Interband optical transitions between confined and unconfined states in quantum wells

We have measured and calculated the interband optical absorption in single quantum wells (SQW’s) due to transitions between confined states and unconfined continuum states (C-U transitions), and compared it to absorption due to transitions involving pairs of confined states (C-C transitions). Our sample was an ${\mathrm{In}}_{0.25}$${\mathrm{Ga}}_{0.75}$As/GaAs SQW with a 61-Å well width as determined by electron microscopy. The computed spectra and the results of polarization-modulated absorption measurements reveal steplike absorption features with gradual turnon due to C-U transitions, as opposed to the sharp steps corresponding to the C-C transitions. The transitions of both types have comparable strength, demonstrating the importance of the C-U transitions, which have been largely neglected in previous calculations of the optical properties of quantum wells. Unlike our SQW results, published absorption spectra of uncoupled multiple quantum wells show that C-U transitions produce steplike absorption features with sharp onset. We use a simple model to demonstrate how the slow-turnon absorption line shapes characteristic of SQW’s evolve into sharp steplike features as more wells are added.