Acoustic plasmons and indirect intersubband excitations in tunneling-coupled ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}-\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ double quantum wells

We investigate tunneling-coupled GaAs-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ double quantum wells (DQWs) by resonant inelastic light scattering. External gates allow us to control the carrier density and the spatial symmetry of the DQWs. By tuning the DQW potential to an asymmetric state with respect to the tunnel barrier, we are able to observe, besides the optical plasmon, an acoustic intraband plasmon of the tunneling-coupled bilayer system. For strongly asymmetric DQW potentials we find a crossover of the intersubband plasmon between the tunneling-split subbands from a direct to an indirect excitation of the bilayer system. Experimental excitation energies compare well with calculations in the random-phase approximation.