Relation between electroabsorption in bulk semiconductors and in quantum wells: The quantum-confined Franz-Keldysh effect

We evaluate the interband optical absorption of a semiconductor quantum well in the presence of a uniform electric field perpendicular to the layer and neglecting excitonic effects. We show that this formally becomes the Franz-Keldysh effect in the limit of an infinitely thick layer. When the potential drop across the layer is small compared to the confinement energy we obtain behavior qualitatively different from the bulk Franz-Keldysh effect and we explain this in terms of a quantum-confined Franz-Keldysh effect; with increasing field we demonstrate numerically for a GaAs-like semiconductor that we recover Franz-Keldysh-like behavior, once the originally ``forbidden'' quantum-well transitions become strong. Our discussion gives an alternative physical picture for the Franz-Keldysh effect, including a simple explanation of the Franz-Keldysh oscillations.