Interband impact ionization and nonlinear absorption of terahertz radiation in semiconductor heterostructures.

We have theoretically investigated nonlinear free-carrier absorption of terahertz (THz) 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 the THz regime.