Physics News Update, Number 490

Number 490, June 22, 2000, 2000 by Phillip F. Schewe and Ben Stein

ATOMIC SCALE LOCOMOTIVES. Miniaturization has produced several examples of nm-sized rotors, ratchets, and gears, but not yet nano-steam engines. But to move raw materials around the freight yards of future nano-factories one needs nano-locomotives.

Scientists at Tel Aviv University in Israel (Markus Porto, 011-972-3-640-7229), have proposed how this can be done. In their scheme the freight yard consists of a lithographically prepared corrugated surface, something like the shape of an egg carton (on the microscopic level, anyway; to the naked eye the surface looks flat). The engine, in its simplest form, consists of three tiny clusters of metal atoms connected by two "springs." Each spring is actually a photochromophore molecule, one whose length can be expanded or shrunk with light. So to get the engine to move, laser light is shot in from above, the molecule expands, and one metal particle moves into depression on the surface.

By careful timing and correlating of the light pulses, the engine can be made to move along like an inchworm (see movie at Physics News Graphics). Cargo consisting of, say, inactive chains of molecules or other atomic material would be coupled to the locomotive and transported to where it is needed. (Porto et al., Physical Review Letters 26 June 2000; Select Article.)

NM-RESOLUTION VISIBLE-LIGHT MICROSCOPY, of a sort, has been accomplished by scientists at the Max Planck Institute for Biophysical Chemistry in Gottingen, Germany (Stefan Hell, 011-49-551-201-1366, shell@gwdg.de). The diffraction of light waves normally limits spatial resolution of nearby objects to no better than the wavelength of the light source. So called near-field microscopy beats this limit by moving the source very close to the subject to be imaged.

But the Gottingen group, whose work is an example of far-field microscopy, in this case does not so much beat the diffraction limit as circumvent it. They split a laser pulse (wavelength of 820 nm) into two parts and illuminate a sample consisting of beads attached to a Langmuir-Blodgett layer, the kind of filmlike layer of water-hating and water-attracting molecules poised back-to-back that forms the membrane of most cells. The sample, positioned close to the place where the two laser beams meet at the same focal point of two lenses, starts to fluoresce. This fluorescence is viewed through filters at two different colors.

This "confocal" microscopy does not exactly "resolve" the objects apart but does measure the distance between them with a precision as high as 1.2 nm. For the process to work, however, the contrasting objects, in this case the layer and a bead, must fluoresce at different colors. This is just what one gets when attempting the co-localization of proteins and organelles or vesicles and membranes, etc. And unlike such imaging techniques as atomic force microscopy (AFM) or transmission electron microscopy (TEM), the use of non-bleaching visible light permits the study of living cells. (Schmidt et al., Review of Scientific Instruments, July 2000; Select Article.)

THE NEXT GENERATION SPACE TELESCOPE (NGST), 100 times more sensitive than the Hubble Space Telescope, sits at the top of the list of desirable future observatories, a list formulated by the National Academy of Sciences. The billion-dollar NGST should possess an 8-m mirror, an orbit 1 million miles from Earth, and an ability to view the most distant (and earliest) stellar objects in the universe at infrared wavelengths.

Next in order of priority is the Giant Segmented Mirror Telescope (GSMT), a 30-m ground based telescope for complementing with superb spectroscopy the sharp imaging of the NGST; the $800 million Constellation-X Observatory, specializing in x rays; an Expanded Very Large Array (EVLA) radio telescope; the Large-aperture Synoptic Survey Telescope (LSST), which would scan the whole sky, every week for faint objects; and the Terrestrial Planet Finder (TPF), "the most ambitious science mission ever attempted by NASA," whose goal is to search for planets around nearby stars. (NAS website: http://www.nationalacademies.org/topnews/.)