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Acoustic Microscopy
A. Historical past: The acoustic microscope was produced as a tool for studying the inner microstructure of nontransparent solids or biological supplies. In acoustic microscopy, a sample is imaged by ultrasound waves, along with the distinction in reflection furnishes a map of the spatial distribution of your mechanical properties. Numerous guides and handbook content give in depth historical outlines. Briefly, the development of your first high-frequency scanning acoustic microscope was motivated from the notion of making use of an acoustic subject to study the spatial versions of the elastic materials properties with nearly optical resolution (The lateral resolution of SAM is dependent about the frequency with the acoustic waves and, at finest, is about 0.seventy five microns). The primary experiments date back again towards the 1940's when high-frequency acoustic pictures had been obtained through the Leningrad scientist Sokolov (Sokolov, S., Doklady Akademia Nauk SSSR, 64, 333, 1949). He observed an acoustical image making use of the tube named after him, in which the acoustic picture was converted into a television display. The primary scanning acoustic microscope was created by Lemons and Quate at Stanford University in 1973 (Lemons, Quate, Appl. Phys. Lett., 24, 163, 1974). It was mechanically driven and operated in the transmission mode. Since then, gradual mechanical and electronic circuit improvements have been made and image recording has been automated. In general, acoustic microscopes now work in the reflection mode.

Left: The schematic diagram of your combined optical and acoustic microscope (Weiss, Lemor et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 54 2257, 2007).Right: A photograph from the combined optical (Olympus IX81) and time-resolved scanning acoustic microscope, SASAM, Fraunhofer-Institute for Biomedical Technology, St. Ingbert, Germany.
B. Principles: The scanning acoustic microscope (SAM) can be characterized by a combination of operating principles distinguishing it from other microscope types (Zinin Weise,Office 2010 Activation Key, “Theory and applications of acoustic microscopy”. in T. Kundu ed., Ultrasonic Nondestructive Evaluation: Engineering and Biological Material Characterization. CRC Press, 654-724, 2004). These principles are (a) image generation by scanning, (b) far-field wave imaging, and (c) the use of acoustic waves. Image generation by scanning is basically different from the functionality of a conventional optical microscope which is the oldest microscope type. The conventional microscope can be considered a parallel processing system in which we can see all points with the object at the same time. In distinction to this, the scanning acoustic microscope is a sequential imaging system in which a piezoelectric transducer emits a focussed ultrasound beam that propagates through a water, for the sample. The beam is scattered through the sample, as well as the scattered ultrasound wave is detected piezoelectrically. The output signal is just one single voltage. As the sample is scanned, the voltage is recorded in each scanning position with the focus and a grey-scale image is generated. The use of a focussed beam leads to your second operating principle. As the focus is formed by converging propagating waves, the size with the focal spot (or focal area) is limited by diffraction. Imaging with ultrasound is the third operating principle. The operating frequencies of SAMs are between 100 MHz and 2 GHz; the substantial frequency provides the opportunity to obtain accurate measurement results for crack and void distributions with a resolution of up to 1 µm at a depth of 10 µm. Pictures made through the SAM are called C-scans. They are obtained when the acoustic microscope mechanically scans sample in a plane parallel to your sample surface Variation of the mechanical properties with depth can be studied by scanning at various defocus values. Collecting photos obtained at various defocus positions allows a three-dimensional image to be constructed, representing the volume of the entire microstructure of your investigated sample.


Time-resolved acoustic microscopy adds an additional degree of freedom for quantitative measurement, namely time. In time-resolved acoustic microscopy a short sound pulse is sent toward a sample (for instance biological cell). The setup for quantitative time-resolved acoustic microscopy: t0 is the arrival time from the echo reflected from the glass substrate outside the cell ("reference echo"), t1 is the arrival time of your echo reflected from the surface with the sample (surface echo), t2 is the arrival time from the echo reflected off the sample/substrate interface (bottom echo), and d is the sample thickness. For layered supplies the reflected signal represents a train of pulses (A-scan). The time delay from the pulses and their amplitudes provides information about the elastic properties and attenuation of sound in the layer. The velocity with the wave can be determined by measuring the time delay of your corresponding pulse. Time resolved photos obtained by mechanical scanning along a line are called B-scans.
C. Development: Considerable progress in the acoustic microscopy of solid structures has been made since then (Briggs, A. Acoustic Microscopy, 1992, Zinin, "Quantitative Acoustic Microscopy of Solids",Microsoft Office 2010 Professional, in Handbook of Elastic Properties of Solids, Liquids, and Gases. Volume I, Levy et al., eds. 2001, 187). Considerable progress in the acoustic microscopy of solid structures has been made since then. Developments in the theory of your image formation of subsurface defects (Lobkis et al. 1995) and three-dimensional objects (Zinin, Weise et al., Wave Motion, 25, 213, 1997) allow size and location of objects inside solids to be determined. Conventionally, SAM photos show variations of the amplitude with the acoustical signal. Reinholdtsen and Khuri-Yakub (Reinholdtsen, Khuri-Yakub, IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 38, 141, 1991) measured amplitude and phase of the SAM signal at low frequency (3 to 10 MHz) to improve subsurface photos. Grill extended this technique to high (1.2 GHz) frequency . This technique permits reconstruction of the surface relief from the sample with submicron resolution (Grill, Hillmann,Office 2007 Serial, et al., Advances in Acoustic Microscopy. Briggs, Arnold, eds. vol. II: 167, 1996). Combining the time-of-flight technique with acoustic microscopy provides a powerful instrument for investigating adhesion problems as well as the microstructure of small superhard samples. An important step has been made in the direction of imaging subsurface structures at substantial temperatures. Ihara et al.(Ihara, Jen, France, Rev. Sci. Instrum., 71, 3579, 2000) formulated a sound imaging technique to see a small steel object immerged in molten zinc at 600oC. With the advancement of the ultrasonic force microscope ( Kolosov,Office 2007 Professional Plus, Yamanaka, Jap. J. Appl. Phys. 32, L1095, 1993) and the atomic force acoustic microscope (Rabe, Arnold, Annalen Der Physik 3, 589, 1994) the capability of your conventional acoustic microscope has been expanded to nanometer resolution.

Application of SAM in Components (Natural and Artificial) Science
Measuring the elastic properties solids and thin films
Measurement and visualization of adhesion in layered structures.
Subsurface imaging: the most common application of the acoustic microscope is the detection of subsurface defects in coatings.
Visualization of stress inside solid materials (Drescherkrasicka, Willis. Nature 384, 52, 1996).
Characterization of carbon-fiber-reinforced composites (Manghnani, Zinin, et al., Acoustical Imaging, Vol. 27, 83, 2004).

Acoustic (left) and SEM (right) photos of concrete sample made with granitic aggregate grains and Portland cement paste. The acoustic image was made at 400 MHz.

Application of SAM for Elastic Characterization of Biological Cells
Mechanical characterization of biological cells and tissue cytoplasm by a conventional acoustic microscope was discussed thoroughly in the following review: Bereiter-Hahn, Blase, Ultrasonic Characterization of Biological Cells, in T. Kundu ed., Ultrasonic Nondestructive Evaluation: 722, 2004. Recently, a new high-frequency (1 GHz) time-resolved acoustic microscope was created at the Fraunhofer-Institute for Biomedical Technology, St. Ingbert, Germany (Weiss, Lemor et al., IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 54 2257, 2007). It is based on an optical microscope from Olympus and it operates in a reflection mode. The design of the new microscope is different from that of conventional acoustical microscopes in that it has a modular structure. The microscope consists of four main modules: acoustical lens; optical module; scanning unit; and high-frequency electronics. This new microscope can be characterized by a combination of operating principles and design features distinguishing it from other high-frequency acoustic microscopes. These principles are: (a) it operates in time-resolved mode; (b) it is designed as an attachment to an inverse optical microscope; (c) it is fully automated ; (d) measurements can be done at 37oC. Such a combination is of importance for learning dynamical processes in biological cells and temperature sensitive materials. This microscope enables us to measure acoustical properties of a single HeLa cell in vivo and to derive elastic parameters of subcellular structures.

From IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 54 2257,Office Professional, 2007, Ultrasound Med. Biol. 33, 1320, 2007.
References

A. Briggs, Acoustic Microscopy Clarendon Press, Oxford, 1992
P. V. Zinin, "Quantitative Acoustic Microscopy of Solids", in M. Levy, H. Bass, R. Stern, V. Keppens eds., Handbook of Elastic Properties of Solids, Liquids, and Gases. Vol. I: Dynamical Methods for Measuring the Elastic Properties of Solids, Academic Press, New York, 187-226 (2001).
P. V. Zinin, "Quantitative Acoustic Microscopy". in M. Levy, H. E. Bass eds., Experimental Methods in the Physical Sciences, Vol. 39: Modern Acoustical Techniques for the Measurement of Mechanical Properties. Academic Press, New York, 135-187 (2001).
P. Zinin and W. Weise, "Theory and applications of acoustic microscopy", in T. Kundu ed., Ultrasonic Nondestructive Evaluation: Engineering and Biological Content Characterization. CRC Press, Boca Raton, chapter 11, 654-724 (2004).
A.Briggs, (ed.), Advances in Acoustic Microscopy, Plenum Press, New York, 1995, pp. 153-208.
A. Briggs, W. Arnold eds), Advances in Acoustic Microscopy Plenum Press, New York, 1996.