Observation of intersubband absorption in the forbidden polarisation for a stepped double barrier quantum well

Intersubband absorption in the conduction band of a series of double barrier multiple quantum wells has been studied. Absorption in the forbidden polarisation was observed for a GaAs/In_x Ga_(1-x) As/AlAs/Al_y Ga_(1-y) As stepped well using 45° waveguide geometry. This is attributed to the relaxation of the polarisation selection rule. This corroborates the authors´ previous observation of enhanced absorption at the Brewster angle in these structures. In 45° waveguide geometry, it is possible to resolve Γ(well)-X(AlAs) transitions in some samples. The observed energies of both Γ-Γand Γ-X transitions are in good agreement with the proposed theoretical model using an X-band effective mass of 0.4m_o.