The refractive index is one of the essential parameters in describing biochemical reactions and biomedical diagnostics, and in situ measurement of the refractive index is very important for the micro total analysis system. We proposed an ultracompact biosensor based on a high numerical aperture metalens that was designed as two parts for generating the Pancharatnam-Berry phase and propagation phase, respectively, by using zinc sulfide nanofins, and nanopillars. Then incident light was modulated into a right-circularly polarized vortex beam and a left-circularly polarized vortex beam in the focal spot. The superposition of both beams can generate a structured vector beam with various polarization states along the focus direction. The polarization is sensitive to the difference of optical path, which has a potential application in index detection and biochemical analysis.