Catalystlike effect of orbital angular momentum on the conversion of transverse to three-dimensional spin states within tightly focused radially polarized beams

We report on the catalystlike effect of orbital angular momentum (OAM) on local spin-state conversion within the tightly focused radially polarized beams associated with optical spin-orbit interaction. It is theoretically demonstrated that the incident OAM can lead to a conversion of purely transverse spin state to a three-dimensional spin state on the focal plane. This conversion can be conveniently manipulated by altering the sign and value of the OAM. By comparing the total OAM and spin angular momentum (SAM) on the incident plane to those on the focal plane, it is indicated that the incident OAM have no participation in the angular momentum intertransfer, and just play a role as a catalyst of local SAM conversion. Such an effect of OAM sheds new light on the optical spin-orbit interaction in tight-focusing processes. The resultant three-dimensional spin states may provide more degrees of freedom in optical manipulation and spin-dependent directive coupling.

Figure 1

Calculated electric field intensity and SAM density distributions on the focal plane of tightly focused radially polarized beams carrying vortex phase $\exp \left(i\mathit{l}\varphi \right)$ with (a) $\mathit{l}=0$ and (b) $\mathit{l}=1$. Insets: phase structures of the corresponding electric field components. Arrows in (${\mathrm{a}}_4$) and (${\mathrm{b}}_4$) depict the orientations of local spin state.

Figure 2

Cross-sectional longitudinal, radial, and azimuthal SAM density distributions along the radial coordinate on the focal plane of tightly focused radially polarized beams carrying different OAMs. Topological charge is (a) $\mathit{l}=1$ and (b) $l=-1$, respectively.

Figure 3

Cross-sectional (a) longitudinal and (b) azimuthal SAM density distributions along the radial coordinate on the focal plane of tightly focused radially polarized beams with $\mathit{l}=0$, 1, 2, 4, and 6.

Figure 4

Calculated (a) intensity and (b) polarization ellipticity distributions for (1) $\mathit{l}=0$ and (2) $\mathit{l}=1$. (c) Cross-sectional longitudinal SAM density distributions along the radial coordinate in the focal plane of tightly focused azimuthally polarized beams with $\mathit{l}=0$, 1, 2, 4, and 6.