Surface acoustic waves are used to actuate and process smallest amounts of fluids on the planar surface of a piezoelectric chip. Chemical modification of the chip surface is employed to create virtual wells and tubes to confine the liquids. Lithographically modulated wetting properties of the surface define a fluidic network, in analogy to the wiring of an electronic circuit. Acoustic radiation pressure exerted by the surface wave leads to internal streaming in the fluid and eventually to actuation of small droplets along predetermined trajectories. This way, in analogy to microelectronic circuitry, programmable biochips for a variety of assays on a chip have been realized.

We have developed a microfluidic device operating at a planar surface instead of a closed channel network. The fluid is transported in single droplets using surface acoustic waves (SAW) on a piezoelectric LiNbO(3) substrate. The surface of the piezo is chemically structured to induce high contact angles of the droplets or enclose areas where the liquid can wet the substrate. Combining the SAW technique with thin film resistance heaters, a biological analysis chip with integrated DNA amplification by PCR and hybridization was designed. To prevent evaporation of the PCR reagents at high temperatures the sample is enclosed in droplets of mineral oil. On this chip the SAW resolves dried primers, shifts the oil capped liquid between the two heaters and mixes during hybridization. The chip is able to perform a highly sensitive, fast and specific PCR with a volume as low as 200 nl. During the temperature cycles an online monitoring of the DNA concentration is feasible with an optical unit, providing a sensitivity of 0.1 ng. After PCR the product is moved to the second heater for the hybridization on a spotted DNA array. With our chip we were able to detect a single nucleotide polymorphism (SNP) responsible for the Leiden Factor V syndrome from human blood.

The miniaturization of chemical and biological processes has made enormous progress driven mainly by genomics and proteomics. Microfluidics is the core technology to realize miniaturized laboratories with feature sizes on a submillimeter scale. Here, we report on a novel microfluidic technology which allows biochips to be programmed so that different biological assays can be performed with only one chip layout. Interdigital transducers integrated on piezoelectric substrates excite surface acoustic waves (SAW) which drive reagents on the surface of the biochip. The reagents can be placed on any desired spot on the chip's surface, they can be merged, split and brought to reaction. SAW technology can also be used to efficiently agitate small volumes of liquids accelerating diffusion limited reactions considerably.

The miniaturization and integration of electronic circuitry has not only made the enormous increase in performance of semiconductor devices possible but also spawned a myriad of new products and applications ranging from a cellular phone to a personal computer. Similarly, the miniaturization and integration of chemical and biological processes will revolutionize life sciences. Drug design and diagnostics in the genomic era require reliable and cost effective high throughput technologies which can be integrated and allow for a massive parallelization. Microfluidics is the core technology to realize such miniaturized laboratories with feature sizes on a submillimeter scale. Here, we report on a novel microfluidic technology meeting the basic requirements for a microfluidic processor analogous to those of its electronic counterpart: Cost effective production, modular design, high speed, scalability and programmability.

A novel technology incorporates pumps and valves without moving parts on a chip surface. Using these building blocks that can be combined in a plug-and-play manner, biochemical reactions on the chip are controlled electronically.

The influence of conduction band non-parabolicity on the cyclotron resonance of a two-dimensional electron system in InAs quantum wells is investigated. We demonstrate that the experimentally determined dependence of the cyclotron mass on the carrier density in the well can be excellently described using a two-band k . p model. In contrast to previously studied systems our experimental results allow us to deduce quantitatively the quantization energy of the first electrical subband for wells of different width.

Collective intersubband resonances in InAs/AlSb single quantum wells are studied in terms of their dependence on the well width and the electron density. The transitions are found to lie within the atmospheric windows of 8–12 μm and 3–5 μm, respectively.

Electron cyclotron resonance is studied in very deep quantum wells consisting of InAs between Also barriers. High electron mobility and strong conduction band non-parabolicity together with the small effective mass and the large effective g factor of this material enable us to observe simultaneous cyclotron transitions between adjacent sets of spin split Landau states. Our experiments resolve spin-conserving transitions involving two or three different Landau levels depending on the filling factor. The results are compatible with a single-particle model.

We study cyclotron resonance (CR) in a two-dimensional electron system on InAs/AlSb-quantum wells observing discrete transitions between adjacent sets of spin-split Landau states. While in three-dimensional electron systems such splittings are well established, no spin-split CR has been observed in a two-dimensional electron system, to date. Theoretical attempts link this fact to the reduced dimensionality and strong electron-electron coupling. Our results, however, can be interpreted quantitatively in a straightforward single-particle model offering new clues to this major puzzle.

Measuring photoconductivity at low temperatures, we investigate the recently observed bipolar behaviour of the persistent photoeffect in InAs/AlSb quantum wells. Depending upon the incident wavelength we observe either a persistent increase or a persistent decrease of the carrier density in the well. We discuss our experimental findings in terms of a simple model based on the band structure as known to date and the growth parameters of the heterostructure.

The electronic and magneto-optical properties of the interesting system InAs/AlSb are investigated at low temperatures and in high magnetic fields. The system yields very deep quantum wells with type II staggered bandstructure and can have very high electron concentrations with high mobilities. Far-infrared spectroscopy reveals very pronounced oscillations in the linewidth of the cyclotron resonance absorption line. In the regions of the reststrahlenbands of the system we observe strong interaction of the cyclotron resonance with optical phonons. The number of electrons in this system can be tuned via persistent photoeffect which unexpectedly can lead to both an increase as well as a decrease of the carrier density depending on the wavelength of the illuminating light.

The interaction between surface acoustic waves and quasi-two-dimensional inversion electron systems on GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterojunctions is investigated in high magnetic fields and at low temperatures. The interaction of the surface acoustic wave with high-mobility inversion electrons leads to strong quantum oscillations in both the transmitted surface wave intensity as well as in the sound velocity, reflecting the quantum oscillations of the magnetoconductivity as a function of an applied magnetic field. We study the dependence of this interaction on the magnetic field and on the surface-acoustic-wave power and frequency, and discuss the results using simple models. The influence of slight spatial inhomogeneities in the carrier density on the line shape of the quantum oscillations is analyzed in detail and related to their influence on the quantum Hall effect. First experimental results on the interaction of surface acoustic waves with two-dimensional electron systems in gated heterojunctions providing an adjustable carrier density are presented.

We describe a simple, contactless method to study the interaction of surface acoustic waves (SAW) with a two-dimensional electron system (2DES) in GaAs/AlGaAs heterostructures at low temperatures and in high magnetic fields. The heterostructure is part of a sandwich structure on a Y-cut Z-propagating LiNbO3-SAW-delay line. The interaction of the SAW with the 2DES leads to quantum oscillations of the SAW amplitude as a function of the applied magnetic field which can be used to characterize the sample and to investigate the transport properties of the 2DES.