We have observed and characterized wavelength-dependent laser damage thresholds in crystalline germanium induced
by trains of high-power infrared picosecond laser pulses at wavelengths ranging from 2.8 μm to 5.2 μm, using the
Vanderbilt Free-Electron Laser. In this wavelength range, photon energies are well below the band-gap energy. As the
wavelength is increased, threshold fluences are observed to increase by a factor of five over the studied wavelength
range. Two- and three- photon absorption is the predominant photon energy absorption mechanism up to 4.4 μm. At
wavelengths above 4.8 μm tunnel absorption appears to be the primary absorption mechanism. Wavelength and fluence
dependent transmission and reflection measurements provide valuable insight into the nature of the damage mechanisms.