Nonlinear detection of ultrasonic vibration of AFM cantilevers in and out of contact with the sample

Ultrasonic vibration can be nonlinearly detected by means of an atomic forc e microscopy cantilever when the tip is in contact with a sample surface ow ing to the so-called (sample-induced) ultrasonic force. The procedure has b een developed as a novel technique, ultrasonic force microscopy (UFM), that provides information about the nanoscale elastic and adhesive properties o f surfaces. Here, we compare differences in the UFM signal when ultrasound is excited from the back of the sample (sample UFM) and from the cantilever base (waveguide UFM). UFM relies on the nonlinear ultrasound-induced canti lever displacement (due to the aforementioned ultrasonic force), and does n ot monitor the linear high-frequency vibration of the cantilever. In this p aper, we discuss the influence of a linear high-frequency cantilever respon se in the UFM measurements and provide experimental evidence of the feasibi lity of nonlinearly detecting the free ultrasonic cantilever vibration when the tip is out of contact with the sample surface using the typical laser- beam deflection method for monitoring cantilever displacements.