  We investigate type I second harmonic generation in III-V semiconductor wire
waveguides aligned with a crystallographic axis. In this direction, because of
the single nonzero tensor element of III-V semiconductors, only frequency
conversion by mixing with the longitudinal components of the optical fields is
allowed. We experimentally study the impact of the propagation direction on the
conversion efficiency and confirm the role played by the longitudinal
components through the excitation of an antisymmetric second harmonic higher
order mode.
