Optical orbital angular momentum from the curl of polarization

We predict a new category of optical orbital angular momentum that is associated with the curl of polarization and a kind of vector field with radial-variant hybrid states of polarization that can carry such novel optical orbital angular momentum. We present a scheme for creating the desired vector fields. Optical trapping experiments validate that the vector fields, which have no additional phase vortex, exert torques to drive the orbital motion of the trapped isotropic microspheres.

Figure 1

Intensity patterns of four radial-variant linearly polarized vector fields passing through a horizontal polarizer and their schematics of SoPs. (a) $n=1.0$ and ${\alpha }_0=0$, (b) $n=1.5$ and ${\alpha }_0=0$, (c) $n=0.5$ and ${\alpha }_0=\pi /2$, and (d) $n=0.5$ and ${\alpha }_0=3\pi /4$.

Figure 2

Intensity patterns of two radial-variant vector fields with hybrid SoPs, schematics of SoPs, and measured Stokes parameters. The 1st and 2nd columns correspond to ($n=0.5$, ${\alpha }_0=0$) and ($n=1.0$, ${\alpha }_0=\pi /4$), respectively. For comparison, a local linearly polarized vector field with ($n=0.5$, ${\alpha }_0=0$) is also depicted in the 3rd column.

Figure 3

Experimental configuration to validate OAM associated with the curl of polarization. The dashed-line box shows the generating unit of radial-variant vector fields. Inset (a) shows an example of simulated ring focus. Inset (b) shows the properties of the ring focus for a radial-variant vector field with hybrid SoPs ($n=10$), where the solid and dashed lines are the radial dependences of intensity and OAM $J_z^{(3)}$, respectively.

Figure 4

Snapshots of the motion of trapped particles around the ring focus generated by radial-variant vector fields with hybrid SoPs, caused by polarization-curl-induced OAM (also see Ref. 35).