There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Accurate measurement of chirality is essential for the advancement of natural and pharmaceutical sciences. We report here a method to measure chirality using non-separable states of light with geometric phase-gradient in the circular polarization basis, which we refer to as spin-orbit beams. A modified polarization Sagnac interferometer is used to generate spin-orbit beams wherein the spin and orbital angular momentum of the input Gaussian beam are coupled. The out-of-phase interference between counter-propagating Gaussian beams with orthogonal spin states and lateral-shear or/and linear-phase difference between them results in spin-orbit beams with linear and azimuthal phase gradient. The spin-orbit beams interact efficiently with the chiral medium, inducing a measurable change in the center-of-mass of the beam, using the polarization rotation angle and hence the chirality of the medium are accurately calculated. Tunable dynamic range of measurement and flexibility to introduce large values of orbital angular momentum for the spin-orbit beam, to improve the measurement sensitivity, highlight the techniques' versatility.

Spin-orbit beams for optical chirality measurement

The more than a century old Sagnac interferometer is put to first of its kind use to generate an achromatic single-charge vortex equivalent to a Laguerre-Gaussian beam possessing orbital angular momentum (OAM) The interference of counter-propagating polychromatic Gaussian beams of beam waist ωλ with correlated linear phase (ϕ
 0
  ≥ 0025 λ) and lateral shear (y
 0
  ≥ 005 ωλ) in orthogonal directions is shown to create a vortex phase distribution around the null interference Using a wavelength-tunable continuous-wave laser the entire range of visible wavelengths is shown to satisfy the condition for vortex generation to achieve a highly stable white-light vortex with excellent propagation integrity The application capablitiy of the proposed scheme is demonstrated by generating ultrashort optical vortex pulses, its nonlinear frequency conversion and transforming them to vector pulses We believe that our scheme for generating robust achromatic vortex (implemented with only mirrors and a beam-splitter) pulses in the femtosecond regime, with no conceivable spectral-temporal range and peak-power limitations, can have significant advantages for a variety of applications

/pdf/ultrashort-vortex-from-a-gaussian-pulse-an-achromatic-55s81v1zw0.pdf

Ultrashort vortex from a Gaussian pulse - An achromatic-interferometric approach.

In recent years, there has been a growing interest in spin-orbit torques (SOTs) for manipulating the magnetization in nonvolatile magnetic memory devices. SOTs rely on the spin-orbit coupling of a nonmagnetic material coupled to a ferromagnetic layer to convert an applied charge current into a torque on the magnetization of the ferromagnet (FM). Transition metal dichalcogenides (TMDs) are promising candidates for generating these torques with both high charge-to-spin conversion ratios, and symmetries and directions which are efficient for magnetization manipulation. Moreover, TMDs offer a wide range of attractive properties, such as large spin-orbit coupling, high crystalline quality and diverse crystalline symmetries. Although numerous studies were published on SOTs using TMD/FM heterostructures, we lack clear understanding of the observed SOT symmetries, directions, and strengths. In order to shine some light on the differences and similarities among the works in literature, in this mini-review we compare the results for various TMD/FM devices, highlighting the experimental techniques used to fabricate the devices and to quantify the SOTs, discussing their potential effect on the interface quality and resulting SOTs. This enables us to both identify the impact of particular fabrication steps on the observed SOT symmetries and directions, and give suggestions for their underlying microscopic mechanisms. Furthermore, we highlight recent progress of the theoretical work on SOTs using TMD heterostructures and propose future research directions.

/pdf/spin-orbit-torques-in-transition-metal-dichalcogenide-2pdbolcwsh.pdf

Spin-Orbit Torques in Transition Metal Dichalcogenide/Ferromagnet Heterostructures

Probing Proximity-Tailored High Spin-Orbit Coupling in 2D Materials

We propose a distinct concept on the generation of optical vortex through coupling between the amplitude and phase differences of the superposing beams. For the proof-of-concept demonstration, we propose a simple free-space optics recipe for the controlled synthesis of an optical beam with a vortex dipole by superposing two transversely scaled Gaussian beams. The experimental demonstration using a Sagnac interferometer introduces the desired amount of radial shear and linear phase difference between the two out-of-phase Gaussian beams to create a vortex pair of opposite topological charge in the superposed beam. Flexibility to tune their location and separation using the choice of direction of the linear phase difference and the amount of amplitude difference between the superposing beams has potential applications in optical tweezers and traps utilizing the local variation in angular momentum across the beam cross section.

Generation of optical vortex dipole from superposition of two transversely scaled Gaussian beams.

A scheme to study the effect of residual phase gradients in an optical interference between two out-of-phase Gaussian beams is proposed. In a Sagnac interferometer configured to provide a null output, a variable linear phase swept across the null point unfolds an optical field rotation due to an apparently negligible residual phase gradient present orthogonal to the linear phase sweep. As the optical beam that rotates around its propagation axis carries orbital angular momentum, the experimental results presented in this Letter could provide an insight into the momentum change associated with the energy redistribution in the fundamental phenomenon of optical interference.

Effect of residual phase gradients in optical null interference.

Spin-orbit interactions (SOI) arising due to the propagation of a paraxial light beam, with spin-angular momentum and intrinsic- and extrinsic-orbital angular momentum degrees of freedom, along a curved trajectory and their mutual interactions is investigated due to several fundamental effects of interest. We demonstrate here all six direct and reciprocal SOI effects due to the propagation of light in an inhomogeneous-anisotropic medium, such as a graded-index rod. We show that each of light’s angular momentum component impacts the other in such a way as to have a unique effect, characterized using interferometry, polarimetry, and weak measurement methods. The results are expected to have significant impact on the basic understanding of light-matter interaction and its applications.

Direct and reciprocal spin-orbit interaction effects in a graded-index medium

A scheme to sense the residual phase gradients in an optical interference of two out-of-phase Gaussian beams is proposed. Its implementation by detecting the residual phase gradient as an optical field rotation in a Sagnac interferometer, configured to provide a null output is presented.

T. Pradeep Chakravarthy

Papers

Probing Proximity-Tailored High Spin-Orbit Coupling in 2D Materials

Generation of optical vortex dipole from superposition of two transversely scaled Gaussian beams.

Effect of residual phase gradients in optical null interference.

Direct and reciprocal spin-orbit interaction effects in a graded-index medium

Residual phase gradients in optical null interference sensing rotating optical field in a nulling Sagnac interferometer