We study the coupling of Landau levels and electric subbands in quasi-two-dimensional electron inversion layers of modulation-doped GaAs/${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions in tilted magnetic fields. We employ far-infrared Fourier-transform spectroscopy with light polarized parallel to the layers. When the subband separation $E_{10}$ is much larger than the cyclotron energy ħ${\omega }_c$, we find an enhancement of the cyclotron mass due to the parallel magnetic-field component. When the subband separation $E_{10}$=rħ${\omega }_c$ is a multiple r=1,2 of the cyclotron energy, we observe a resonant splitting of the cyclotron resonance. The mass enhancement as well as the magnitudes and energetic positions of the splittings are described by perturbation theory and by the triangular-well approximation of the space-charge potential. From the experimental data we determine intersubband energies and dipole matrix elements. Both depend on electron density, which we adjust by illumination with a light-emitting diode or by a gate voltage applied between the inversion layer and a front gate.

• Received 20 September 1988

DOI:https://doi.org/10.1103/PhysRevB.39.3785