Compensation effects on optically pumped intersubband absorption in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum-well structures

Measurements of intersubband transitions and cyclotron resonance (CR) have been carried out on lightly donor-doped GaAs/${\mathrm{Al}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$As multiple-quantum-well structures. Absorption coefficients of CR and intersubband resonances combined with calculations of coupling efficiency given an oscillator strength of about 1. Measurements of CR absorption versus pump intensity show clear saturation at high intensities. Results demonstrate that the observed intersubband transitions are due to free electrons and that the large photogenerated excess free-electron density is due to compensation by acceptors.