Polarization of gain and symmetry breaking by interband coupling in quantum well lasers

We have studied the influence of conduction band-valence band coupling on the polarization of gain in quantum well (QW) lasers. As a reference we used the eight-band k ∙ p description of the gain polarization. Our eight-band k ∙ p model accounts for the crystal orientation, lack of inversion symmetry, strain induced deformation potentials, and piezoelectricity. We have studied both strained and unstrained (001) and (111) QWs. The results are compared with the transition dipole model of the gain polarization [ M. Asada , IEEE J. Quantum Electron. 20 , 745 ( 1984 ) ], which is based on a phenomenological generalization of Kane's [ J. Phys. Chem. Solids 1 , 249 ( 1957 ) ] linear k ∙ p model of bulk crystals. We found a quantitative difference between our multiband model and the transition dipole model of Asada The difference is addressed to lack of orthogonality between the transition dipole and the electron wave vectors. The orthogonality is broken outside the Γ point by both the QW heterostructure geometry and the interband coupling. Results obtained by the complete eight-band model are also compared with restricted multiband models excluding the conduction band.