Picosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium

An ellipsometer has been used with picosecond pulses to measure the time evolution of optically generated plasmas in intrinsic Ge. By measuring the change in optical ellipticity of a weak probe beam following the absorption of an intense excitation pulse, the time evolution of the plasma density can be determined with a precision of a few picoseconds. At a density of 1.7 × ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$, the ambipolar diffusivity of the plasma was observed to be a factor of 3 greater than the low-density value, consistent with a simple theoretical model of diffusion in the degenerate regime.