Physica E 7 (2000) 274–278 www.elsevier.nl/locate/physe Interminiband spectroscopy of biased superlattices M. Helm a ; * , W. Hilber a , G. Strasser b , R. De Meester c , F.M. Peeters c , A. Wacker d a Institut f ur Halbleiter- und Festk orperphysik, Universit at Linz, A-4040 Linz, Austria b Institut f ur Festk orperelektronik, TU Wien, A-1040 Wien, Austria c Department of Physics, University of Antwerp, B-2610 Antwerpen, Belgium d Institut f ur Theoretische Physik, TU Berlin, D-10623 Berlin, Germany Abstract We study the interminiband absorption in a GaAs= AlGaAs superlattice under the application of an electric eld along the growth axis. While transport shows domain formation and next-nearest tunneling of electrons, the infrared absorption spectrum is strongly modied and reveals transitions to Zener-coupled Wannier–Stark ladders far in the continuum. ? 2000 Elsevier Science B.V. All rights reserved. Keywords: Superlattice; Wannier–Stark ladder; Interminiband absorption; Tunneling Upon application of an electric eld along the growth axis, intersubband transitions exhibit Stark shifts [1,2], transfer of oscillator strengths in cou- pled quantum wells [3], or localization-induced line-narrowing of transitions into the continuum [4]. Strongly coupled superlattices (SLs) behave dif- ferently in several respects. First, the intersubband absorption is known to extend over a certain spec- tral width, which corresponds to the widths of the minibands involved in the transition. It has been shown that the oscillator strength is distributed over the mini-Brillouin zone, with the maximum at the zone-edge [5,6]. This has found application in super- lattice cascade lasers [7]. In a vertical electric eld, * Corresponding author. Tel.: +43-732-2468-9602; fax: +43- 732-2468-650. E-mail address: m.helm@hlphys.uni-linz.ac.at (M. Helm) they show negative dierential resistance (NDR) [8], Bloch oscillations [9] and the formation of a Wannier–Stark ladder (WSL) [10,11]. These phe- nomena have been studied using transport [8] and infrared emission (in doped SLs) [12] as well as by optical methods (in undoped SLs) [9]. The evolution of the inter-miniband absorption spectrum in a ver- tical electric eld, however, has not been studied to date. Yet it could yield new insight into the above phenomena, especially when transport and infrared optical techniques are combined. In the mid-infrared one can use the inter-miniband absorption spectrum to observe the modication of the electron distribu- tion function in the electric eld [13] or the break-up of the minibands into Wannier–Stark ladders. In the far-infrared (or THz) it has been predicted that, under conditions of NDR and Bloch oscillations, optical gain occurs below the Bloch frequency [14]. 1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII:S1386-9477(99)00275-1