Interaction of Wannier-Stark ladders and electrical breakdown in superlattices

We study optical transitions of strongly coupled GaAs/(Ga,Al)As superlattic es in an electric field. With progressing held strength we observe (i) the interference of Wannier-Stark and Franz-Keldysh oscillations; (ii) Wannier- Stark ladders belonging to the first and second allowed transition; (iii) n ondiagonal Wannier-Stark transitions and anticrossings of different levels; and (iv) a Zener breakdown with removal of ail sharp transitions. We demon strate that the standard approximation of the eigenstates by Kane functions is valid only for small fields; for intermediate fields quantitative and q ualitative differences to the exact solution are noticed; and the electrica l breakdown at high fields cannot be described by Kane functions. [S0163-18 29(99)04648-2].