The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.

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Using a model structure for a two-dimensional ͑2D͒ biexciton confined in a quantum well, it is shown that the form of the Hamiltonian of the 2D biexciton reduces into that of an exciton. The binding energies and Bohr radii of a 2D biexciton in its various internal energy states are derived analytically using the fractional dimension approach. The ratio of the binding energy of a 2D biexciton to that of a 2D exciton is found to be 0.228, which agrees very well with the recent experimental value. The results of our approach are compared with those of earlier theories. ͓S0163-1829͑96͒02623-9͔

We investigate the dynamics of the organic laser guest-host composite of tris-(8-hydroxy quinoline) aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) embedded in a high-Q (Q ≈ 4500) double distributed Bragg reflector microcavity (2DBR) using sub-picosecond upconversion techniques. Lasing is observed at a threshold of 0.4 nJ/20 µJcm −2 with a line-width of 0.05 nm (resolution limit). We observe a strongly nonlinear intensity-dependent delay of the emitted radiation burst. All experimental results are successfully modeled by a set of nonlinear rate equations, emphasizing the importance of a feedback mechanism for lasing.

We demonstrate high-efficiency top emitting organic light-emitting diodes employing silver (Ag) for both anode and cathode. Following the p-i-n doping and double emission layer concepts, the devices show a very high efficiency of 50cd∕A at 1000cd∕m2 with a driving voltage of only 2.85V. The efficiency can be further improved to 78cd∕A by tuning the optical structure with an organic capping layer. A simple explanation based on the transmittance of the top contact cannot explain this efficiency enhancement. Instead, we theoretically show that this capping effect is dependent on the overall optical structure of the device.