that the sound waves move without dissipation, thus constituting an example of self-induced acoustic transparency. The researchers, who are presently testing their scheme, also hope to combine this ability to position electrons or deliver them selectively to quantum dots with other processes, such as the conversion of light waves into electron-hole (exciton) objects useful for processing optically-encoded information (see www.nano.physik.uni-muenchen.de/research.hml http://www.aip.org/enews/physnews/1997/split/pnu321-1.htm ). (Govorov et al., Physical Review Letters, 26 November 2001)

SINGING LIKE A CANARY
requires little thought, but simple actions, to yield complex vocal physics, researchers have found, yielding potential insights into how humans generate speech sounds. Human speech and the songs of many bird species share a central similarity: the skills are not present at birth, but are only learned through early-life experiences. To determine how brain activity leads to the production of sound, scientists strive to understand how much of the sound comes from complicated instructions from the brain and how much comes from complex physics of vocal organs. Now, a US-Argentina research collaboration (Gabriel Mindlin, University of Buenos Aires, Gabriel@birkhoff.df.uba.ar) has designed a simple physical model that accurately reproduces notes of a canary song. The researchers modeled the canary's vocal organ, called the syrinx. According to previous experimental evidence, the syrinx generates sound through vibrations of its labial "folds"---flaps of tissue which open and close the air passage between the throat and the lungs. In their model, the researchers make the key assumption that these labial folds behave like a simple spring, moving back and forth to change the size of the air passage. They further assume that a canary controls its vocalizations through two actions: changing the pressure of the air from the lungs and using muscles to modify the stiffness of the folds. By varying these two parameters, the researchers found that the spring-like labia could produce faithful recreations of three canary notes. Therefore, simple changes to a basic system, rather than sophisticated instructions from the brain, can reproduce the rich, complex vocal physics which give rise to complicated sounds. (Gardner et al., Physical Review Letters, 12 November 2001)

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