Observation of normal-incidence intersubband absorption in n-type ${\mathrm{Al}}_{0.09}$${\mathrm{Ga}}_{0.91}$Sb quantum wells

Normal-incidence intersubband absorption has been observed in n-type ${\mathrm{Al}}_{0.09}$${\mathrm{Ga}}_{0.91}$Sb quantum wells, as predicted by our previous calculation. The quantum wells were grown on (100)-GaSb and (100)-GaAs substrates and contain an electron sheet density of approximately 7×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The electrons occupy a two-dimensional L-point subband having ellipsoidal Fermi surfaces. For 7.0-nm-wide quantum wells, the absorption peak occurs at 870 ${\mathrm{cm}}^{\mathrm{-}1}$ (11.5 μm), the peak fractional absorption per quantum well is 0.06%, and the full width at half maximum of the absorption profile is 30 meV. The measured absorption strength is within 12% of the theoretical calculation, which was based on the effective-mass method.