  We for the first time report the truncated diffraction with a triangular
aperture of the SU(2) geometric modes and propose a method to detect the
complicated orbital angular momentum (OAM) of an SU(2) wave-packet, to the best
of our knowledge. As a special vortex beam, a nonplanar SU(2) mode carrying
special intensity and OAM distributions brings exotic patterns in truncated
diffraction lattice. A meshy structure is unveiled therein by adjusting the
illuminated aperture in vicinity of the partial OAM regions, which can be
elaborately used to evaluate the partial topological charge and OAM of an SU(2)
wave-packet by counting the dark holes in the mesh. Moreover, through
controlling the size and position of the aperture at the center region, the
truncated triangular lattice can be close to the classical spot-array lattice
for measuring the center OAM. These effects being fully validated by
theoretical simulations greatly extend the versatility of topological
structures detection of special beams.
