  We report on simulations and measurements of the optical absorption of
silicon nanowires (NWs) versus their diameter. We first address the simulation
of the optical absorption based on two different theoretical methods : the
first one, based on the Green function formalism, is useful to calculate the
scattering and absorption properties of a single or a finite set of NWs. The
second one, based on the Finite Difference Time Domain (FDTD) method is
well-adapted to deal with a periodic set of NWs. In both cases, an increase of
the onset energy for the absorption is found with increasing diameter. Such
effect is experimentally illustrated, when photoconductivity measurements are
performed on single tapered Si nanowires connected between a set of several
electrodes. An increase of the nanowire diameter reveals a spectral shift of
the photocurrent intensity peak towards lower photon energies, that allows to
tune the absorption onset from the ultraviolet radiations to the visible light
spectrum.
