A low-drive voltage GaAs/AlGaAs coupled quantum well modulator exhibiting blue and red absorption edge shifts

Optical quantum well modulators, formed from isolated wells, have been fabricated and subsequently used in a spatial light modulator (4×4) exhibiting a contrast ratio of 7.8:1 at a drive voltage of 25 V. In order to improve this contrast ratio together with the advantage of lowering the drive voltage in the electroabsorption modulators, coupled quantum well devices were studied. The barriers in these structures are thin enough to allow the well to be coupled by resonant tunnelling of the wavefunctions, leading to the formation of sub-bands and thus a narrower bandgap. However on the application of an electric field the energy levels are misaligned, turning off this resonant tunnelling. The bandgap becomes larger leading to a blue shift of the absorption edge