Surface Acoustic Waves

SAWs were first described in combination with earthquakes (4). Reduced to a significantly smaller nanoscale, they found their way into friendlier fields: SAW devices are widely used for radio frequency (RF) signal processing and filter applications and also play a large part in mobile communication. SAW devices have been around for years in communication circuitry—every cell phone has filters using the effect. An electrical signal fed into so-called transducers on the surface of a piezoelectric chip is converted into a deformation of the crystal underneath. Given the right frequency of the signal, a mechanical wave is launched across the chip. In Fig. 1, we sketch a snapshot of a SAW propagating on a solid. A SAW is characterized by subsequent regions of compressed and expanded material, as indicated in grayscale. All physical quantities related to the SAW-like material deformation, stress, and strain, (as well as the piezoelectric fields and potentials associated with the mechanical part of the wave) decay in an exponential manner with the substrate and vanish at approximately one wavelength depth. Typical wavelengths of technically exploited SAWs range from about X = 30|m at f ~ 100 MHz down to X = 0.5 |m at f ~ 6 GHz.

It is especially convenient to excite SAWs on piezoelectric substrates. Such materials undergo a defined deformation if they are subjected to an electric field, and, vice versa, they produce an electric field if they are deformed. The reason is the lack of inversion symmetry of the respective crystal structures.

Passage Pietons

Fig. 2. Detail of an interdigitated transducer (IDT) for exciting a surface acoustic wave on a piezoelectric substrate. In its simplest form, it consists of two comb-like metallic electrodes on top of the crystal. A high-frequency signal applied to the transducer is effectively converted into a surface wave once the resonance condition is met, as explained in the text.

Fig. 2. Detail of an interdigitated transducer (IDT) for exciting a surface acoustic wave on a piezoelectric substrate. In its simplest form, it consists of two comb-like metallic electrodes on top of the crystal. A high-frequency signal applied to the transducer is effectively converted into a surface wave once the resonance condition is met, as explained in the text.

The piezoelectric effect is well known from cigarette lighters, or, more high tech, from actuators in scanning probe microscopes. A SAW of a well-defined wavelength and frequency can be excited on such a piezoelectric substrate if a specially formed pair of metal electrodes is deposited on top of the substrate. Such electrodes are usually referred to as interdigitated transducers (IDTs), as shown in Fig. 2. A high-frequency signal applied to such an IDT is then converted into a periodic crystal deformation, and, if fed with the right frequency f = v/A, a monochromatic and coherent SAW is launched. Here, v denotes the sound velocity of the respective substrate, and the wavelength A is given by the lithographically defined periodicity of the IDT. If a second IDT is placed downstream from the substrate surface, a so-called delay line is formed. An electrical high-frequency signal fed into one of the transducers is converted into a surface sound wave, which travels along the substrate surface until it reaches the receiving transducer. There, it is reconverted into an electrical high-frequency signal. Both transducers, their design, and the substrate properties thus act as a high-frequency filter with a predetermined frequency response. They are light weight, relatively simple, and low cost and can be produced very reproducibly, which explains their massive use in high-frequency signal processing like mobile telephony.

In the recent past, however, SAWs have also been used in a completely different way than for filtering and signal processing by converting electrical signals into mechanical vibrations and vice versa. Excited on piezoelectric substrates, they are accompanied by large electric fields. These electric fields travel at the speed of sound of the substrate (~3000 m/s), having the same spatial and temporal periodicity as their mechanical companions. Charges at or close to the surface couple to these electric fields, and currents are induced withing a conducting layer.

Nearly 20 yr ago, we introduced SAWs to study the dynamic conductivity c(ro,k) of low-dimensional electron systems (LDES) in high magnetic fields and at low temperatures. It turns out that the interaction between a SAW and the mobile charges in a semiconductor is strongest for very low sheet conductivities, such as those observed, e.g., in the regime of the quantum Hall effect (5). However, SAWs can be used not only to probe the properties of quantum systems but also to deliberately alter some of them. SAWs represent a spatially modulated strain and stress field accompanied by strong electric fields in a solid, propagating at the speed of sound. Such an interaction between SAWs and the optical properties of a semiconductor quantum well led us to the discovery that photogenerated electron hole pairs in a semiconductor quantum well can be spatially separated under the influence of a SAW-mediated electric field. This, in turn, has an enormous impact on the photoluminescence (PL) of the semiconductor. We were able to show that the PL not only is quenched under the influence of a SAW, but also can be reestablished at will at a remote location on the sample and after a certain delay time (6). Further studies include the acoustic charge transport and the creation of dynamically induced electron wires (7), as well as the study of nonlinear acoustic interaction with low-dimensional electron systems in semiconductors (8).

However, the piezoelectric effect is usually only a small contribution to the elastic properties of a solid: most of the energy propagating in a SAW (usually more than 95%) is of a mechanical nature. Hence, not only electrical interactions as described above, but also mechanical interactions present possibilities for experimental investigations. Because they have wavelengths of a few microns and amplitudes of about only a nanometer, however, the forces and electric fields within the SAW "nanoquake" are sufficient to have a macroscopic effect. Any piece of matter at the surface along the way of a SAW experiences their vibrating force: viscous materials like liquids absorb a lot of their energy. It turns out that the interaction between a SAW and a liquid on top of the substrate surface induces an internal streaming, and, as we point out in Subheading 3.2. below, at large SAW amplitudes this can even lead to a movement of the liquid as a whole.

Fig. 3. Schematic representation of the diffraction of a surface acoustic wave on an elastic solid into a fluid layer on top of the substrate. As the sound velocities in both materials differ considerably, a wave is launched under an angle into the fluid.

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