This PDF file contains the front matter associated with SPIE Proceedings Volume 10347, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Optical Tweezing is a non-invasive technique that can enable a variety of single cell experiments or cell-cell communication experiments. To date, optical tweezers tend to be based on a high numerical aperture microscope objective to deliver the tweezing light and image the sample, which introduces restrictions in terms of flexibility. A single optical fibre-based probe able to manipulate microparticles independently from the imaging system is demonstrated. The working principle of the probe is based upon two crossed beams that can be used to trap a microparticle in the area where the two beams overlap. The two deflected beams are produced by incorporating fibre-end facet mirrors onto a multicore fibre using a Focused Ion Beam fabrication technique. The light from the two cores overlaps close to the end of the fibre and has been demonstrated to be capable of trapping particles in the area where the beams intersect. By using a multicore fibre instead of separate fibres glued together results in simplified probe manufacture and alignment and offers a smaller probe that is suitable for use in a wider range of applications, including on-chip manipulation

During the anaphase stage of mitosis, a motility force transports genetic material in the form of chromosomes to the poles of the cell. Chromosome deformations during anaphase transport have largely been attributed to viscous drag force, however LaFountain et. al. found that a physical tether connects separating chromosome ends in crane-fly spermatocytes such that a backwards tethering force elongates the separating chromosomes. In the presented study laser microsurgery was used to deduce the mechanistic basis of chromosome elongation in rat-kangaroo cells. In half of tested chromosome pairs, laser microsurgery between separating chromosome ends reduced elongation by 7±3% suggesting a source of chromosome strain independent of viscous drag. When microsurgery was used to sever chromosomes during transport, kinetochore attached fragments continued poleward travel while half of end fragments traveled towards the opposite pole and the remaining fragments either did not move or segregated to the proper pole. Microsurgery directed between chromosome ends always ceased cross-polar fragment travel suggesting the laser severed a physical tether transferring force to the fragment. Optical trapping of fragments moving towards the opposite pole estimates an upper boundary on the tethering force of 1.5 pN.

We design and demonstrate multi-trap tug-of-war (TOW) optical tweezers with object-adapted optical potentials for trapping and manipulating asymmetric particles and biological samples such as mutant bacterial cells. While dual TOW tweezers can effectively trap rod-shaped objects and even stretch them laterally, triangular TOW tweezers enable in-plane trapping of larger asymmetric objects which do not necessarily have mirror symmetry. When trapping with the dual TOW tweezers, we previously demonstrated that they are more stable than Gaussian beam-based dual traps, and the strong lateral pulling forces from the TOW optical tweezers can stretch and even break apart cellular clusters. Here we show multi-trap TOW (with 3 and 4 arms) optical tweezers can be employed to control and manipulate mutant Sinorhizobium meliloti bacterial cells, which are typically multi-pronged. We discuss the advantage of such TOW beam-based optical tweezers over traditional Gaussian beam-based holographic tweezers, and the potential applications of these TOW tweezers in studying cellular viscoelasticity, biomechanics, motility, and intercellular interactions.

We demonstrate that the optical stretcher, a fully automated dual-beam laser trap for probing single-cell mechanics, can also be used to trap pairs of cells and manipulate them. More specifically, we can press cells against each other and tear them apart again, enabling us to measure cell adhesion / dissociation dynamics. We show that we can see differences in adhesion behaviour between cell lines and we see single-molecule dissociation processes. We calculate the forces which we exert on the cells, which are in the pN range. This "optical micromanipulator" provides high-throughput adhesion measurements with about 50 cell pairs per hour, while allowing for full optical inspection of the cell dissociation process. The method can be combined with other characterization methods readily available in the stretcher such as fluorescence imaging and cell rheology evaluation.

Lightsheet fluorescence microscopy (LSFM) has rapidly progressed in the past decade from an emerging technology into an established methodology. This progress has largely been driven by its suitability to developmental biology, where it is able to give excellent spatial-temporal resolution over relatively large fields of view with good contrast and low phototoxicity. In many respects it is superseding confocal microscopy. However, it is no magic bullet and still struggles to image deeply in more highly scattering samples. Many solutions to this challenge have been presented, including, Airy and Bessel illumination, 2-photon operation and deconvolution techniques. In this work, we show a comparison between a simple but effective Gaussian beam illumination and Bessel illumination for imaging in chicken embryos. Whilst Bessel illumination is shown to be of benefit when a greater depth of field is required, it is not possible to see any benefits for imaging into the highly scattering tissue of the chick embryo.

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. We propose a model for studying TBI using laser-induced shockwaves (LISs). An optical system was developed that allows single cells to be studied in response to LISs. Our system utilizes an optically-coupled force measurement component that allows for the visualization of shockwave dynamics. Here, the force measurement system is characterized by imaging stages over the period of violent expansion and collapse of microbubbles responsible for shockwave generation.

We investigate holographic optical tweezing combined with step-and-repeat projection micro-stereolithography for fine control of live cell positioning within a three-dimensional (3D) hydrogel microstructure. Samples are fabricated using NT2 cells, which have been pre-differentiated into NT2-N human neurons. A twisted nematic 256x256 pixel SLM is used to pattern the supporting hydrogel structures. Neurons are shown to grow along printed hydrogel channels, demonstrating that the structure can be used to pre-determine the path of cellular growth. Sample viability is assessed for a variety of hydrogel geometries. This work demonstrates biocompatibility of the printing method. The samples fabricated with this system are a useful model for future studies of neural circuit formation, neurological disease, cellular communication, plasticity, and repair mechanisms.

Nanohole optical trapping is a tool that has been shown to analyze proteins at the single molecule level using pure samples. The next step is to detect and study single molecules with dirty samples. We demonstrate that using our double nanohole optical tweezing configuration, single particles in an egg white solution can be classified when trapped. Different sized molecules provide different signal variations in their trapped state, allowing the proteins to be statistically characterized. Root mean squared variation and trap stiffness are methods used on trapped signals to distinguish between the different proteins. This method to isolate and determine single molecules in heterogeneous samples provides huge potential to become a reliable tool for use within biomedical and scientific communities.

Optical tweezers integrated with Raman spectroscopy allows analyzing a single trapped micro-particle, but is generally less effective for individual nano-sized objects in the 10-100 nm range. The main challenge is the weak gradient force on nanoparticles that is insufficient to overcome the destabilizing effect of scattering force and Brownian motion. Here, we present standing-wave Raman tweezers for stable trapping and sensitive characterization of single isolated nanostructures with a low laser power by combining a standing-wave optical trap (SWOT) with confocal Raman spectroscopy. This scheme has stronger intensity gradients and balanced scattering forces, and thus is more stable and sensitive in measuring nanoparticles in liquid with 4-8 fold increase in the Raman signals. It can be used to analyze many nanoparticles that cannot be measured with single-beam Raman tweezers, including individual single-walled carbon nanotubes (SWCNT), graphene flakes, biological particles, polystyrene beads (100 nm), SERS-active metal nanoparticles, and high-refractive semiconductor nanoparticles with a low laser power of a few milliwatts. This would enable sorting and characterization of specific SWCNTs and other nanoparticles based on their increased Raman fingerprints.

We report on the development and application of coherent Rayleigh-Brillouin scattering for the in situ detection of large molecules and nanoparticles. This four wave mixing diagnostic technique relies on the creation of an electrostrictive optical lattice in a medium due to the interaction between polarized particles and the intense electric field gradient created by the optical interference of two intense pulsed laser beams. Though this interaction, we can detect the temperature, pressure, relative density, polarizability and speed of sound of a gas and gas mixture. This diagnostic was already successfully demonstrated in atomic and molecular gaseous environments, where the different gas polarizabilities and pressures were successfully measured. We are currently conducting in situ measurements with large molecules and nanoparticles produced in an arc discharge, the results of which will be presented in this meeting.

The reflection matrix (RM) measured from a rough-surface reflector contains the phase information of the light from each spatial light modulator (SLM) segment to every segment in the observation plane. This phase infor- mation can be used to produce phase maps that can refocus light to any segment in the observation plane. The measurement of an RM requires the optical system to be completely static; any disturbances result in degraded ability to refocus light. Diffraction based simulations show that RMs contain redundant phase information that can be exploited. A method is presented that allows control of the refocused light in the observation plane from a single reference phase map. This allows for the continuous optimization of the reference phase map, that compensates for system disturbances, while preserving the ability to control the location of the refocused light and eliminate the need to measure the entire RM.

Scattering is one of the simplest light mater interactions possible. For spherical particles, this process can be described using the Lorenz-Mie theory, which makes use of vector spherical harmonic solutions of Maxwell’s equations to represent the fields involved. Using these solutions it is possible to describe the light field scattered from microscopic spherical particles and thus represent the field around a scattering object as a function of the incident fields. These solutions also allow us to determine the optical momentum transfer to the scattering object. This can be calculated using Maxwell stress tensor. Here, we use this approach to calculate the quadratic relationship between the incident field and the optical forces acting on the scattering objects. This relationship defines also a set of orthogonal optical eigenmodes, which delivers a natural basis to describe momentum transfer in light-matter interactions. Using this natural description of the momentum transfer it is possible to define, for each numerical aperture, particle size or geometry the optimal trapping beam in 1, 2 or 3 dimensions. We present a study as a function of the particle parameter and conclude on the maximum achievable trapping stiffness enhancement factor as a function of these parameters.

Beam shaping of powerful multimode fiber lasers, fiber-coupled solid-state and diode lasers is of great importance for improvements of industrial laser applications. Welding, cladding with millimetre scale working spots benefit from “inverseGauss” intensity profiles; performance of thick metal sheet cutting, deep penetration welding can be enhanced when distributing the laser energy along the optical axis as more efficient usage of laser energy, higher edge quality and reduction of the heat affected zone can be achieved. Building of beam shaping optics for multimode lasers encounters physical limitations due to the low beam spatial coherence of multimode fiber-coupled lasers resulting in big Beam Parameter Products (BPP) or M² values. The laser radiation emerging from a multimode fiber presents a mixture of wavefronts. The fiber end can be considered as a light source which optical properties are intermediate between a Lambertian source and a single mode laser beam. Imaging of the fiber end, using a collimator and a focusing objective, is a robust and widely used beam delivery approach. Beam shaping solutions are suggested in form of optics combining fiber end imaging and geometrical separation of focused spots either perpendicular to or along the optical axis. Thus, energy of high power lasers is distributed among multiple foci. In order to provide reliable operation with multi-kW lasers and avoid damages the optics are designed as refractive elements with smooth optical surfaces. The paper presents descriptions of multi-focus optics as well as examples of intensity profile measurements of beam caustics and application results.

Optical vector beams arise from point to point spatial variations of the electric component of an electromagnetic field over the transverse plane. In this work, we present a novel experimental technique to generate arbitrary vec- tor beams, and provide sufficient evidence to validate their state of polarization. This technique takes advantage of the capability of a Spatial Light Modulator to simultaneously generate two components of an electromagnetic field by halving the screen of the device and subsequently recombining them in a Sagnac interferometer. Our experimental results show the versatility and robustness of this technique for the generation of vector beams.

Fluorescence imaging provides a powerful approach to study fundamental life processes and has become an integral part of the toolbox for biologists. In this talk, I will present optical nanosensors as a new tool; and how we push it to the limit for the applications of translational medicine. Two examples will be given to represent two distinct architectures to solve medical problems at single molecule and single cell level, respectively. The first example is a “lab-on-a-chip” device that can measure binding kinetics between two single molecules without fluorescent labeling. The second example is a “lab-on-a-tip” device that monitors protein expressions in single living cells over time. These nanosensor approaches, complementary to fluorescent imaging, will broaden our understanding of basic life processes at molecular level and will provide new ways for drug discovery and disease diagnostics.

We assembled an ultra-fast infrared optical trapping system to detect mechanical events that occur less than a millisecond after a ligand binds to its filamentous substrate, such as myosin undergoing its 5 – 10 nm working stroke after actin binding. The instrument is based on the concept of Capitanio et al.1, in which a polymer bead-actin-bead dumbbell is held in two force-clamped optical traps. A force applied by the traps causes the filament to move at a constant velocity as hydrodynamic drag balances the applied load. When the ligand binds, the filament motion stops within 100 μs as the total force from the optical traps is transferred to the attachment. Subsequent translations signal active motions, such as the magnitude and timing of the motor’s working stroke. In our instrument, the beads defining the dumbbell are held in independent force clamps utilizing a field-programmable gate array (FPGA) to update the trap beam positions at 250 kHz. We found that in our setup, acousto-optical deflectors (AODs) steering the beams were unsuitable for this purpose due to a slightly non-linear response in the beam intensity and deflection angle vs. the AOD ultra-sound wavelength, likely caused by low-amplitude standing acoustic waves in the deflectors. These aberrations caused instability in the force feedback loops leading to artefactual ~20 nm jumps in position. This type of AOD non-linearity has been reported to be absent in electro-optical deflectors (EODs)². We demonstrate that replacement of the AODs with EODs improves the performance of our instrument. Combining the superior beam-steering capability of the EODs, force acquisition via back-plane interferometry, and the dual high-speed FPGA-based feedback loops, we smoothly and precisely apply constant loads to study the dynamics of interactions between biological molecules such as actin and myosin.

Techniques to observe and track single unlabelled biomolecules are crucial for many areas of nano-biotechnology; allowing to shed light on important nanoscale biological processes. Impressive progress has been made over the past few years to extend the sensitivity of such techniques, primarily via evanescent field enhancement. However, such approaches expose the biological system to greatly increased optical intensity levels. Here, we introduce an evanescent biosensor that operates at the fundamental quantum limit. This allows a five order-of magnitude reduction in optical intensity whilst maintaining state-of-the-art sensitivity and enabling quantum noise limited tracking of single biomolecules as small as 3.5 nm.

Feedback traps are tools for trapping single charged objects in solution. They periodically measure an object’s position and apply a feedback force to counteract Brownian motion. The feedback force can be calculated as a gradient of a potential function, effectively creating a “virtual potential.” Its flexibility regarding the choice of form of the potential gives an opportunity to explore various fundamental questions in stochastic thermodynamics. Here, we review the theory behind feedback traps and apply it to measuring the average work required to erase a fraction of a bit of information. The results agree with predictions based on the nonequilibrium system entropy. With this example, we also show how a feedback trap can easily implement the complex erasure protocols required to reach ultimate thermodynamic limits.

In the field of laser ablation, especially in the field of micro-structuring, the current challenge is the improvement of productivity. While many applications, e.g. surface fictionalization and structuring, drilling and thin film ablation, use relatively low pulse energies, industrial laser sources provide considerably higher average powers and pulse energies. The main challenge consist of the effective energy distribution and depositions. There are essential two complementary approaches for the up-scaling of (ultra) short pulse laser processes: Higher repetition frequency or higher pulse energies. Using lasers with high repetition rates in the MHz region can cause thermal issues like overheating, melt production and low ablation quality. In this paper we pursuit the second approach by using diffractive optics for parallel processing. We will discuss, which technologies can be used and which applications will benefit from the multi-beam approach and which increase in productivity can be expected. Additionally we will show, which quality attributes can be used to rate the performance of a diffractive optic and and which limitations and restrictions this technology has.

Scalar fields can be propagated through non-paraxial systems using the Gaussian beam decomposition method. However, for high NA objectives, this scalar treatment is not sufficient to correctly describe the electromagnetic fields inside the focal region due to high ray bendings, which result in a significant change in the polarization state of light. To model these vectorial effects, the Gaussian beam decomposition method has to be extended to include the polarization state of light. In this work we have combined it with the three dimensional polarization ray tracing in order to propagate vectorial fields through high NA optical systems. During the Gaussian beam decomposition, the polarization state of each individual beamlet is represented by a polarization vector [𝐸_𝑥, 𝐸_𝑦, 𝐸_𝑧 ] associated with its central ray. Individual Gaussian beams are then propagated through the system using the complex ray tracing method. The effect of the optical system on the polarization state of each beam is computed by applying the three dimensional polarization ray tracing of the corresponding central rays. Finally the individual beams are superposed coherently in the plane of interest resulting in the complete vectorial field. We apply the proposed method to compute the vectorial field inside the focal region of a high NA microscope objective lens and compare our result to the vectorial Debye integral method. We find that the Gaussian beam decomposition method overcomes serious limitations of algorithms relying on Fourier transforms, i.e. the field sampling requirements are less critical in high NA focusing and in the presence of large aberrations. However, sharp edges in the amplitude profile are difficult to handle as they should be replaced with smooth Gaussian edge.

Optical tweezers is an increasingly important technique for controlling and probing particles since computer-generated holography (CGH) make steering of multiple traps individually possible. In addition, the dark focus of orbital angular momentum (OAM) beams is increasingly widely used in trapping reflecting, absorbing or low-dielectric-constant objects. In this paper, we present a method to create arbitrary three-dimensional configurations of orbital angular momentum modes to achieve manipulation of micro-particles. Compared with conventional optical tweezers, this method can steer mixed arrays of traps individually and randomly by producing three-dimensional structure of optical vortices. These optical traps we used was formed by a CGH generated complex phase mask, which has three components: 1) a helical phase mask to change the transverse phase structure, 2) a blazed grating phase mask to vary the propagation direction of the incident beams, and 3) a modulated grating phase mask to divert the focal plane from the planar configurations. The latter one ensure that we can form threedimensional trapping patterns. The trap patterns can also be generated dynamically by holographic display system based on liquid crystal on silicon (LCoS). The experimental results show that the refresh frequency of reconfiguring achieves 24fps. Our method is effective and promise an exciting new opportunity to be used as a valuable non-contact manipulation tool in various applications.

For paraxial propagation of scalar waves classic electromagnetic theory definitions of transverse linear (TLM) and orbital angular (OAM) momenta of beam waves are simply related to the wave coherence function in the coherent wave case. This allows the extension of the TLM and OAM density concepts to the case of partially coherent waves when phase is indeterminate. We introduce a general class of Radial Irradiance-Angular Phase (RI-AP) waves that includes the Laguerre-Gaussian (LG) beams, and similar to LG beams have discrete OAM to power ratio, but have more complex phase shape than simple helices of LG beans. We show on several examples that there no direct connection between the intrinsic OAM and optical vorticity. Namely, neither the presence of the optical vortices is necessary for the intrinsic OAM, nor the presence of the optical vortices warrants the non-zero intrinsic OAM. We examine OAM for two classes of partially-coherent beam waves and show that the common, Schell-type coherence, does not add variety to the TLM and OAM in comparison to coherent waves. However, Twisted Gaussian beam has an intrinsic OAM with per unit power value that can be continuously changed by varying the twist parameters. This analysis suggests an intrinsic OAM creation method based on rotation of tilted Gaussian beam. Using the parabolic propagation equation for the coherence function, we show that both total TLM and OAM are conserved for the free space propagation. We discuss the application of the Shack-Hartman wave front sensor for the OAM measurements.

We present a few simple examples to illustrate certain fundamental properties of the EM field. Using elementary physical concepts, we explain the nature of interactions that involve exchanges of energy, linear momentum, and angular momentum between EM fields and material media. First, the radiation force experienced by a small, polarizable particle which has a predetermined dielectric susceptibility will be examined. The dielectric susceptibility of small spherical particles will be related to their refractive index (with proper accounting for the effects of radiation resistance). We describe the relation between the energy and orbital angular momentum of a cylindrical harmonic EM wave trapped inside a hollow cylindrical cavity, and explore the relations among the energy, linear momentum, and angular momentum picked up by a small particle under illumination by a cylindrical harmonic EM wave. In light of this analysis, it becomes clear why a small particle spins around its own axis when illuminated by a light beam that carries spin angular momentum, whereas the same particle tends to orbit around an axis of vorticity when exposed to a beam (such as a vector cylindrical harmonic) that possesses orbital angular momentum.

Combining the multiple degrees of freedom of photons has become topical in quantum communication and information processes. This provides advantages such as increasing the amount of information that is be packed into a photon or probing the wave-particle nature of light through path-polarisation entanglement. Here we present two experiments that show the advantages of using hybrid entanglement between orbital angular moment (OAM) and polarisation. Firstly, we present results where high dimensional quantum key distribution is demonstrated with spatial modes that have non-separable polarisation-OAM DOF called vector modes. Secondly, we show that through OAM-polarisation entanglement, the traditional which-way experiment can be performed without using the traditional physical path interference approach.

Luminescence of a single upconverting particle (NaYF₄:Er³⁺,Yb³⁺) can be used to determine the optical trap temperature due to the partial absorption of the trapping beam either by the medium (water) or the optically trapped particle itself. This fact is an important drawback can be reduced by shifting the trapping wavelength out of the water absorption band, or by using time-modulated laser trapping beams. Both approaches have been studied and the results have shown that the thermal loading due to the trapping radiation can be minimized.

Accurate centroid position measurements of a light spot are vital for optical tweezers. To get quantitative measurements we can find the optical force by measuring the change in momentum of the trapping beam and track the position of the trapped particle using its image. We propose to use a filter with a linear transmittance to modulate the amplitude of the input beam in such a way that the transmitted intensity of the light will be proportional to the displacement of the spot. Our method is compared with existing methods and we find that it provides accurate measurements at high bandwidth.

We will present the multi-modal photonic platform including Optical Tweezers, linear and non linear optics techniques in a single instrument to allow parallel information gathering during single cell processes. The platform includes the following techniques: multipoint Optical Tweezers; Laser cutting; multi/single photon fluorescence, Fluorescence Lifetime Imaging (FLIM); Förster Resonant Energy Transfer (FLIM-FRET); Fluorescence Correlation Spectroscopy (FCS); Raman; Second/Third Harmonic Generation (SHG/THG); Coherent AntiStokes Raman Scattering (CARS) and cascade CARS; Near field tip-enhancement and 1 and 2 photons Photoluminescence Excitation Spectroscopy (1-2 PLE). Next, we will discuss the issue of spherical wave vectors decomposition of any optical beam in the Fourier space without any approximation solving the problem of spherical Bessel functions cancellation in both sides of the expansion. This expansion is the necessary first step to perform optical forces, as well as optical signals intensities, of scattering/absorbing particles. The limit of Rayleigh regime is easily obtained.

NaYF4:(Er,Yb,Gd) nanorods of different size were trapped using our original optical tweezers consisting of two fiber tips facing each other. Trapping properties were found to depended drastically on the actual particle size. Small rods were efficiently trapped whereas long rods were strongly attracted by the fiber tips and their stable trapping position was situated at the apex of one single fiber tip. In the case of the long particles the trapped particle modified the fiber tip emission properties and trapping of a second nanorod at distances of some microns from the first one is observed. These experimental results will be explained by numerical simulations using the exact Maxwell Stress Tensor approach.

Regenerative medicine has the capability to revolutionise many aspects of medical care, but for it to make the step from small scale autologous treatments to larger scale allogeneic approaches, robust and scalable label free cell sorting technologies are needed as part of a cell therapy bioprocessing pipeline. In this proceedings we describe several strategies for addressing the requirements for high throughput without labeling via: dimensional scaling, rare species targeting and sorting from a stable state. These three approaches are demonstrated through a combination of optical and ultrasonic forces. By combining mostly conservative and non-conservative forces from two different modalities it is possible to reduce the influence of flow velocity on sorting efficiency, hence increasing robustness and scalability. One such approach can be termed "optically enhanced acoustophoresis" which combines the ability of acoustics to handle large volumes of analyte with the high specificity of optical sorting.

One of the challenges for the Optical Vortex Scanning microscope is to find the effective procedures for surface topography reconstruction. We proposed an experimental setup to support solution of this problem. The Spatial Light Modulator (SLM) is used as a phase object. SLM allows to generate phase disturbance in the range 0-2π, which can be easily introduced into the beam carrying optical vortex. Our system gives an opportunity to measure optical vortex response due to phase modifications introduced by the SLM and investigate vortex sensitivity. We tested how the object position, size affects vortex and position of the vortex point inside the beam.

In the last few years a remarkable field of study has emerged, in which we study the interaction of light with the mechanical motion of massive objects: optomechanics. With the introduction of an optical resonator, cavity optomechanical systems have been cooled to their motional ground state. This means that objects many microns in size have been observed exhibiting quantum behaviour. Optomechanical systems can be used to coherently transduce quantum signals, or store them for long times, indicating that in the future they will be essential components in quantum networks. They also offer the potential for ultra-precise sensing, and the on-chip nature of many nanomechanical systems points the way to technological integration. In this lecture I will introduce the basic mathematical structure of optomechanical systems, how they can be cooled, several detailed examples, and the major results from the field. I will also discuss the future implications, and the relevance to studying fundamental physics and the limits of quantum mechanics. There will be a particular focus on systems where the nanomechanical oscillator is levitated and isolated from the environment, due to its relevance to the Optical Trapping and Micromanipulation community.

Since the early work by Ashkin in 1970,¹ optical trapping has become one of the most powerful tools for manipulating small particles, such as micron sized beads² or single atoms.³ Interestingly, both an atom and a lump of dielectric material can be manipulated through the same mechanism: the interaction energy of a dipole and the electric field of the laser light. In the case of atom trapping, the dominant contribution typically comes from the allowed optical transition closest to the laser wavelength while it is given by the bulk polarisability for mesoscopic particles. This difference lead to two very different contexts of applications: one being the trapping of small objects mainly in biological settings,⁴ the other one being dipole traps for individual neutral atoms⁵ in the field of quantum optics. In this context, solid state artificial atoms present the interesting opportunity to combine these two aspects of optical manipulation. We are particularly interested in nanodiamonds as they constitute a bulk dielectric object by themselves, but also contain artificial atoms such as nitrogen-vacancy (NV) or silicon-vacancy (SiV) colour centers. With this system, both regimes of optical trapping can be observed at the same time even at room temperature. In this work, we demonstrate that the resonant force from the optical transition of NV centres at 637 nm can be measured in a nanodiamond trapped in water. This additional contribution to the total force is significant, reaching up to 10%. In addition, due to the very large density of NV centres in a sub-wavelength crystal, collective effects between centres have an important effect on the magnitude of the resonant force.⁶ The possibility to observe such cooperatively enhanced optical force at room temperature is also theoretically confirmed.⁷ This approach may enable the study of cooperativity in various nanoscale solid-state systems and the use of atomic physics techniques in the field of nano-manipulation and opto-mechanics.

Nitrogen-vacancy (NV) centers in diamond provide a platform for room temperature spin manipulation, making them a strong candidate for inclusion in optical levitation experiments seeking to couple mechanical and spin degrees of freedom. Here, we report progress on the coherent manipulation of single NV center spins contained within optically levitated nanodiamond in a free-space optical dipole trap. The NV center spin is coherently manipulated at both atmospheric pressure and low vacuum, and while the trapping beam causes a reduction in the fluorescence emitted by the center, no reduction in the spin coherence is observed. Further, after an initial exposure to low vacuum, the nanodiamond remains at near room temperatures at all pressures and trapping powers considered in these experiments.

Cavity optomechanics has been used to cool the center-of-mass motion of levitated nanospheres to millikelvin temperatures. Trapping the particle in the cavity field enables high mechanical frequencies bringing the system close to the resolved-sideband regime. Here we describe a Paul trap constructed from a printed circuit board that is small enough to fit inside the optical cavity and which should enable an accurate positioning of the particle inside the cavity field. This will increase the optical damping and therefore reduce the final temperature by at least one order of magnitude. Simulations of the potential inside the trap enable us to estimate the charge- to-mass ratio of trapped particles by measuring the secular frequencies as a function of the trap parameters. Lastly, we show the importance of reducing laser noise to reach lower temperatures and higher sensitivity in the phase-sensitive readout.

We present an optically driven colloidal microscopic rheometer based on the recent work of Williams et al. The interplay between magnetic and optical forces allows us to experimentally build a two dimensional Taylor-Couette cell and to explore the rheological properties of confined colloidal suspensions at a microscopic scale. Despite the discrete nature of the system, we observe local instabilities in the response of the layers’ flow to the applied shear, which is a characteristic of shear banding in larger systems. Besides the rheological phenomena, these type of experiments can be useful to develop the understanding of non-equilibrium many body systems.

Rheological parameters (viscosity, creep compliance and elasticity) play an important role in cell function and viability. For this reason different strategies have been developed for their study. In this work, two new microrheometric techniques are presented. Both methods take advantage of the analysis of the polarized emission of an upconverting particle to determine its orientation inside the optical trap. Upconverting particles are optical materials that are able to convert infrared radiation into visible light. Their usefulness has been further boosted by the recent demonstration of their three-dimensional control and tracking by single beam infrared optical traps. In this work it is demonstrated that optical torques are responsible of the stable orientation of the upconverting particle inside the trap. Moreover, numerical calculations and experimental data allowed to use the rotation dynamics of the optically trapped upconverting particle for environmental sensing. In particular, the cytoplasm viscosity could be measured by using the rotation time and thermal fluctuations of an intracellular optically trapped upconverting particle, by means of the two previously mentioned microrheometric techniques.

Optical measurements of acoustic oscillations in metal nanoparticles provide a sensitive probe into the mechanical properties of materials at GHz frequencies and nanometer length scales. In these experiments, an incident pump laser heats the nanoparticles, leading to their expansion and the excitation of mechanical vibrations. The vibrations produce oscillations in the plasmon resonance frequency of the nanoparticles, which are monitored by measuring the change in transmission through the sample of a second, probe laser pulse. By making these measurements on a highly monodisperse sample of bipyramidal gold nanoparticles, we were able to determine both the frequency and the decay rate of the vibrations. Measurements on nanoparticles in different solvents made it possible to determine the portion of damping and the vibrational frequency shift that are due to coupling to the surrounding liquid environment. Viscous damping could account for results at low viscosities, but significant discrepancies were observed for higher viscosities. The discrepancies were ultimately resolved by accounting for the viscoelastic nature of the surrounding liquids. For more viscous liquids, relaxation times are higher, and thus more of the vibrational energy is stored as elastic energy in the surrounding liquid. This reduces damping, and the restoring force provided by the stored energy increases the vibrational frequency, the opposite of what would occur for an ordinary Newtonian fluid. These measurements demonstrate that metal nanoparticles can serve as nanoscale rheometers, with the picosecond response times required to reveal viscoelastic effects in conventional liquids.

Precise temperature measurements in optical traps are crucial for biological objects as they are highly sensitive to the temperature. We shift the trapped particle from the equilibrium position by moving the stage and record the position and the optical force synchronously. The absolute nature of the force detection method makes it suitable for measuring optical forces in a non-linear region of the optical trap increasing the precision to 0.2K. Moreover, the proposed method does not require a precalibration of the force detector and, therefore, can be used for temperature independent calibration.

Emulsion droplets with ultralow interfacial tensions can be pulled apart by a pair of optical traps into daughter droplets that remain connected by an oil thread of nanoscopic thickness. This thread is stabilized by the bending modulus of the oil-water interface, which opposes the necking that leads to break up into droplets in the Rayleigh-Plateau instability. Variation in the pressure exerted on the droplets by the optical traps leads to a flow of liquid between the droplets via the nanothread. The flow has two components: (i) Poiseuille flow within the thread, and (ii) transport of the entire thread from one droplet to the other, with interface being created on one droplet and destroyed on the other. For typical viscosities of the oil and water, the dominant contribution to flow arises from the transport of the whole thread with the flow internal to the thread making a minor contribution.

Superfluid helium having extremely low temperature, negligibly small viscosity, and huge thermal conductivity provides us a unique opportunity to generate a novel cryogenic space for the fabrication of nanostructures and the manipulation of their motion. Here we fabricated metallic nano- and micro-particles by laser ablation in superfluid helium and selectively trapped superconducting particles with a quadrupole magnetic field utilizing perfect diamagnetism caused by Meissner effect. We also discuss the size dependence of the superconducting transition temperatures of the trapped metallic particles by changing the temperature of liquid helium.

Theoretical analyses of radiation pressure and photon momentum in the past 150 years have focused almost exclusively on classical and/or quantum theories of electrodynamics. In these analyses, Maxwell’s equations, the properties of polarizable and/or magnetizable material media, and the stress tensors of Maxwell, Abraham, Minkowski, Chu, and Einstein-Laub have typically played prominent roles [1-9]. Each stress tensor has subsequently been manipulated to yield its own expressions for the electromagnetic (EM) force, torque, energy, and linear as well as angular momentum densities of the EM field. This paper presents an alternative view of radiation pressure from the perspective of thermal physics, invoking the properties of blackbody radiation in conjunction with empty as well as gas-filled cavities that contain EM energy in thermal equilibrium with the container’s walls. In this type of analysis, Planck’s quantum hypothesis, the spectral distribution of the trapped radiation, the entropy of the photon gas, and Einstein’s 𝐴𝐴 and 𝐵𝐵 coefficients play central roles.

We investigate dynamics and interactions of particles levitated and trapped by the thermophoretic force in a vacuum cell. Our analysis is based on footage taken by orthogonal cameras that are able to capture the three dimensional trajectories of the particles. In contrast to spherical particles, which remain stationary at the center of the cell, here we report new qualitative features of the motion of particles with non-spherical geometry. Singly levitated particles exhibit steady spinning around their body axis and rotation around the symmetry axis of the cell. When two levitated particles approach each other, repulsive or attractive interactions between the particles are observed. Our levitation system offers a wonderful platform to study interaction between particles in a microgravity environment.

This paper reviews our experiments and numerical calculations on short timescale Brownian motion and its applications. We verified the modified Maxwell-Boltzmann distribution using micrometer-sized spheres in liquids at room temperature. In addition, we proposed using Brownian particles as probes to study boundary effects imposed by a solid wall, wettability at solid-fluid interfaces, and fluid compressibility. The experiments rely on the use of tightly focused laser beams to both contain and probe the Brownian motion of microspheres in fluids. A dielectric sphere near the focus of a laser beam scatters some of the incident photons in a direction which depends on the particle’s position. Changes in the particle’s position are encoded in the spatial distribution of the scattered beam, which can be measured with high sensitivity. Lastly, we discuss the proposed studies on fluid compressibility and non-equilibrium physics using a short duration pulsed laser.

We have demonstrated chemical transformation in single microscopic-sized aerosol droplets localised in optical tweezers. Droplets in situ are measured during chemical transformation processes of solvent exchange and solute transformation through an ion exchange reaction. Solvent exchange between deionised water and heavy water in aerosol droplets is monitored through observation of the OH and OD Raman stretches. A change in solute chemistry of aerosol is achieved through droplet coalescence events between calcium chloride and sodium carbonate to promote ion exchange. The transformation forming meta-stable and stable states of CaCO₃ is observed and analysed using Gaussian peak decomposition to reveal polymorphs.

Photoacoustics have been widely used for the study of aerosol optical properties. To date, these studies have been performed on particle ensembles, with minimal ability to control for particle size. Here, we present our singleparticle photoacoustic spectrometer. The sensitivity and stability of the instrument is discussed, along with results from two experiments that illustrate the unique capabilities of this instrument. In the first experiment, we present a measurement of the particle size-dependence of the photoacoustic response. Our results confirm previous models of aerosol photoacoustics that had yet to be experimentally tested. The second set of results reveals a size-dependence of photochemical processes within aerosols that results from the nanofocusing of light within individual droplets.

Optical pulling is the attraction of objects back to the light source by the use of optically induced “negative forces”. The light-induced photophoretic force is generated by the momentum transfer between the heating particles and surrounding gas molecules and can be several orders of magnitude larger than the radiation force and gravitation force. Here, we demonstrate that micron-sized absorbing particles can be optically pulled and manipulated towards the light source over a long distance in air with a collimated Gaussian laser beam based on a negative photophoretic force. A variety of airborne absorbing particles can be pulled by this optical pipeline to the region where they are optically trapped with another focused laser beam and their chemical compositions are characterized with Raman spectroscopy. We found that micron-sized particles are pulled over a meter-scale distance in air with a pulling speed of 1-10 cm/s in the optical pulling pipeline and its speed can be controlled by changing the laser intensity. When an aerosol particle is optically trapped with a focused Gaussian beam, we measured its rotation motion around the laser propagation direction and measured its Raman spectroscopy for chemical identification by molecular fingerprints. The centripetal acceleration of the trapped particle as high as ~20 times the gravitational acceleration was observed. Optical pulling over large distances with lasers in combination with Raman spectroscopy opens up potential applications for the collection and identification of atmospheric particles.

In this paper a versatile experimental system for optical levitation is presented. Microscopic liquid droplets are produced on demand from piezo-electrically driven dispensers. The charge of the droplets is controlled by applying an electric field on the piezo-dispenser head. The dispenser releases droplets into a vertically focused laser beam. The size and position in 3 dimensions of trapped droplets are measured using two orthogonally placed high speed cameras. Alternatively, the vertical position is determined by imaging scattered light onto a position sensitive detector. The charge of a trapped droplets is determined by recording its motion when an electric field is applied, and the charge can be altered by exposing the droplet to a radioactive source or UV light. Further, spectroscopic information of the trapped droplet is obtained by imaging the droplet on the entrance slit of a spectrometer. Finally, the trapping cell can be evacuated, allowing investigations of droplet dynamics in vacuum. The system is utilized to study a variety of physical phenomena, and three pilot experiments are given in this paper. First, a system used to control and measure the charge of the droplet is presented. Second, it is demonstrated how particles can be made to rotate and spin by trapping them using optical vortices. Finally, the Raman spectra of trapped glycerol droplets are obtained and analyzed. The long term goal of this work is to create a system where interactions of droplets with the surrounding medium or with other droplets can be studied with full control of all physical variables.

Optical matter is a unique class of materials formed by pure electrodynamic interactions of colloidal particles in an optical field, yet previous research on optical matter was almost limited to microparticle systems. Some recent experimental studies, including ours, have extended the boundary of optical matter into nanometer regime, but it remains a significant challenge to build large-scale optical matter with even more than 10 nanoparticles. Here we report our ongoing work on light-driven self-organization of plasmonic nanoparticles into mesoscale clusters and arrays. We use advanced laser beam shaping techniques and the significant electrodynamic interactions among strongly scattering Ag nanoparticles to stabilize the self-organization. By simultaneously controlling the intensity, phase and polarization of light, we can design and tailor the optical field to assemble stable optical matter with more nanoparticles, and reveal new structures arising from optical binding interactions.

Our experiments have used arrayed traps in the near field of a photonic crystal to selectively capture nanoparticles by size. The photonic crystal is designed to support a guided resonance mode, and the incident laser is tuned to the resonance wavelength. Each hole of the photonic crystal acts as a trapping site. In this work, we use simulations of particle dynamics to determine the optimal experimental conditions for size selection. We include the effects of optical forces, fluid flow, and Brownian motion and explicitly track the trajectories of particles near an array of trapping sites. We study the effects of varying chamber height, flow rate, and particle concentration on size selectivity and trapping yield. We further present photonic crystal designs for selectively trapping smaller or larger particles out of mixed-particle solutions.

Particle jumping between optical potentials has attracted much attention owing to its extensive involvement in many physical and biological experiments. In some circumstances, particle jumping indicates escaping from the optical trap, which is an issue people are trying to avoid. Nevertheless, particle jumping can facilitate the individual trap in each laser spot in the optical lattice and enable sorting and delivery of nanoparticles. Particle hopping has not been seen in fluid because Fluidic drag force dramatically reduce the dwell time of particle or break the potential well. Here, we observe particle hopping in the microchannel by three reasons, e.g., particle collision or aggregation, light disturbing by pretrapped particle and fake trapping position. We show that commonly ignored particle influence to the light could create a new isolated trapping position, where particle hops to the adjacent potential well. The hopping happens in an optofluidic fishnet which is comprised of discrete hotspots enabling 2D patterning of particles in the flow stream for the first time. We also achieve a 2D patterning of cryptosporidium in the microchannel. Our observed particle hopping in the flow stream completes the family of particle kinetics in potential wells and inspires new interests in the particle disturbed optical trapping. The 2D patterning of particles benefits the parallel study of biological samples in the flow stream and have potential on cell sorting and drug delivery.

We present a pulsed laser activated cell sorter (PLACS) integrated with novel sheathless size-independent dielectrophoretic (DEP) focusing. Microfluidic fluorescence activated cell sorting (μFACS) systems aim to provide a fully enclosed environment for sterile cell sorting and integration with upstream and downstream microfluidic modules. Among them, PLACS has shown a great potential in achieving comparable performance to commercial aerosol-based FACS (>90% purity at 25,000 cells sec⁻¹). However conventional sheath flow focusing method suffers a severe sample dilution issue. Here we demonstrate a novel dielectrophoresis-integrated pulsed laser activated cell sorter (DEP-PLACS). It consists of a microfluidic channel with 3D electrodes laid out to provide a tunnel-shaped electric field profile along a 4cmlong channel for sheathlessly focusing microparticles/cells into a single stream in high-speed microfluidic flows. All focused particles pass through the fluorescence detection zone along the same streamline regardless of their sizes and types. Upon detection of target fluorescent particles, a nanosecond laser pulse is triggered and focused in a neighboring channel to generate a rapidly expanding cavitation bubble for precise sorting. DEP-PLACS has achieved a sorting purity of 91% for polystyrene beads at a throughput of 1,500 particle/sec.

This paper presents the optical fractionation of nanoparticles in silicon waveguide arrays. The optical lattice is generated by evanescent coupling in silicon waveguide arrays. The hotspot size is tunable by changing the refractive index of surrounding liquids. In the experiment, 0.2-μm and 0.5-μm particles are separated with a recovery rate of 95.76%. This near-field approach is a promising candidate for manipulating nanoscale biomolecules and is anticipated to benefit the biomedical applications such as exosome purification, DNA optical mapping, cell-cell interaction, etc.

We implemented an integrated time sharing multiple optical trapping system through the synchronisation of high speed voice coil scanning lens and laser pulsing. The integration is achieved by using commonly available optical pickup unit (OPU) that exists inside optical drives. Scanning frequencies of up to 2 kHz were showed to achieve arbitrary distribution of optical traps within the one-dimensional scan range of the voice coil motor. The functions of the system were demonstrated by the imaging and trapping of 1 μm particles and giant unilamellar vesicles (GUVs). The new device circumvents existing bulky laser scanning systems (4f lens systems) with an integrated laser and lens steering platform that can be integrated on a variety of microscopy platforms (confocal, lightsheet, darkfield).

We present a biophotonics platform based on the optical manipulation of photonic membranes via holographical tweezers. We review the fabrication and manipulation protocol which grants full six-degrees-of-freedom control over these membranes. This is despite the membranes having extreme aspect ratios, being 90 nm in thickness and 15 - 20 μm in side length. The photonic properties of the trapped membranes can be tailored to very specific applications, by structuring their topology carefully. Our method merges the flexibility of photonic design of optical meta-surfaces with the advanced manipulation capability offered by holographic optical tweezers. Here we demonstrate the validity of our approach, discussing the peculiar mechanical properties of trapped photonic membranes. Specifically, we focus on imaging and surface-enhanced Raman spectroscopy applications.

In this contribution, we will report on a new adventure in the field of photonics, combining the optical control of photonic materials with that of true micro meter scale robotics. We will show how one can create complex photonic structures using polymers that respond to optical stimuli, and how this technology can be used to create moving elements, photonic skin, and even complete micro meter size robots that can walk and swim. Using light as the only source of energy. The materials that we have developed to that end can also be used to realize tunable photonic components that respond to light and adapt their photonic response on the basis of the illumination conditions.

The measurement of the topological charge of laser beams with orbital angular momentum (OAM) is key to many applications like deciphering information encoded in several channels. Current techniques useful for that purpose are interferometry, diffraction through different poligonal apertures like triangular or pentagonal and, azimuthal and radial decomposition. A less explored issue is the diffraction of OAM beams through circular sectors. Jack et al. studied the angular diffraction of Gaussian beams (whose OAM is null) through a circular sector. By means of a Fourier transform of the truncated Gaussian beam they showed that the orbital angular momentum spectrum of the transmitted beam has a sinc-shaped envelope centered at zero orbital angular momentum, the width of which increases as the central angle of the circular sector decreases. We analyze here the spectrum of a laser beam with integer OAM that has been diffracted by a circular sector. We present results for circular sectors of different central angles. For circular π-sector, we also study the influence of the transmittance in the OAM spectra of the transmitted beam, using straight borders of nanometric thin films of titanium oxide with different thicknesses. We use a spatial light modulator with a fork hologram placed on to generate the incoming OAM beam and measure the evolution of the intensity profile of the diffracted beam as it propagates away from the circular sector. The spectra of the diffracted OAM beams are shown numerically and experimentally to have a sinc shaped envelope centered at the OAM value of the incoming OAM wave.

Calcite crystals trapped in an elliptically polarized laser field exhibit intriguing rotational motion. In this paper, we show measurements of the angle-dependent motion, and discuss how the motion of birefringent calcite can be used to develop a reliable and efficient process for determining the polarization ellipticity and orientation of a laser mode. The crystals experience torque in two ways: from the transfer of spin angular momentum (SAM) from the circular polarization component of the light, and from a torque due to the linear polarization component of the light that acts to align the optic axis of the crystal with the polarization axis of the light. These torques alternatingly compete with and amplify each other, creating an oscillating rotational crystal velocity. We model the behavior as a rigid body in an angle-dependent torque. We experimentally demonstrate the dependence of the rotational velocity on the angular orientation of the crystal by placing the crystals in a sample solution in our trapping region, and observing their behavior under different polarization modes. Measurements are made by acquiring information simultaneously from a quadrant photodiode collecting the driving light after it passes through the sample region, and by imaging the crystal motion onto a camera. We finish by illustrating how to use this model to predict the ellipticity of a laser mode from rotational motion of birefringent crystals.

Beams carrying orbital angular momentum (OAM) are ubiquitous in many experiments carried out today and cover a wide range of research, from surface microstructure processing to optical tweezers and communications. It follows that these beams are a significant factor in the outcome of these research areas. They are often generated through the use of phase-only modulation with elements such as SLMs and q-plates due to the simplicity of the approach. Interesting consequences result from this generation principal which include the introduction of radial modes as they propagate. We experimentally demonstrate how this effects the distribution of power where a notable decrease in the desired fundamental mode power occurs with higher OAM beams in addition to an expansion in the power across these radial modes. This research additionally affirms their mathematical description as the recently introduced Hypergeometric- Gaussian beams.

DNA undergoes a dramatic condensation in sperm nuclei. During this condensation, the DNA rapidly folds into a series of toroids when protamine proteins replace histone proteins. Measuring the mechanics and folding pathway for this incredible condensation is an important goal. Here, we report on progress to use an in vitro, optical trapping assay to measure the DNA folding dynamics for this process. In this assay, a single DNA molecule with its associated histone proteins is attached to a cover slip and to an optically trapped bead. Movement of the optical trap applies a force on the bead, stretching the DNA to a particular extension. When protamine is added, the extension changes, allowing us to measure the preliminary folding dynamics for the process.

Basic reactions involving water molecules are essential to understand the interaction between radiation and the biological tissue because living cells are composed mostly by water. Therefore, the knowledge of ionization of the latter is crucial in many domains of Biology and Physics. So, we study theoretically the photoionization of water molecules by extreme ultraviolet attopulse trains assisted by lasers in the near-infrared range. We use a separable Coulomb-Volkov model in which the temporal evolution of the system can be divided into three stages allowing spatial and temporal separation for the Coulomb and Volkov final state wavefunctions. First, we analyze photoelectron angular distributions for different delays between the attopulse train and the assistant laser field. We compare our results for water and Ne atoms as they belong to the same isoelectronic series. Moreover, we contrast our calculations with previous theoretical and experimental work for Ar atoms due to the similarities of the orbitals involved in the reaction. Second, we study the effect of varying the relative orientations of the attopulse and laser field polarizations and we compare our predictions with other theories and experiments. We expect these studies contribute to the improvement of polarization experiments and the development of the attopulse trains and assistant laser fields technologies. Finally, we hope our work promote progress on the control of the chemical reactivity of water molecules since this could be useful in different fields such as radiobiology and medical physics.

We have developed an inverted microscope optical tweezers for trapping and manipulation of microscopic gas bubbles. Trapping is achieved by a time-averaged optical trap using a rapidly-scanning Gaussian laser beam. Unlike holographic optical tweezers for microbubbles that employ a Laguerre-Gaussian beam, in this configuration the backwards-directed optical gradient force is sufficient to confine a microbubble against both the optical scattering force and the microbubble buoyancy. We have calibrated the optical trapping forces for microbubbles with a range of sizes, and determined the scanning trap configuration that produces the strongest confinement. Our system also includes a real-time “point-and-click” user interface for interactive selection, capture and isolation of individual microbubbles with optimal trap stiffness.

Controllable rotation of the trapped microscopic objects has traditionally been thought of one of the most valuable optical manipulation techniques. The controllable rotation of a microsphere chain was achieved by the dual-beam fiber-optic trap with transverse offset. The experimental device was made up of a PDMS chip housing two counter-propagating fibers across a microfluidic flow channel. Each fiber was coupled with different laser diode source to avoid the generation of coherent interference, both operating at a wavelength of 980 nm. Each fiber was attached to a translation stage to adjust the transverse offset distance. The polystyrene microspheres with diameter of 10 μm were chosen as the trapped particles. The microfluidic flow channel of the device was flushed with the polystyrene microspheres solution by the mechanical fluid pump. At the beginning, the two fibers were strictly aligned to each other. Five microspheres were captured as a chain parallel to the axis of the fibers. When introducing a transverse offset to the counter-propagating fibers by adjusting the translation stages, the microsphere chain was observed to rotating in the trap center. When the offset distance was set as 9 μm, the rotation period is approximately 1.2s. A comprehensive analysis has been presented of the characteristics of the rotation. The functionality of rotated chain could be extended to applications requiring microfluidic mixing or to improving the reaction speed in a localized environment, and is generally applicable to biological and medical research.

The design, fabrication and characterization of space-variant Pancharatnam-Berry phase optical elements is presented for the terahertz regime (THz). These PBOEs are made out of polystyrene and were fabricated by commercially available three-dimensional printers, providing a simple and inexpensive solution for the generation of THz vector beams. The polarization structure was characterized by using a THz time-domain imaging system. These devices can find applications in future THz technologies and provide new tools for the study of polarization morphologies

In this work we provide a practical formulation to evaluate both, dynamical an geometrical phases, for any polarization state entering an optical system characterized by a Jones matrix. By employing an automated and robust interferometric experiment, we observe characteristic behaviors depending on whether the system is homogeneous, with orthogonal eigenpolarizations, or inhomogeneous, with nonorthogonal eigenpolarizations. The results apply either for classical or quantum states of light and can be used for the design of Pancharatnam- Berry phase optical elements.

We demonstrate a simple scheme for generation the internal energy flows in optical fields based on a biaxial crystal. Such fields offer a variety of possibilities for microparticles’ trapping and control, for example: the intensity minima (maxima) due to the gradient forces; phase singularities are coupled with the vortex-like orbital flows; both the orbital and spin flows; the spin angular momentum density of the field may induce controllable spinning motion of particles; the output field pattern can be easily modified via the controllable input and output polarization, which provides suitable means for fine spatial positioning of the trapped particles.

Laser trapping of 100nm diameter polystyrene bead under high repetition rate ultrafast pulsed excitation is studied theoretically as well as experimentally. In our theoretical analysis, we explore the role of optical Kerr effect at 50mW average power under pulsed excitation. In our experiment, we use a CMOS camera to record two-photon fluorescence signal from the trapped particle which decays with time due to photo-bleaching.

Gold nanoparticles have sparked strong interest owing to their unique optical and chemical properties. Their sizedependent refractive index and plasmon resonance are widely used for optical sorting, biomedicine and chemical sensing. However, there are only few examples of optical separation of different gold nanoparticles. Only separating 100-200 nm gold nanoparticles using wavelength selected resonance of the extinction spectrum has been demonstrated. This paper reports an optofluidic chip for sorting single gold nanoparticles using loosely overdamped optical potential wells, which are created by building optical and fluidic barriers. It is the first demonstration of sorting single nanoparticles with diameters ranging from 60 to 100 nm in a quasi-Bessel beam with an optical trapping stiffness from 10⁻¹⁰ to 10⁻⁹ N/m. The nanoparticles oscillate in the loosely overdamped potential wells with a displacement amplitude of 3–7 μm in the microchannel. The sizes and refractive indices of the nanoparticles can be determined from their trapping positions using Drude and Mie theory, with a resolution of 0.35 nm/μm for the diameter, 0.0034/μm and 0.0017/μm for the real and imaginary parts of the refractive index, respectively. Here we experimentally demonstrate the sorting of bacteria and protozoa on the optofluidic chip. The chip has high potential for the sorting and characterization of nanoparticles in biomedical applications such as tumour targeting, drug delivery and intracellular imaging.

This paper presents a near-field approach to align multiple rod-shaped bacteria based on the interference pattern in silicon nano-waveguide arrays. The bacteria in the optical field will be first trapped by the gradient force and then rotated by the scattering force to the equilibrium position. In the experiment, the Shigella bacteria is rotated 90 deg and aligned to horizontal direction in 9.4 s. Meanwhile, ~150 Shigella is trapped on the surface in 5 min and 86% is aligned with angle <; 5 deg. This method is a promising toolbox for the research of parallel single-cell biophysical characterization, cell-cell interaction, etc.

A brief description of the long-standing problem of the optical momentum in media and its resolution is given. The method of force tracing to trace optical force fields along the trajectories of light rays is reviewed and a few illustrative examples are shown. Then, based on the method of force tracing, several graded-index devices performing interesting optical manipulations are reviewed.

Radiation pressure is an observable consequence of optically induced forces on materials. On cosmic scales, radiation pressure is responsible for the bending of the tails of comets as they pass near the sun. At a much smaller scale, optically induced forces are being investigated as part of a toolkit for micromanipulation and nanofabrication technology [1]. A number of practical applications of the mechanical effects of light–matter interaction are discussed by Qiu, et al. [2]. The promise of the nascent nanophotonic technology for manufacturing small, low-power, high-sensitivity sensors and other devices has likely motivated the substantial current interest in optical manipulation of materials at the nanoscale, see, for example, Ref. [2] and the references therein. While substantial progress toward optical micromanipulation has been achieved, e.g. optical tweezers [1], in this report we limit our consideration to the particular issue of optically induced forces on a transparent dielectric material. As a matter of electromagnetic theory, these forces remain indeterminate and controversial. Due to the potential applications in nanotechnology, the century-old debate regarding these forces, and the associated momentums, has ramped up considerably in the physics community. The energy–momentum tensor is the centerpiece of conservation laws for the unimpeded, inviscid, incompressible flow of non-interacting particles in the continuum limit in an otherwise empty volume. The foundations of the energy–momentum tensor and the associated tensor conservation theory come to electrodynamics from classical continuum dynamics by applying the divergence theorem to a Taylor series expansion of a property density field of a continuous flow in an otherwise empty volume. The dust tensor is a particularly simple example of an energy–momentum tensor that deals with particles of matter in the continuum limit in terms of the mass density ρm, energy density ρ_mc ² , and momentum density ρ_mv. Newtonian fluids can behave very much like dust with the same energy–momentum tensor. The energy and momentum conservation properties of light propagating in the vacuum were long-ago cast in the energy–momentum tensor formalism in terms of the electromagnetic energy density and electromagnetic momentum density. However, extrapolating the tensor theory of energy–momentum conservation for propagation of light in the vacuum to propagation of light in a simple linear dielectric medium has proven to be problematic and controversial. A dielectric medium is not ”otherwise empty” and it is typically assumed that optically induced forces accelerate and decelerate nanoscopic material constituents of the dielectric. The corresponding material energy–momentum tensor is added to the electromagnetic energy–momentum tensor to form the total energy–momentum tensor, thereby ensuring that the total energy and the total momentum of the thermodynamically closed system remain constant in time.