Picosecond optical measurement of free-carrier, intervalence-band, and indirect absorption in germanium at high optically created carrier densities

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Abstract
It has been suggested that enhanced intervalence-band and Coulomb-assisted indirect-absorption effects may be significant at the high optically created carrier densities encountered in the recent excite and probe experiments performed in germanium using intense picosecond optical pulses with a wavelength of 1.06 μm, and that these processes may result in an absorption versus carrier-density curve containing a minimum. Such a curve could then be combined with a recombination process to explain the results of the excite and probe experiments. Here, we report measurement of the combined free-carrier, intervalence-band, and indirect absorbance in thin germanium samples during these excite and probe experiments by exciting at 1.06 μm and by probing both at 1.06 μm and with a Raman-generated probe at 1.55 μm. The measurements suggest that these processes are significant at the high optically created carrier densities encountered in the present excite and probe experiments; however, they do not introduce a minimum in the absorption versus carrier-density curve as originally suggested.