  Optical measurements of nanostructured GaSb prepared by sputtering is
presented. The optical response is studied by Mueller Matrix Ellipsometry (MME)
in the visible range (430--850nm), and by spectroscopic ellipsometry in the
range 0.6--6.5eV. The nano-structured surfaces reported in this work, consist
of densely packed GaSb cones approximately 50nm high, on bulk GaSb. The
nanostructured surfaces are here shown to considerably modify the optical
response of the surface, hence giving a strong sensitivity to the far field
spectroscopic (Mueller matrix) ellipsometric measurements. The off-specular
scattering and the depolarization is found to be low. The anisotropic response
is particularly emphasized by studying nano-structured GaSb cones approximately
45 degrees tilted with respect to the surface normal. In the latter case, one
observes upon rotating the sample around the surface normal, that the Mueller
matrix elements $m_{13}$ and $m_{14}$ oscillate as a function of the rotation
angle. Finally, Mueller matrix techniques have been applied to the measured
data, in order to analyze the acquired Mueller matrix in terms of physical
realizability and noise.
