Nanoscale Coherent Acoustic Phonon Imaging

An ultrafast optical pump and probe technique known as picosecond ultrasonics is used to generate and detect coherent acoustic phonon pulses in nanostructured films grown on Si wafers. By detecting the phonons after they have diffracted across a millimeter thick wafer, it is possible to measure the scattered phonons in the acoustic far field. Numerical backpropagation algorithms can then he used in order to reconstruct the object which scattered the acoustic phonon pulses. We describe measurements and simulations of experiments performed on surface and sub-surface nanostructures. Results with similar to 500 nm image resolution are shown, and plans for improving that resolution by an order of magnitude will be described.