This PDF file contains the front matter associated with SPIE Proceedings Volume 12436, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

A final frontier of optical micromanipulation of mesoscopic particles in high-numerical aperture microscopes is the combination of displacement and rotation of single particles. In addition to the interesting challenge this presents, it is useful for the investigation of microscopic systems. Vaterite is a spheroidal birefringent particle used in optical micromanipulation for its ability to cause microscopic flow fields and measure the microenvironment surrounding it. It spins by changing the polarization of beam used to trap it. Typically, spinning vaterites can create a flow field with rotation axis in the direction of the beam axis. We show how light can be structured to allow optical trapping and rotation of vaterite—and other birefringent particles—around arbitrary axes, including those orthogonal to the average beam transmission axis! To achieve this effect, we use vector beam shaping produced using an interferometric technique resistant to changes in air currents and mechanical vibrations.

It is well understood that circularly polarized light possesses optical chirality. This optical chirality can engage with material chirality, leading to optical activity, the underlying physics of chiroptical spectroscopy. Optical vortex beams with azimuthal phase possess helical wavefronts that are chiral. Under spatial confinement (e.g. tight focusing), the polarization state of the input beam and the geometrical wavefront chirality are not independent of one another, and their interplay significantly influences the optical chirality density of the beam. Here we highlight how the state of input beam polarization, e.g. linear, elliptical, and circular, influences the optical chirality of vortex beams. We show that the spatial distributions of the optical chirality density are acutely dependent on the polarization of the input beam, alongside other characteristics like the topological charge and radial index. Most striking is that even an unpolarized optical vortex beam possesses non-zero optical chirality density.

Coupling and mutual influence of spin angular momentum (SAM), orbital angular momentum (OAM), and linear momentum of light have led to a number of photonic spin-orbit interaction (SOI) effects in various light-matter interactions[1-2]. These have provided new insights on the universal SOI phenomena and also have opened up a new paradigm of spin-orbit photonic devices. Despite the considerable promises of these spin-orbit photonic effects, there remain several outstanding challenges to address. Besides the enhancement of these weak SOI effects, developing novel experimental systems for efficient probing, interpreting their underlying physics and manifestation are highly sought after in the context of developing photonic SOI based technologies. Among the various SOI effects, photonic spin-momentum locking has attracted particular attention due to its profound origin and potential device applications. Usually the spin-directional coupling is obtained in spatially inhomogeneous anisotropic metamaterials or metasurfaces through appropriate tailoring of the geometric phase gradient leading to the breaking of the spatial inversion symmetry. The photonic spin-momentum locking has also been demonstrated in planar interfaces without any structures. In this scenario, the spin-directional coupling of the transversly propagating surface waves or waveguide modes arises due to the presence of transverse spin angular momentum (SAM) of the evanescent waves.

We review highlights of our recent contributions to understanding the propagation dynamics and transverse orbital angular momentum of optical pulses carrying spatiotemporal optical vortices (STOVs). STOVs, which were first observed as an emergent phenomenon in nonlinear self-focusing, were first linearly generated using a 4𝑓 pulse shaper and measured using transient-grating single-shot supercontinuum spectral interferometry (TG-SSSI). That STOV-based transverse orbital angular momentum (OAM) is carried at the single photon level was then confirmed in measurements of OAM conservation in second harmonic generation. Our recent theory for the electromagnetic mode structure and transverse OAM of STOV-carrying pulses in dispersive media predicts half-integer OAM and the existence of a transverse OAM-carrying quasiparticle: the bulk medium STOV polariton.

The particular capabilities of selected combined adaptive optical systems for laser beam shaping in near infrared were studied. Fast switching sequences were obtained by combining an LCoS-SLM with angular-tuning, piezo-driven MEMS-axicons. By flexibly programming highly localized beams into SLM phase maps, the illumination of MEMS microaxicons, tunable spiral phase plates and Fresnel mirrors was optimized to enable for fast, variable and nondiffracting shaping performance. The approach can be applied to advanced types of optical processors like adaptive autocorrelators, or in micromachining. By varying the divergence of an illuminating beam with a liquid lens, spatio-temporal self-imaging of array patterns known as nondiffracting Talbot effect was demonstrated in adaptive mode with tunable Talbot distances.

We numerically model the propagation of light through a Kerr medium and through a far-red-detuned Bose Einstein condensate (BEC) inside an optical cavity using Lugiato-Lefever and Gross-Pitaevskii equations. We demonstrate the formation of light-atomic ring lattices with rotation speeds and direction that can be controlled by the orbital angular momentum of the light. In the BEC, we show the possibility of moving from a lattice to a ring-shaped atomic circuit by changing the atomic scattering length, and explore the potential generation of rotating BEC cavity solitons. Our results may be of interest as slow light pulses with fully controllable speed and structure for use in optical quantum memories, for particle manipulation and trapping, and for the novel realization of highly controllable, tightly confined, rotating atomic lattices.

We numerically model the propagation of far-detuned optical beams through a Bose-Einstein condensate (BEC) using coupled nonlinear Schr¨odinger and Gross-Pitaevskii equations. For red atom-field detuning, we show that light carrying orbital angular momentum (OAM) can lead to coupled light-atom solitons carrying angular momentum. We demonstrate the formation of azimuthally rotating wavepackets, toward an atomic persistent current without trapping requirements. For blue atom-field detuning, we show highly exotic BEC distributions, with the atoms trapped in dark regions of the optical field. Our results provide a novel means of atomic transport and of sculpting a BEC into unique transverse distributions.

We report on HoloTile, a new approach to speckle-reduced and rapidly generated holograms. By way of an analytical approach to Gaussian beam shaping, we modify the Point Spread Function (PSF) of a holographic system into flat-top squares. The width of each flat-top output “pixel” is determined from the inter-spectral distance arising from designed tiling on a Spatial Light Modulator (SLM). The resultant hologram consists of the product of a once-calculated PSF shaping hologram and a tiled object hologram. With HoloTile, we achieve speckle-reduced reconstructions and extremely fast hologram generation, due to the reduced resolution of the object hologram.

In-line Digital Holographic Microscopy (DHM) is an imaging technique that creates 3D images from holographic data. We categorize DHM reconstruction methods into direct, iterative, and machine learning based algorithms. Direct methods can be fast but often suffer from noise and low contrast due to the inherent twin-image aberration present in holographic reconstruction. Iterative methods improve accuracy and reduce noise but are generally more time consuming and computationally complex. Finally, machine learning methods can be extremely fast at inference but require large amounts of resources and training data. We provide a literary and illustrative summary of these methods (Figure 1), allowing for easy comparison.

Here we introduce a free-space optical communication (FSOC) system that is capable of adjustment to alignment drift, varying atmospheric turbulent conditions of multiplexed spatial structured laser beams such as orbital angular momentum (OAM) beams. The detection system is based on heterogeneous convolutional neural network with first Fourier convolution neural network layer implemented in optics as a 4f system driven by kilohertz-fast reprogrammable high-resolution digital micromirror devices (DMDs). We utilize this optical-filtering-based convolutional neural network to realize the training and identification of two co-propagating OAM beams among 12 different multiplexed modes under simulated turbulent condition using modified von K´arm´an atmospheric model. The current implementation shows test accuracy of 95.04% (under weak turbulence) and 87.52% (under strong turbulence).

Einstein beams are coherent optical beams generated by the conditions of gravitational lensing. In the ray picture, Einstein beams are formed by the intersection of light rays deflected by a lensing mass, similar to non-diffracting Bessel beams, but with the difference that adjacent rays diverge slightly. When accounting for the wave properties of light, they form beam-like diffraction patterns that preserve their shape but expand as the light propagates. The addition of a topological charge to the light, leads to more complex patterns carrying orbital angular momentum. For symmetric lensing conditions, Einstein beams carry modes described by confluent hypergeometric functions, which can be approximated by Bessel functions. A theoretical analysis of this is presented here. In astrophysical observations, many of these features can only be inferred because conditions of coherence and alignment make them difficult to observe. Studies of Einstein beams in the laboratory can be used to inform astrophysical observations and discover new non-astrophysical laboratory applications.

In the interactions of optical vortices with matter, principles of fundamental symmetry determine the viability of observing various forms of chiral discrimination. To elicit the underlying principles requires careful analysis of the geometrically chiral structure of the beam. The scale of observation becomes especially important for local, intra-beam measurements, as for example when quantum transitions occur. Approaching such issues invites a deeper analysis in terms of fundamental symmetry, in order to ascertain conditions in which chiral discrimination may, or may not, be permissible. These methods provide a straightforward and reliable basis for judgement on whether previously unrecognized forms of interaction may be worth pursuing in experiment.

Optical beams reflected or transmitted from an interface experience polarization-dependent subwavelength deflection in the centroid. We demonstrate the inherent non-separability of longitudinal and transverse beam shift taking an example of the partial reflection of a Gaussian beam from an air-glass interface. The specific regimes in the parameter space manifesting the observable non-separability are pointed out. Such non-separability leads to a classical position-position entanglement from a simple partial reflection of a Fundamental Gaussian beam. The experimental parameters offer control over the degree of entanglement. Such a non-separable structure of light and its dependence on the system parameters might have potential applications in sensing and metrology.

The design of a dynamically tunable, dual-axicon beam expander for coupling into higher order modes in standard, large core step-index fiber is presented. A single mode of the optical fiber is excited, allowing bend-resistant propagation and efficient perseveration of brightness for a variety of applications. The coupling from the dual-axicon system is shown to be equivalent to utilizing the standard single axicon coupling system, with improved ease of use. The described optical train utilizes all high-purity fused silica optical components to allow very high peak and average powers for fiber coupling.

Much research has been performed into applications of visible vortex Laguerre-Gaussian modes, including chiral material fabrication. These will demand compact, robust sources at a range of emission wavelengths. We have developed an ultracompact vortex laser source with a blue (442nm) diode pumped Pr3+ doped water-proof fluoro-aluminate glass (Pr:WPFGF) fibre laser, emitting red (638nm) and, for the first time, orange (612nm) vortex modes, in addition to balanced dual wavelength emission. The laser uses a standard microscope slide as an interferometric output coupler to convert the internal Gaussian mode into a vortex output, yielding high quality first order vortex modes in a compact setup. We additionally demonstrate the broad (∼5 nm) emission bands in the orange (610 nm), red (637 nm) and infrared regions (698 nm), with the potential for green (530 nm) and cyan (485 nm) emission.

Capabilities to structure X-ray light are developing fast and now allow to generate and characterize light carrying orbital angular momentum (OAM). The use of X-rays to design novel spectroscopic signals sensitive to molecular chirality will be presented. Their differences and similarities with optical techniques will be briefly outlined and we then introduce X-ray helical dichroism, the dichroic measurement of absorption spectra with positive and negative OAM. An experimental implementation of helical dichroism developed at the Swiss Light Source (SLS) synchrotron using spiral Fresnel zoneplates on iron chiral complexes will be discussed.

A new polarization state generator (PSG) is presented based on two geometric-phase diffraction gratings, useful to generate partially polarized light in an efficient and controlled way. The PSG is used to experimentally analyze the generation of vector and vortex beams which lie, respectively, inside the generalized higher-order (HOPS) and the orbital angular momentum (OAMPS) Poincaré spheres. This way the mapping relations between polarization states, vector beams and vortex beams are analyzed and the classical concept of degree of polarization (DoP) is extended to these other Poincaré spheres. Experimental results that validate the theory are presented.

Optical binding occurs when light scattered from a microscopic particle induces forces on other microscopic particles, which includes attractive, binding, forces. Since the original demonstration using dielectric spheres, there has been much interest in altering the shape and symmetry of the bound particles, leading to complex optically driven behaviour typically involving uniform illumination in a restricted region e.g. an extended or line focus or the axis of a Bessel beam. However, in the present computational study we consider confinement in complex optical landscapes comprising one or many focussed beams. In this survey we will explore the behaviours of systems of spherical beads with various refractive indices, as well as systems of dimers, or dumbbells, consisting of pairs of linked spheres and active janus particles comprising pairs of beads with different refractive indices. The relative importance of optical trapping versus optical trapping between the particles will be assessed.

The curvature and intensity of photonic hooks depend on the size, shape, and refractive index of the particles and can be used as optical tweezer that can guide cells/nanoparticles around obstructions. We propose the generation of on-chip photonic hook from a TiO2 hemispherical dome on SiO₂ substrate. We further found that the curvature of the photonic hook can be continuously tuneable as a function of angle of illumination. For illumination angles below 20°, the photonic hook bends towards the optical axis and for angles above 20°, the beam bends away from the optical axis, forming a tuneable photonic hook.

We study here the amount of helicity exchange of scattered light from the monolayer molybdenum disulfide (MoS₂) or graphene on different structures by using spin angular momentum of light in Raman spectroscopy. Our in-house modified Raman spectroscopy setup is used for analyzing the optical helicity by calculating the degree of circular polarization (DoCP) of circularly polarized incident light when scattered from the 2D materials on different substrates. We observe Raman intensity variation when controlling phonon vibration of the 2D materials under the polarized excitation. The experimental results show that the layered materials deposited on SiO₂/Si substrates has maximum DoCP as compared to other substrates with different structures. All the findings are analyzed by numerical simulation that incorporate the related Raman tensors and optical Jones calculus, which is worth exploring the phonon-electron interaction.

Laser-induced surface modifications self-organization are alternatives to lithographic techniques for the functionalizing of surfaces on the micro- and nanoscale. Recently the fabrication of LIPSS with circularly polarized light gained increased attention. 2D patterning may be achieved on a variety of surfaces. We demonstrate the structuring of thin metal films on silicon, obtaining stripe, honeycomb, and hexagonal nanoparticle array patterns through laser-driven self-organization. Patterning is demonstrated on different metals, all showing individual pattern preferences and pattern developments dependent on the energy dose. The metal thin film thicknesses, the free energy of oxide formation and alloying, and the individual irradiation parameters affect the pattern formation and can be tuned to obtain the desired metal patterns.

Surface metrology in the nanometric regime is of increasing relevance in many scientific and industrial applications. Here we present two techniques using structured light to measure topographic features, both being all-digital and contactless and offering improved stability. The first one uses a common path interferometer, scanning the sample to obtain a surface map. The second one takes advantage of the modal description of light to determine the height of a step-like feature. In both cases, we obtained excellent agreement between our measurements and the known reference surfaces, thus demonstrating the good performance of our approaches.

We discuss studies on optical trapping dynamics of particles at different axial planes and effect of optical nonlinearity on the equilibrium position of stable trap under femtosecond pulsed excitation.

HoloTile aims to solve the challenge of rapid and speckle-free light sculpting without the need for time-averaging techniques — a challenge that exists in several fields of optics, photonics and display technology. HoloTile works by multiplexing the phase-shaped point spread function of the diffractive system to match the inter spatio-spectral spacing in the far field that occurs due to tiling on a high-resolution phase-only modulating device.

It is known for a while now that a focused circularly-polarized Gaussian beam produces vortex mode of topological charge ±1 in the longitudinal field component. While the remaining transverse field component in the focal region is paid little attention to, it is important to note that it is spatially inhomogeneous due to superposition of parallel and orthogonal to the input field components. Reflecting the focused light beam using a highly reflecting mirror, kept at the focal plane, followed by polarization filtering of the beam in the Fourier plane leads us to realize high-quality and high-conversion efficiency optical vortex beam of charge ±2, which depends on the input beam spin, σ = ±1. We present here the theoretical formulation to understand the generation of vortex beam followed by simulation and experimental demonstration to ascertain the anticipation.

We investigate the joint spectral-spatial dispersion effects on the spin-orbit interaction of light. The effect arises due to propagation of weakly focused linearly-polarized light beam through an elliptically birefringent quartz crystal plate (XP). We demonstrate via simulation and experiment a continuous and full rotation of the two-lobe intensity pattern around a phase singular point in the parameter space as a function of the incident beam wavelength (λ) and the fast-axis orientation (ϕ) of the XP. The joint spectral-spatial dispersion effect on the spin-orbit interaction of light further enhances our understanding of light-matter interaction and the emergence of topological behavior from a single crystal plate.

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.

While the spin-to-orbital angular momentum (AM) conversion and the spin-Hall effect are widely investigated effects of the spin-orbit interaction of light, the inverse orbital-to-spin AM conversion has rarely been paid attention to. The pair-wise first-order effects concerning the influence of the intrinsic and extrinsic orbital AM on the spin AM of light are a part of the orbital-to-spin AM conversion. These effects manifest as changes in the spin content under symmetric and broken symmetric conditions of the optical system. Furthermore, the second-order effects of the combined influence of the intrinsic and extrinsic orbital AM on the spin AM of light still remain unexplored. To this end, we demonstrate the orbital-to-spin AM conversion for off-axis propagation of spin-free optical vortex (OV) beam incident at different input positions on a GRIN rod and as a function of the topological charge of the OV beam. The changes in the output beam polarization measured and quantified through the S3 Stokes parameter is seen to increase as a function of the charge and off-axis position of the OV beam and exhibits asymmetry for + and – charges of the OV beam. The differential trajectory and the orbital-Berry phase for + and – charges OV beams lead to the measured variation of the S3 parameter. Understanding such intricate effects can potentially be helpful in various applications including optical communication, particle manipulation, light-guiding, and finding the signature patterns of fundamental interaction of light related to higher-order OV, HG, Poincaré, and vector-vortex beams.