LASER COOLING OF BULK MATTER. For many years laser light has been used to slow down and thereby cool individual atoms in traps. But laser light can now also be used to cool bulk matter. This happens by a process called anti-Stokes fluorescence. In the solid certain molecules reside in vibrational states which make them somewhat warmer than their neighbors. If now specially tuned laser light hits these molecules, they will emit a photon whose energy is greater than the one they absorbed. Thus heat energy is carried away as light energy (New Scientist, 18 January 1997). In this way gases and liquids have been cooled, and now researchers at Los Alamos have cooled a solid ytterbium-doped optical fiber from a temperature of 298 to 282 K, a difference of 16 K. Applied on a large scale, this principle could lead to a laser refrigerator. (C.E. Mungan et al., Physical Review Letters, 10 February 1997.)

SONOLUMINESCENCE BUBBLES COLLAPSE AT MORE THAN FOUR TIMES THE SPEED OF SOUND, new experiments have shown. Sonoluminescence is the still- mysterious process in which sound waves aimed at a water tank cause bubbles to collapse and generate ultrashort light flashes which represent a trillionfold concentration of the original sound energy. UCLA researchers (Seth Putterman, 310-825-2269) determine the speed of bubble collapse by measuring the amount of laser light scattered from a bubble during different points of its implosion. The amount of scattered light is proportional to the square of the bubble radius. Previous experiments could only establish that the bubble collapsed faster than the speed of sound; using ultrashort (100-fsec) laser pulses has now enabled the researchers to ascertain that the bubble collapses at a speed greater than Mach 4 (more than 1 km/s for this tiny bubble). This confirms a major prediction of the leading explanation for sonoluminescence known as the shock-wave model but does not rule out competing explanations because this and other experiments to date can only probe the outer surface of the bubble, not what is happening inside. In addition, the UCLA team determined that the bubble accelerates by at least 10^11 g when the bubble stops compressing and starts expanding; amazingly, the bubble remains intact during this massive acceleration. (K.R. Weninger et al., upcoming article in Physical Review Letters.)

POLYMER QUANTUM WIRES. By restricting the spatial motions of electrons in semiconductors, one also begins to restrict the electron's allowable energies. With this comes greater control over the way in which the electrons' wave properties can be used in practical devices such as diode lasers, the ones used in CD players. So far "quantum confinement" structures that restrict motion in one dimension (quantum wells), two dimensions (quantum wires), and all three dimensions (quantum dots) have been made with inorganic semiconductors such as GaAs. But now scientists at Rochester (Samson Jenekhe, jenekhe@che.rochester.edu) report that they have made light- emitting quantum wires using a blend of two polymers. The linear chain structure and electronic properties of polymers make them a sort of natural quantum wire to begin with. It's rather early to compare to GaAs optical devices, but the polymer versions might have better stability in high electric fields. Furthermore, as an organic material, the polymer wires could be "grown" rather than built up in an expensive epitaxial process using particle beams in vacuum chambers. (Chen and Jenekhe, Applied Physics Letters, 27 January 1997.)