Scalar-aperture control of spin angular momentum in vector vortex beams.

We investigate how a polarization-independent sector aperture modifies the beam-integrated normalized spin angular momentum (SAM) of vector vortex beams. For beams containing a single vortex mode in each circular component, the aperture reduces both transmitted powers by the same fractional factor, and the normalized SAM remains invariant. For coherent vortex superpositions, sector truncation breaks azimuthal modal orthogonality and produces different interference contributions to the powers of the two circular components. Their unequal attenuation enables continuous variation, including sign reversal, of the integrated normalized SAM without direct conversion between circular polarizations. The model is extended to complex modal coefficients and independent sector orientation and is verified experimentally by Stokes polarimetry. Analysis of the transmitted powers and transverse-displacement sensitivity confirms that the observed SAM variation originates from polarization-independent spatial filtering of the structured field.