  As one fundamental property of light, the orbital angular momentum (OAM) of
photon has elicited widespread interest. Here, we theoretically demonstrate
that the OAM conversion of light without any spin state can occur in
homogeneous and isotropic medium when a specially tailored locally linearly
polarized (STLLP) beam is strongly focused by a high numerical aperture (NA)
objective lens. Through a high NA objective lens, the STLLP beams can generate
identical twin foci with tunable distance between them controlled by input
state of polarization. Such process admits partial OAM conversion from linear
state to conjugate OAM states, giving rise to helical phases with opposite
directions for each focus of the longitudinal component in the focal field.
