Coupling of intersubband charge-density excitations to longitudinal-optical phonons in modulation-doped GaAs quantum wells

We have investigated the interaction between the longitudinal-optical (LO) phonon and the lowest intersubband charge-density excitation of the two-dimensional (2D) electron gas formed in a modulation-doped single quantum well structure. The intersubband spacing is continuously increased by applying dc voltages up to about 200 V between a top gate and a back contact. The frequency of the LO-phonon mode confined to the quantum well exhibits an electron-density-dependent redshift which is attributed to enhanced screening effects by the 2D electron gas. Inelastic light-scattering spectra reveal a clear anticrossing behavior of the energies of LO-phonon and charge-density excitation, for which a coupling constant of 1.5(3) meV is obtained.