The Franz-Keldysh effect and free carrier dispersion effect in germanium

We calculated the changes of refractive index and absorption coefficient in Ge induced by the Franz-Keldysh (FK) effect and free carrier dispersion (FCD) effect, taking advantage of reported absorption spectra.The FK effect is weak near the indirect absorption edge of Ge. FCD effect can induce a much larger change of refractive index. In the wavelength range of 2–10 μm, theoretical formulas about dependences of the changes of absorption coefficient and refractive index on the change of carrier concentrations are presented. In addition, FCD effects in N-type and P-type Ge are compared.