Energy-level localization in Bragg-confined asymmetric coupled quantum wells studied by electric field modulation spectroscopy

Electronic Bragg mirrors are used to confine carriers at energy levels above the barrier height in asymmetric coupled quantum wells. An electric field inside the quantum structure is created by transferring carriers from a wide quantum well into a narrow one. Two classes of above-the-barrier states are resolved by using modulated resonant Raman spectroscopy. The first level is the resonant one, which is highly localized by the Bragg reflector above the asymmetric quantum well and is redshifted when a photogenerated local electric field is created in the asymmetric quantum well region. The second class of levels, which extend mainly above the reflectors region, is seen by photoluminescence and photoluminescence excitation measurements. It is less shifted than the resonant level, when the photogenerated local field is applied, which is due to the smaller localization of these states in the asymmetric coupled quantum wells. We used modulated photoluminescence and Raman spectroscopy to resolve the Stark shifts of the bound and continuum levels as a function of the infrared photoexcitation intensity. Our results indicate that because of the photoinduced electric field, the shifts of the above the barrier levels are linked to their degree of localization. These shifts are much stronger than those of the bound states inside the well and a model is proposed to explain the shifts of the continuum and bound levels using perturbation theory.