Grazing-angle intersubband absorption inn-doped GaAs multiple quantum wells

We present a study of the mid-infrared intersubband absorption in n-doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ multiple quantum wells (MQWs) for various internal incident angles. Above 45° incidence we show that the traveling-wave approximation is not valid: redshifts of the absorption peak are observed, strikingly additional peaks appear for thick structures, and the strength of absorption does not necessarily increase with the incident angle. The understanding of such complex absorption behavior requires considering the quantum well as an absorbing uniaxial material and taking into account the multiple reflections of the light inside the MQW.