Acoustic microscopy

The notion of acoustic microscopy dates back to 1936 when S. Ya. Sokolov proposed a device for producing magnified views of structure with 3-GHz sound waves. However, due to technological limitations at the time, no such instrument could be constructed, and it was not until 1959 that Dunn and Fry performed the first acoustic microscopy experiments, though not at very high frequencies. The scientific literature shows very little progress toward an acoustic microscope following the Dunn and Fry experiments up until about 1970 when two groups of activity emerged, one headed by C.F. Quate (Stanford University) and the other by A. Korpel and L.W. Kessler (Zenith Radio Research Labs). The first efforts to develop an operational acoustic microscope concentrated upon high-frequency adaptations of low-frequency ultrasonic visualization methods. One early system employed Bragg diffraction imaging, which is based upon direct interaction between an acoustic-wave field and a laser light beam. Another example was based on variations of the Pohlman cell. The original device is based upon a suspension of asymmetric particles in a thin fluid layer which, when acted upon by acoustic energy, produce visual reflectivity changes. Cunningham and Quate modified this by suspending tiny latex spheres in a fluid. Acoustic pressure caused population shifts which were visually detectable. Kessler and Sawyer developed a liquid crystal cell that enabled sound to be detected by hydrodynamic orientation of the fluid. In 1973, the Quate group began the development of a concept, which utilized a confocal pair of acoustic lenses for focusing and detecting the ultrasonic energy. Advancements of this instrument, a scanning acoustic microscope, have to do with achieving very high resolution, novel modes of imaging, and applications. The SAM was commercially introduced by Leitz Corp and by Olympus Corp. In 1970, the Korpel and Kessler group began to pursue a scanning laser detection system for acoustic microscopy. In 1974, the activity was shifted to another organization under Kessler (Sonoscan Inc), where practical aspects of the instrument were developed. This instrument, the scanning laser acoustic microscope (SLAM), was made commercial available in 1975. In 1984, Kessler’s group completed development of the C-SAM concept instrument which operated in the reflection mode, as well as the through-transmission (only) mode of the SLAM. Using the same transducer to pulse ultrasound and receive the return echoes meant that the acoustic image could easily be constrained to a depth of interest. This design was the precursor of essentially all of the acoustic microscopes in use today, and was the development that made possible numerous later advances such as cross-sectional acoustic imaging, three-dimensional acoustic imaging, and others. In the half-century since the first experiments directly leading to the development of acoustic microscopes, at least three basic types of acoustic microscope have been developed. These are the scanning acoustic microscope (SAM), confocal scanning acoustic microscope (CSAM), and C-mode scanning acoustic microscope (C-SAM). More recently acoustic microscopes based around picosecond ultrasonics systems have demonstrated acoustic imaging in cells using sub-optical wavelengths working with ultrasonic frequencies into the multi-GHz. Since the vast majority of acoustic microscopes in use today are C-SAM type instruments, this discussion will be limited to these instruments.