  We have measured the terahertz response of oriented germanium nanowires using
ultrafast optical-pump terahertz-probe spectroscopy. We present results on the
time, frequency, and polarization dependence of the terahertz response. Our
results indicate intraband energy relaxation times of photoexcited carriers in
the 1.5-2.0 ps range, carrier density dependent interband electron-hole
recombination times in the 75-125 ps range, and carrier momentum scattering
rates in the 60-90 fs range. Additionally, the terahertz response of the
nanowires is strongly polarization dependent despite the subwavelength
dimensions of the nanowires. The differential terahertz transmission is found
to be large when the field is polarized parallel to the nanowires and very
small when the field is polarized perpendicular to the nanowires. This
polarization dependence of the terahertz response can be explained in terms of
the induced depolarization fields and the resulting magnitudes of the surface
plasmon frequencies.
