The basic physics of intersubband transitions – from the simple to the subtle

I will describe the basic properties of intersubband transitions in quantum wells, partly along the lines of the now ten-years-old review paper [1]. I will proceed from the very basic to the discussion of some more specific systems, like superlattices or transitions within the valence band, and to some recently discovered, rather subtle effects.
An interesting interplay is found with transitions between shallow impurity states, which becomes particularly striking in superlattices and coupled quantum wells [2]. Based on a calculation that treats the heterostructure potential and the random-impurity potential on the same footing [3], we can successfully explain or re-interpret some old, hitherto poorly understood data (see Fig. 1).
Another subtle effect emerges, if intersubband spectra are recorded using time-domain terahertz spectroscopy, i.e. fully resolving the phase of the electric field. We show that transmission and absorption spectra are not equivalent, and whereas the absorption line has a standard Lorentzian shape, the transmission spectrum exhibits a Fano-type lineshape (see Fig. 2). This has been explained with the influence of the so-called ponderomotive current [4].
Most of the more recent experimental work discussed here was performed mainly by Dominik Stehr and Martin Wagner (both FZD), most of the samples used were provided by Gottfried Strasser (TU Vienna) and his group, and for part of this work the collaboration with the theory group of Univ. Marburg (D. Golde, M. Kira, S. W. Koch) was indispensable.
[1] M. Helm, “The basic physics of intersubband transitions”, in “Semiconductors and Semimetals”, Vol. 62, p. 1-99 (2000).
[2] D. Stehr, C. Metzner, M. Helm, T. Roch, and G. Strasser, “Resonant impurity bands in semiconductor superlattices”, Phys. Rev. Lett. 95, 257401 (2005).
[3] D. Stehr, M. Helm, C. Metzner, and M. C. Wanke, “Microscopic theory of impurity states in coupled quantum wells and superlattices”, Phys. Rev. B 74, 085311 (2006).
[4] D. Golde, M. Wagner, D. Stehr, H. Schneider, M. Helm, A.M. Andrews, T. Roch, G. Strasser, M. Kira, and S.W. Koch: “Fano signatures in the intersubband terahertz response of optically excited semiconductor quantum wells”, Phys. Rev. Lett. 102, 127403 (2009).