Study of elementary surface acoustic wave phenomena

– Many attempts have been made in acoustic microscopy to both achieve nanometer lateral resolution and sub-Å wave amplitude detection. Employing a scanning acoustic force microscopy technique, acoustic wave properties of arbitrarily polarized modes can be measured with sub-wavelength resolution and high sensitivity. Surface acoustic wave fields of elementary model systems like a single scatterer and a single wave source are analysed in detail. We are able to observe radiation patterns, revealing the influence of the anisotropy of the GaAs substrate and the angular distribution of the piezoelectric coupling coefficient. Surface acoustic waves (SAWs) [1] have long been studied and appear in our everyday life in areas ranging from earthquake waves [2] to high-frequency filters [3] with wavelengths covering a range from kilometres to submicrons. A broad area of technological application relies also on the high degree of localization in the vicinity of the wave-carrying surface. Being sensitive to surface layers makes them especially suitable for materials science related measurements of thin films [4,5], or gas and liquid phase sensor applications [6]. Typical tools for analysing these waves are optical methods [7], electron probes [8], X-rays [9], and mechanical contact transducers [10]. However, all these methods lack lateral resolution or sensitivity. Through the use of a sub-wavelength probe or source (or both), acoustic wave fields can be mapped with a spatial resolution that is solely determined by the size of the probe. Applying scanning probes to the detection of ultrasound enables us, in principle, to detect acoustic waves with nanometer resolution. A variety of methods have been developed [11–13]. The obstacle for the detection of ultrasonic (10 MHz–10 GHz) surface phenomena with scanning force microscopy is given by the mechanical frequency limitations of the cantilever. One way of going beyond the frequency range of typically 100 kHz is the excitation of higher harmonics of the cantilever oscillation [14]. To explore the frequency range beyond some 100 MHz, however, different approaches are required. Utilizing the non-linearity of the force-distance curve, or a non-linear interaction in general, opens up new ways of accessing fast phenomena on the nanoscale [15,16]. Analogous to a crystal radio detector diode which demodulates amplitude-modulated rf signals and delivers the modulation signal in the audible frequency range, the non-linear force-distance (∗) E-mail: [email protected].