Coherence in real space: the transition range from bulk to confined states studied by the Franz–Keldysh effect

The reduction of the thickness of samples to less than the mean free path in bulk material leads to confinement effects which can be resolved as deviation of field-modulated spectra from the Franz–Keldysh effect of bulk samples. We demonstrate such changes in a 100 nm thick GaInAs layer by using differential electroabsorption spectroscopy. The confinement effects are best seen at small fields and lead to reduction of the range of Franz–Keldysh oscillations, the appearance of oscillations related to the quantum-confined Stark effect at higher energy and finestructure of Franz–Keldysh oscillations near the gap. These features are reproduced by model calculations based on envelope functions.