  The monochromatic and geometrically anisotropic acoustic field generated by
400 nm and 120 nm diameter copper nanowires simply dropped on a 10 $\mu$m
silicon membrane is investigated in transmission using three-dimensional
time-resolved femtosecond pump-probe experiments. Two pump-probe time-resolved
experiments are carried out at the same time on both side of the silicon
substrate. In reflection, the first radial breathing mode of the nanowire is
excited and detected. In transmission, the longitudinal and shear waves are
observed. The longitudinal signal is followed by a monochromatic component
associated with the relaxation of the nanowire's first radial breathing mode.
Finite Difference Time Domain (FDTD) simulations are performed and accurately
reproduce the diffracted field. A shape anisotropy resulting from the large
aspect ratio of the nanowire is detected in the acoustic field. The orientation
of the underlying nanowires is thus acoustically deduced.
