The strong piezoelectric fields accompanying a surface acoustic wave on a semiconductor quantum well structure are employed to dissociate optically generated excitons and efficiently trap the created electron hole pairs in the moving lateral potential superlattice of the sound wave. The resulting spatial separation of the photogenerated ambipolar charges leads to an increase of the radiative lifetime by orders of magnitude as compared to the unperturbed excitons. External and deliberate screening of the lateral piezoelectric fields triggers radiative recombination after very long storage times at a remote location on the sample. [S0031-9007(97)03194-3] PACS numbers: 73.50. Rb, 77.65.Dq, 78.20.Hp, 78.55.Cr The dynamics of photogenerated carriers in semiconductor structures with reduced dimensionality has been the subject of intensive investigations in recent years [1,2]. State-of-the-art band-gap engineering technologies enable us to tailor low-dimensional semiconductor systems with desirable optoelectronic properties and study the fundamental aspects of carrier dynamics. This has increased tremendously our fundamental understanding of the dynamic properties of artificial semiconductor structures and has also resulted in a wide range of novel devices such as quantum well lasers, modulators, and detectors, as well as all-optical switches. Nevertheless, the bulk band structure of semiconductors seems to dominate optoelectronic properties since the strength of interband transitions is largely governed by the atomiclike Bloch parts of the wave function [3]. Thus it appears at first glance unavoidable that strong interband optical transitions are linked to direct band-gap semiconductors with short radiative lifetimes such as GaAs, whereas long radiative lifetimes of photogenerated carriers imply utilization of semiconductors with indirect band gaps such as Si and correspondingly reduced interband absorption. Initial attempts to employ band-gap engineering in order to combine strong interband absorption with long radiative lifetimes have focused on so-called doping superlattices [4]. There, alternate n and p doping along the growth direction is utilized to combine a direct gap in momentum space with an indirect gap in real space which causes a spatial separation of photogenerated electron-hole ͑e-h͒ pairs and hence considerably prolonged lifetimes.Here, we introduce a new way of band-gap engineering in which we expose a semiconductor quantum well of a direct gap material to a moving potential superlattice modulated in the plane of the well. We show that the confinement of photogenerated e-h pairs to two dimensions, together with the moving lateral superlattice, allows reversible charge separation [5]. We demonstrate that the combination of both the advantages of strong interband absorption and extremely long lifetimes of the optical excitations is achieved without affecting the superior optical quality of the quantum well material.The spatial separation of the electron-hole pairs is achieved via the piezoelectric pot...

Aligned liquid crystalline nanowires within ordered porous alumina templates show a pronounced texture on a macroscopic scale. We have investigated the influence of the geometric confinement and the nature of the pore walls on the mesophase formation by means of X-ray diffraction. The apparent texture is the result of a complex interplay of the pore geometry, interfacial phenomena, and the thermal history. Pores with a diameter of a few hundred nm guide the mesophase formation more efficiently than those with a diameter below 100 nm.

The present paper explores the possibility of orienting columnar discotic liquid-crystalline (LC) materials and causing a uniaxial "in-plane" orientation of the columns along the orientation direction. The discotic LCs employed were based on 2-hydroxy-3,6,7,10,11-penta-(1-butoxy)triphenylene, which was derivatized with 1-adamantenecarboxylic acid (Ada-PBT) and 10-undecenoic acid (ω-undecenoyl-PBT), respectively. As unequivocally shown by optical microscopy, uniaxial alignment of columnar Ada-PBT structures was readily achieved by spin-coating this material onto a poly(tetrafluoroethylene) orientation layer. Annealing of the films at temperatures at which the Ada-PBT was in its hexagonal plastic or hexagonal ordered mesophase slightly increased the degree of orientation. The optical absorption and photoluminescence spectra of oriented Ada-PBT films were significantly polarized, with absorption and emission dichroic ratios of 6.5 and 3, respectively. The optical anisotropy is unambiguously related to the uniaxial "in-plane" orientation of the cylindrical stacks of the LC molecules rather than an alignment of individual Ada-PBT molecules. Interestingly, this orientational behavior could not be mirrored when attempting to orient blend films of ultrahigh molecular weight polyethylene (UHMW-PE) and Ada-PBT or ω-undecenoyl-PBT by tensile deformation.

We demonstrate the localisation of quantum well excitons in a periodic array of linear traps using photoluminescence experiments. The excitonic traps are induced by applying spatially alternating external voltages via interdigitated metal gates. The localisation originates from the periodical modulation of the strength of the quantum-confined Stark effect in the plane of the quantum well. In our experiments, the trap depth is easily tuned by the voltages applied to the interdigitated gates. Furthermore, we find that a perpendicular magnetic field reduces the exciton diffusion length. In short-period lateral superlattices, we observe a magnetic-field-induced stabilisation of excitons in the presence of strong in-plane electric fields.Comment: including 11 figure