Local acoustic probing using mechanical and ultrafast optical techniques

Any set-up with local excitation or detection of acoustic waves at a solid surface provides the basis for acoustic microscopy and the mapping of sub-surface elastic properties. One method of improving the spatial resolution, independent of the acoustic frequency, is to mechanically localise the acoustic excitation or detection region by means of a fine tip, as small as atomic in size, held or vibrated very close to or in contact with a sample. Another approach is to improve the temporal resolution through local excitation with picosecond light pulses. Although limited at present by diffraction to micron-order lateral spatial resolution, the optical excitation of acoustic waves in the 100-GHz region allows resolution in the depth direction of atomic-scale elastic inhomogeneities. In this paper I describe and compare two complementary techniques for local acoustic probing with micron-order lateral resolution: mechanical probing using a microscopic tip vibrating at few kilohertz and ultrafast probing using focused optical excitation and detection of ultrasonic waves. I conclude by discussing the possibility for atomic-scale acoustic probing on ultrashort time scales.