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 CH3F molecular laser at λ=496 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.
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