Depolarization shift in coupled quantum wells with tunable level spectrum

The degeneracy of electronic subband levels in spike-inserted wide parabolic quantum wells is investigated by far-infrared spectroscopy and magnetotransport measurements. A narrow potential barrier within the parabolic well divides the electron system into two well-separated but strongly coupled layers, leading to a drastical change of the subband system and the collective intersubband excitations. In contrast to a single robust resonance, which, obeying Kohnʹs Theorem is usually seen in a purely parabolic quantum well, the collective intersubband transitions now recover the complex coupling and splitting of the single-particle states. By applying two different experimental methods being sensitive to either the single-particle spectrum or the collective excitation spectrum, we are able to confirm the theoretical predictions that the size of the observed splitting caused by resonant subband coupling is not altered by collective effects. Rather, the corresponding intersubband resonance lines are shifted to higher energies in a depolarization like manner.