Scanning near-field ultrasound holography

Non-invasive nanoscale characterization is becoming exceedingly important to study complex nanosystems, such as dynamic biological processes controlled by nanosized subcellular components and microelectronic devices with nano-sized components that control their operation. Scanning Probe Microscopy (SPM) is a technique that has been shown to have the capability to adapt to surface characterization of several material properties in a non-destructive manner. Scanning Near-Field Ultrasound Holography (SNFUH) is a Scanning Probe Microscopy (SPM) based technique that employs ultrasonic waves for characterization of elastic properties of the surface as well as subsurface materials in a studied system. The characterization method involves launch of ultrasonic plane waves through the bottom of a sample and ultrasonic excitation of the SPM cantilever probe tip with piezoelectric transducers. Phase signal of the difference in excitation frequency between the sample and tip is recorded as an image. The phase contrast in the image is contributed to by both the near-field ultrasonic wave scattering and surface tip-sample interactions. SNFUH has already been demonstrated to be able to characterize elastic properties of not only hard but also soft material systems. Development of this method by further understanding its operation principles and contrast mechanisms will help to integrate it as a mainstream nanoscale characterization method for any system with hard and soft materials.