Depopulation of subbands by magnetic and electric fields in gated ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As-GaAs quantum wells

We have investigated the cyclotron resonance (CR) of selectively doped n-type ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As-GaAs quantum wells in which the carrier density could be tuned via a front-gate voltage $V_g$. With increasing $V_g$ we observe a second CR which was found to represent the CR of a second subband $E^1$. The onset voltage $V_{t1}$ and the occupation of the $E^1$ subband depend on the strength B of a perpendicular magnetic field. We found that the CR in the $E^1$ subband exhibits a lower effective mass and a significantly higher dynamic mobility as compared with the $E^0$ subband.