Spin-orbit coupling in evanescent waves results in complex topological vector field structures and influences nonlinear optical interactions, optical forces and is important for high-resolution imaging and metrology. This coupling is mediated by the transverse spin carried by evanescent waves and depends on the presence of orbital angular momentum of the field. In this talk we will overview the properties of the transverse spin, its dynamics and applications, including counterintuitive transverse spin of unpolarised light and formation of photonic spin textures in guided waves, such as skyrmion and merons, in the presence of spin-orbit interactions.

Physical analogies are particularly useful to study not easily accessible systems. This is the case of the quantum pendulum, whose evolution in time is not easily observed. Fortunately, its mathematical description is shared by the Pendulum beams, which can be readily created in optical laboratories. We engineered mode superpositions that upon propagation mimic the time evolution of the quantum pendulum. We stablished experimental conditions in which the analogy is valid and present results for several relevant cases, finding excellent agreement between experiment and theoretical predictions. We expect to explore other superpositions to extend our analogy to pendulums with more complex dynamics.

In this talk, a new type of beam shaper will be discussed, capable of generating arbitrary vector spatiotemporal beams, where the user can define the amplitude, phase, and polarization independently for each point in space and time. This beam shaper was recently used to demonstrate time reversed optical waves. Such waves propagate through complex media, as if watching a traditional scattering process in reverse - starting as a complicated ‘pre-scattered’ wave, which then becomes a desired target field at the distal end of the complex media.

Optical bottle beams (OBBs) are beams of light where a dark region is completely surrounded by light, but so far, this is not necessarily the case. In modern OBBs, created by Bessel and vortex beams, nodal surfaces are present leading to a three-dimensional lobed structure. Must this be the case? We use a combination of computational modeling with novel wavefront retrieval techniques and polarization dependent wavefront manipulation with twin spatial light modulators to explore the creation of computer-generated holographic bottle beams using various methods and whether it is possible to achieve a uniform perfectly enclosed dark region.

Skyrmions are topologically stable fields that cannot be smoothly deformed into any other field configuration that differs topologically or possesses a different integer topological invariant, the Skyrme number. They have been studied as full skyrmions in magnetic spin systems, and more recently as baby skyrmions in optical systems. Here, we introduce an optical analogy to magnetic skyrmions and demonstrate their dynamics within a magnetic field. Our optical skyrmions and synthetic magnetic field are both engineered using superpositions of Bessel-Gaussian beams, with time dynamics observed over propagation distance. We show that the Skyrmionic form changes during propagation, exhibiting controllable periodic precession over a well defined range, analogous to time varying spin precession in homogeneous magnetic fields. This local precession manifests as the global beating between skyrmion types, while still maintaining the invariance of the Skyrme number.

Topological complex electromagnetic waves give access to nontrivial light-matter interactions and provide additional degrees of freedom for information transfer. An important example of such electromagnetic excitations are space-time non-separable single-cycle pulses of toroidal topology. Here we introduce an extended family of super-toroidal electromagnetic excitation, which exhibit skyrmionic structure of the electromagnetic fields, multiple singularities, and fractal-like energy backflow. By further introducing bandlimited effect into super-toroidal pulses, we show that space-time non-separable band-limited light fields can exhibit superoscillations simultaneously in the spatial and temporal domains, i.e. can oscillate faster that the highest harmonics of their spectra. The super-toroidal pulses with space-time superoscillation are of interest for transient light-matter interactions, ultrafast optics, spectroscopy, and toroidal electrodynamics.

Spatiotemporal orbital angular momentum (ST-OAM) of light is an emergent, spatiotemporally sculptured light. Such spatiotemporal optical vortices carry transverse OAM and exhibit novel properties. However, the lack of a simple and straightforward characterization method substantially slows its progress and potential adaptions for future applications. Here we demonstrated a simple, stationary, single-frame method to quantitatively characterize ST-OAM pulses. Our new method can measure the presence of ST-OAM, space-time topological charge numbers, OAM helicity, pulse dispersion, and beam divergence. We also investigated the nonlinear properties of ST-OAM pulses, uncovering the conservation of space-time topological charges in a second-harmonic generation process.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

Optical conveyors of airborne particles were previously demonstrated using different techniques allowing particles to be conveyed along straight trajectories either down or upstream the laser radiation direction. Here, the operation of an optical conveyor which has the geometry of an Archimedes screw whose central axis is a predefined arbitrary trajectory in 3D space allowing to convey particles along any desired paraxial trajectory, is demonstrated. The beams' trajectory in space can be chosen arbitrarily as long as it is paraxial. With this beam, we convey airborne particles along centimeter-scale arbitrary trajectories in adjustable velocities and directions.

We present sequential trapping and positioning of 20 nm polystyrene particles into an array configuration by using metamaterial plasmonic tweezers. The polystyrene nanoparticles suspended into a heavy water solution were trapped on adjacent plasmonic hotspots with a very low excitation power of 3.8 mW, creating a large trap stiffness of about 3.5 fN/nm. This high trapping stiffness kept the particles trapped into the nanocavities’ hotspots achieving almost 80% occupancy of the excited hotspots.

Speckle, the granular interference pattern produced when coherent light is scattered by a rough surface, is commonly thought to be detrimental to optical systems. But the scattering processes are entirely linear, and speckle can therefore be used as a surprisingly sensitive probe of the properties of the light, the environment and the scatterer. We review current progress in measuring the wavelength, polarisation and topological charge of the light; the refractive index of the environment; and displacement of the scattering surface. In particular, we show how the sensitivity can be optimised by tailoring the scattering geometry and input light field.

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 a variety of complex channels, demonstrating that the inhomogeneous nature of the polarisation remains unaltered from the near-field to far-field, even as the structure itself changes. We go on to show how this can be used for noisy free optical communication across noisy channels.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

We experimentally and theoretically study nonlinear vortex dynamics with propagation by encoding vortices into a laser beam and propagating the beam through a tank filled with methanol and carbon dust. The fluid medium allows variable path length through the tank and provides a repulsive third-order nonlinear photon-photon interaction via its thermo-optic coefficient. The emerging mode is imaged onto a CCD and interfered with a reference beam allowing measurement of amplitude and phase information by single-shot interferometry. We observed core sharpening and suppression of the annihilation of oppositely charged vortices consistent with our theoretical predictions. The repulsive third-order nonlinearity of the medium gives a direct mathematical analogy between nonlinear propagation of light and mean-field dynamics of quantum fluids.

Structured light is routinely used in free-space optical communication channels, both classical and quantum, where information is encoded in the spatial structure of the mode for increased bandwidth. Unlike polarisation, the spatial structure of light is perturbed through such channels by atmospheric turbulence, and consequently, much attention has focused on whether one mode type is more robust than another, but with seemingly inconclusive and contradictory results. Both real-world and experimentally simulated turbulence conditions have revealed that free-space structured light modes are perturbed in some manner by turbulence, resulting in both amplitude and phase distortions. Here, we present complex forms of structured light which are invariant under propagation through the atmosphere: the true eigenmodes of atmospheric turbulence. We provide a theoretical procedure for obtaining these eigenmodes and confirm their invariance both numerically and experimentally.

Space-time wave packets (STWPs) are pulsed fields with non-separable spatio-temporal structure that yields exotic and useful behavior. To date, STWPs have been synthesized using bulky free-space optical systems that require precise alignment. Here, we describe a compact system based on an alternative strategy for spatially resolving the spectrum, which makes use of a volume chirped Bragg grating with Bragg structure rotated by 45degree with respect to device facets. Consequently, the field exits spectrally-resolved from a facet orthogonal to the entrance. We synthesize STWPs using a pair of such devices occupying a volume of 25×25×8mm3, which is orders-of-magnitude smaller than previously.

Free-space propagating optical modes are acutely affected by aberrations such as those caused by atmospheric turbulence. These optical distortions are due to variations in optical density that arise from changes in temperature and pressure along an optical path. These distortions don’t only change in time, but along the entire optical path. Such aberrations results in bulk optical turbulence that cannot be readily corrected using single plane optical correction. We present an approach, using a 7 plane multiple light converter that can correct optical aberrations arising from 1km optical paths with up to a D/r0=20 and with less than -9dB combined crosstalk for 11 spatial modes.

Entangled two-photon dynamics offer a rich manifestation of the interplay between geometry and information. Such states accumulate a geometric phase that is not equal to the sum of those of the associated single-photon states, the difference amounting to a Geometric Phase of Entanglement (GPE). A functional relationship is derived between the GPE and reduced state entropies, showing that geometric phase measured for one photon will exhibit a quantum correlation with that of the other photon. This is explored within the setting of Type-I, collinear spontaneous parametric down-conversion, where two-photon states can be represented as a sum of spatially-entangled Laguerre-Gaussian modes.

The notion of a skyrmion as a topologically stable field configuration has proven to be highly versatile, manifesting in spintronics, condensed matter physics and more recently optics. Yet despite its origin in quantum field theory, skyrmions have only ever been observed as classical fields and particles. Here we report the first quantum skyrmion, existing as a non-local quantum entangled state that can be engineered and controlled through its wavefunction. We show that the topology of the quantum wavefunction makes this skyrmionic state robust to entanglement decay, remaining intact until the entanglement vanishes completely.

If light interacts with media, it will naturally transform into structured light, spatially varying in its properties as amplitude, phase, and/or polarization and carrying information about the interacting medium. Going to the nano-regime, media information is typically hidden in the non-paraxial properties of light, including its 3D polarization states. However, with standard imaging techniques, this information is not directly accessible and stays invisible. We present how customized non-paraxial laser light fields can serve as a sophisticated tool for revealing hidden media information by selective dipole excitation. Additionally, we further advance our system by implementing a dielectric metasurface for information extraction.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

Optical mode sorters are a key enabling technology that have been widely used in communication and sensing systems. Conventionally, these are designed for a particular set of optical modes based on specific geometric transformation. We present a new approach for free-space optical mode sorting that can be tailored to measure any set of spatial modes or arbitrary superpositions of modes. Our approach is enabled by the combination of a reconfigurable photonics integrated circuit (PIC) interfaced with a Multiple-Plane-Light-Converter (MPLC) for pure optical mode processing. We have successfully sorted a range of high order spatial modes with crosstalk with a mean value -16dB and a reconfigurability greater than 12kHz.

The rapid development of optical technologies, such as optical manipulation and trapping, data processing, optical sensing and metrology, enhanced imaging and microscopy, as well as classical and quantum communications necessitates fundamental studies of the new degrees of freedom for sculpting optical beams in space and time beyond conventionally used amplitude, phase, and polarization. Topological structured optical fields have emerged as one of the most promising candidates for such degrees of freedom. We will discuss our theoretical and experimental studies of “structured light and darkness” generation, detection, and linear and nonlinear light-matter interactions of optical links and knots in complex media such as optical metasurfaces and highly scattering media.

Video microscopy has a long history of providing insights and breakthroughs for a broad range of disciplines, from physics to biology. Image analysis to extract quantitative information from video microscopy data has traditionally relied on algorithmic approaches, which are often difficult to implement, time consuming, and computationally expensive. Recently, alternative data-driven approaches using deep learning have greatly improved quantitative digital microscopy, potentially offering automatized, accurate, and fast image analysis. However, the combination of deep learning and video microscopy remains underutilized primarily due to the steep learning curve involved in developing custom deep-learning solutions. To overcome this issue, we have introduced a software, currently at version DeepTrack 2.1, to design, train and validate deep-learning solutions for digital microscopy.

We propose a method to controllably create free-space, “hard-trapped” two-dimensional fluids with Bessel-Gaussian beam superpositions. We use these systems to experimentally demonstrate several vortex dynamics that resemble dynamics numerically predicted in weakly-interacting quantum fluids. Theoretically, we perform a Bessel-mode decomposition on an initial state containing an arrangement of optical vortices. The experiment is performed using a laser beam that is structured holographically with a spatial light modulator that contains the initial condition of the reconstructed field from the theoretical decomposition. This works allows for the experimental verification of many numerically predicted dynamics or the discovery of new dynamics from experiment.

Optical binding in liquid and in vacuum continues to unveil and enable new physics. We present two results. Firstly, we show how, for particles beyond the Rayleigh limit, surprising asymmetric forces can arise in counter-propagating beams. These can cause particle transport or instability. Secondly, we show how optical binding between two microspheres levitated in vacuum can be used for sympathetic cooling of the centre of mass motions of the spheres to sub-Kelvin temperatures.

We overview and further methods for using structured light to fabricate microscopic structures for optical trapping and cell study experiments through two-photon-photopolymerisation (2PP). We utilise a spatial light modulator (SLM) to correct wavefront distortions in-situ from gold nanoparticle reflections, which are shown to offer significant improvements for in-house nanofabrication. Aberration corrected 2PP offers a mechanism for using a simple, modified holographic optical tweezers apparatus to produce complex 3D micro-structures with as many as 50 independent foci. Custom-designed structures are used to induce and study the collective behaviour of bacteria allowing us to test otherwise simulated theories of cell dynamics.

We present electrically-driven generation and dynamics of the real-space optical vortices based on the high-dimensional gradient-thickness optical cavity. The structure consists of a metal-dielectric multi-layer that supports non-trivial topological phases, generating optical vortex lines and rings in three-dimensional generalized parameter space. We experimentally demonstrated the high-dimensional gradient-thickness optical cavity by liquid crystal-based multi-layer structure, which bijectively projects a surface slice of generalized parameter space into the real space. By electric control of the alignment of liquid crystal molecules, we successfully spanned the architecture of the high-dimensional optical singularity in real space, which actively generates and manipulates the optical vortex-antivortex pair.

When light propagates through aberrated optical systems, the resulting degradation in amplitude and phase has deleterious effects, for example, on resolution in imaging, spot sizes in focusing, and the beam quality factor of the output beam. Traditionally, this is either pre- or post-corrected by adaptive optics or phase conjugation. Here, we consider the medium as a complex channel and determine the corresponding eigenmodes which are impervious of the channel perturbation. We employ a quantum-inspired approach and apply it to the tilted lens as our example channel, a highly astigmatic system that is routinely used as a measure of orbital angular momentum. We find the eigenmodes analytically, show their robustness in a practical experiment, and outline how this approach may be extended to arbitrary astigmatic systems.