TE- and TM-polarized roughness-assisted free-carrier absorption in quantum wells at midinfrared and terahertz wavelengths

A free-carrier absorption mechanism, in which a photon transition is accompanied by an elastic interface-roughness scattering event, is considered for a few representative quantum well structures. Interface-roughness scattering determines the low-temperature mobility in undoped narrow quantum wells such as those used in infrared lasers. It is found that in spite of the contribution due to scattering to higher subbands, the absorption coefficient for TE-polarized light remains appreciably less than the semiclassical value at shorter wavelengths. The implications of these results for the recently proposed interband far-infrared and terahertz lasers based on “W” antimonide structures are discussed. In the case of roughness-assisted absorption of TM-polarized light, transitions forbidden by symmetry and exclusion arguments become allowed. However, the largest second-order contribution relative to first-order intersubband absorption tends to occur at wavelengths where the total absorption coefficient is rather small.