Figure 2. Experimental apparatus.
A TEM0,0 beam is obtained by filtering the output of a Ti:Sapphire laser through a single mode fiber (SMF). The initial polarization state is selected by using a polarizing beam splitter (PBS), a half-wave plate (HWP) and a quarter-wave plate (QWP). Then the beam passes through an electrically tunable q-plate, whose optical retardation is controlled by applying an adjustable electric field. When exiting the q-plate the beam has acquired an inhomogeneous polarization pattern that can be experimentally reconstructed by employing a point by point Stokes polarimetry, as discussed in the main text. The projection over the six polarization states H, V, A, D, L, R is implemented through a QWP and a HWP followed by a PBS. The intensity of the analyzed field component is recorded on a CMOS camera. A lens (focal length f = 10 cm) and a 20X microscope objective placed on a translation stage are used to study the polarization pattern at different distances z from the q-plate.