Dispersion of the nonlinear optical susceptibility of n-type germanium

We observe a strong impurity-induced resonance in the third-order nonlinear optical susceptibility of n-type germanium. Our observations are used to obtain the cross-sections for electronic Raman scattering between the valley-orbit split 1s states of phosphorus and arsenic donors near 10 μm. Good agreement is found between measurements and calculations. The possibility of obtaining tunable impurity Raman laser action in n-type germanium is discussed.