Gain and absorption: Many-body effects

For the theoretical analysis and modeling of semiconductor lasers, it is desirable to have access to reliable absorption/gain and refractive index calculations. Whereas some generic device features may be understood on the basis of rate equations with empirical gain coefficients or using free-carrier theory, most quantitative predictions require realistic gain models that are based on microscopic calculations.Many applications critically depend on fine details of the optical material properties. For example, in most vertical-cavity surface-emitting lasers (VCSELs), the Bragg-mirrors select a well-defined operating wavelength. For optimal use of the laser medium, this cavity wavelength should be at the position of the gain maximum, which shifts as a function of carrier density and temperature. Other examples are devices that are designed to be polarization insensitive. Here, different regions of the device are usually made of different materials. Part of them influence predominantly transverse electric (TE) polarized light-i.e. have stronger TE gain or absorption-others influence transverse magnetic (TM) polarized light stronger. For true polarization insensitivity, the TE and TM gains or absorptions have to be as similar as possible, which can only be achieved if the material composition is chosen properly. Furthermore, if one wants to use an optical amplifier to amplify several wavelengths at the same time, one would like the gain to be almost wavelength insensitive over a given spectral region. Thus, one has to know the spectral lineshape of the gain very precisely. Generally, not only for the design and optimization of such advanced devices, but also for more conventional structures, the optical material properties have to be known very precisely if one wants to avoid the expensive and time-consuming process of fabricational trial and error. In this chapter, we review our approach to compute optical material properties of semiconductor-based optoelectronic devices. The theory is based on a fully microscopic model in which scattering and dephasing processes are calculated explicitly using generalized quantum Boltzmann equations. As input to the calculations, we need material parameters such as the Luttinger parameters, bulk band gaps, bulk dipole matrix elements, band offsets for heterostructure interfaces, and bulk material constants like the background refractive index or phonon energies. As illustrative examples, we show some theory-experiment comparisons and demonstrate that the calculations reliably predict the optical properties of good quality systems.