  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 spin state can occur in homogeneous
and isotropic medium when a line-variant locally linearly polarized (LVLLP)
beam is strongly focused by a high numerical aperture (NA) objective lens. The
high-NA objective lens here acts as a modulator that enables the
spin-to-orbital OAM conversion of the two components of left and right circular
vibrations of the input SDLLP beam spatially separated. Hence, partial
conversion from linear state to conjugate OAM states takes place, resulting in
helical phases with opposite directions for the longitudinal component of the
two foci. Furthermore, such customized light field can be used to identify and
separate chiral objects in optical tweezers.
