A thorough discussion of the various features of the photoluminescence spectra of undoped, p‐doped and n‐doped Al_xGa_1−xAs (0≤x≤1) alloys is given. This review covers spectral features in the energy region ranging from the energy band gap down to ≂0.8 eV, doping densities from isolated impurities to strongly interacting impurities (heavy‐doping effects) and lattice temperatures from 2 to 300 K. The relevance of photoluminescence as a simple but very powerful characterization technique is stressed also in comparison with other experimental methods. The most recent determinations of the Al concentration dependence of some physical properties of the alloy (energy gaps, carrier effective masses, dielectric constants, phonon energies, donor and acceptor binding energies, etc.) are given. The main physical mechanisms of the radiative recombination process in semiconductors are summarized with particular emphasis on the experimental data available for Al_xGa_1−xAs. The effects of the nature of the band gap (direct or indirect) on the features of the photoluminescence spectra are discussed in detail. Particular attention is devoted to the consequences of the band structure of Al_xGa_1−xAs (both the multivalley conduction band or the degenerate valence band) on the impurity states by summarizing the theoretical predictions and by detailing the behavior of a number of shallow impurities. Heavy doping effects are also analyzed. A systematic presentation of the photoluminescence related to deep defects and impurities (vacancies, antisites, DX centers, Si‐Si self‐compensating pairs, transition metals, and rare‐earth ions) is carried out after a brief introduction to the terminology used to describe the deep states in semiconductors.

The propagation of light in nonperiodic quasicrystals is studied by ultrashort pulse interferometry. Samples consist of multilayer dielectric structures of the Fibonacci type and are realized from porous silicon. We observe mode beating and strong pulse stretching in the light transport through these systems, and a strongly suppressed group velocity for frequencies close to a Fibonacci band gap. A theoretical description based on transfer matrix theory allows us to interpret the results in terms of Fibonacci band-edge resonances.

We report on the observation of Bloch oscillations in light transport through periodic dielectric systems. By introducing a linear refractive index gradient along the propagation direction the optical equivalent of a Wannier-Stark ladder was obtained. Bloch oscillations were observed as time-resolved oscillations in transmission, in direct analogy to electronic Bloch oscillations in conducting crystals where the Wannier-Stark ladder is obtained via an external electric field. The observed oscillatory behavior is in excellent agreement with transfer matrix calculations.