Intersubband optical absorption in heavily doped n-type GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple quantum wells

Intersubband optical absorption between the ground and first excited states and the first and second excited states is reported for n-type doped GaAs/${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As multiple quantum wells. This phenomenon is shown to be due to high doping, which causes more than one subband to be populated. Experimental results supported by theoretical calculations are presented.