Semiconductor electrooptic medium shows behavior different from a medium based on
quantum confined
Stark Effect. A preferred embodiment has a type-II
heterojunction, selected such, that, in zero
electric field, an
electron and a hole are localized on the opposite sides of the
heterojunction having a negligible or very small overlap of the wave functions, and correspondingly, a zero or a very small
exciton oscillator strength. Applying an
electric field results in squeezing of the wave functions to the
heterojunction which strongly increases the overlap of the
electron and the hole wave functions, resulting in a strong increase of the
exciton oscillator strength. Another embodiment of the novel electrooptic medium includes a heterojunction between a layer and a
superlattice, wherein an
electron and a hole in the zero
electric field are localized on the opposite sides of the heterojunction, the latter being effectively a type-II heterojunction. Yet another embodiment has a heterojunction between two superlattices, wherein an electron and a hole in a zero electric field are localized on the opposite sides of the heterojunction, the latter operating effectively as a type-II heterojunction. A further embodiment has an ultrathin
quantum well layer confined by barrier
layers, having an essentially different barrier heights and a thick layer, wherein, in a zero electric field, a
charged particle of one sign having a large effective
mass is localized in this ultrathin layer, and a particle having a different sign of the charge, having a small effective
mass is not localized in this ultrathin layer, but is localized mainly in the neighboring thick layer. Thus, the heterojunction between the two
layers operates effectively as a type-II heterojunction. Applying an electric field to all types of the electrooptic medium of the present invention results in a dramatic increase of the
exciton oscillator strength and, therefore, in a large positive
refractive index change at the
photon energies below the exciton absorption peak. A very strong increase in the
optical transition photon energy can be achieved, when necessary.