Behzad Khajavi

High-order disclinations in space-variant polarization

We present the investigation of high-order disinclination patterns in the spatially variable polarization of a light beam. The beam was prepared by encoding two distinct high-order optical vortices on each of the circular polarization components of the beam. As a consequence, we were able to produce high-index lemon and star patterns, which have positive and negative indices, respectively. By varying the asymmetry of one of the vortices we were able to transform one symmetric pattern (lemon or star) into another (lemon or star). With one exception, monstar patterns always appear for specific ranges of asymmetry regardless of the end symmetric patterns. Mapping of all disclinations within each case is contained in a spherical space, where monstar regions are cusp-shaped. We found that high-order monstar patterns can have positive or negative index.

Preparation of Poincaré beams with a same-path polarization/spatial-mode interferometer

Abstract. We present a same-path polarization interferometer that uses two spatial light modulators to encode the most general type of Poincaré beam. We demonstrate this design by presenting new results on the encoding of symmetric C-point polarization singularities using spatial modes with high-order topological charges. We also present new results on composite C-points. These are cases where there are multiple C-points in a single beam obtained by combining two modes with composite optical vortices in orthogonal states of polarization. The measurements show good agreement with the simulations.

Monstar disclinations in the polarization of singular optical beams

The control of spatial and polarization modes of optical beams enables the production of topological singularities encoded on the polarization of the light. This allows the study of topological disclinations not easily found in the natural setting. In this article we report on the observation of new features in disclinations realized with singular optical beams. They were prepared using three spatial modes bearing optical vortices in non-separable superpositions with circular polarization states. The disclinations involve asymmetric rotational dislocations, whose optical counterparts in the optical far field are known as C-points, and which are classified as monstars. They have been known to have a singularity index that can be positive, or negative as reported by us recently. Here we report on monstars with an index of zero. Monstars are characterized by having sectors bound by radial lines that involve curved lines radiating from the singularity. We found that kinks in otherwise smooth line patterns of asymmetric disclinations are scars of a separate but related pattern of line-slope discontinuities, carried optically by C-lines in the far field. These scars are indicative of the underlying structure or symmetry of the pattern. We present a general formalism to understand and generate monstars, along with measurements: the experimental results are in excellent agreement with theoretical modelings.

High-order disclinations in the polarization of light

We present modelings of high-order line singularities encoded in space-variant polarization of light. This involves calculating the line patterns produced by the superposition of light beams in orthogonal states of circular polarization, with each beam carrying an optical vortex, and where one of them is asymmetric. This setting allowed us to study the case of monstars of high order. We find that monstars can have positive or negative singularity indices, modifying the previous understanding of the pattern, which was based on the case of lowest-order C- points. Monstars then remain characterized only by their own unique feature: sectors with patterns of mostly curved lines that radiate from the center. Given this definition, we propose that the case where the index is +1 be classified as a monstar. We also found that the asymmetric modes contain kinks that appear in the C-lines of a distinct but related pattern that contains line orientation discontinuities.

Multitwist Möbius Strips and Twisted Ribbons in the Polarization of Paraxial Light Beams

The polarization of light can exhibit unusual features when singular optical beams are involved. In 3-dimensional polarized random media the polarization orientation around singularities describe 1/2 or 3/2 Möbius strips. It has been predicted that if singular beams intersect non-collinearly in free space, the polarization ellipse rotates forming many-turn Möbius strips or twisted ribbons along closed loops around a central singularity. These polarization features are important because polarization is an aspect of light that mediate strong interactions with matter, with potential for new applications. We examined the non-collinear superposition of two unfocused paraxial light beams when one of them carried an optical vortex and the other one a uniform phase front, both in orthogonal states of circular polarization. It is known that these superpositions in 2-dimensions produce space-variant patterns of polarization. Relying on the symmetry of the problem, we extracted the 3-dimensional patterns from projective measurements, and confirmed the formation of many-turn Möbius strips or twisted ribbons when the topological charge of one of the component beams was odd or even, respectively. The measurements agree well with the modelings and confirmed that these types of patterns occur at macroscopic length scales and in ordinary superposition situations.

Determining topological charge of an optical beam using a wedged optical flat

The topological charge of a beam carrying an optical vortex is an important parameter that specifies the amount of orbital angular momentum carried by the beam and the azimuthal order of the beam mode. We present an experimental method to determine the sign and magnitude of the topological charge using a wedged optical flat as a lateral shearing interferometer. When the curvature of the wavefront is adjusted to be planar, the fringe pattern generated by the shearing interferometer consists of two conjoined forks that unambiguously identify the topological charge of the beam. We also investigated the changes in the pattern when the wedged flat is rotated.

Monstar polarization singularities with elliptically-symmetric q-plates

Space-variant polarization patterns present in the transverse mode of optical beams highlight disclination patterns of polarization about a singularity, often a C-point. These patterns are important for understanding rotational dislocations and for characterizing complex polarization patterns. Liquid-crystal devices known as q-plates have been used to produce two of the three types of disclination patterns in optical beams: lemons and stars. Here we report the production of the third type of disclination, which is asymmetric, known as the monstar. We do so with elliptically-symmetric q-plates. We present theory and measurements, and find excellent agreement between the two.

Determination of the topological charge of complex light beams by shearing interference from a wedged optical flat

The topological charge (l) is the parameter that defines the amount of orbital angular momentum (OAM) of beams carrying optical vortices. In this article we expand on a robust experimental method to determine the topological charge that we previously developed [B. Khajavi & E. J .Galvez, Opt. Lett. 42, 1516-1519 (2017)]. This method consist of using a pair of confocal lenses and a wedged optical flat as a shearing interferometer. The interference pattern produced by a pure OAM mode directly reveals the magnitude and sign of the topological charge: two conjoined forks with |l| + 1 number of tines pointing away from (towards) one another for l > 0 (l < 0). Superpositions of two modes with the same value of |l| but with the opposite sign produce shearing interference patterns that can be used the infer the value of l and the relative weight of the component modes.

Monstar disclinations in high-order singular optical beams (Conference Presentation)

We investigate disclinations in the orientation of space-variant polarization patterns produced by collinear non-factorizable superpositions of high-order spatial modes and polarization. Asymmetric disclination patterns were formed by superpositions of spatial modes with asymmetric optical vortices. They give rise to monstar patterns of high order that can have a negative or positive disclination index. This has led to an examination of what constitutes a monstar. We present theoretical as well as experimental results.

Searching for the helical-gradient force on chiral molecules

We investigate a force that has been predicted to discriminate molecules by their chirality when they are in the presence of an optical field with a polarization-helicity gradient. We investigate several experimental geometries for observing evidence of this force via enantiometer separation in racemic mixtures. We do this with singular-optical beams carrying a polarization helicity gradient across their transverse mode. Molecular diffusion and the dipole force – an intensity-gradient force – have so far precluded measurements of this force.