NEWS: contrast is based on friction at material boundaries, see paper!!!
The nondestructive imaging of subsurface structures on the nanometer scale has been a longstanding desire in both science and industry. A few impressive images were published so far that demonstrate the general feasibility by combining ultrasound with an Atomic Force Microscope.
From different excitation schemes, Heterodyne Force Microscopy seems to be the most promising candidate delivering the highest contrast and resolution. However, the physical contrast mechanism is unknown, thereby preventing any quantitative analysis of samples.
Recently, we showed that friction at material boundaries within the sample is responsible for the contrast formation!
This result is obtained by performing a full quantitative analysis, in which we compare our experimentally observed contrasts with simulations and calculations. Surprisingly, we can rule out all other generally believed responsible mechanisms, like Rayleigh scattering, sample (visco)elasticity, damping of the ultrasonic tip motion, and ltrasound attenuation. Our analytical description paves the way for quantitative SubSurface-AFM (3D) imaging.

Publications (click the link):
8) Friction as Contrast Mechanism in Heterodyne Force Microscopy
7) Resonance Frequencies of AFM cantilevers in Contact with a Surface
6) A Subsurface Add-On for standard Atomic Force Microscopes
5) Beating beats Mixing in Heterodyne Detection Schemes
4) Heterodyne Force Microscopy: Nondestructive SubSurface Characterization on the Nanoscale
4) Cantilever dynamics in heterodyne force microscopy
3) Subsurface Atomic Force Microscopy: Towards a Quantitative Understanding
2) Subsurface-AFM: Sensitivity to the Heterodyne Signal
1) Cantilever Dynamics in Heterodyne Force Microscopy