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.

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 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.

Poincaré-beam patterns produced by nonseparable superpositions of Laguerre-Gauss and polarization modes of light.

We present a study of Poincaré-beam polarization patterns produced by collinear superposition of two Laguerre-Gauss spatial modes in orthogonal polarization eigenstates (circular or linear). We explore theoretically and experimentally the combinations that are possible. We find that the resulting patterns can be explained in terms of mappings of points on the Poincaré sphere onto points in the transverse plane of the beam mode. The modes that we produced yielded many types of polarization singularities.

Laser imaging polarimetry of nacre.

Nacre is a complex biomaterial made of aragonite-tablet bricks and organic mortar that is considerably resilient against breakage. Nacre has been studied with a wide range of laboratory techniques, leading to understanding key fundamentals and informing the creation of bio-inspired materials. In this article, we present an optical polarimetric technique to investigate nacre, taking advantage of the translucence and birefringence of its microcomponents. We focus our study on 3 classes of mollusks that have nacreous shells: bivalve (Pinctada fucata), gastropod (Haliotis asinina and Haliotis rufescens) and cephalopod (Nautilus pompilius). We sent polarized light from a laser through thin samples of nacre and did imaging polarimetry of the transmitted light. We observed clear distinctions between the structures of bivalve and gastropod, due to the spatial variation of their birefringence. The patterns for cephalopod were more similar to bivalve than gastropod. Bleaching of the samples disrupted the transmitted light. Subsequent refilling of the bivalve and gastropod nacre samples with oil produced optical patterns similar to those of unbleached samples. In cephalopod samples, we found that bleaching produced irreversible changes in the optical pattern.

Polarimetry of Pinctada fucata nacre indicates myostracal layer interrupts nacre structure.

The inner layer of many bivalve and gastropod molluscs consists of iridescent nacre, a material that is structured like a brick wall with bricks consisting of crystalline aragonite and mortar of organic molecules. Myostracal layers formed during shell growth at the point of muscle attachment to the shell can be found interspersed within the nacre structure. Little has been done to examine the effect the myostracal layer has on subsequent nacre structure. Here we present data on the structure of the myostracal and nacre layers from a bivalve mollusc, Pinctada fucata. Scanning electron microscope imaging shows the myostracal layer consists of regular crystalline blocks. The nacre before the layer consists of tablets approximately 400 nm thick, while after the myostracal layer the tablets are approximately 500 nm thick. A new technique, imaging polarimetry, indicates that the aragonite crystals within the nacre following the myostracal layer have greater orientation uniformity than before the myostracal layer. The results presented here suggest a possible interaction between the myostracal layer and subsequent shell growth.

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.

Photon Entanglement Through Brain Tissue.

Photon entanglement, the cornerstone of quantum correlations, provides a level of coherence that is not present in classical correlations. Harnessing it by study of its passage through organic matter may offer new possibilities for medical diagnosis technique. In this work, we study the preservation of photon entanglement in polarization, created by spontaneous parametric down-conversion, after one entangled photon propagates through multiphoton-scattering brain tissue slices with different thickness. The Tangle-Entropy (TS) plots show the strong preservation of entanglement of photons propagating in brain tissue. By spatially filtering the ballistic scattering of an entangled photon, we find that its polarization entanglement is preserved and non-locally correlated with its twin in the TS plots. The degree of entanglement correlates better with structure and water content than with sample thickness.