Number 76 (Story #1), April 17, 1992 by Phillip F. Schewe and Ben Stein|
NON-OPTICAL DIFFRACTION PATTERNS in solids have been imaged. It has been known for almost a decade that phonons, packets of acoustical or thermal energy in solids, can interfere to form diffraction patterns, but these patterns had not clearly been seen prior to a new experiment performed by Richard Weaver (217-333-3656) and co-workers at the University of Illinois (Matt R. Hauser et al., Physical Review Letters, 27 April 1992). In their experiment, an ultrasonic wave source produces long-wavelength phonons which can yield much larger, easy-to-discern patterns than earlier methods, which employed laser pulses that engendered much smaller-wavelength phonons. Since the arrangement of atoms in solids is anisotropic, certain preferred directions for phonon travel exist. As a consequence, even though phonons may enter the material from many different directions, the material's for more, geometry will cause phonons to converge, interfere, and ultimately form diffraction patterns which can be picked up by an external acoustical detector. Such patterns may eventually help workers detect defects in materials and help them to determine the structures of unknown materials non-invasively. Unlike previous methods of imaging phonons, the ultrasonic imaging method can be performed at ambient temperatures with all types of materials, including non-crystalline solids.