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/Inx Ga(1-x) As/AlAs/Aly 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 0.4mo.