Kerr nonlinearity and multi-photon absorption in germanium at mid-infrared wavelengths.

Multiphoton absorption coefficients and nonlinear refractive indices of germanium in the near and mid-infrared (2-5 µm) are reported. The nonlinear coefficients are measured by open and closed aperture Z-scan with 150 fs pulses at a repetition rate of 1 kHz. The nonlinear refractive index of Ge has a peak value of 9:1 x 10^-5 cm²/GW at a wavelength of 3 µm. The effect of free electrons generated by multiphoton absorption is discussed by investigating the variation of multiphoton absorption coefficients at different input powers. Kramers-Kronig relations are also discussed with regard to the relationship between nonlinear refractive index and two photon absorption coefficient. [ABSTRACT FROM AUTHOR]