PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institut

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PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institute of Physics Number 186 July 5, 1994 NEAR-FIELD OPTICAL MICROSCOPES can achieve a spatial resolution of a fraction of a light wavelength by positioning the optical source only tens of nm from the sample. By working in this "near field," scientists can greatly reduce the resolution-limiting effects of diffraction. Physicists at AT&T Bell Labs were able to image dye molecules with a resolution of less than 100 nm using laser light with a wavelength of 600 nm brought to the sample in an optical fiber tapered at the tip to only 20 nm. (Physics Today, May 1994.) Using the incident light to excite photoluminescence in a sample, the microscope becomes a spectrometer with potentially high spatial and energy resolution. In this way, the Bell Labs researchers are beginning to resolve individual centers of luminescence in quantum wells, structures in which electrons are confined to an essentially 2- dimensional GaAs region sandwiched between AlGaAs layers. The study of quantum wells in such fine (spatial) detail is important since they play a key role in certain high-tech lasers and transistors. (H.F. Hess et al., Science, 17 June 1994.) THE EVIDENCE FOR TOP QUARK PRODUCTION, announced by the Collider Detector at Fermilab (CDF) collaboration in April, is now officially published in Physical Review Letters (PRL). An abbreviated form of a much longer article that will appear later in the journal Physical Review D, the PRL account does not much add to the salient facts established in April, namely the observation of 12 events consistent with top production, with an estimated cross section of 13.9 picobarns and an estimated top mass (based on analysis of 7 of the 12 events) of 174 GeV. (F. Abe et al., Phys. Rev. Lett., 11 July.) INTEGRATED CIRCUITS ARE MOSTLY TWO-DIMENSIONAL, whereas nature more efficiently works in three dimensions. The human retina, for example, is a massively-parallel, 3D imaging system consisting of a layer of sensing cells (rod and cone cells), two layers of processing cells (bipolar and ganglion) and two layers of interconnection cells. As for manmade circuits, one method for stacking two chips is to flip one over and attach it to the other using raised metal bumps which serve as a support and as electrical connectors. Such "flip chips" have been used in military infrared- detecting "smart Pixel" arrays. Some stacks with more than two layers have been made using metal interconnections that go all the way through the silicon wafer. Efforts are also underway to link up several stacked circuit layers with optical signals that pass through the wafers. One of the problems here is the lattice mismatch between light-emitting materials and the silicon substrate. One solution may be the use of "epitaxial liftoff" (ELO), a technique in which a specially-grown "epilayer" can be separated from an underlying growth substrate by etching away an intermediate sacrificial layer. The microns-thick epilayer can then be transferred to a different host substrate for further processing. (Optics & Photonics News, April 1994.) -END OF FILE- =============


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