  We have theoretically investigated nonlinear free-carrier absorption of
terahertz radiation in InAs/AlSb heterojunctions. By considering multiple
photon process and conduction-valence interband impact ionization (II), we have
determined the field and frequency dependent absorption rate. It is shown that
(i) electron-disorder scatterings are important at low to intermediate field,
and (ii) most importantly, the high field absorption is dominated by II
processes. Our theory can satisfactorily explain a long standing experimental
result on the nonlinear absorption in THz regime.
