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 force microscopy cantilever when the tip is in contact with a sample surface owing to the so-called (sample-induced) ultrasonic force. The procedure has been developed as a novel technique, ultrasonic force microscopy (UFM), that provides information about the nanoscale elastic and adhesive properties of 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 cantilever displacement (due to the aforementioned ultrasonic force), and does not monitor the linear high-frequency vibration of the cantilever. In this paper, we discuss the influence of a linear high-frequency cantilever response in the UFM measurements and provide experimental evidence of the feasibility 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.