Minibands in the continuum of multi-quantum-well superlattices

We report the first observation of well defined minibands and minigaps between them, at energies above the barriers in multi-quantum-well structures. Using intersubband absorption spectroscopy, we have observed the optical transitions between confined electronic states of isolated quantum wells to minibands in the continuum of the periodic structure. By comparison with a theoretical model we spectroscopically identify all the observed transitions and explain their strength and spectral shape. In addition, we estimate the coherence length of the electronic wave function in these minibands comparing the absorption spectrum with that of a finite array of quantum wells.