This PDF file contains the front matter associated with SPIE Proceedings Volume 10723, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Photophoresis can stably hold opaque microscopic particles in a laser focus surrounded by room air with strength sufficient to enable centimeter-scale patterns to be drawn by sweeping the laser beam. The resulting images rely on visual persistence as laser light scatters from the particle, which is rapidly swept through the 3-D pattern. Control can be maintained while moving the particle with air speeds up to 2 m/s. A desire to greatly increase the sweep speed motivates a re-examination of the fundamentals of photophoresis-based laser-particle traps. Most explanations offered are qualitative, with differing opinions as to whether, for example, asymmetric heating or asymmetric thermal accommodation is primarily at work. Which particles become trapped in the beam is typically based on self-selection, as a variety of particles with possible differing shapes and sizes are offered to the laser focus for capture. Characteristics that make some particles preferred over others are especially relevant. There is broad consensus that structure in the laser focus greatly aids in stable trapping. Nevertheless, it is still possible for even a relatively smooth TEM00 beam to capture and hold particles. Moreover, even in a structured focus (i.e. with aberrations and local intensity minima and maxima), questions remain as to exactly how a particle becomes stably trapped in certain beam locations. A zoomed-in look at trapped particles reveals oscillations or orbits with excursions over tens of microns and accelerations up to 10 gs. We trapped particles in zero-gravity as well as 2-g environments with no noticeable difference in stability.

When an optical lens is illuminated by a plane wave, the generated focal spot is given by the Abbe diffraction limit. However, arbitrary small spots, surrounded by additional lobes, can be obtained by illuminating the lens with a suitable light pattern. This is a manifestation of super-oscillation (SO), since the far field intensity pattern is band-limited by the ratio of the lens numerical aperture and the wavelength, but nevertheless the light beam at the focal plane can oscillate locally at much higher frequency. Here, we investigate a systematic method to structure the small lobes of SO function, by using Gaussian, Hermite-Gaussian, Laguerre-Gaussian and Airy functions. After experimentally realizing the subwavelength focusing of these structured super-oscillating optical beams we showed their capabilities to achieve high localization of nano-meter sized particles and observed unprecedented localization accuracy and trapping stiffness, significantly exceeding those provided by standard diffraction limited beams. Further, we envisage that the method of structuring super-oscillating functions shown here can be used in other fields, e.g. STED microscopy, nonlinear frequency conversion, lithography, plasmonics as well as in the time domain for structuring light pulses for supertransmission and for time-dependent focusing

In this work, we develop a general method for customizing the intensity statistics of speckle patterns, on a target plane. By judiciously modulating the phase-front of a monochromatic laser beam -with a spatial light modulator- we experimentally generate speckle patterns possessing either arbitrarily-tailored intensity probability density functions (PDFs) or non-local spatial intensity correlations. Relative to Rayleigh speckles, our speckles with customized intensity PDFs exhibit radically different topologies yet maintain the same spatial correlation length. Furthermore, they are fully developed, ergodic, and stationary: with circular non-Gaussian statistics for the complex field. Furthermore, we can tailor the spatial intensity-correlations of speckle patterns: in particular, the non-local intensity correlations so that the speckle field and intensity spatially fluctuate on different length scales. We can even synthesize speckles with non-isotropic long-range intensity correlations, while the spatial field-correlation remains short-ranged and isotropic. Propagating away from the target plane, however, all of the customized speckles revert back to Rayleigh speckles. This work provides a versatile framework for tailoring speckle patterns with varied applications in microscopy, imaging and optical manipulation.

The spectacular facets of light have made light ubiquitous in all fields of science. Light’s interaction with matter allows for accurate manipulation of atomic and molecular structures that enabled fundamental breakthroughs in physics, chemistry, and biomedical research. The transfer of light’s energy on molecules and genetically expressed proteins can be used to stimulate cells and emulate cellular processes such as synaptic inputs spatially distributed along the neuron’s dendritic tree. Here, we show basic neuronal functions derived via numerical modelling and describe how we can use light to emulate these functions in order to provide a systematic study of the neuron’s response. We focus on cortical pyramidal neurons and use the NEURON simulation environment to analyze how spatio-temporal stimulation patterns along various dendritic locations sets the neuron to fire an output. We then show an equivalent response from experiments via complex spatial light patterns for stimulating across different regions along the dendritic tree. Furthermore, we use the same spatial light patterns to simultaneously visualize neuronal responses via functional calcium imaging predicted via the same neuron model. Visualizing dendritic responses from back-propagating action potentials can provide new insights to some important features of dendritic computation.

Understanding the fundamental processes of life and disease ultimately requires an understanding of interactions at the molecular scale. While breakthroughs in structural biology have led to a wealth of images of molecular interactions at near-atomic detail (e.g. motor proteins interacting with microtubules or DNA replication complexes) these images are static snapshots. To truely understand complex processes we need to be able to study dynamical interactions at the single molecule scale. Optical tweezers are an ideal tool to manipulate and study these dynamical interactions and especially when combined with advanced imaging techniques allow scientists to directly observe complex biomolecular processes in real-time. We will present the latest developments in correlative tweezers fluorescence microscopy (CTFM) and illustrate how at Lumicks we develop turn-key instruments that can become standard tools in the biologist toolkit. By carefully analysing market segments we identified what are the key specifications for our customers. With measurements and simulations we will illustrate how precisely key specifications such as force detection drift, trap-trap distance drift and acquisition speed translate into impact on data quality.

This study aims to address the question of fluctuation-dissipation relationship in a non-equilibrium system comprising active Brownian particles. Specifically, the fluctuation power spectral densities (PSD) of an Induced-ChargeElectrophoresis-driven metallic Janus particle in an optical trap was measured and compared the PSDs to that of nondriven particles. Unlike the PSDs of Brownian particles where there exists only one characteristic frequency, the PSDs of active Brownian particles have two with the second frequency characterized by the particles’ rotation diffusion. Energy dissipation of such particles, defined as the integrated PSDs were examined. Finally, Effective temperatures defined by various means, including the probability distribution of the particle's positions in the trap, and the zero-frequency limit of the fluctuation power spectral densities, are examined for the active Brownian particles.

Nonequilibrium processes of small systems are ubiquitous in physics, biology, and chemistry. Optical tweezers provide an ideal tool for controlling small systems to investigate nonequilibrium thermodynamics. Recently, we performed the first experimental test of the differential fluctuation theorem, using an optically levitated nanosphere in air in both underdamped and overdamped regimes, and in both spatial and velocity spaces [Phys. Rev. Lett., 120, 080602 (2018)]. We also experimentally realized the Feynman’s ratchet, using a colloidal particle in water confined in an optical tweezer array under feedback control. Feynman's ratchet is a microscopic machine in contact with two heat reservoirs that was proposed by Richard Feynman in 1960’s to illustrate the second law of thermodynamics. Despite broad interests, an experimental realization of Feynman's ratchet has not been reported before our work.

Otoliths are calcium carbonate crystals located in fish ears. They play an important role in zebrafish for hearing, its sense of balance and acceleration. Many studies have been conducted to understand its structure, function but also development conditions. However the encoding in the brain as a movement sensor remains unknown. Here we developed a non-invasive system capable of manipulating one or two otoliths simultaneously in different directions to simulate movement or acceleration and sound. Our system uses optical traps created with an infra-red laser at different positions on the otoliths creating forces in chosen directions. However, as the optical traps need to go through brain tissue in a live fish, it becomes difficult to determine the exact forces applied. In this study we investigate the limits of forces determination. We will present the theory and experimental measurements of optical tweezers applied to otoliths which we mostly published in Nature Communications (doi:10.1038/s41467-017-00713-2). We will also present our latest result on brain imaging in response to artificial acceleration and sound.

Traumatic Brain Injury (TBI) occurs when an external force injures the brain. While clinical outcomes of TBI can vary widely in severity, few mechanisms of neurodegeneration following TBI have been identified for treatment. Understanding mechanotransduction in cells is key to understanding cellular response to injury. This has been previously studied using a variety of optical techniques such as laser tweezers, laser ablation, and others. We propose a model utilizing photodisruption for studying the early pathogenesis of TBI in primary neuron cultures by generating laser-induced shockwaves (LISs). Photodisruption allows for the generation of spatiotemporally defined shear stress against cells. The shear stress exerted by the shockwave is between 0 - 50 kPa depending on the distance from the shockwave epicenter. Cells typically situated at a distance from the epicenter of 50 m undergo necrosis while viability is preserved for those located at a distance of 100 m. An optical system was developed that allows single cells to be selectively studied in response to LISs. Approximate timescales of each of the effects culminating in shockwave generation span several orders of magnitude from nanoseconds to milliseconds. Thus, our system utilizes Pockels cells — a high-speed, electro-optical shutter — to capture shockwave dynamics. The force measurement system is characterized by imaging stages over the period of cavitation then, violent expansion and collapse of microbubbles responsible for shockwave generation. Here, we visualize LISs and observe subsequent, morphological responses elicited by cells under a range of forces generated from optical breakdown.

Due to their unique properties, magnetic nanoparticles, often made of iron oxides, have received significant attention in chemistry, solid state physics, and the life sciences. Although a magnetic field is the most obvious mean by which one can manipulate magnetic nanoparticles, we here demonstrate that magnetic nanoparticles can be individually controlled by optical manipulation. We quantify the interaction of optically trapped individual magnetic nanoparticles with the electrical field by determining the spring constant. Also, by finite element modeling we determine the extinction, scattering and absorption cross sections of magnetic nanoparticles as well as the real and imaginary parts of their complex polarizability. In comparison to magnetic manipulation, optical manipulation has the advantage, due to the tight focusing of the laser beam, that it allows for manipulation of a single particle at a time. Also, one can imagine applications where it is advantageous to employ both magnetic and optical manipulations simultaneously.

The transfer of mechanical signals through cells is a complex phenomenon. To uncover a new mechanotransduction pathway, we study the frequency-dependent transport of mechanical stimuli by single microtubules and small networks in a bottom-up approach using optically trapped beads as anchor points. We interconnect microtubules to linear and triangular geometries to perform micro-rheology by defined oscillations of the beads relative to each other, which are measured with 3D back focal plane interferometry. We find a substantial stiffening of single filaments above a characteristic transition frequency of 1-30 Hz depending on the filament’s molecular composition. Below this frequency, filament elasticity only depends on its contour and persistence length. Interestingly, this elastic behavior is transferable to small networks, where we found the surprising effect that linear two filament connections act as transistor-like, angle dependent momentum filters, whereas triangular networks act as stabilizing elements. These observations implicate that cells can tune mechanical signals by temporal and spatial filtering stronger and more flexibly than expected. In addition, we integrate a novel label-free microscopy techniques, capable of imaging freely-diffusing microtubules in real-time and independent of their orientation. We show that rotating coherent scattering (ROCS) microscopy in dark-field mode provides strong contrast also for structures far from the coverslip such as arrangements of isolated MTs and networks. We could acquire thousands of images over up to 30 minutes without loss in image contrast or visible photo damage.

Optically levitated masses can provide extremely sensitive force sensors that are decoupled both thermally and electrically from their environment. Due to these features, they provide ideal sensors for a number of searches for new fundamental interactions, which may appear as tiny deviations in the forces acting on massive or charged objects. These forces are motivated by a number of models of new physics beyond the Standard Model, allowing tests of certain models accounting for dark matter, dark energy, or the microscopic properties of gravity in a table-top scale experiment.

Optical traps for dielectric particles have become an exceptional tool in testing optomechanics as well as fundamental physics. We report on our use of optical trapping in a program searching for non-Newtonian gravity, as well as recent tests that demonstrate control of the rotational degrees of freedom of optically trapped particles via electrostatic interactions with the dipole moments of test particles. The technique levitates individual micrometer-sized dielectric spheres and measures their three-dimensional position by optical heterodyne detection, making use of a single laser for both trapping and imaging. The two radial degrees of freedom are measured by interfering light transmitted through the microsphere with a reference wavefront, while the axial degree of freedom is measured from the phase of the light reflected from the surface of the microsphere. This method pairs the simplicity and accessibility of single-beam optical traps to a measurement of displacement that is intrinsically calibrated by the wavelength of the trapping light and has exceptional immunity to stray light. A theoretical shot noise limit of 1.3e-13 m/rt{Hz} for the radial degrees of freedom, and 3.0e-15 m/rt{Hz} for the axial degree of freedom can be obtained in the system described. The measured acceleration noise in the radial direction is 7.5e-5 (m/s^2)/rt{Hz}.

Gravimeters measure small changes in the local gravitational acceleration. They are applied for environmental monitoring, oil and gas prospecting and defence and security. Gravimeters used in these applications have a remarkable sensitivity but at a cost of being bulky and very expensive. Recently, a micro-electrical mechanical system (MEMS) gravimeter has been developed, which was cheap, had a comparable sensitivity to commercial gravimeters and maintained its stability over long timescales (10−6 Hz). In this paper we discuss to replace the current shadow sensor readout with an on-chip interferometer. This new readout has a higher sensitivity so that the device can be more robust and reduces the system size. The design of this readout is discussed and the first experimental results are presented. The new readout improves the imaging capabilities of density anomalies of the device.

We report the development of a phonon laser based on the center-of-mass oscillation of an optically levitated silica nanosphere in a free-space optical dipole trap. A parametric feedback scheme based on the detection of the oscillator’s center-of-mass is used to provide a cooling signal that intrinsically depends on the oscillator’s mean phonon occupation. When an amplification signal is added to the feedback at the mechanical resonance, these two signals produce center-of-mass dynamics that are analogous to those of a single-mode optical laser. Observed phenomena include a threshold in oscillation amplification, a transition from Brownian motion below threshold to coherent oscillation above threshold, reduction in the linewidth of the oscillation spectrum, and gain saturation. We also analyze the statistical phonon number distributions above and below threshold. The observed dynamics are described by a model that includes both stimulated and spontaneous emission of center-of-mass phonons. Importantly, the operation of this phonon laser relies on externally controllable, feedback-based parameters and therefore allows tuning of the threshold via these parameters. We also explore the use of the levitated nanoparticle phonon laser as a detector of weak external forces via injection locking.

We experimentally study acoustic excitation of a radiation pressure coupled optomechanical resonators above and below self-sustained oscillation threshold. First we demonstrate injection locking of a radiation pressure driven microtoroidal optomechanical oscillator (OMO) via acoustic waves by locking its phase and frequency to a piezoelectric transducer (PZT). We characterize the injection locking process and show that even without proper acoustic impedance matching, the OMO can be locked to the PZT and tuned over 17 kHz with only -30 dBm of RF power driving the PZT. As opposed to the previously reported techniques, injection locking of OMO via acoustic waves does not require optical power modulation or physical contact with the OMO and it can be easily implemented on various platforms to lock different types of OMOs independent of their size and structure. The high efficiency, simplicity and scalability of the proposed approach paves the road toward a new class of photonic systems that rely on synchronization of several OMOs to a single or multiple RF oscillators with applications in optical communication, metrology and sensing. Next we study the acousto-optical response of the same optomechanical resonator below oscillation threshold optical power level. We show that in this regime the reduced damping due to radiation pressure may be used for high sensitivity detection of acoustic waves. We analyze the performance of radiation pressure coupled optomechanical resonators as acousto-optical transducers and explore their potential advantages over other optical and piezoelectric transducers in applications where high sensitivity, low power consumption and small size are critical.

Van der Waals (vdW) forces play an integral role in the binding and interaction energies of molecules in condensed phases; their long-range, many-body nature can modify phonons in molecular crystals and thereby impact thermodynamic stability. Typical macroscopic descriptions of such optical forces tend to ignore important atomistic effects arising at short (nanometric) scales, while microscopic treatments tend to ignore long-range, geometry-dependent electromagnetic effects. We describe an ab-initio approach to model such fluctuation-induced forces in mesoscopic systems comprising large molecules in the vicinity of macroscopic bodies, conjoining atomistic treatments of electronic and vibrational fluctuations derived from density functional theory in the former, with continuum descriptions of electromagnetic response in the latter, thereby accounting for many-body and multiple scattering effects to all orders. Such long-range electromagnetic effects become particularly important in situations where the finite sizes and shapes of the molecules and continuum bodies combine to create phonon polaritons with highly delocalized (nonlocal) charge distributions. We find that even in small molecules, but especially in elongated low-dimensional molecular systems, these effects modify van der Waals forces by orders of magnitude and produce qualitatively different behavior compared to predictions based on simple dipolar or pairwise approximations, valid only in atomically small or dilute molecular systems. In particular, we focus on the interactions of fullerenes, carbyne wires, and graphene sheets with one another and with a gold surface. We compare forces with and without phonon and at multiple temperatures, and compare our predictions to those obtained from commonly used dipolar and continuum treatments. In particular, we show that phonons can delocalize molecular charge distributions from a few angstroms to several nanometers, in ways that depend strongly on the shape of the molecules and their proximity to the surface. Even for small fullerenes, phonons can lead to force deviations at tens of nanometer separations from the surface compared to treatments lacking phononic effects, while for higher dimensional molecules such as elongated carbyne wires and graphene sheets, the nonlocality of these interactions produces nonmonotonic power laws that cannot be qualitatively captured by dipolar and/or continuum models.

Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a black art. Although there are now numerous examples of control using laser-induced nucleation, a physical understanding is absent and preventing progress. We will show that concentration fluctuations in the neighborhood of a liquid-liquid critical point can be harnessed by an optical-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in liquid mixtures using a low-power laser diode confirm the effect. Phase separation and nucleation through an optical-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter.

Optically induced forces are under intensive investigation as part of a toolkit for micromanipulation and nanofabrication technology. The long-disputed Abraham-Minkowski momentum controversy remains a significant issue for the theoretical treatment of optically induced forces on and within a linear optical medium. We disclose the existence of a theoretical paradox in which an easily verifiable identity of the macroscopic Maxwell field equations violates special relativity and other fundamental physical principles. We derive a new theory of continuum electrodynamics from Lagrangian field theory and we discuss how the new theory relates to fundamental physical principles applied to a linear magneto-dielectric medium including flat non-Minkowski material spacetime, dielectric special relativity, and spacetime conservation laws.

Invoking Maxwell’s classical electrodynamics in conjunction with expressions for the electromagnetic (EM) energy, momentum, force, and torque, we use a few simple examples to demonstrate the nature of linear and angular momentum exchange between a wave-packet and a small spherical particle. The linear and angular momenta of the EM field, when absorbed by the particle, will be seen to elicit different responses from the particle.

Solar sailcrafts make use of radiation pressure to propel a payload through space. Modern diffractive structures such as broadband single order gratings, polarization diffraction gratings, and related metamaterials offer the potential to replace reflective sails with efficient diffractive sails for either solar or laser driven space travel. We have experimentally verified the radiation pressure force on a transmissive diffraction grating, demonstrating a large component of force parallel to the surface of the grating. This component is important for orbit-raising types of maneuvers. Unlike a reflective sail with force components only normal to the surface, we also measure a near-vanishing normal force component around the Littrow angle

Counter to conventional methods of measuring laser optical power, radiation pressure-based power meters operate by reflection rather than absorption. This provides an opportunity for in situ, non-destructive total beam power measurement. Compact radiation pressure power meters designed to operate between a few tens and a few thousands of watts consist of a planar millimeter-scale spring-electrode-mirror component that deflects under radiation pressure from an incident beam. Spring constant, resonant frequency, and quality factor of microfabricated springs as well as coatinginduced straining of the spring are the focus of this manuscript. We compare finite element models of the mechanical component with various measurements to inform future designs.

Optical binding of plasmonic nanoparticles offers a unique route to assemble mesoscale clusters and chains. However, stability is an issue that prevents assembling large-scale optical matter from nanoparticles. Here, we report a new method to study and improve the spatiotemporal stability of optical matter chains consisting of gold nanospheres by modulating the polarization direction of a linearly polarized optical line trap. The optical binding strengths of gold nanoparticles with parallel and perpendicular polarized light are different, resulting in versatile oscillation properties of the nanoparticles with polarization modulation. We show that the optical binding strength is spatially inhomogeneous along the nanoparticle chains depending on the total number and relative positions of particles, and it is temporally variable depending on the frequency of polarization modulation. In particular, the average oscillation amplitude of the particles can be tuned by increasing the frequency of polarization modulation. The spatiotemporal stability of the optically bound nanoparticles can be improved when the polarization modulation speed is fast and the optical binding is strong enough to suppress thermal motion. This study represents a new way to manipulate optical forces at mesoscale, and provides important information for assembling large-scale optical matter with nanoparticles.

Fluctuating isotropic electromagnetic fields are obtained by considering a large group of plane waves with wave vectors, polarizations and phases randomly distributed and fluctuating on time. Due to the isotropic character of this electromagnetic field, the optical force induced on an electric dipole is, in average, equal to zero. However, the dynamics of electric dipoles on these kind of systems are far from being trivial. In this work we analyze the dynamics of two dipoles using molecular dynamics simulations. In particular, we consider two silver nanoparticles of 5nm radius at Fr¨ohlich resonance. Under these conditions a gravity-like interaction among the two particles is induced. The molecular dynamics numerical simulations show how Keplerian-like trajectories are obtained under these particular conditions

Ultra-high sensitivity sensors can be achieved with optically levitated particles in ultra-high vacuum (UHV). Trapped particles act as high-Q harmonic oscillators, whose amplitude, position, and frequency can be monitored to provide high sensitivity measurements of the particle’s acceleration. Larger particles (10-30 microns in diameter) provide higher sensitivity, but they are difficult to trap in UHV without particle loss. To overcome the radiometric forces that lead to particle loss, rare earth (RE) ion dopants can be incorporated into the particles to enable solid-state laser cooling of the particle’s internal temperature. The laser used for optical trapping can be tuned to a wavelength on the lower energy side of the ion absorption band, and thus also serve as the pump laser for solid-state laser cooling. Internal cooling occurs when the average energy of the photons emitted is larger than the average energy of the photons absorbed. Ions will rapidly thermalize while in the ground and the excited states to create the energy difference. Solid-state laser cooling has been realized in bulk host materials and is well understood. This technique of internal cooling for reducing loss pressure is currently being tested.

Optical trapping at high vacuum of a nanodiamond containing a nitrogen vacancy centre (NVC) would provide a new test bed for several phenomena in fundamental physics. Progress has been made towards this goal but it has not yet been possible to optically levitated nanodiamonds at pressures below a few mbar. We demonstrated that the problem is the absorption of the trapping light by the nanodiamond, which heats them to destruction (above 800 K) except at pressures above a few mbar where air molecules dissipate the excess heat. Here we solve this problem by showing that milling diamond of 1000 times greater purity creates nanodiamonds that do not heat up even when the optical intensity is raised above 700 GW/m2 below 5 mbar of pressure [1]. The large quantities of high purity nanodiamonds made in this way may also find applications in nanoscale sensing such as magnetometry. We have also proposed an analytical model to describe the interferometric balanced detection which is commonly used to sensitively measure the position of a levitated nanoparticle [2]. [1] AC Frangeskou, ATMA. Rahman, L Gines, S Mandal, OA. Williams, PF Barker, and GW Morley, in press at New Journal of Physics, arXiv:1608.04724 (2016). [2] ATMA Rahman, AC Frangeskou, PF Barker & GW Morley, Review of Scientific Instruments 89, 023109 (2018)

The nitrogen-vacancy (NV) centre in diamond gained popularity as a probe for nanoscale sensing applications in research thanks to the array of sensing modalities available and the bio-compatibility of the material itself, however the utility of NV sensing is limited by the lack of suitable strategies to control them spatially. By confining single nanodiamonds using an optical tweezers (OT) we are able to combine the sensing opportunities of the NV centre with the precision control of position and orientation afforded by OT. This work is an investigation of the interaction of the trapping laser with the spin-based photoluminescence of the NV centre, further it is a demonstration of an all-optical sensing protocol which eliminates the spin depolarisation effects of the trapping laser and allows for NV spin relaxometry in an optically trapped nanodiamond. This relaxometry protocol can determine spin lattice relaxation times on the order of ms and requires relatively low trapping powers < 50 mW, making it particularly applicable to biological systems.

A single vertical laser beam is used to trap SiO₂ and vaterite spheres with diameters ranging from 5 to 32um. The center of mass acceleration sensitivity for the SiO₂ spheres is as low as 4 × 10⁻⁷ g/Hz^−1/2 for the largest spheres. This system also allows high-bandwidth modulation of the polarization of the trapping beam, enabling control of the rotation of the microspheres. The maximum rotation speeds exceed a few MHz for both types of spheres, while the damping time exceeds several hours at 2 × 10⁻⁷ mbar. This setup can be used to measure forces and torques acting on the microspheres

A microfluidic device consisting of a 3D network of buried microchannels and integrated waveguides has been fabricated and used to controllably manipulate particles within the micro channels. The channels and waveguides were made using the direct laser writing technique of ultrafast laser inscription, followed by selective chemical etching to fabricate channels. Particles flowing through the device undergo hydrodynamic flow focusing into a narrow stream within a main channel due to the geometry of the channel network. 3D hydrodynamic focusing performance was validated using polystyrene microspheres, coloured dye and cells by visualizing the focusing within the device. A focusing width of 4 um was achieved, reducing the risk of particles sticking to walls, clogging the channel and ensuring all particles pass the beam. Particles are irradiated by 1064 nm light, in a direction perpendicular to the flow, from the embedded waveguide, causing a lateral displacement of the particle due to the optical scattering force. 5 and 10 micron beads in water were focused to a narrow stream. Lateral displacement was evaluated for 5 different laser powers for particles flowing at a constant velocity >1 mm/s. A linear increase in displacement of the particles with laser power was observed. Bacteria, yeast, microalgae and mammalian cells have been flow-focused and optically manipulated within the device. The device is capable of both passive and active separation of particle species, and the routing of particles to required outlets demonstrates potential for cell sorting.

We will review recent progress in on-chip particle trapping and manipulation using liquid-core waveguides.

Optical imaging combined with single-molecule force spectroscopy broadens the horizons for the spatiotemporal localization and the mechanical details of target molecules. We developed a simple optical method to extend the depth of field in a high numerical aperture objective (NA = 1.2 and 1.4) that requires to visualize a single fluorophore. This method enables us to obtain an optical signal outside the focal plane without unintended interruption of the force signal in single-molecule optical imaging-force spectroscopy. By axial scanning, using an electrically tunable lens with a fixed sample, we were successfully able to visualize the diffusion of proteins along DNA that is three-dimensionally stretched by optical tweezers. We also demonstrated the performance of the ETL system in the single particle tracking analysis of epidermal growth factor receptor (EGFR) on filamentous actin bundles connecting cells, in the presence of the mechanical force applied to the intercellular nanotube, using an optical trap combined with a line scanning confocal microscope. In fact, ETL has been exploited in various microscopes. These microscopes are intended to have a wide axial scan range of hundreds of micrometers in an objective lens with low NA (< 1.0) for deep-tissue imaging, except for one study, which used a high NA objective lens (NA = 1.4) in wide-field microscopy for the static image of protein-complexes in HeLa cells. Our study is a first demonstration of an ETL being used for a single-molecule force-optical imaging system in extended depth of field.

Optical trapping using nanoapertures in metal films has advanced significantly in recent years, allowing for the trapping of nanoparticles in the single digit nanometer range, including proteins. It has been recognized previously by theoretical studies coaxial that apertures with small gaps in a metal film can provide extremely large trapping potentials for such nanoparticles. However, past approaches to nanofabrication, such as focussed ion beam milling, do not reliably produce sub-10 nm features. Here we demonstrate the use of a combined electron-microscopy and atomic layer deposition approach to reliably fabricate sub-10 nm gaps on the wafer scale. We achieve trapping of polystyrene nanoparticles and proteins using these apertures. Numerical simulations show the steep trapping potential achieved in a resonantly tuned coaxial structure. The coaxial structures fabricated are also measured to ensure the wavelength of the resonance is close to the trapping laser wavelength. As nanogap structures are also promising for surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption, our devices can act as a multifunctional platform to integrate single-molecule manipulation and spectroscopic analysis.

We propose to engineer the excitation light in photo-induced force microscopy that enables the nanoscale detection of longitudinal and transverse (with respect to the propagation direction) components of chirality of samples. We employ an achiral tip in the vicinity of a chiral specimen, and illuminate the tip-sample interactive system with appropriate engineered structured light to explore both components. Particularly, we discuss using circularly polarized light to detect the transverse chirality, and the superposition of specifically-engineered radially and azimuthally polarized beams to detect longitudinal chirality. We obtain all the mentioned results through a rigorous theoretical analysis with several numerical examples. The proposed technique may have a high impact especially in biomedicine and pharmaceutics.

Over the past few decades, “microrheology” has emerged as a widely-used technique to measure the mechanical properties of soft viscoelastic materials, many of which are derived from biology. These methods offer an alternative to conventional “bulk” rheology, which can require prohibitively large sample volumes, can damage fragile materials, and cannot resolve microscale heterogeneities or deformations of individual macromolecules. Optical tweezers offer a powerful platform for performing microrheology measurements that can uniquely measure rheological properties at the level of single molecules out to near macroscopic scales. Unlike passive microrheology methods, which use diffusing microspheres to extract steady-state rheological properties, optical tweezers can access the nonlinear mechanical response of materials and measure the space- and time-dependent rheological properties of heterogeneous, non-equilibrium materials. I will describe the basic principles underlying how optical tweezers can be used to perform microrheology measurements. I will discuss instrumentation requirements, benefits over other methods, and material systems that are most amenable to the method. I will also describe several novel approaches that include coupling optical tweezers with fluorescence microscopy and microfluidics, and using single molecules as stress and strain probes. These novel configurations can characterize non-continuum mechanical properties, nonlinear viscoelasticity, strain-field heterogeneities, stress propagation, force relaxation dynamics, and time-dependent active matter mechanics. Finally, I will show examples of applications of these methods to widely-studied soft biological materials including entangled DNA, cytoskeleton protein networks, and mucus.

Two-point microrheology measurement from widely separated colloids can reveal the bulk rheological property of a fluid. We develop such a technique which measures the effective viscosity using two trapped particles in a dual trap optical tweezers by exploiting the motional resonance excited in the probe particle when the control particle is driven externally. We carry out the measurement both from the amplitude and the phase response of the resonance and show that the zero-crossing of the phase with respect to the drive signal at the resonance frequency gives more accurate results when the particles are separated widely. Later on, we compare our measured viscosity values with that measured using a commercial rheometer and obtain an agreement within ∼1 %. In future, this method can be extended to a linear viscoelastic fluid enabling high accuracy measurements.

Antibiotic resistance kills an estimated 700,000 people each year worldwide and experts predict that this number could hit 10 million by 2050. Rapid diagnostics would play an essential role in the fight against this alarming phenomenon by improving the way in which antibiotherapy is used, notably by stopping the unnecessary use of antibiotics. Clinical microbiology has relied on culture as the standard method for characterizing pathogens over the past century. This process is time-consuming and requires large biomasses. In this context, single-cell monitoring would be a significant breakthrough compared to Petri dishes culture. A first step was achieved by the demonstration of single bacterium trapping by optical tweezers and integrated photonics. Here, the nondestructive real-time state monitoring of a single alive trapped bacterium is demonstrated. In order to achieve this, a two-laser setup was developed to simultaneously trap and monitor a single bacterium in the near-field of a nanobeam microcavity. While the first laser is used to excite the optical field tweezing the bacterium, the second laser probes the cavity resonance spectrum. The bacterium optical interaction with the resonant cavity mode allows to assess the bacterium state in real time when subjected to an antibacterial agent (antibiotics, alcohol, temperature). Confronted to standards culture-based methods, this optical label-free approach yields relevant information about bacterial viability, without time-consuming culture or staining. Those results evidence that on-chip devices operating at telecom wavelength may greatly enhance the monitoring of bacteria in the near future leading to major improvements in health care diagnosis and patient treatments.

Cell adhesion forces have been of interest since the 1920’s. These forces are estimated to be in the nano-Newton range, which is inaccessible to the majority of optical tweezers systems, so most studies use direct contact methods such as atomic force microscopy or micropipettes. In the case of weakly adhered cells such as biflagellated plant cells attached to a coverglass surface, separation does seem possible with the use of an optical trap system. We report the detachment of microalgae Dunaliella tertiolecta from the top and bottom of a coverglass sample chamber using a single nearinfrared CW trapping laser focused with an underfilled high numerical aperture microscope objective. We show that the time required for detachment of a cell is dependent on the incident laser power, taking 3 seconds for a power of 319 mW, and about 90 seconds for a power of 41 mW. We are able to hold the cell in the optical trap after being detached using the same laser beam. The cells detached from the bottom were levitated at an equilibrium position determined by the incident power, while the ones on the top were held in the trap against the top surface. After the cells were released, they were seen to be unaffected in their motion. One advantage of our described method is that, from the large range of sizes of the microalgae within the prepared sample chamber, we are able to select a specific size for further studies.

We demonstrate dynamic trapping and computer-controlled manipulation of nanoparticles with plasmonic holograms. Research into plasmonic trapping has been motivated by its potential for enhanced optical forces. However, while holographic optical tweezing has become an indispensible tool to researchers across a wide range of disciplines, the benefits of full computer control over focused trapping sites has not yet been realized for plasmonic tweezing. In this work, by tailoring the illumination pattern of an incident laser beam with a spatial light modulator, the location of a focused plasmonic hotspot can be moved to arbitrary locations across a surface. The trapping hotspot is purely plasmonic, i.e. the incoming laser beam does not directly illuminate the trapped particles and it is constructive and destructive interference of converging plasmon waves that form the mobile trapping sites. Specifically, a computergenerated hologram illuminating around the edges of a silver Bull’s Eye nano-structure generates surface plasmon waves that propagate towards the center. Shifting the phase of the plasmon waves as a function of space around the Bull’s Eye gives complete control over the location of the focus. We show that 200-nm diameter fluorescent nanoparticles trapped in this focus can be moved in arbitrary patterns in the center of the Bull’s Eye structure. This allows, for example, circular motion of the trapped nanoparticle using linearly-polarized light. These results show the versatility of holographically-generated surface plasmon waves for the trapping and manipulation of nanoparticles under full computer control, combining the many benefits of plasmonic tweezing and holographic optical tweezing.

The optical tweezers technique has attracted extensive interest and is playing an important role in manipulating nano/micro-objects in many fields. However, number of the trapped particles are always imprecise in classical techniques, to form uncontrollable clusters at the focus. Yet, precise manipulation of a specific objective is of vital importance in many situations. Many approaches have been proposed and implemented to assist pulling or pushing forces for specific traps. They could modulate a hybrid plasmonic field to achieve a single trap while rejecting other particles, however, control of the trapping dynamics is still lacking. A simple and direct way to achieve selective trapping is still urgently needed. As trapping commences with the distribution of the optical field, tailoring the field distribution is a direct way to modulate the force and trapping results. Recently, the sharply developed metasurface technique provides a novel approach for this. We designed a chiral sensitive metalens to tailor the optical field by focusing the left-circular polarization but produces a diverging right-circular polarization beam with opposite focal length. Consequently, two independent plasmonic fields are excited at the surface plasmon resonance angle, and propagate oppositely to assist trapping and anti-trapping forces simultaneously. Combining the effects of two orthogonal circular polarizations, single target particle was stably trapped in the center while all other particles were repelled. Both theoretical simulations and experimental results validate the effectiveness of the proposed method. This points toward targeted manipulations that may find applications in single-particle assistant molecular Raman detection and assembly of plasmonic structures.

A charged nanoparticle that is confined and cooled in an ion trap can, in principle, be expelled from the trap and directed onto a substrate with high positional accuracy using an electrostatic lens. This deposition technique could provide a complement to studies of trapped nanoscale objects by allowing examination of the object outside the trap. It may also be used to assemble new types of structures (for example, by depositing a 2D material onto a reactive surface in high vacuum). In our system, a charged nanoparticle held in a quadrupole electric field trap is released from the trap and directed toward a removable indium tin oxide (ITO) coated substrate in ultrahigh vacuum (UHV), using an Einzel lens to focus the particle’s trajectory. We have worked with a variety of materials: graphene nanoplatelets around 1 micron in diameter, as well as three-dimensional nanoparticles (including gold, silver, tin, silica, polystyrene, and graphite) with diameters of 200-800 nm. We have consistently detected particles striking the substrate by means of a charge sensor connected to the conductive substrate coating. Some particles, but not others, are observed to stick to the substrate; we are currently working to increase the chance of adhesion for metal nanoparticles by raising their temperature before deposition. We have had some success in locating the deposited particles using a camera positioned above the substrate; efforts to improve the imaging method are ongoing.

Electron beams carrying orbital angular momentum (OAM), or electron vortex beams (EVBs), can be produced in a Transmission Electron Microscope (TEM) with forked diffraction gratings.^1,2 Just as optical vortex beams can be used to trap and rotate particles, EVBs have been reported to transfer their OAM to a nanoparticle on a dry substrate and cause it to spin.^3,4 However the results have not been reproduced, perhaps due to contact friction. It has been suggested that a more dramatic effect could be observed by imaging particles levitated in an optical trap or in a liquid environment. ^3,5,6 To reproduce these results and demonstrate a more pronounced response, we are performing experiments to transfer OAM from an EVB to a nanoparticle suspended in a fluid. TEM liquid cells consist of a liquid sample sealed against the vacuum of the standard TEM column between two electron-transparent windows. These cells have enabled the study of liquid samples at nanometer to atomic resolution and have applications in the study of microfluidics, electrochemical processes, and biological samples. Electron vortex beams could provide a useful new tool to manipulate nanoparticles and liquids themselves inside such cells. Here we describe an experimental investigation of EVB-induced rotation in a liquid and show that initial results are inconclusive. A theoretical consideration using the fluctuation-dissipation theorem suggests that, unlike in an optical trap, viscous forces and rotational Brownian motion may overwhelm the subtle torqueing effect the EVB has on the particle.

It is well-known that continuous-wave lasers can accelerate small particles to high velocities, and, more recently, it has been observed that laser damage of optical materials occurs at laser intensities that are orders of magnitude lower in the presence of accelerated absorbing particles. At these high powers the primary interactions are ablative, and a complete set of force balance equations in this regime have only recently been described. In our experiments, small groups of 35-41m diameter stainless steel particles are accelerated using a high-power continuous wave (CW) Yb-doped fiber laser illuminating at intensities between 1MW/cm2 and 2MW/cm2. The trajectories of the particles are tracked using a high-speed camera until they leave the camera field of view or evaporate, a process typically occurring on timescales of a few milliseconds. The lifetimes and trajectories of the superheated particles are calculated using a system of coupled equations to track particle velocity, mass, momentum, radius, vapor opacity, temperature, and temperature distribution. Upon illumination, the particles heat to their vaporization temperatures, with evaporating atoms transferring momentum to the remaining particle. Significant laser attenuation occurs due to an opaque glow region around the particle consisting of dense evaporated atoms and ions. Since the particle is not uniform in temperature, the number of evaporating atoms varies with position and imparts a net acceleration to the particle until a terminal velocity on the order of a few tens of meters per second is reached where drag forces offset further acceleration. Based on the calculated interplay between temperature gradient and acceleration, heat transfer within the evaporating particles must be dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars. It is further observed that absorbing particles accelerated by continuous wave (CW) lasers initiate catastrophic failure in the form of micromachined drill holes. This process was tested using stainless steel, PMMA, and silica particles with fused silica, sapphire, and spinel substrates. Hole drilling occurred at laser power densities as low as 250kW/cm2. A potential dependence of accelerated particle breakdown on substrate bandgap may suggest that the underlying physical process is similar to that seen for CW laser breakdown of contaminated optical coatings.

Nonspherical particles play a key role in the atmosphere by affecting processes such as radiative forcing, photochemistry, new particle formation and phase transitions. In this context, measurements on single particles proved to be very useful for detailed investigations of the properties of the particles studied and of processes affecting them. However, measurements on single nonspherical particles are limited by the difficulties and lack of understanding associated with the optical trapping of such particles. Here, we aim at better understanding the optical trapping of nonspherical particles in air by comparing the motion of an observed nonspherical particle with simulated optical forces and torques. An holographic microscope is used to retrieve the 6D motion of a trapped peanut-shaped particle (3D for translation and 3D for rotation). Optical forces and torques exerted by the optical trap on the peanut-shaped particle are calculated by using FDTD simulations. Most of the main features of the particle motion are in agreement with the calculations while some specific aspects of the particle motion cannot yet be explained.

Optical tweezers are perhaps most well-known for their ability to make precise measurements of small forces and displacements, but they are also capable of high-speed and long-distance motion. High speed and long distance optical manipulation is necessary for high throughput in applications such as tissue engineering, cell sorting, and the assembly of 3D structures and materials. Here we present the greatest speeds that we have achieved using 3D optical traps to manipulate a variety of particle materials and sizes across millimeter-scale translation distances [1]. In general, higher laser powers enable faster manipulation speeds, and we investigate the high-speed / high-power limit of this relationship. For polystyrene microscale particles with diameters in the range 0.5 µm – 5 µm, we find that we are limited by mechanical stage vibrations to maximum speeds of ~220 µm/s, while for nanoscale gold, silver, and polystyrene particles, we are limited by thermal absorption effects to maximum speeds of 150 µm/s – 170 µm/s. In the low-power regime, we find good agreement with standard theory based on the balance of the optical gradient force with Stokes’ drag. Our results are, to the best of our knowledge, one of the most comprehensive studies of maximum particle manipulation speed, and we have attained the fastest published submicron particle manipulation speed. We think that these results will establish and highlight the high throughput potential for automated pick-and-place processes based on optical tweezers. [1] J. E. Melzer and E. McLeod, ACS Nano, in press (2018), doi: 10.1021/acsnano.7b07914.

In this paper we will look at the relative merits of assembling particles in the mesoscopic to macroscopic size range with Optical Tweezers and Optoelectronically enhanced Tweezers. Optical tweezers provide an elegant method for controlling the position of microscopic particles in three dimensions, allowing their assembly into desired patterns. The technique works well when moving particle similar in size to the diffraction limited spots of the laser tweezers. In comparison to this Optoelectronic Tweezers (OET) use a light patterned photoconductive device to move particles in two dimensions through light patterned electrical fields. At light/dark boundaries on the biased photoconductor of an OET device high electrical gradients are created which move particles by dielectrophoresis forces in a similar manner to how the optical gradients move particles in a standard optical tweezers system. Xerox recently reported the manipulation of 150 and 300 micron silicon chips with optoelectronic tweezers, with coarse assembly of 1000 objects per second with the goal of creating a printer system for electronics assembly. We have demonstrated the alignment of commercial 250 micron InP stripe laser dies, 50 micron diameter solder beads for the creation of conductive paths and SMT components up to 600x300x300 microns in dimension. In this paper we will discuss the remaining challenges including different strategies for fixing the assembled components into place. Finally, we will look at the assembly of particles at the small end of the size range and discuss the potential uses for the large area patterning of mesoscopic particles.

Optoelectronic tweezers (OET) is an opto-electro-fluidic micromanipulation technology that uses light-induced dielectrophoresis (DEP) for touch-free actuation of micro-scale objects in physical, chemical and biomedical studies. In this work, we introduce a new method to evaluate the behavior of particles trapped in OET traps and used this technique to study their escape mechanism. Particles experiencing negative DEP were made to move in a circular path on a microscope stage, such that the particles’ velocities and trajectories could be observed under different conditions. At high velocities, particles were observed to escape the trap vertically (into the suspending medium) before settling back onto the surface. Three-dimensional numerical simulations of electrical field distribution indicated that the vertical displacement phenomenon occurs when the particle experiences the strongest DEP force at the boundary of the light pattern, lifting the trapped particle to a region where viscous drag exceeds the local horizontal DEP force, thereby forcing it to escape OET confinement. Similar ‘hopping’ phenomena were also observed for cells and particles of different sizes. We propose that the escape mechanism clarified in this work is a general one for objects manipulated by negative DEP in an OET trap, which will be important to consider in the future design of OET-enabled micromanipulation tools for a wide range of applications.

Particle patterning and hopping has attracted much attention owing to their extensive involvement in many physical and biological studies. Here, by configuring an intriguing Optofluidic, we are able to pattern 500 nm particles into a 2D array in the flow stream. We also achieve a 2D patterning of cryptosporidium in the microchannel. By investing particle-particle interactions, we studies the long ignored new particle hopping mechanisms, and used them to screen antibodies. Our observed particle hopping in the flow stream completes the family of particle kinetics in optofluidic potential wells and inspires new minds in the develop new light fields in the microchannel. The 2D patterning of particles facilites the parallel culture and study of multiple biological samples in the flow stream.

The development of methods for the rapid analysis of pathogenic bacteria or viruses is of crucial interest in the clinical diagnosis of infectious diseases. In the last decade, optical resonators integrated with microfluidic layers arose as promising tools for biological analysis, notably thanks to their ability to trap objects with low powers, beneath the damage threshold of biological entities, and with a small footprint. Moreover, the resonant nature of optical cavities allows for the simultaneous acquisition of information on the trapped objects, thanks to the feedback effect induced by the specimen on the trapping field itself. Here we report on the trapping and on the Gram-type differentiation of seven types of living bacteria in an optofluidic system based on an optical cavity consisting in a large hole in a 2D silicon photonic crystal membrane. The hollow nature of the resonant cavity results in a large overlap between the confined field and the hollow volume, allowing for a maximum interaction between the trapping field and the trapped cell. The optical cavity was excited at the resonance wavelength and the shift induced by the trapped bacteria was analysed. To test the trapping capabilities of our structure, we investigated seven types of bacteria, featuring different morphologies, Gram-types and mobilities (presence or absence of flagella). The analysis of the resonance shift yielded Gram typing in a label-free and not destructive way, due to differences in the refractive index and in the deformability of the cell wall. In particular, Gram negative bacteria showed a larger shift.

Optothermal manipulation, which exploits photon-phonon conversion and matter migration under a light-controlled temperature gradient, is one of the emerging techniques. Elucidation of the underlying physics of opto-thermo-fluidics and rational engineering of fluidic environments are required to realize diverse optothermal manipulation functionalities. This talk covers working principles, design concepts and applications of a series of our newly developed optothermal manipulation techniques, including bubble-pen lithography, opto-thermophoretic tweezers, opto-thermoelectric tweezers, optothermal assembly, and opto-thermoelectric printing.

Active control technology of particles in a microchannel using optical force, is an area of interest to scientists in various field today. The optical force is generated by momentum change caused by refraction and reflection of light. The generated optical force changes the surface of the particle to change the angle of incidence of light, so that the force generated by the change in the momentum of light changes. Interpreting this technique through simulation is a complex task. The deformation of a particle, interaction between the surrounding fluid and particle, and reflection and refection of light need to be analyzed simultaneously to be simulated. In this study, a deformable particle in a microchannel with optical sources was simulated with a three-dimensional lattice Boltzmann immersed boundary method. The beam originating from the optical source is analyzed by dividing it into individual ray. To calculate the optical forces acting on the particle, the intensity, momentum and direction of ray were calculated. First, the optical separator problem with one optical source was analyzed by measuring the distance traveled by the optical force. Second, the optical stretcher problem with two optical sources was studied by analyzing the relationship between the intensity of the optical source and the deformation of the particles.

Materials containing suspended micro- or nanomaterials are used extensively in multiple fields of research and industry. In order to understand the behavior of nanomaterials suspended in a liquid, the knowledge of particle stability and mobility is fundamental. For this reason, it is necessary to know the nanoscale solid-solid interaction and the hydrodynamic properties of the particles. In the presented research we used a hybrid Atomic Force Microscope coupled with Optical Tweezers system to measure the femtonewton scale interaction forces acting between single particles and the walls of a microchannel at different separation distances and environmental conditions. We show an important improvement in a typical detection system that increases the signal to noise ratio for more accurate position detection at very low separation distances.

Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Such microstructures have been optimized for optical manipulation and are finding application elsewhere, including micro-optics, mechanical microstructures and polymer crystallography. Furthermore, due to self-focusing, photopolymerization can form a waveguide, which develops into an optical fibre as long as submillimeters. Importantly, to date virtually all photopolymerization studies have been performed with incident light fields possessing planar wavefronts and simply exploit the beam intensity profile. Here we investigate photopolymerization of ultraviolet curing resins with a light field possessing orbital angular momentum (OAM). We show that the annular vortex beam breaks up via modulation instability into the m-microfibers, depending on the azimuthal index m of an incident optical vortex. These microfibers exhibit helical structures with chirality determined by the sign of m and mirror the helical nature of the incident vortex beam wavefront. We have developed a numerical model based on the Beam Propagation Method that captures the key experimental observations for a variety of optical vortices characterized by their azimuthal index m. This research opens up a range of new vistas and has broad consequences for the fields of structured light, new approaches to writing novel mesoscopic structures and applications such as in detecting or sorting the OAM mode (e.g. photonic lanterns) in areas including optical communications and manipulation.

Simulation of optical tweezers involves the calculation of optical and non-optical forces and torques, modelling the Brownian motion of the particle and combining these components to calculate the overall dynamics of the system. Here we describe two new toolboxes: an improved optical force/torque calculation toolbox and a full dynamics simulation toolbox which combines all the individual parts to simplify the process of calculating particle dynamics. The new toolbox will provide functions for simulating particle dynamics, estimating trap stiffness and calculating trap depths. The toolbox will be able to calculate optical forces/torques in different regimes including the geometric optics limit, Rayleigh limit, and intermediate regime with Vector Spherical Wave Functions/Tmatrix,VSWF/T-matrix, Discrete Dipole Approximation, DDA, and the Final Difference Time Domain method, FDTD. Using these tools, it is possible to model different types of trapping configurations that can be used to study motile particles held in optical tweezers.

Using absolutely calibrated optical tweezers, we make quantitative measurements of the motility force of Escherichia coli (E. coli) by measuring the change in momentum of the deflected beam. By tracking the position of the particle, in addition to the optical force measurements, it should be possible to simultaneously calculate the motility force and drag. In a simple Gaussian beam optical trap away from the sample chamber E. coli tends to align and swim along the beam axis, which can make tracking the particle position and measuring the force difficult. We use a 3-D optical force detection system to measure the absolute force on the particle, allowing us to measure the motility force of E. coli in a simple Gaussian beam. By using a line-shaped trap, it is possible to align E. coli transverse to the beam axis, facilitating easy particle position measurement. The investigated methods are not specific to E. coli and could be applied to other motile organisms, the study of wall effects and bio-films.

One important aspect of the complete physical characterization of novel viscoelastic materials is the assessment of their response on short timescales. Optical tweezers, equipped with a fast quadrant photodiode, aid in fulfilling this task by providing high-frequency viscoelastic information about the sample. In passive microrheology, this is normally achieved by extracting rheological information from the thermal motion of an optically trapped bead embedded in a test fluid. Here we present the calibration and use of optical tweezers to study the formation of thermally reversible DNA hydrogels. We complement our results with rheological data from dynamic light scattering, video microscopy and conventional bulk rheology. Merging experimental data from different techniques allows us to study the viscoelastic behavior of these DNA networks over a wide frequency-band and the scaling of the complex viscoelastic modulus at the two frequency extremes. By analyzing the high-frequency behavior of our transient network, we prove the semi-flexible polymer nature of DNA and provide an estimate of its persistence length.

Optically Trapped Probe Microscopy (OTPM) is an emerging imaging technique using optically trapped objects as near-field probes, able to sense a variety of local effects by utilizing different probe materials and geometries. However, the quest for super-resolution in OTPM presents an almost insurmountable barrier; the ever-present stochastic Brownian motion of the trapped object, which serves to geometrically broaden the acquired signal and degrade resolving capacity. A reduction of Brownian motion can be achieved through active trap control or increased laser power, but these approaches carry increased system costs and complexities, or could jeopardize the life-safe nature of IR optical trap experiments. In this poster, we present a post-processing solution to the Brownian motion problem; by splitting measurements into microsecond-length integrations and simultaneously measuring the position of the trapped probe, deconvolution of the effect of Brownian motion on transmitted signal is possible. We conduct an experimental and simulated investigation into this post-processing, and are able to reduce the noise and increase resolution of a variety of excitation patterns in a trapped nanodiamond fluorescence microscope.

We study the effects of the optical forces acting on nanoparticles when they are illuminated with structured light. We developed a computational toolbox that calculates optical forces. The results allow us to obtain the trajectory described by the particles depending on their initial conditions and physical properties. As an example, we generate structured light beams by superimposing optical vortices. The resulting beam generates gradient, scattering and curl forces. We show the dynamics of dielectric and metallic nanoparticles. This work may stimulate further research on controlling nanometer-size particles using light beams with space-variant polarization.

Feedback traps can manipulate particles arbitrarily. In a feedback trap, a position detector detects the particle’s position, a computer calculates the necessary force to be applied based on the position in the “virtual potential,” which is applied to the particle. The process is repeated with as fast a loop rate as practical. Previous feedback traps have used electrokinetic or hydrodynamic forces to manipulate particles. Here, an optical trap creates the force used by the feedback trap to impose arbitrary potentials. We create feedback forces on optically trapped particles by moving the trap position rapidly in response to observed fluctuations with the help of an acoustooptic deflector (AOD). In preliminary experiments, we have confined a 1.5 μm silica bead in a virtual potential that is 35-40 times stiffer than the underlying optical trap, whose laser power is kept constant. We also create a virtual double-well potential with independent control over the well separation and barrier height, which is impossible to do with time-sharing optical tweezers.

We have generated a vortex beam with spiral polarization using a few-mode optical fiber. The excitation of the beam is controlled by coupling conditions of incident light beam with the fiber and the state of polarization of input Gaussian beam. The constituent orthogonal linearly polarized modes that contribute to the generation of such spiral vortex beam by inherent mode-mixing are also selectively excited by launching orthogonally polarized light into the fiber input end. Our experimental results suggests a strong dependency of polarization and phase of the fiber output beam on the state of polarization of input beam and are expected to be useful in mode conversion and controlled OAM beam generation. The obtained experimental results are best matched with simulation results.

Here we report the creation and manipulation of colloidal crystals by inducing temperature gradients in a colloidal suspension of silica microparticles. A colloidal crystal is an ordered array of colloid particles analogous to their atomic or molecular counterparts with proper scaling considerations. The generation and properties of colloidal crystals have been of great interest for diverse science applications such as photonic crystals, chemical sensors among others. We report a technique that utilizes particles of silica of different diameters to form colloidal crystals by temperature gradients produced by light absorption at a metallic thin film deposited on one of the substrates. Moreover, we study the behavior of the particles by having different number of hot zones.

Nonlinear absorption of laser radiation focused on a volume of a highly absorbing thermal medium creates a temperature difference and induces a refractive index change inside the medium. It has been shown that gas bubbles can be generated by increasing the focused laser power due to thermal blooming. In our previous work, all possible forces acting on a microbubble which is confined within a horizontally oriented thin glass container have been incorporated in the force model. The thermo-capillary force is the dominant attractive force, while the optical force is repulsive. It is experimentally shown that the microbubble can thus be trapped by a focused laser beam and they can be manipulated by steering the laser beam. In this work, the 2D trapping is extended to 3D by generating a microbubble inside a vertical thick glass cuvette. A 514.5 𝑛𝑚, Ar-ion laser beam, tightly focused using a microscope objective, is used to generate and trap the microbubble. Once again, the force model is developed: unlike the 2D case, there are no contributions from surface tension and friction.

Micro bubbles can be used to generate and manipulate flows in the ambient fluid in optical tweezers. Here, we develop a novel technique of generating micro bubbles in a controlled manner in thermo-optical tweezers, where the temperature variation of the surface tension at the liquid-gas interface generates tangential thermo-capillary stress. This produces flows in the surrounding fluid, which can be controlled using the bubble. An analytical solution of the thermocapillary problem, based on the Stokes and heat equations, yields the flow profile around the bubble, which is realized experimentally in single and two-bubble systems.

Generation and 3D manipulation of microbubbles by means of temperature gradients induced by low power laser radiation is presented. Photodeposited silver nanoparticles on the distal end of two optical fibers act as thermal sources after light absorption. The temperature rises above liquid evaporation temperature generating a microbubble at the optical fibers end in non-absorbent liquids. Alternatively, switching the thermal gradients between the fibers, it is possible to generate forces in opposite directions, causing the migration of microbubbles from one fiber optic tip to another. Marangoni force induced by surface tension gradients in the bubble wall is the driving force behind the manipulation of microbubbles

We demonstrate a scheme based on a biaxial crystal for generation of the orbital and spin flows in optical fields. Such fields offer a possibility for microparticles’ rotation. By using the spin flow of beams with a gradient trap, refractive dielectric particles can be rotated. We achieved the orbital rotation of such particle due to the combination of an optical vortex trap and a gradient trap in orthogonal linearly polarized incoherent beams. We quantitatively compared the action of the spin and orbital momenta on the same particle of gamboge for its rotational speed.

We demonstrate planar nano-fabrication and integration of a plasmonic nano-hole on the tip of a single-mode optical fiber using a template-stripping approach. Our proposed nanoaperture fiber tweezer (NAFT) combines the fiber light guiding properties with the field localization of the nanoaperture in a gold film. Furthermore, a key advantage of the NAFT is its potential to replace the cumbersome optics used in conventional optical trapping setups, e.g. microscopes and free space optics. Upconverting nanoparticles (UCNPs) of size 30 nm were trapped in order to show the optical trapping ability of the NAFT, and the results were very promising. This modular trapping technique can be very beneficial in applications such as optical sensors, single photon sources and in studies concern with luminescence from nanoemitters.

The paper deals with the influence of an evanescent wave on the dynamics of motion of erythrocytes into blood plasma. Computer simulation of erythrocytes moving into evanescent field and experimental demonstration of the forecasted motion argue the feasibilities for control of position of cells into blood plasma. The range of velocities of transversal motion of erythrocytes due to action of the optical force of generated evanescent field is determined in a function of the angle of illumination of a cell by the linearly polarized wave with the azimuth of polarization 45°.

The influence of an evanescent field formed by two evanescent waves under the total internal reflection on the dynamics of motion of separate erythrocyte into blood plasma is demonstrated. Computer simulation of red blood cell motion into evanescent field and experimental demonstration of rotational and rectilinear motion expand the possibilities of using optical evanescent waves in applied tasks of nanophysics and biomedicine. The vertical spin produced by the illumination of a cell by the linearly polarized wave with the azimuth of polarization 45º demonstrates unique ability to control transverse motion of the nanoobject that is not characterized to the action of spin momentum inherent to the classical circular polarized optical beam.

In this work the improved nanonewton force facility of the Physikalisch-Technische Bundesanstalt (PTB), the German national metrology laboratory, and its possibility of measuring a theoretical predicted negative light pressure between two metallic plates with subwavelength distance is presented. The work includes presentation of upgrades to the existing nanonewton force facility for the realization of the experiment, the measuring methods and the first results obtained with the experimental setup together with comparisons to theoretical calculations.

Direct optical trapping of single viral particles allows characterization of individual particles in suspension with singlemolecule sensitivity. Alternative to direct optical trapping of particles, individual particles may be tethered specifically in suspension for manipulation by optical tweezers indirectly, which could be useful for studies of virus-cell interactions. One specific example is the interactions between cell surface receptors and the envelope glycoproteins (Env) on the surface of human immunodeficiency virus type 1 (HIV-1). Env binds to cellular receptors and undergoes a series of conformational changes, culminating in fusion of the viral and cellular membranes that mediates viral entry into cells. In addition to being required for cellular infection, Env is also the sole target for neutralizing antibodies. Thus, significant research has focused on elucidating the structure of Env and the mechanism of HIV-1 entry. However, current methods are unable to resolve the dynamics and stoichiometry of Env binding to cellular receptors during the entry process. Fluorescence and electron microscopy have visualized Env clusters in the viral membrane, but the extent to which these clusters actually bind to cellular receptors, and the mechanism of cluster formation, remain unclear. We describe the development of an optical tweezers technique that can potentially address these questions by delivering a single HIV-1 virion to a live cell with minimal perturbation to the system. Our method can be used to quantitatively probe the physical interactions between Env and cellular receptors in their native environment, which may reveal critical parameters in HIV-1 entry. Furthermore, our method can be used to investigate other protein-protein interactions in the context of live cells, such as the recognition of particulate antigens by B cells, thus offering insight into fundamental features of proteinmediated receptor activation.