Valence-band mixing effects on the gain and the refractive index change of quantum-well lasers

The effects of valence-band mixing on the gain and on the refractive index change of the quantum-well laser and the effect of an applied electric field perpendicular to the quantum wells for gain switching are studied theoretically. Our calculations are based on the multiband effective-mass theory (k⋅p method) and the density-matrix formalism with the intraband relaxation taken into account. First, we calculate the nonparabolic valence-band structure by the finite difference method after making a unitary transformation of the Luttinger-Kohn Hamiltonian. The calculated gain for our model shows remarkable differences in both spectral shape and peak amplitude as compared with those for the conventional model of the parabolic valence band. The peak gain is reduced considerably and the gain spectrum is more symmetric in our model compared with that for the conventional model. The refractive index change shows a negative increment in the active region for both the TE and TM polarizations resulting in the antiguidance. However, the TM polarization shows more negative change which would result in the suppression of the TM modes as compared with the TE modes. As another example, the perpendicular electric field effects on the quantum-well laser with lateral current injection are also calculated. A red shift and a reduction of the gain spectrum are shown when an electric field is applied. This may have applications as a tunable and high-speed switching quantum-well laser.