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The exponential growth in the rate at which information can be communicated through an optical fibre is a key element in the 'information revolution'. However, as for all exponential growth laws, physical limits must be considered. The nonlinear nature of the propagation of light in optical fibre has made these limits difficult to elucidate. Here we use a key simplification to investigate the theoretical limits to the information capacity of an optical fibre arising from these nonlinearities. The success of our approach lies in relating the nonlinear channel to a linear channel with multiplicative noise, for which we are able to obtain analytical results. In fundamental distinction to linear channels with additive noise, the capacity of a nonlinear channel does not grow indefinitely with increasing signal power, but has a maximal value. The ideas presented here may have broader implications for other nonlinear information channels, such as those involved in sensory transduction in neurobiology. These have been often examined using additive noise linear channel models but, as we show here, nonlinearities can change the picture qualitatively.

We discuss the evolution of optical properties of semiconductor quantum wells, as the quasi-two-dimensional electronic states are further confined into quasi-zero dimensions by a perpendicular magnetic field. We show that confinement in all three directions strongly modifies both linear and nonlinear optical response. In particular, quasi-zero-dimensionality makes an ensemble of magneto-excitons a unique many-body system, distinct from higher-dimensional excitons and the one-component Coulomb system in the fractional quantum Hall regime or Wigner crystal.

Capacitance-voltage (CV) profiling measurements on delta-doped n-type GaAs reveal extremely narrow peaks with a full-width at half-maximum of 40 A. Comparison of experimental with self-consistently calculated CV profiles demonstrates that Si impurities are localized on a length scale of a lattice constant in delta-doped GaAs. Diffusion and segregation are of minor importance. The basic theory of CV measurements on quantummechanical systems such as delta-doped semiconductors is developed and presented.

Spatial localization and diffusion of atomic Si is studied inσ-doped GaAs by employing capacitance-voltage (CV) measurements and rapid thermal annealing. It is found that diffusion and segregation are irrelevant inσ-doped GaAs grown at temperatures below 550° C. Combination of rapid thermal annealing and capacitance-voltage profiling is a novel method which is most sensitive to diffusion and it is shown that this method is able to detect diffusion on a length scale of 10A. CV-profile widths broaden from⪯40A to 137A upon rapid thermal annealing at 1000° C for 5 sec. The diffusion coefficient and the activation energy of atomic Si-diffusion in GaAs are determined to beDo = 4 × 10−4 cm2/s andEa = 2.45 eV, respectively. The basic theory of CV-measurements on a quantum-mechanical electron system is developed.