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 present both theoretical and experimental results on the intersubband resonance in InAs/AlSb quantum wells. From a Kane (k•p) description of the band structure we investigate the effect of the large nonparabolicity and of the high Fermi wave vector on the selection rules and matrix elements. The 1-2 transition in parallel excitation (x) is shown to be very weak from simple parity arguments; in perpendicular excitation (z) the matrix element ͗z͘ is shown to be largely unaffected by nonparabolicity. The 1-3 transition turns out to be very weak in both geometries. Two band-gap engineering approaches to enhancing the parallel excitation of 1-2 are considered but the effect remains small as compared to the conventional z excitation. In z excitation the depolarization field condenses all the oscillator strength into essentially one sharp line despite the broadening expected from the nonparabolicity in the band dispersions. Inclusion of the depolarization field in the theory gives us good agreement with both the experimentally determined line shape and ͗z͘ matrix element.

The Influence of conducllcn band non-parabcliclty on tho cyclotron resonance of a two-d mensional o.octrcn system In lnAs quantum well6 16 Investlgatod. WQ dQmOnSlratQ that the OXperlmOntally dQtQrmlnGd dopondonco of the cyclotron mess on the carrler oonslty In tho well can OB exco lenlly described us.ng a two.band k p model. In contrasl to previously stuolcd systems o L r exporlmental rasdts allow us to doouce qLantltatlvely the quantization energy of tne first eloctrlca. subcand for wells of d.ffQrent wldln.

Intersubbandoptical resonances in InAs/AlSb quantum wells have been studied in the Voigt configuration.Both spin-conserving and spin-Hip transitions in the midinfrared spectral range were observed simultaneously.We demonstrate that the in-plane magnetic field mediates the spin-conserving transitions, whereas the bulk inversion asymmetry of InAs induces spin-Hip excitations. The difference in the observed resonance energies allows us to deduce directly the depolarization shift. Moreover, we show that this depolarization strongly reduces the linewidth of the spin-conserving resonance.

Measuring photoconductivity at low temperatures, we investigate the recently observed bipolar behaviour of the pemistent photoeffect in InAs/AISb 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.