**Part 2**

**Acoustic Waves as Investigative Tools**

122 Acoustic Waves – From Microdevices to Helioseismology

[16] M. S. Sodha and S. Guha, *Advances in Plasma Physics*, edited by A. Simon and W. B.

[19] H. Thomas, G. E. Morfill, V. Demmel, J. Goerce, B. Feuerbacher and D. Mohlmann, *Phys.* 

[21] L. Boufendi, A. Bouchoule, P. K. Porteous, J. Ph. Blondeau, A. Plain and C. Laure, *J.*

[23] B. N. Kolbasov, A. B. Kukushkin, V. A. Rantsev Kartinov, and P. V. Romanov, *Phys.* 

[25] P. K. Shukla, *Waves in dusty, solar wind and space plasmas,* (AIP Conference proceedings,

[20] G. S. Selwyn, J. E. Heidenreich and K. L. Haller, *Appl. Phys. Lett*. 57, 1876 (1990).

[22] A. Gondhalekar, P. C. Stangeby and J. D. Elder, *Nucl. Fusion* 34, 247 (1994).

[24] N. N. Rao, P. K. Shukla and M. Y. Yu, *Planet. Space Sci.* 38, 345 (1990).

[28] J. P. Goedbloed, R. Keppens and S. Poedts, S*pace Sci. Rev.* 107, 63 (2003).

[32] I. Ballai, E. Forgacs and A. Marcu, *Astron. Nachr*. 328 (8), 734 (2007).

[34] M. Khan, S. Ghosh, S. Sarkar and M. R. Gupta, *Phys. Scr.* T116, 53 (2005).

[30] K. Itoh, K. Hallatschek and S-I Itoh, *Plasma Phys. Control. Fusion* 47, 451 (2005).

[39] E. Marsch, *Liv. Rev. Solar Phys.* 3, 1 (2006) (http: //livingreviews.org/lrsp-2006-1).

[44] S. C. Tripathy, F. Hill, K. Jain and J. W. Leibacher, *Astrophys. J.* 711, L84 (2010).

[41] A. Nordlund, R. F. Stein and M. Asplund, *Living Rev. Solar Phys.* 6, 2 (2009) (http:

[43] V. M. Nakariakov and E. Verwichte, *Living Rev. Solar Phys.* 2, 3 (2005) (http:

[29] S. P. Kuo and D. Bivolaru, *Phys. Plasmas* 8 (7), 3258 (2001).

[33] P. K. Shukla and A. A. Mamun, *New J. Phys.* 5, 17.1 (2003).

[38] J. Lemaire and V. Pierrard, *Astrophys. Space Sci.* 277, 169 (2001).

[40] U. Narain and P. Ulmschneider, *Space Sci. Rev.* 75, 453 (1996).

[35] M. S. Ruderman, *Phi. Trans. R. Soc. A*, 364, 485 (2006). [36] W. J. Chaplin, *Astron. Nachr*. 331 (9), 1090 (2010). [37] J. C. Dalsgaard, *Astron. Nachr*. 331 (9), 866 (2010).

//livingreviews.org/lrsp-2009-2).

//livingreviews.org/lrsp-2005-3).

[42] R. Gunn, *Phys. Rev.* 37, 983 (1931).

[14] C. B. Dwivedi, *Pramana-J. Phys*. 55, 843 (2000). [15] C. B. Dwivedi, *Phys. Plasmas* 6, 31 (1999).

[17] C. K. Goertz, *Rev. Geophys*. 27, 271 (1989). [18] F. Verheest, *Space Sci. Rev*. 77, 267 (1996).

*Rev. Lett*. 73, 652 (1994).

*Appl. Phys*. 73, 2160 (1993).

Leuven, Belgium) 537, 3 (2000). [26] P. K. Karmakar, *Pramana- J. Phys.* 68, 631 (2007). [27] P. K. Karmakar, *Pramana- J. Phys.* 76 (6), 945 (2011).

*Lett. A* 269, 363 (2000).

[31] I. Ballai, *PADEU* 15, 73 (2005).

Thompson (Wiley, New York, 1971) vol. 4.

**6** 

*Italy* 

**Acoustic Waves:** 

Marco G. Beghi

**A Probe for the Elastic Properties of Films** 

Films and thin films are exploited by an ever increasing number of technologies. The properties of films can be different from those of the same material in bulk form, and can depend on the preparation process, and on thickness. Specific techniques are needed for their measurement. Whenever films or thin layers have structural functions, as in micro electro-mechanical systems (MEMS), a precise characterization of their stiffness is crucial for the design of devices. The same can be said for the design of devices which exploit thin layers to support surface acoustic waves (surface acoustic wave filters). More generally, knowledge of the elastic properties is interesting because such properties depend on the

The most widespread technique for the mechanical characterization of films, instrumented indentation, induces both elastic and inelastic strains. It also characterizes irreversible deformation, but the extraction of the information concerning the elastic behaviour is non trivial. To overcome this difficulty, several measurement methods have been developed, which exploit vibrations as a probe of the material behaviour. These methods intrinsically involve only elastic strains, and are non destructive. This is true at any length scale, and is peculiarly useful at micrometric and sub-micrometric scales, where the exploitation of other

Both propagating waves and standing waves can be exploited, with excitation which can be either monochromatic (e.g. resonance techniques) or impulsive, therefore broadband, requiring an analysis of the response either in the time domain (the so called picoseconds ultrasonics) or in the frequency domain (the so called laser ultrasonics). The propagation velocities of vibrational modes are obtained, from which the stiffness is derived if an

Older resonance techniques have been developed to be operated with thin slabs, also exploiting optical measurements of displacement. In the measurement of films and small structures, the advantages of light, a contact-less and inertia-less probe, are substantial, and are increasingly exploited. The laser ultrasonics technique, commercially available since some years ago, measures waves travelling along the film surface. The so called picosecond ultrasonics technique measures waves travelling across the film thickness; it is a relatively sophisticated optical technique, which exploits femtosecond laser pulses in a pump-and-probe measurement scheme. For best performance it needs the deposition of

**1. Introduction** 

structural properties.

types of probes can become critical.

independent value of the mass density (the inertia) is available.

*Politecnico di Milano, Energy Department and NEMAS Center, Milano* 
