  We report on controlling the bi-photon orbital angular momentum (OAM)
eigenmodes in the spontaneous parametric down conversion process by simply
adjusting the asymmetry of the pump vortex beam. Adjusting the optic axis of
the spiral phase plate (SPP) of phase winding corresponding to OAM mode, l,
with respect to the beam propagation axis, we have transformed a Gaussian beam
into an asymmetric vortex beam with OAM modes, l, l-1, l-2, 0 with different
weightages. Pumping the nonlinear crystal with such asymmetric vortices and
controlling their asymmetry we have tailored the spiral spectrum of the
biphoton OAM eigenmodes. Calculating the Schmidt number of the biphotons we
observe the increase in the spiral bandwidth of the OAM eigenmodes and hence
the dimensionality of the system. Although we have restricted our study to show
the increase in spiral bandwidth of the biphotons by simply controlling the
asymmetry of the pump vortices, we can, in principle, further enhance the
dimensionality of the entangled states by manipulating the pump beam size and
crystal length.
