Detection of ultrasound using a tunneling microscope

We have developed a scanning tunneling microscope capable of detecting high frequency (1.8-12.5 MHz) acoustic waves. Acoustic pulses arriving at a conducting surface are detected as a high frequency perturbation of the tunneling current. Near and below the rolloff frequency of the current to voltage amplifier, the instantaneous displacement of the surface can be reconstructed. For higher frequencies, the nonlinear (rectifying) properties of the tunneling behavior allow the detection of the envelope (and hence the time of arrival and total amplitude) of the acoustic burst. The technique's sensitivity is comparable to that of optical detection schemes but offers much better lateral resolution.