Far-Infrared Two-Photon Transitions in n-GaAs

Two-photon transitions in the far-infrared spectral range have been observed for the first time. The photoconductivity of n-GaAs as a function of the applied magnetic field exhibited two-photon transitions between 1s and 2s shallow donor states and two-photon cyclotron resonance. The experimental results indicate that two-photon cyclotron resonance is caused by impurities.