Polarization-singularity C-points, a form of line singularities, are the vectorial counterparts of the optical vortices of spatial modes and fundamental optical features of polarization-spatial modes. Their generation in tailored beams has been limited to lemon and star C-points that contain symmetric dislocations in state-of-polarization patterns. In this article we present the theory and laboratory measurements of two complementary methods to generate isolated asymmetric C-points in tailored beams, of which symmetric lemons and stars are limiting cases; and we report on the generation of monstars, an asymmetric C-point with characteristics of both lemons and stars. So far, the study of line singularities relies on the diagnosis of natural occurrences. Non-separable superpositions of polarization and spatial mode of light can provide a vehicle for deliberately creating line-singularity patterns for their study. This polarization-spatial-mode hybridization also adds a new dimension to imaging, where polarization provides additional sensing information [10,11]. Different species exploit polarization-spatial combinations for their survival [12], and line singularities may provide the means to characterize them. At the quantum level, these hybrid modes provide larger Hilbert spaces for encoding information [13]. An investigation of polarization-spatial light modes is also essential for understanding this type of imaging at a deeper level [14].C-points are the umbilical points of the line singularities in the polarization of light because they connect the apex of two opposite cones (a diabolo): of semi-major and semi-minor axis lengths [2,15]. They consist of a state of circular polarization surrounded by a field of polarization ellipses in the optical field, with orientations that rotate about the C-point [16]. C-points are singular points of ellipse orientation. They are intimately linked to the optical vortices of scalar fields, but encode the optical dislocations in the state of polarization instead of the phase [17]. The production of the full spectrum of C-points is of interest in its own right, as it reveals a new domain of complex light not investigated before. The production and analysis of C-points are the basis for new techniques to produce and diagnose optical vortices, which are of interest in metrology due to their high sensitivity to perturbations [18].The two symmetric types of C-point singularities, known as lemons and stars, correspond to dislocations where the ellipse orientation varies linearly with and counter to the angle about the singularity, respectively. Yet, the generation of beams bearing isolated C-points (i.e., alone in a light beam) has been limited to these two cases [19][20][21]. The larger class of asymmetric C-points containing orientations evolving nonlinearly have been produced only in speckle patterns [6][7][8][9], or as C-point pairs (dipoles) in tailored beams [22].The two symmetric cases are the ends of a spectrum of C-points where the pattern of orientations in the ellipse f...

We present a geometric-analytic introductory treatment of polarization based on the circular polarization basis, which connects directly to the Poincaré sphere. This enables a more intuitive way to arrive at the polarization ellipse from the components of the field. We also present an advanced optics lab that uses Poincaré beams, which have a polarization that is spatially variable. The physics of this lab reinforces students' understanding of all states of polarization, and in particular, elliptical polarization. In addition, it exposes them to Laguerre-Gauss modes, the spatial modes used in creating Poincaré beams, which have unique physical properties. In performing this lab, students gain experience in experimental optics, such as aligning and calibrating optical components, using and programming a spatial light modulator, building an interferometer, and performing polarimetry measurements. We present the apparatus for doing the experiments, detailed alignment instructions and lower-cost alternatives.

We image optical singularities by exploiting the connection between scalar and vector fields. We examine the sensitivity of optical vortices to perturbations and suggest a method of study via imaging polarimetry of the optical field. This is possible by converting optical vortices to polarization-singularity C-points. We present the deliberate creation of C-points using a superposition of two circularly polarized beams of opposite helicity, with a phase vortex in one and a planar wavefront in the other. We present a theoretical analysis and measurements of the transformation of C-points from lemon to star, going through the monstar stage. We do this by varying the phase gradient of the optical vortex.

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