By Elias Burstein, Claude Weisbuch

The optical houses of semiconductors have performed an immense position because the identity of semiconductors as "small" bandgap fabrics within the thinies, due either to their primary curiosity as a category of solids baving particular optical propenies and to their many very important functions. at the former point we will cite the elemental part absorption and its project to direct or oblique transitions, many-body results as printed via exciton formation and photoconductivity. at the latter point, large-scale functions sucb as LEDs and lasers, photovoltaic converters, photodetectors, electro-optics and non-linear optic units, are evoked. The eighties observed a revitalization of the complete box as a result of introduction of heterostructures of lower-dimensionality, in most cases two-dimensional quantum wells, which via their greater photon-matter interplay yielded new units with unsurpassed functionality. even supposing some of the uncomplicated phenomena have been evidenced throughout the seventies, it was once this impression on functions which in flip resulted in the sort of tremendous funding in fabrication instruments, because of which many new buildings and fabrics have been studied, yielding funher advances in basic physics.

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In bulk crystals, where the sample dimensions are much larger than the exciton Bohr radius a~, the natural unit for e-h pair density is the reciprocal exciton volume. 1017 cm -3 in a typical semiconductor like GaAs with a~ = 13 nm. Taking a radiative recombination time of 1 ns, and converting 32 it to a generation rate, we find that at least 2· 104 W/ cm 2 of cw pump power are needed to maintain such a density in a layer of I ţlm thickness, 1 Il " .. 1 ,~ I which is roughly the absorption length.

This effect is usually called excitation induced band gap shrinkage. 12]. d) Another effect is due to the build-up of phonon populations which are driven by the optically injected carriers and their dissipative energy relaxation through e-phonon coupling. The crystal lattice is heated on a 100 fs ... 100 ps time scale by the generation of nonequilibrium phonons, their subsequent anharmonic decay and final approach to equilibrium. 13a]. 00 = d cv . Eo is an appropriate measure for the fi. coupling strength between the driving electric field Eo and the two-level system.

E. 15]. Provided that the ion cores do not move far from their equilibrium positions their motion can be regarded as simpIy harmonic. 16], so that the problem reduces to a system of independent harmonic oscillators. For every normal mode of wave vector q there is an effective harmonic oscillator whose energy is quantized with integer multiples of liooq • The frequencies OOq as a function of q represent the dispersion relation of the lattice vibrations in harmonic approximation. The quanta liooq are the "phonons", their occupation number nq =O, 1,2, ...