Effects of interwell coupling on plasmon modes in symmetric double square quantum well structures

Using self-consistent-field and linear response theories, we calculate the collective charge-density excitations of modulation-doped $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ double square quantum well structures in which the subband structures are determined self-consistently. We show that the effects of many-body renormalization on the subband energies are profound. We systematically investigate the dependence of the plasmon spectrum on the coupling between the two wells. In the Coulomb coupling regime, the optical and acoustic modes move close to each other when the interwell coupling is decreased. They both approach the intrasubband mode of a single quantum well as the barrier becomes quite wide. When the barrier becomes narrower, tunneling coupling plays an important role. Our numerical results show that the intersubband collective modes strongly depend on the tunneling coupling strength, while the intrasubband mode is insensitive to this tunneling coupling. The evolution of the collective excitation modes with tunneling coupling strength can be clearly observed by decreasing the width or height of the barrier. The behavior of the depolarization shift shows that the dynamical many-body effects become stronger with the increase of the tunneling coupling strength. Hence, the characteristics of the plasmon modes in different tunneling regimes provide a powerful tool for studying many-body effects.