4. Focusing systems

4.1 Focusing lens

The basis of the acoustic microscope remains the acoustic lens, which must be carefully cut to ensure the best possible image quality. A piezoelectric transducer provides electromechanical signal conversion, using a thin layer of zinc oxide (ZnO), a few µm thick, with a perfectly controlled crystallographic orientation, deposited on one of the flat, polished faces of a small, metallized synthetic sapphire rod. The power efficiency of such a transducer can reach 50% for frequencies in the gigahertz range.

The high attenuation in the transmission fluid means that lens curvature radii of the order of a tenth of a millimetre are required. The manufacture of spherical surfaces of this size, polished to an optical quality, is a tricky business,...