Number 359, February 18, 1998 by Phillip F. Schewe and Ben Stein
BIG LIGHT THROUGH LITTLE HOLES. Baffles with apertures smaller than the wavelength of a particular light wave aren't supposed to transmit much of that light. So it came as a surprise to Thomas Ebbesen at the NEC Research Institute in New Jersey when he shone light through sub-wavelength arrays (150-nm holes in a film of silver, coating a quartz substrate); at selected wavelengths, plentiful amounts of light (with wavelengths up to 10 times the size of the holes) came out the other side. Taking into account the area of the holes relative to the size of the light beam, the light was in some instances being transmitted with an efficiency of greater than 1. The leading explanation is that the light is making its way through the holes in the form (or with the assistance) of surface plasmons, non-radiating electromagnetic disturbances arising from the collective movements of electrons at conductor- insulator interfaces. The researchers believe that for device applications their arrays, which transmit light at special wavelengths, will complement so-called photonic crystals, which exclude light at special wavelengths. (Ebbesen et al., Nature, 12 Feb 1998.)
RECORD HIGH POWERS FROM QUANTUM CASCADE LASERS, devices which produce technologically important light in the mid-infrared range, were described by Federico Capasso of Bell Labs (908-582-7737) at the AAAS meeting in Philadelphia last week. In the Bell Labs design, an electron travels through a sandwich of ultrathin semiconductor layers (grouped into 25 "active regions") and drops to a successively lower energy every time it enters an active region, releasing a photon with each energy drop. (Update 322) Unlike other semiconductor-based lasers, in which an electron combines with a positively charged "hole" to release but a single photon (with a color that is determined by the semiconductor's chemical makeup), Capasso said that their latest, higher-power-output designs enable virtually all electrons injected into the sandwich (above and beyond the electric current needed to initiate the laser process) to contribute their maximum of 25 photons apiece (with colors that can be easily tailored by modifying the layer thicknesses). At temperatures of 80 K, quantum cascade lasers can now produce continuous streams of 5.2-micron-wavelength light at powers of 200 milliwatts, whereas other kinds of semiconductor-based lasers can at best provide only 1-2 milliwatts in this range (J. Faist et al., IEEE J. Quantum Electronics, February 1998). At room temperature the Bell Labs group produced 50-ns pulses of 8.2-micron light with powers of 170 milliwatts. Since this mid-infrared light transmits excellently through clean air but scatters from or gets absorbed by greenhouse gases and other pollutants, researchers at Pacific Northwest National Laboratories (Jim Kelly and Steve Sharpe, 509-375-2699) are testing these lasers with the objective of developing systems that can detect pollutants at concentrations of 10 parts per billion or lower.
MOLECULAR INDIVIDUALISM, the idea that physically identical molecules can behave differently under seemingly identical conditions, has been observed by a Stanford group (Steven Chu, 415-723-3571). Inside a microscopic fluid cell, coiled DNA strands experiencing the same flow currents unravel in a host of radically different ways--sometimes forming kinks in the middle, others forming knots at one or both ends, and others getting caught up in a folded shape as they try to unfold. Speaking at the AAAS meeting, Chu attributes this (non-chaotic) phenomenon to tiny fluctuations in the starting conditions--such as a small temperature change in the fluid cell. (Stanford news release, February 11, 1998.)