Acoustic images at the nanometer scale—Why and how?

For years, researchers have attempted super-resolution acoustical images. Using a subwavelength detector one can map acoustical fields with a spatial resolution determined by the size of the detector. A breakthrough approach is to use an atomic force microscope (AFM) in order to detect ultrasound. The lateral resolutions that have been achieved by various groups are better than 50 nm. Different methods are in use to generate and detect the signal. Three main setups have been developed: (i) Contact-mode medium-frequency (typically 500 kHz) low-amplitude methods (SLAM), (ii) ‘‘mechanical diode’’ high-frequency (typically 5 MHz) high-amplitude pulsed setups (UFM), and (iii) contact-mode high-frequency (typically 5–400 MHz) methods using mixing of two ultrasonic waves through the nonlinearity of the tip–sample interaction. All three systems map different properties of the sample and have advantages and disadvantages. An overview of these methods will be illustrated by images and measurements.