This PDF file contains the front matter associated with SPIE Proceedings Volume 11701 including the Title Page, Copyright information and Table of Contents

Introduction to SPIE Photonics West OPTO conference 11701: Complex Light and Optical Forces XV.

Spatial Light Modulators (SLMs) have advanced the fields of complex and structured light. These Liquid-Crystal-on-Silicon (LCoS) based devices allow for the dynamic modulation of both the amplitude and phase of light. This tutorial-type talk will provide an overview of the working principle of Spatial Light Modulators (SLMs). We will first look at the mechanism as to how these devices operate, followed by how to setup your own SLM, as well as the motivation to calibrate these devices. We will look at how digital holograms can be generated, as well as some common problems that the user might encounter. Finally, we will discuss some applications which make use of SLMs.

Digital Micromirror Devices (DMDs) can be used as alternatives to liquid crystal spatial light modulators, and due to their high speed and resolution, relatively low cost as well as wavelength and polarization invariance, are gaining popularity in the structured light community. We briefly summarise several practical considerations when using DMDs in structured light, explain how to generate holograms to modulate the amplitude and phase of an incident laser beam, as well as how to generate vector modes with spatially varying polarisation.

When situations make it difficult for students to be physically present in the laboratory, there is a need to provide remote instructional offerings. This is a particularly acute problem in upper-level physics laboratories because they involve the use of sophisticated equipment for the investigation of advanced topics. The possibility of automating such experiences presents itself as a possible solution. In this article I present the offering of an automated quantum optics lab for advanced physics students. I do so by automating the laboratory components via actuators and sensors controlled through serial connections. Live images of the laboratory provide visual inspection of the apparatus and sensors. All of these components are connected to a personal computer that students can control by remote access. The experience provides a new paradigm for experimentation, giving students experience on laboratory work with a remote apparatus at flexible times, making the experiment a form of homework assignment.

Free space optical communication is a promising means to establish versatile, secure and high-bandwidth communication for many critical point-to-point applications. While the spatial modes of light offer an additional degree of freedom to increase the data rate of an optical link, atmospheric turbulence imposes a serious threat to the data fidelity as it can introduce severe inter-mode crosstalk and consequently data degradation. Here, we introduce a high-dimensional free space communication protocol using vector beams. Our protocol uses the spatial polarization profiles of orthogonal vector vortex beams to represent information levels, and we show both numerically and experimentally that such a protocol can be resilient to moderately strong atmospheric turbulence without the need of any adaptive optics.

In this presentation we will outline our recent results utilising support vector network machine learning approaches to determine independently variations in Reynolds number and Fried Parameter over an atmospheric channel by analysing optical degradations in optical beams carrying Orbital Angular Momentum (OAM). Through numerical modelling of cascaded optical perturbations a comprehensive training set of OAM mode spatial spectra was produced over a simulated 1.5km's free-space optical channel in an urban environment. Our results indicate this machine learning approach will determine independently the Reynolds number and Fried Parameter with over 90.4% accuracy. These results indicate potential new methods for determination of variation in material properties that could be used for the detection of environmental contamination and weather monitoring.

Noise is an unavoidable feature in most optical systems and many techniques exist to minimize its adverse effects. In this regard, spatial filtering is a commonly deployed technique to improve the quality of laser beams by optically filtering the noise. In the “textbook” example, the noise is usually assumed to be high frequency and the laser beam, Gaussian. In this case, the filtering is achieved by a simple pinhole placed at the common focal plane of two lenses. In this work, we describe how this process can be generalized to arbitrary beam profiles: spatial filtering of complex light.

Longitudinal fields of quantized Laguerre-Gaussian modes are derived, revealing their importance even for light that may be considered to be propagating in the paraxial regime. This is in contrast to unstructured laser light, e.g. a Gaussian beam, where the magnitude of longitudinal components only become important under strong-focusing of the source. The unique effects stem specifically from the optical angular momentum, both orbital and spin, of optical vortex light, and include spin-orbit interactions in freely-propagating circularly-polarized vortices in free-space. The contribution that longitudinal fields make to the rate of single-photon absorption is calculated, highlighting that for optical vortices they cannot be neglected in general.

Light-matter interactions involving structured optical field (SOF) and meta-atoms have been recently investigated. SOF allows a large degree-of-freedom in the interaction, and includes vortex beams with helical wavefront, cylindrical vector beams with spatially inhomogeneous polarization, and non-diffracting Bessel beams. Meta-atoms even further extend the degree-of-freedom. Therefore, accurate and efficient description of both SOFs and arbitrary meta-atoms is required to study this problem, for which multipole approach can be a versatile framework. Here, we discuss description of SOFs and their implementation into multipole approach, and demonstrate dichroisms involving chiral meta-atoms and SOFs with spatially inhomogeneous phase and polarization distributions.

There is a fruitful interplay in the literature between quantum information theory and the study of vector beams, inspired by the mathematical structure shared by quantum entanglement and classical beams of light. As an example, measures of entanglement, notably concurrence, are widely used as measures of the quality of a vector beam. In addition, there is a close analogy, previously exploited in the literature, between polarimetry and qubit state tomography, which become identical tasks in the single photon limit. In this talk I will discuss the use of so-called POVM measurements in spatially-resolved polarimetry to characterise structured light beams.

High-dimensional entanglement of structured light offers the potential for noise-robust, high-capacity quantum communication protocols. However, the generation, measurement, and transport of high-quality entanglement presents some unique challenges. We demonstrate the generation and measurement of two-photon macro-pixel entanglement with a record dimensionality, quality, and measurement speed. We then discuss an experiment where we unscramble high-dimensional pixel entanglement through a commercial multimode fibre. In contrast with classical techniques, entanglement is also used to measure the transmission matrix of the fibre. Interestingly, we are able to regain entanglement without manipulating the fibre or the photon that entered it.

Quantum secret sharing is the procedure of securely distributing information between multiple parties by exploiting the features of quantum mechanics. Many variants exist, but in this work, we report a high-dimensional realization of a single-photon secret sharing scheme for distributing classical keys amongst many nodes. The implementation, which makes use of twisted light, is realized for as high as 11 dimensions and for as many as 10 participants: the highest reported to date and which is easily extendable to even higher dimensions and many participants. Such a result is an important first step towards a future quantum network.

In this work we generate optical fields whose polarisation structures not only rotate about the propagation axis, but can be tailored to accelerate, independently from their spatial profiles. Here we will demonstrate how this can be achieved with orthogonal, scalar fields, represented by weighted superpositions of oppositely charged Bessel beams, through path interference with a beam-splitter. In addition to their creation, we investigate various aspects of these generated modes, such as their angular accelerating Stokes vectors and optical current or intensity transport between various local positions within the field. Finally, we present a digital analogy to measure such fields in order to reconstruct their state of polarisation via Stokes polarimetry. undefined In this work we generate optical fields whose polarisation structures not only rotate about the propagation axis, but can be tailored to accelerate, independently from their spatial profiles. Here we will demonstrate how this can be achieved with orthogonal, scalar fields, represented by weighted superpositions of oppositely charged Bessel beams, through path interference with a beam-splitter. In addition to their creation, we investigate various aspects of these generated modes, such as their angular accelerating Stokes vectors and optical current or intensity transport between various local positions within the field. Finally, we present a digital analogy to measure such fields in order to reconstruct their state of polarisation via Stokes polarimetry.

A full-Poincare beam is an optical beam that contains every polarization state on the Poincare sphere over its cross-section. We present an interferometric method to measure the polarization state of a test beam by interfering it with a full-Poincare beam and analyzing it in the Fourier domain. This results in a Fourier peak whose shape and phase structure changes with the test polarization; with proper calibration, we are able to recover the test beam polarization. A Stress-Engineered Optic (SEO) is used to generate the full-Poincare beam. SEOs, when used at the pupil of an imaging system, have been used to create polarization-dependent PSFs. In this work, we are interferometrically creating a similar PSF in the Fourier domain while retaining the phase information. The effect of having a low photon number in the test beam is also investigated.

Light is a vector field often displaying a complex structure. Parameters such as amplitude, phase and the polarization, i.e., the direction of the electric field vector may change drastically in space even over sub-wavelength dimensions, giving rise to exotic properties such as phase singularities, polarization vortices and exotic angular momenta. These properties of light can be measured by letting it interact with sub-wavelength particles and detecting the scattered light. In reverse, by illuminating a small particle with properly designed light one can steer the scattered light and sensitively measure the particle position. These and related effects will be reviewed.

Spherical aberration of light is commonly known as an undesired effect in optical trapping. We demonstrate how to take advantage of this effect in order to shape extended light landscapes in a counter-propagating trapping configuration, fully-structured in three-dimensional (3D) space and embedding transverse as well as longitudinal field contributions. We numerically analyze the aberration, presenting controlled intensity and 3D polarization ellipse structures. Experimentally, we demonstrate the versatility of the structured light field using optically guided particles as sensors, analyzing their dynamics by velocimetry and scattered light. Finally, we elucidate the potential of the complex light field for 3D particle assemblies.

The study and application of structured light is mature in the transverse extent, whereas the third spatial dimension is rather unexploited though bearing many promising advancements with respect to super resolution and information content. We present complex light volumes that are shaped on-demand in their amplitude, phase and polarization in all spatial extents. By holographic beam shaping, we embed singularities in phase as well as polarization as skeleton of these structure. By transferring the self-imaging property of scalar beams to self-replicating vectorial light fields, we form 3d singular fields that include the creation and annihilation of singularities upon propagation. Furthermore we generate fields of sub-wavelength complexity by counter-propagation of structured light.

Spatial light modulators produce beams with a desired phase pattern, which directly affects the beam’s power spatial distribution. We demonstrate an approach for creating holographic spatial light modulators, which are permanently recorded in a photosensitive glass and do not require electric power in order to operate. The essence of the approach is that by means of holographic recording, an arbitrary phase pattern is encoded into a transmitting volume Bragg grating. Upon Bragg reconstruction, the phase pattern is reconstructed in the diffracted beam. These spatial light modulators are tunable and could be achromatic in broad spectral range.

Space-time wave packets -- a new class of pulsed optical beams in which the spatial and temporal degrees of freedom are inextricably intertwined -- provide a new platform for the study of structured light. Space-time wave packets can be designed to be propagation-invariant (diffraction-free and dispersion-free) and travel at arbitrary group velocities in free space and transparent dielectrics. I will discuss recent results obtained with space-time wave packets including the observation of anomalous refraction, tunable axial acceleration or deceleration, and novel Talbot effects observed in space-time rather than in space or time separately.

We use a spatial light modulator (SLM) to mimic the effect of gravity and steer the light from a laser to observe Einstein rings with a laboratory camera. The derived programming of the phase of the SLM follows a logarithmic dependence with impact parameter. As expected, we also observe arcs when the source and lensing object are not in line with the observer. Measurements for distinct parameters are consistent with the expectations. The coherent optical beams that are programmed to follow gravitational lensing trajectories have a transverse mode consistent with Bessel functions, yet they do not exhibit the non-diffracting properties of Bessel beams: they expand linearly with the propagation distance. The addition of a vortex phase also produces patterns that coincide with Bessel modes of order given by the topological charge of the vortex..

Previously we studied the spectral Gouy rotation as a specific rotational phenomenon of conical polychromatic light fields shaped by spiral gratings. The rotation of spectral anomalies around singularities results from accumulated spectrally dependent Gouy phase shift. We proposed to apply radially chirped spiral structures to obtain an axial modulation of the rotational characteristics. Here we present related experimental results with non-uniform spiral gratings which were programmed into a 10-Megapixel, phase-only, liquid-crystal-on-silicon (LCoS) spatial light modulator (SLM). A propagation-dependent variation of the Gouy rotation was indicated. More complex non-uniform geometries are considered.

We show that a class of vector vortex beams possesses a topological property that derives both from the spatially varying amplitude of the field and its varying polarization. This property arises as a consequence of the inherent skyrmionic nature of such beams and is quantified by the associated skyrmion number. We illustrate this idea for some of the simplest vector beams and discuss the physical significance of the skyrmion number in this context.

Surface plasmons are three-dimensional vector-field bound to a two dimensional surface. As a result, structuring or interfering surface plasmons gives rise to new topologies and textures of electro-magnetic waves, typically not found in propagating waves which are often assumed paraxial. Using near-field investigation with well controlled boundary condition, we discover new phenomena arising solely from the interference of discrete number of surface plasmon: we demonstrate the first optical skyrmions - topological defects in 3D vector field bound to a 2D surface, show plasmonic-based spin effects in systems with discrete symmetries and discover a 4D topological charge in 2D plasmonic quasi-lattices.

In this study, interactions between optical vortex light and partially capped core-shell (PCCS) nanostructures simulated by a discrete dipole approximation (DDA) method are described. The plasmonic characteristics of the PCCS structure, in which a part of the surface of dielectric nanoparticles is covered with a metal cap, can be tuned by the core-shell ratio and the coverage condition. It was found that the topological charge and the polarization degree indicating the state of the optical vortex determine the resonance order, peak wavelength, direction and magnitude of optical torque of the excited pseudo-plasmon resonance of PCCS structures. It was also found that under certain incident conditions, higher-order resonance modes, such as hexapole and octupole modes are excited. Likewise, distorted resonance modes due to the asymmetry of the structure are excited. These plasmonic characteristics cannot be realized by scalar beam excitation, it becomes possible by an asymmetrical PCCS structure with optical vortex excitation.

vBack Focal Plane interferometry (BFPI) is a widely used technique in particle tracking, providing high-speed, highresolution measurements. We present Structured Back Focal Plane Interferometry (SBFPI), a modification to BFPI utilizing structured beams and structured detectors [1]. Utilising Conical Wavefront Gaussian Beams and Annular Quadrant Photodetectors, SBFPI significantly increases tracking range while maintaining sensitivity and robustness to aberrations. We studied the variation in detection range and sensitivity over our parameters. A maximum increase in axial and lateral tracking range of 4.25-fold and 1.83-fold was found respectively, concomitant with sensitivity decreases of 53% and 61%. We also found parameter combinations with smaller range improvements but which retain sensitivity, in particular a 2.31 fold axial range increase with a 22.4% reduction in sensitivity, and these can be tuned to each experiment. We also studied the effect of aberrations on signal integrity.

FORMA (force reconstruction via maximum-likelihood-estimator analysis) addresses the need of measuring the force fields acting on microscopic particles. Compared to alternative established methods, FORMA is faster, simpler, more accurate, and more precise. Furthermore, FORMA can also measure non-conservative and out-of-equilibrium force fields. Here, after a brief introduction to FORMA, I will present its use, advantages and limitations. I will conclude with some recent work where we exploit Bayesian inference to expand the scope of application of FORMA.

Artificial active nano- and microparticles driven by light are of special interest since their propulsion can easily be controlled via the illumination. Recently, we have established a propulsion mechanism based on asymmetric light refraction caused by a refractive-index gradient. We will introduce into this new approach based on two-photon polymerization of the active particles, and demonstrate active propulsion under illumination as well as pattern formation in colloidal suspensions of these active particles controlled by light.

We demonstrated optical trapping of 20 nm particles using a Fano-resonance-assisted plasmonic tweezers based on arrays of asymmetrical split nanoapertures on a 50-nm gold thin film. By transmission and reflection spectra measurements, the close-mode Fano-type excitation peak was estimated at 928 nm. We investigated the trapping performance through power- and wavelength-dependent characterization. We determined the trap stiffness using the transient time method and a linear dependence of the trap stiffness for low incident laser intensities under off-resonance conditions was observed. For the on-resonance condition, a large normalized trap stiffness of 8.65 fN/nm/mW was obtained which enables our system to improved motion control of the trapped nanoparticle. Furthermore, the trap stiffness on-resonance was enhanced by a factor of 63 compared to that of off-resonance. We conclude that this enhancement is due to the ultrasmall mode volume and a cavity effect contribution. Our approach opens new avenues for steady and dynamic optical trapping, making a variety of lab-on-chip applications possible.

We fabricated an array of nanorings of various inner diameters on a 50 nm thick Au film. Herein, we present nanoparticle trapping using this plasmonic array and we investigate the trap stiffness as a function of the inner diameter and the intensity of the trapping laser.

Computer Generated Holography (CGH) promises unprecedented capabilities for a variety of applications in Optics and Photonics. However, one of the biggest challenges for CGHs is the fundamental tradeoff between algorithm runtime and achieved reconstruction fidelity and efficiency while maintaining light projections at real-time frame rates. In addition, the light projection quality achieved by most CGH-modalities are rather low due to the mismatch between the optical wave propagation of the applied Spatial Light Modulator (SLM) and its simulated model. A promising new avenue of CGH, neural holography, utilizes machine learning models in the generation of single and multi plane holograms. Neural network generated holograms have the distinct advantage that inference is performed in constant time without the need for iterative calculations of the phase SLM pattern. This allows the networks to generate holograms 3-500 times faster than traditional iterative algorithms, which enables the applications dependent on real-time holography. State-of-the-art implementations of neural holography [1, 2] furthermore achieve higher accuracy than traditional iterative algorithm, when compared to target images. Applications of these SLM-encoded CGHs include all areas where a fast and parallel one- or two-photon light excitation is needed such as in Laser Material Processing, Additive Manufacturing and 3D printing, Neurophotonics and Optogenetics, Laser Image Projection and many more.

Triggered by the need for arrays of individually resolvable excitation foci or trapping potentials in photonics applications, coherent lattice theory describes a unique approach to design structured interference patterns. Typically, large periodicity lattices remain unexplored due to limitations in the theoretical description. Here, we present a method for efficient computation of coherent lattices, successfully covering all periodic and quasi-periodic lattices. The previously unrelated moiré theory and prime number factorization are the foundation of the proposed method. Additionally, we experimentally verify key optical coherent lattices and propose broadening their applicability towards structured light microscopy and optical trapping using photonic integrated circuits.

Trapping microparticles in vacuum has led to the burgeoning area of levitated optomechanics. Here we will describe recent work on rotating particles in vacuum. This offers a rich playground for dynamics. In particular we describe how we can attain very high rotation rates by spin angular momentum transfer, optical binding of particle sin vacuum. For a single particle stochastic, thermal fluctuations combine with nonconservative optical forces to break detailed balance and produce increasingly coherent, apparently deterministic motion. In turn this leads to the highest Q value ever reported for a vacuum traded microparticle

Optical tweezers based on single optical fibers are typically fabricated with methods that grant only limited design flexibility in the trapping geometry. Here, we present a novel hybrid micro-optical structure 3D printed in a single step on a standard single-mode optical fiber, to create an ultra-compact fiber tweezer. The structure is made of stacked refractive, reflective, and waveguiding optical elements. Our approach allows an easy and independent tuning of trapping parameters as the numerical aperture and the working distance of the optical trap. We experimentally illustrate the 3D trapping capabilities of the proposed fiber optical tweezers.

Optical tweezers are versatile tools capable to separate microparticles, yet present formidable challenges in the separation of nanoparticles smaller than 200 nm. The difficulties arise from the controversy on the requirement of a tightly focused light spot in order to create strong optical forces while a large area is kept for the sorting. To overcome this problem, we create a near-field potential well array with connected tiny hotspots in a large-scale. This can separate nanoparticles with sizes from 100 to 500 nm, based on the differentiated energy depths of each potential well. In this way, nanoparticles of 200, 300 and 500 nm can be selectively trapped in this microchannel by appropriately tuning the laser power. Our approach provides a unprecedent solution for optical trapping and separation of nanoparticles and biomolecules, so that it presents a huge potential in the physical and biomedical sciences.

We set out to construct an experimental system in which we control the fluctuations in the random motion of particles in a system. We subject colloidal particles to periodically applied random optical forces using holographic optical tweezers. As a result, the particles are pushed or pulled in random directions. We calculate the effective temperature (𝑇_eff) of the particles from their trajectories using the Stokes-Einstein relation. We find that 𝑇_eff depends non-monotonously on the frequency of switching of the optical landscape. We show that particles flow from high-temperature regions to low temperatures regions. A striking observation is that particles subjected to two regions with the same 𝑇_eff but differing in driving frequencies occupy both regions equally. This implies that in some way 𝑇_eff is a relevant thermodynamic function.

Within recent years, optical micromanipulation has opened a broad field of applications in biomechanics, microrheology and nanoparticle assembly. In these fields, a highly desired action is sorting of the objects under consideration by their size. In this contribution, we employ Laguerre-Gaussian beams with helical phase fronts as trapping potential for dielectric particles and investigate the dynamics and size-dependent assembly of silica microspheres, induced by light-matter interactions. We demonstrate both experimentally and numerically, that by adjusting the parameters of the tailored trapping potential, spatial segregation of particles by size is possible.

The powerful discipline of optogenetics can be described as “the branch of biotechnology which combines genetic engineering with optics to observe and control the function of genetically targeted groups of cells with light”. Using advanced laser beam-shaping such as Generalized Phase Contrast (GPC) and derived holographic modalities makes it possible to take advantage of cutting-edge two-photon technology to develop an unprecedented ‘circuit optogenetics’ platform with both high spatio-temporal selectivity and high penetration depth without disturbing speckle-noise. We will report on the latest laser shaping GPC-modalities with temporal focusing to uniquely sculpt patterns of one- and two-photon excitations for living neural-circuits and genetically modified light-sensitive cells.

In this work, we illustrate a simple approach of generating polarization singularities using cylindrical vector beams. We use a superposition of two free propagating Gaussian beams that modulated with optical vortices. The two cylindrical vector beams possess radially and azimuthally polarized components, respectively. By evaluating the Stokes parameters at the total beam focus of the total superimposed beam and tracking the null values of the Stokes parameters, various polarization singularities observed, such as star, lemon, spider-web, and high order polarization singularities.