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 magnetophotoconductivity of $n$-GaAs measured by a pulsed high-power C${\mathrm{H}}_3$F molecular laser at $\lambda =496\mathrm{}$ μm exhibited two-photon transitions between $1s$ and $2s$ shallow donor states and two-photon cyclotron resonance. The experimental results and symmetry considerations indicate that the observed two-photon cyclotron resonance is caused by impurities.