  We propose a scheme to controllably convert the wavefront of an arbitrary
incident beam into a helical one by compact transformation slabs, thus enabling
the output beam to carry desirable orbital angular momentum (OAM). First, based
on transformation optics, a three-dimensional (3D) phase transformation between
any two wavefronts by {\deg}at transformation media is established and then
used to mold a wavefront of Gaussian beam into a helical one. Second, 3D FDTD
simulations are performed to confirm the spiraling evolutions of the resultant
field and phase, clearly demonstrating OAM generated. Further theoretical
analyses show that the refractive index exhibiting a helical distribution leads
to the oppositely spiral phase front and that it is feasible to produce
desirable OAM by generators of unit OAM. The results not only provide an
additional way to manipulate phase and photon OAM, but reciprocally shed
further light on the phase structure of helical beams, which leads to a new
transformation way by a surface.
