It has been stressed throughout the previous sections that metamaterials usually work for a certain specific single frequency (or a very narrowband frequency range) or for specific discrete set of frequencies. This is mainly due to their geometrical arrangement from which their extraordinary properties arise. Being able to realize them in such a way that they can be exploited along a broad-frequency range would allow them to be used on real applications, especially in the ultrasonic region for ultrasonic non-destructive testing applications and medical imaging. Moreover, since chirps and coded signals provide the well-described properties and gains described deeply in Chapter 2, they would be successfully used in combination with such broadband devices. As a consequence, metamaterials energy losses can be counterbalanced by using such coded excitation. Moreover, phase-time information can be restored after acoustic wave travel into the metamaterial by using such coded excitations.
3.6 Conclusions
In this chapter, a literature review which considers electromagnetic metamaterials, phononic crystals and acoustic metamaterials has been given. Moreover, a deep mathematical description of metamaterials working principles, as well as the way to overcome the diffraction limit has been given. In particular, since this thesis research focuses
its effort on a specific class of acoustic metamaterials, i.e. holey-structured acoustic metamaterial, their peculiar working principles and main properties have been described in detail. The next chapter will describe a Finite Element Analysis that has been carried out to validate the exotic features promised by the use of such acoustic metamaterials for sub- wavelength imaging purposes.
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