Scanning Near Field Ultrasound Holography (SNFUH) combines the non-destructive nature of acoustic waves for depth information, high lateral spatial resolution of near-field scanning probe microscopy (SPM) platform, and a holography paradigm for phase sensitive detection. It relies on spatial and phase monitoring of the scattered specimen ultrasound wave, reflected in perturbation to the surface acoustic wave. It measures the phase variation across the surface acoustic wave pattern that has been modified as ultrasound passes through the features buried into specimen, while another highfrequency ultrasound wave the sample.

As depicted in the figure, a high frequency acoustic wave (of the order of MHz or higher, and significantly more than the resonance frequency f0 of the typical cantilever, f0 ~ 10-100 KHz) is launched from the bottom of the sample, while another high frequency acoustic wave is launched on the AFM cantilever, albeit at a slightly different frequency. The interference of these two waves at the surface of the sample forms a surface acoustic wave. The variations of the phase and amplitude of the surface acoustic wave are locally monitored by the AFM tip acting as an acoustic antenna. When the sample acoustic wave is modified by scattering from buried features (which have different acoustic impedance than the bulk of the sample), the resultant alteration in the surface acoustic wave, especially its phase, can be experimentally extracted from the tip deflection of the AFM, via the conventional lock-in detection method. Thus, within the near-field regime (which enjoys superb spatial resolution, beyond the diffraction limit), the surface acoustic wave (which is essentially a map, on the surface of the sample, of its internal mechanical and elastic variations) is fully analyzed, point-by-point, by the AFM acoustic antenna, in terms of its phase and amplitude. As the AFM tip scans across the specimen, a pictorial representation is fully recorded and displayed, offering a “quantitative” account of the internal microstructure of the specimen.