Electroabsorption spectroscopy of intersubband transitions in multiple-quantum-well superlattices

We apply intersubband electroabsorption spectroscopy to a current-carrying multiple quantum-well structure device, in order to simultaneously measure the steady state carrier density and the average time carriers spend in eac h quantum well. By comparing our measurements with a simple quantum mechani cal tunneling model, we show directly that electrons sequentially tunnel th rough the periodic quantum structure. In addition, we use photoinduced abso rption spectroscopy to measure the effect of the applied electric field on electron states within the minibands in the energy continuum. We show that already at relatively small fields, the optical transitions into the contin uum minibands merge into a single spectral band. Using an eight-band model, we explain this observation in terms of field-induced miniband state local ization.