Fig. 5

Optical microscope images of removed gold at different transducer-sample distances for the three transducers. Colours represent the true appearance of the sample: the intact gold layer appears gold and gold-depleted regions appear silver due to exposed nickel. The focus of each transducer, defined as the position at which the recorded echo is maximized, is indicated with red borders. No values are given for the defocus distances due to differing step sizes between frequencies (200 μm for 4.2 MHz and 7.3 MHz, and 100 μm for 11.8 MHz). Removal increases with transducer-sample distance up to a maximum (indicated with blue borders), after which removal tapers off. For all frequencies maximal removal is observed slightly beyond the focus (Fig. 6).