Nanoscale imaging with picosecond ultrasonic computed tomography

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The chosen sample for GHz CT testing is a glass fibre containing two holes along the main axis of its cylinder. Our approach is to excite acoustic waves from one side of the sample and to rotate the probe beam on the opposite side to evaluate the propagated acoustic field for various angles (with a fixed pump excitation spot). Then the fibre is rotated along its axis and the operation is repeated.We realised 3D time-domain acoustic simulations using PZFLex software in a similar geometry. The simulated silica fibre is 5 μm diameter. The two axial air holes of diameter around 1 μm are considered to be vacuum so that no acoustic wave can propagate. This allowed us to demonstrate the detection of both holes with their correct position and also get an estimate of the hole size and shape.The experimental setup was developed to match the particular needs of controlling probe and sample orientations. The setup has been validated on flat glass, and experiments are still ongoing in order to detect the acoustic signal and experimentally demonstrate acoustic computed tomography down to sub-micrometre scale.