Acoustic Microscopy: Recollections

Abstract

W HEN I was first asked to write for this special issue, I was occupied with the reorientation of our entire research program and another article on acoustics seemed like a diversion. I resisted. But the editors were very persuasive, and the special issue has proved to be irresistible. The articles of this issue form a set that is complete, but but there are a few points that can be added to fill in some of the background. This is an opportunity to talk with a special audience, to acknowledge the contributions from those who have worked with us along the way, and to express our appreciation for the people who have provided the funds. Those people in Washington and in the foundations who work so hard to fund research programs make it all possible. There is little that could have been accomplished without them. In London during the month of July 1984, manufacturers exhibited their prototypes of the commercial instruments. 1984 was a milestone year for acoustic microscopy, for the information gathered with these new models outweighs anything that has appeared before. Each of the prototypes will incorporate the essential concept that we have been exploiting for several years; the concept of a single spherical lens concentrating the energy from a large source to a narrow waist with a size determined by the acoustic wavelength. This type of lens does not suffer from spherical aberrations; it is diffraction limited . The original motivation for our work came from the biologists. We were told that it was a matter of some importaye to improve the resolution beyond the limit of 3000 A set by the optical instrument with a water immersion lens. They suggested a factor of two for the improvement. It will be some time before we know whether that is true. In the meantime, the instruments will be used in fields other than biology. I suspect that polymer chemists will be the first to acquire the commercial acoustic instruments, just as they were first to acquire the scanning electron microscopes (SEM). Why did it take so long to reach this stage? That is the interesting question. Certainly the technology was available. Sokolov’s suggestion goes back 35 years. I think that part of it may have been our own inhibitions or misconceptions. One misconception is that the image must be presented in real time, 30 frames per second, as defined by the tel-