arXiv subjects

Nirmal K. Viswanathan

Publications and source records attributed to Nirmal K. Viswanathan.

15 recordsLinked to original sources

arXiv2026

Enhanced Spin-Orbit Interaction of Light through Complex Fresnel Reflection

Spin-orbit interaction (SOI) of light manifests as the generation of a spin-dependent vortex beam and the spin-Hall effect when a spin-polarized beam interacts with an optical interface at normal and oblique incidence, respectively. The weak SOI limits its widespread use for quantifying physical quantities in various applications. We demonstrate a significantly enhanced SOI effect, approaching the strong-coupling regime, by retroreflecting a nonparaxial focused beam of light at a multilayer dielectric mirror. The large difference in angle-of-incidence-dependent Fresnel reflection coefficients (r-TE, r-TM ) introduced by the mirror simultaneously enhances the spin-orbit-converted vortex-beam amplitude and the spin-Hall effect in the reflected beam. Near-field superposition of the orthogonal polarization components in the reflected beam yields a spatially nonuniform beam field with C-, V-, and L- polarization singularities. These topological features are mapped and are used to demonstrate the transverse and 3D spin-Hall effect in the focal region. The superposition of enhanced optical near-fields and the lossless broadband nature of the mirror reflectivity are expected to enable a variety of polarization topologies and to tailor their features via designed mirror reflectivities.

physics.optics↗

arXiv2025

Generic Optical Singularities in Brewster-reflected Post-paraxial Beam-fields

Brewster-reflection of a post-paraxial optical beam at a plane dielectric interface unravels fundamentally significant optical singularity dynamics. We express the simulated field-component profiles of a Brewster-reflected post-paraxial beam-field via empirical functions, using which we demonstrate optical beam-shifts and formation of phase singularities. These occurrences naturally reveal the presence and complex transitional-dynamics of generic polarization-singularities -- which we observe via simulation and experiments. A single reflection being the core process, our method becomes a fundamentally appealing way to generate optical singularities and to study their dynamics.

physics.optics↗

arXiv2024

Spin-orbit Interaction-mediated Measurement of Surface Chirality

The spin-orbit interaction in a focused-reflected beam of light results in spatially non-uniform polarization in the beam cross-section due to the superposition of orthogonal field components and polarization-dependent interface reflection coefficients. Polarization filtering the output beam leads to an interchangeable transformation of - or + 2 charge vortex into two - or + unit charge vortices, for + or - circular polarization of the input Gaussian beam. This transformation follows a trajectory, named optical vortex trajectory, that depend on the input beam spin and hence the vortex charge and reflecting surface characteristics. The OVT is used here to quantify both the sign and the magnitude of the chiral parameter of a quartz crystal. The Jones matrix-based simulation anticipates the chirality-dependent OVT that matches with experimental measurements.

physics.optics↗

arXiv2024

Experimental Measurement of Transverse Spin Dynamics in the Nonparaxial Focal Region

The superposition of complex optical fields in three-dimension (3D) is the basis of several non-trivial wave phenomena. Significant among them are the non-uniform polarization distribution and their topological character, leading to the emergence of transverse spin, transverse momentum, and their dynamics. These aspects are experimentally measured in the nonparaxial focal region of a circularly-polarized Gaussian input beam. A dielectric mirror, kept in the focal region, is axially scanned to obtain the phase and polarization variations in the retro-reflected output beam using an interferometer and spatially-resolved Stokes parameter measurements. The identification of phase and polarization singularities in the beam cross-section and their behaviour as a function of the mirror position enabled us to map and study the phase-polarization variations in the nonparaxial focal region. The lemon-monstar type polarization patterns surrounding the C-point singularity in the output beam are identified and tracked to study the transverse spin dynamics for right- and left- circular polarization of the input beam. Direct measurement of the input beam polarization helicity-independent and helicity-dependent aspects of the transverse and longitudinal spin dynamics in the nonparaxial focal region are the significant findings reported here. The proposed and demonstrated measurement method allows us to investigate the nonparaxial focal region in more detail and has the potential to unravel other intricate optical field effects.

physics.optics↗

arXiv2024

Orbital Angular Momenta and Geometric Phase Characteristics of Spin-polarized Components of a General Paraxial Beam-field

We explore a subtle and fundamental nature of spin-orbit interaction (SOI) in a general paraxial beam-field by mathematically characterizing the orbital angular momentum (OAM) flux densities of the spin-polarized component fields via Barnett's flux determination formalism. We show the application of our formalism by considering the special case of a Brewster-reflected paraxial beam, for which we explore specific details such as non-canonical vortex phase structures and single-photon interpretation of the field function. The phases of the spin-component fields are interpreted as geometric phases; and we devise an experimental method of direct measurement of these phases by transferring the spatial phase information to the polarization domain -- thus demonstrating a classical analog von Neumann measurement. Our complete mathematical characterization of SOI and its relevance to the direct measurement of geometric phase can find potential application in future methods exploring OAM and geometric phase characteristics in other general optical systems.

physics.optics↗

arXiv2024

Complex far fields and optical singularities due to propagation beyond tight focusing: combined effects of wavefront curvature and aperture diffraction

All optical systems, which involve the collimation of a reflected, transmitted or scattered wave subsequent to tight focusing, are subject to two kinds of deviations. One is the wavefront curvature due to inaccurate focal placement of the interface or scatterer particle under consideration, and the other is the diffraction caused by the finite lens aperture. In the present paper we explore these phenomena in detail by considering a rigorous simulated model and an appropriate experimental setup. We hence demonstrate the complicated intensity profiles and optical singularity characteristics of the observed far field. Then we describe ways to minimize these deviations in a general experiment. But more importantly, our analysis proves that these deviations by themselves are significant optical phenomena of fundamental interest. The observed complex field profiles have similarities to standard diffraction-limited tight focal fields, though our field detection is different from the standard schemes. This indicates the relevance of these complex fields to a larger class of systems involving wavefront curvature and aperture diffraction. The detailed analysis and results of the present paper already serve as core explorations of these optical phenomena; and we also suggest future research directions where these system aspects can be purposefully created and explored further.

physics.optics↗

arXiv2023

Measurement of Orbital Angular Momentum of Light using Stokes Parameters and Barnett's Formalism

We present a formalism for experimental determination of the orbital angular momentum (OAM) of a paraxial optical beam-field by intertwining Barnett's formalism and Stokes parameter measurements. Using Barnett's formalism we calculate the OAM flux density of a suitably general 2-dimensional beam-field; and re-express it in terms of experimentally measurable quantities. Considering a demonstrative optical system, we obtain the OAM flux density profiles, total OAM flux and OAM per photon -- leading towards the experimental characterization of OAM using Barnett's formalism. Our method will find potential applications in nano-optical processes involving the transfer of electromagnetic OAM to particles and across interfaces.

physics.optics↗

arXiv2022

Optical Singularity Dynamics and Spin-Orbit Interaction due to a Normal-Incident Optical Beam Reflected at a Plane Dielectric Interface

The degenerate case of normal incidence and reflection of an optical beam (both paraxial and non-paraxial) at a plane isotropic dielectric interface, which is azimuthally symmetric in terms of the momentum-spatial variation of Fresnel coefficients but not in terms of the fundamental polarization inhomogeneity of the incident field, requires in-depth analyses. In this paper, we use the reflection and transmission coefficient matrix formalism to derive an exact field expression of a normal-reflected diverging beam. The availability of the exact field information allows controlled variations of the system parameters, leading to significant dynamics of phase and polarization singularities hitherto unanticipated in the literature. We carry out a detailed exploration of these dynamics in our simulated system, and also verify them experimentally by using an appropriate setup. We then use Barnett's formalism to determine the associated orbital angular momentum (OAM) fluxes, leading to a subtle interpretation and mathematical characterization of spin-orbit interaction (SOI) in the system. Our work thus represents a non-trivial unification of the most fundamental electromagnetic reflection/transmission problem at a plane dielectric interface and the emerging areas of optical singularity dynamics with their understanding in terms of OAM flux and SOI. The normal-incidence--retro-reflection geometry being especially amenable to applications, these beam-field phenomena are anticipated to have applications in interface characterization, particle rotation/manipulation and other nano-optical processes.

physics.optics↗

arXiv2022

Generic Optical Singularities and Beam-field Phenomena due to General Paraxial Beam Reflection at a Plane Dielectric Interface

A single paraxial beam reflection at a plane dielectric interface, configured appropriately, can lead to the formation of a polarization singularity in the inhomogeneously polarized output beam-field for any central angle of incidence. In this paper we derive the necessary condition to realize this effect. We explore the phase singularity characteristics associated to this polarization-singular field; and explore the dynamics of the singularities due to controlled variations of the input polarization. The simulation-generated exact field information lead to the exploration of the unique Goos-Hänchen, Imbert-Fedorov and spin shifts of the optical-singular fields and the anticipation of an exact mathematical characterization of spin-orbit interaction phenomena involved therein. The formation of a phase singularity independent of a polarization singularity is explained subsequently. Interrelating these seemingly unconnected beam-field phenomena and generic optical singularities can lead to a significant and fundamental understanding of the inhomogeneously polarized beam-field; and additionally, our singularity generation method can find potential application in experimental characterization of the involved dielectric media.

physics.optics↗

arXiv2022

High-Sensitivity Characterization of Ultra-Thin Atomic Layers using Spin-Hall Effect of Light

Magnetic/non-magnetic/heterostructured ultra-thin films' characterisation is highly demanding due to the emerging diverse applications of such films. Diverse measurements are usually performed on such systems to infer their electrical, optical and magnetic properties. We demonstrate that MOKE-based spin-Hall effect of light (SHEL) is a versatile surface characterization tool for studying materials' magnetic and dielectric ordering. Using this technique, we measure magnetic field dependent complex Kerr angle and the coercivity in ultra-thin films of permalloy (Py) and at molybdenum disulphide (MoS$_2$) - permalloy (MSPy) hetero-structure interfaces. The measurements are compared with standard magneto-optic Kerr effect (MOKE) studies to demonstrate that SHEL-MOKE is a practical alternative to the conventional MOKE method, with competitive sensitivity. A comprehensive theoretical model and simulation data are provided to further strengthen the potential of this simple non-invasive optical method. The theoretical model is applied to extract the optical conductivity and susceptibility of non-magnetic ultra-thin layers such as MoS$_2$ .

physics.optics↗

arXiv2020

Generalized Matrix Transformation Formalism for Reflection and Transmission of Complex Optical Waves at a Plane Dielectric Interface

We describe a generalized formalism, addressing the fundamental problem of reflection and transmission of complex optical waves at a plane dielectric interface. Our formalism involves the application of generalized operator matrices to the incident constituent plane wave fields to obtain the reflected and transmitted constituent plane wave fields. We derive these matrices and describe the complete formalism by implementing these matrices. This formalism, though physically equivalent to Fresnel formalism, has greater mathematical elegance and computational efficiency as compared to the latter. We utilize exact 3D expressions of the constituent plane wavevectors and electric fields of the incident, reflected and transmitted waves, which enable us to seamlessly analyse plane waves, paraxial and non-paraxial beams, highly diverging and tightly focused beam-fields as well as waves of miscellaneous wavefront-shapes and properties using the single formalism. The exact electric field expressions automatically include the geometric phase information; while we retain the wavefront curvature information by using appropriate multiplicative factors. We demonstrate our formalism by obtaining the reflected and transmitted fields in a simulated Gaussian beam model. Finally, we briefly discuss how our generalized formalism is capable of analysing the reflection-transmission problem of a very large class of complex optical waves -- by referring to some novel works from the current literature as exemplary cases.

physics.optics↗

arXiv2019

Ultra-sensitive Single-beam Atom-optical Magnetometer using Weak Measurement Method

Ultra-sensitive measurement of the magneto-optical rotation, due to interaction of linearly-polarized light passing through room-temperature Rb 85 atoms, in response to change in longitudinal magnetic field (δB_z ) is demonstrated using the weak measurement method. The polarization rotation angle measurement sensitivity (δψ) of 16 μ~rad and hence of the magnetometer of 1 nT, achieved using the weak measurement method is better than the balanced optical polarimetry results by a factor of three. The improvement in the measurement sensitivity is realized via optical amplification of the polarization rotation angle via spin-orbit coupled light beam-field. The method is devoid of external rf modulation, allows for optimal tunability of sensitivity depending on the dynamic range of the applied magnetic field and the sensitivity can be further enhanced by operating in the Spin Exchange Relaxation Free regime of alkali spin polarization.

physics.ins-det↗

arXiv2017

Measuring correlations in non-separable vector beams using projective measurements

Doubts regarding the completeness of quantum mechanics as raised by Einstein, Podolsky and Rosen(EPR) have predominantly been resolved by resorting to a measurement of correlations between entangled photons which clearly demonstrate violation of Bell's inequality. This article is an attempt to reconcile incompatibility of hidden variable theories with reality by demonstrating experimentally a violation of Bell's inequality in locally correlated systems whose two degrees of freedom, the spin and orbital angular momentum, are maximally correlated. To this end we propose and demonstrate a linear, achromatic modified Sagnac interferometer to project orbital angular momentum states which we combine with spin projections to measure correlations.

quant-ph↗

arXiv2015

Quantifying classical entanglement using polarimetry: Spatially-inhomogeneously polarized beams

Generation of spatially inhomogeneously polarized (SIP) beams due to polarization spatial mode nonseparability is demonstrated using a polarization Sagnac interferometer. Counter propagating horizontal (H) and vertical (V) polarized Laguerre Gaussian beams of opposite topological charges(LG01)corresponding to the two binary degrees of freedom (DoF) are coherently superposed in the interferometer. Quantum inspired entanglement witness and quantification such as Bell state measurement, violation of CHSH form of Bells inequality and concurrence measurements are experimentally measured to establish nonseparability between the two DoF. As the SIP beams are equivalent to bipartite pure states, point wise Stokes polarimetry measurements are used to calculate the reduced coherency matrix corresponding to polarization DoF. Using these we calculate the linear entropy and maximized Bells measure to quantify the degree of entanglement of the SIP beams.

physics.optics↗

arXiv2014

Generation of isolated asymmetric umbilics in light's polarization

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.

physics.optics↗