Optical nonlinearity due to carrier separation in tensile strained InGaAs/InP quantum wells

We report on nonlinear optical absorption at the band edge of tensile strained In_1-xGa_xAs/InP MQW structures. While at low strain the well known exciton bleaching is observed, a new nonlinearity dominates at high tensile strain, where the conduction band discontinuity becomes very small. Calculations of the spatial band diagram with and without optical pumping show that charge-carrier-induced band bending and localization of the previously poorly confined n=1 electron wavefunction leads to enhanced absorption at the band edge. We find a good agreement between experimental results and theory.