Keshaan Singh

Bio: Keshaan Singh is an academic researcher from University of the Witwatersrand. The author has contributed to research in topics: Physics & Polarization (waves). The author has an hindex of 2, co-authored 7 publications receiving 21 citations.

Digital Stokes polarimetry and its application to structured light: tutorial.

Abstract: Stokes polarimetry is a mature topic in optics, most commonly performed to extract the polarization structure of optical fields for a range of diverse applications. For historical reasons, most Stokes polarimetry approaches are based on static optical polarization components that must be manually adjusted, prohibiting automated, real-time analysis of fast changing fields. Here we provide a tutorial on performing Stokes polarimetry in an all-digital approach, exploiting a modern optical toolkit based on liquid-crystal-on-silicon spatial light modulators and digital micromirror devices. We explain in a tutorial fashion how to implement two digital approaches, based on these two devices, for extracting Stokes parameters in a fast, cheap, and dynamic manner. After outlining the core concepts, we demonstrate their applicability to the modern topic of structured light, and highlight some common experimental issues. In particular, we illustrate how digital Stokes polarimetry can be used to measure key optical parameters such as the state of polarization, degree of vectorness, and intra-modal phase of complex light fields.

Revealing the invariance of vectorial structured light in complex media

Abstract: Optical aberrations have been studied for centuries, placing fundamental limits on the achievable resolution in focusing and imaging. In the context of structured light, the spatial pattern is distorted in amplitude and phase, often arising from optical imperfections, element misalignment, or even from dynamic processes due to propagation through perturbing media such as living tissue, free-space, underwater and optical fibre. Here we show that the polarisation inhomogeneity that defines vectorial structured light is immune to all such perturbations, provided they are unitary. By way of example, we study the robustness of vector vortex beams to tilted lenses and atmospheric turbulence, both highly asymmetric aberrations, demonstrating that the inhomogeneous nature of the polarisation remains unaltered from the near-field to far-field, even as the structure itself changes. The unitary nature of the channel allows us to undo this change through a simple lossless operation, tailoring light that appears robust in all its spatial structure regardless of the medium. Our insight highlights the overlooked role of measurement in describing classical vectorial light fields, in doing so resolving prior contradictory reports on the robustness of vector beams in complex media. This paves the way to the versatile application of vectorial structured light, even through non-ideal optical systems, crucial in applications such as imaging deep into tissue and optical communication across noisy channels.

Accelerating polarization structures in vectorial fields.

Abstract: We generate optical fields whose polarization structures not only rotate about their propagation axis but also can be controlled to accelerate independently from their spatial profile. We show that by combining accelerated intensity transport with orthogonal polarization states, we can produce a vector beam that displays optical activity with periodical acceleration and deceleration of the Stokes vector during propagation. We achieve this with orthogonal, scalar fields, represented by weighted superpositions of oppositely charged Bessel beams. In addition to their creation, we show that the Stokes vector can be made to accelerate or decelerate at specific locations along the Poincare sphere by tailoring the generating basis. We also witness an optical current, or intensity transport, between local positions in the field that corresponds with the occurrence of the state-of-polarization accelerating or decelerating.

Demonstrating Arago–Fresnel laws with Bessel beams from vectorial axicons

Abstract: Two-dimensional Bessel beams, both vectorial and scalar, have been extensively studied to date, finding many applications. Here we mimic a vectorial axicon to create one-dimensional scalar Bessel beams embedded in a two-dimensional vectorial field. We use a digital micro-mirror device to interfere orthogonal conical waves from a holographic axicon, and study the boundary of scalar and vectorial states in the context of structured light using the Arago–Fresnel laws. We show that the entire field resembles a vectorial combination of parabolic beams, exhibiting dependence on solutions to the inhomogeneous Bessel equation and asymmetry due to the orbital angular momentum associated rotational diffraction. Our work reveals the rich optical processes involved at the interplay between scalar and vectorial interference, opening intriguing questions on the duality, complementarity, and non-separability of vectorial light fields.

A Robust Basis for Multi‐Bit Optical Communication with Vectorial Light

Abstract: Increasing the information capacity of communication channels is a pressing need, driven by growing data demands and the consequent impending data crunch with existing modulation schemes. In this regard, mode division multiplexing (MDM), where spatial modes of light form the encoding basis, has enormous potential but is impeded by noise due to imperfect channels. Here, this challenge is overcome by breaking the existing MDM paradigm of using the modes themselves as a discrete basis and instead exploiting the polarization inhomogeneity (vectorness) of vectorial light as the information carrier. It is shown that this vectorness communication basis is completely impervious to channel noise, which is verified by near perfect data fidelity maintained in multi‐bit information transfer through atmospheric turbulence, with negligible changes on the order of 1%. This allows for demonstration of a new state‐of‐the‐art of 50 vectorial modes in a communications channel with little cross‐talk. This approach replaces conventional amplitude modulation with a novel modal alternative for potentially orders of magnitude channel information enhancement, offering a new paradigm to exploiting the spatial mode basis for optical communication.

Superpositions of the orbital angular momentum for applications in quantum experiments : Special issue on atoms and angular momentum of light

Abstract: Two different experimental techniques for preparing and analysing superpositions of Gaussian and Laguerre-Gaussian modes are presented. These involve exploiting an interferometric method in one case and using computer-generated holograms in the other. It is shown that by shifting a hologram with respect to an incoming Gaussian beam, different superpositions of the Gaussian and the Laguerre-Gaussian beam can be produced. An analytical expression connecting the relative phase, the amplitudes of the modes and the displacement of the hologram is given. The application of such orbital angular momenta superpositions in quantum experiments such as quantum cryptography is discussed.

Relationship between DNA damage and sperm head birefringence

Abstract: Sao Paulo State Univ UNESP, Botucatu Med Sch, Ctr Human Reprod Prof Franco Jr,Res Unit, Paulista Ctr Diag Res & Training,Dept Gynecol & O, Ribeirao Preto, Brazil

Detection of Bessel beams with digital axicons

Abstract: We propose a simple method for the detection of Bessel beams with arbitrary radial and azimuthal indices, and then demonstrate it in an all-digital setup with a spatial light modulator. We confirm that the fidelity of the detection method is very high, with modal cross-talk below 5%, even for high orbital angular momentum carrying fields with long propagation ranges. To illustrate the versatility of the approach we use it to observe the modal spectrum changes during the self-reconstruction process of Bessel beams after encountering an obstruction, as well as to characterize modal distortions of Bessel beams propagating through atmospheric turbulence.

Nonlinear optics with structured light

Abstract: The interest in tailoring light in all its degrees of freedom is steadily gaining traction, driven by the tremendous developments in the toolkit for the creation, control and detection of what is now called structured light. Because the complexity of these optical fields is generally understood in terms of interference, the tools have historically been linear optical elements that create the desired superpositions. For this reason, despite the long and impressive history of nonlinear optics, only recently has the spatial structure of light in nonlinear processes come to the fore. In this review we provide a concise theoretical framework for understanding nonlinear optics in the context of structured light, offering an overview and perspective on the progress made, and the challenges that remain.