The new hot papers on strings (#6, #8, #9) both illustrate the richness of the theory and give insights into the interactions between the major players. The string theory community is close-knit, its members working in constructive harmony rather than aggressive competition. In #8 Amihay Hanany and Edward Witten of the Institute for Advanced Study, Princeton, make a connection between discoveries in supersymmetric gauge theories and configurations in 10-dimensional superstring theory. The formalism also gives an explanation of recently discovered mirror symmetry in 3 dimensions. Paper #6, from Washington Taylor IV of Princeton University, describes the construction of a low-energy field theory for D-branes, which are an important new tool for studying many aspects of string theory.

   Sometime early in the next century when you buy Wagner's Ring Cycle on a single audio disk, remember you read about it first in Science Watch! Advances described in #3 and #4 are set to lead to dramatic increases in the information stored on a compact disk (CD).

   The first light-emitting diodes (LEDs) were red. Progress in the growth technology to make the diodes has led to the realization of blue, green, yellow, and magenta LEDs. Very soon you will see these LEDs being used commercially in traffic signals and indicator lights, because the diode is brighter and lasts much longer than a filament light bulb. However, light from the diode is incoherent. That's not a problem when the detector, the human eye, cannot distinguish coherence or polarization in light. But incoherent light is messy for information technology applications. For those, the coherent light of a laser is essential.

   The tiny laser diodes in CD drives rely on red light semiconductors. The use of light at a shorter wavelength would mean a tighter focus of the beam and therefore more data points for a given area on the disk.

   Shuji Nakamura's group at Nichia Chemical Industries, Tokushima, Japan, reported in 1995 pulsed operation at room temperature of an InGaN-based laser-diode that produced bluish-purple light at 406 nm. For Science Watch, coauthor Yoichi Kawakami observes that "emission colors in these quantum-well devices can be controlled by changing the In compositions in the InGaN active layers. However, the intrinsic nature of InGaN ternary alloys makes a perfect crystal difficult to grow." The Nichia physicists have cracked a lot of difficult problems in thin film physics to deposit metalorganic layers on a sapphire substrate in order to realize the blue-purple laser.

   Nakamura tells Science Watch, "The first room-temperature continuous operation of the InGaN laser-diode is reported in #3. The paper gives the first observation of beautiful longitudinal modes with a peak wavelength of 406 nm. Following those results, we achieved a lifetime of 8,000 hours this year. That means the commercialization of those laser-diodes is imminent. The speed of progress in the research and development of these InGaN laser-diodes is extremely rapid. That's probably due to the unusual characteristics of the InGaN semiconductors, such as their robustness."

   Industrial production of these laser-diodes will require advances in applied physics. Thin film semiconductor physics requires the matching of the lattice structures, atom for atom, of the different layers. The angle of cleavage through a perfect crystal affects the spacing of atoms on the surface. Paper #7 gives details of an InGaN laser diode which uses a cleaved facet on a sapphire substrate. No coating was used to achieve the mirror properties of the sapphire facet. A mirror system is fundamental to every kind of laser. In this quantum well device the mirror is an intrinsic property of the substrate, which in principle simplifies commercial production.