This PDF file contains the front matter associated with SPIE Proceedings Volume 9164, including the Title Page, Copyright information, Authors, Table of Contents, Introduction (if any), and Conference Committee listing.

We describe recent experiments that have demonstrated cavity optomechanical cooling of the center-of-mass motion of nanospheres. The naturally charged silica spheres are levitated within vacuum using an overlapping optical and electrodynamical trap. Using this system we have cavity cooled trapped nano-spheres of radius 200nm from above room temperature to less than 10K in a vacuum of 10^-4 mbar.

We describe cooling of the center-of-mass (c.o.m.) motion of silica microspheres using the morphology dependent whispering gallery mode (WGM) resonances excited by light coupled from a tapered optical fibre. This scheme uses passive cooling via the velocity dependent scattering force from the excitation of WGM resonances in one direction1 and active feedback cooling via cavity enhanced optical dipole forces (CEODF)2 along a perpendicular axis. Initial experiments have shown successful laser frequency locking to a WGM using relatively high coupled powers despite thermal bistability and thermally induced frequency shifts in the WGM. We also demonstrate the optomechanical transduction required for feedback by monitoring the transmission through the tapered fibre, demonstrating the ability to resolve displacements of less than a nanometer and velocities less than 40X10^-6 ms^-1.

We discuss the current progress on study of cooling of levitated microparticles under high vacuum conditions (6×10^-7 Torr). Particle levitation as well as feedback cooling are done using an electrical AC trap (quadrupole ion trap). It is shown that we can cool particle motion in all three degrees of freedom to 50-100K and increase their trapping time from a few hours without feedback to times exceeding weeks when the particles are cooled. The cooling is achieved by FM modulation of the AC trap frequency, and data on its dependence on gain settings is presented.

To simplify applications that rely on optical trapping of cold and ultracold atoms, ColdQuanta is developing techniques to incorporate miniature optical components onto in-vacuum atom chips. The result is a hybrid atom chip that combines an in-vacuum micro-optical bench for optical control with an atom chip for magnetic control. Placing optical components on a chip inside of the vacuum system produces a compact system that can be targeted to specific experiments, in this case the generation of optical lattices. Applications that can benefit from this technology include timekeeping, inertial sensing, gravimetry, quantum information, and emulation of quantum many-body systems. ColdQuanta’s GlasSi atom chip technology incorporates glass windows in the plane of a silicon atom chip. In conjunction with the in-vacuum micro-optical bench, optical lattices can be generated within a few hundred microns of an atom chip window through which single atomic lattice sites can be imaged with sub-micron spatial resolution. The result is a quantum gas microscope that allows optical lattices to be studied at the level of single lattice sites. Similar to what ColdQuanta has achieved with magneto-optical traps (MOTs) in its miniMOT system and with Bose- Einstein condensates (BECs) in its RuBECi^(R) system, ColdQuanta seeks to apply the on-chip optical bench technology to studies of optical lattices in a commercially available, turnkey system. These techniques are currently being considered for lattice experiments in NASA’s Cold Atom Laboratory (CAL) slated for flight on the International Space Station.

There are many ways to calculate the optical forces acting on scattering particles such as Maxwell’s stress tensor, Lorentz forces, gradient and scattering forces, Lorenz-Mie formalism, T-matrix. All these approaches use the electromagnetic field and define the amount of linear momentum transferred to the scattering particles. The resulting momentum transferred is proportional to the intensity of the incident fields, however, the single photon momentum (hbar k) does not naturally appear in these classical expressions. In this paper, we discuss an alternative Maxwell’s stress tensor based formalism that renders the classical electromagnetic field momentum compatible to the quantum mechanical one.

We show how measurements of forces through the analysis of light momentum changes can be combined with holographic optical tweezers (HOTs) to leverage the potential of this force detection method. As the magnitude is not derived from the sample displacement, no in situ calibration is required, and measurements are not restricted to specific conditions. In particular, we show that forces on irregular particles and beams can also be measured with optical traps by simultaneously applying a force in the same direction to multiple holographically-trapped particles through a constant flow. Finally, we measure forces exerted on micro-cylinders in order to assess their transversal and longitudinal drag coefficients.

The Lorentz force law of classical electrodynamics requires the introduction of hidden energy and hidden momentum in situations where an electric field acts on a magnetic material. In contrast, the Einstein-Laub formulation does not invoke hidden entities. The total force and torque exerted by electromagnetic fields on a given object are independent of whether the force and torque densities are evaluated using the law of Lorentz or that of Einstein and Laub. Hidden entities aside, the two formulations differ only in their predicted force and torque distributions throughout material media.

Artificial micro-swimmers are fast emerging as models to mimic and thereby understand the movement patterns of microorganisms and biological cells which self-propel themselves by generating fields or gradients that cause fluid flow around their surface by phoretic surface effects, such as thermophoresis or electrophoresis. In this paper, we demonstrate that radiation pressure can lead to spontaneous revolution of a micron-sized asymmetric particle inside an annular potential formed due to geometrical aberrations of a Gaussian beam focused into a stratified medium using a high numerical aperture microscopic objective. The rate of revolution can be controlled from a few Hz to tens of Hz by changing the intensity of the trapping light which can be achieved either by modifying the laser power or the annular trap diameter. Theoretical simulations performed using Finite-difference time- domain method in Lumerical verify the experimental observations. The electric field distribution confirms that the particle revolution is the effect of asymmetrical scattering by the particle in the annular potential that gives rise to a tangential force. A proper Maxwell stress-tensor analysis of the problem demonstrates this uniform tangential force acting on the particle inside the ring. The model also shows that particles could be custom designed in order to spontaneously revolve in such annular trapping potentials. Thus, such systems could be used in place of LG beams to apply torque on DNA strands in order to study protein-DNA interaction, or to study the hydrodynamic synchronization among multiple rotating objects.

Optical tweezers have been widely used in physics, chemistry and biology to manipulate and trap microscopic and nanoscopic objects. Current optical trapping techniques rely on carefully engineered setups to manipulate nanoscopic and microscopic objects at the focus of a laser beam. Since the quality of the trapping is strongly dependent on the focus quality, these systems have to be very carefully aligned and optimized, thus limiting their practical applicability in complex environments. One major challenge for current optical manipulation techniques is the light scattering occurring in optically complex media, such as biological tissues, turbid liquids and rough surfaces, which give rise to apparently random light fields known as speckles. Here, we discuss an experimental implementation to perform optical manipulation based on speckles. In particular, we show how to take advantage of the statistical properties of speckle patterns in order to realize a setup based on a multimode optical fiber to perform basic optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these “speckle optical tweezers” will greatly broaden the perspectives of optical manipulation for real-life applications.

The linear Doppler shift forms the basis of various sensor types for the measurement of linear velocity, ranging from speeding cars to fluid flow. Recently, a rotational analogue was demonstrated, enabling the measurement of angular velocity using light carrying orbital angular momentum (OAM). If measurement of the light scattered from a spinning object is restricted to a defined OAM state, then a frequency shift is observed that scales with the rotation rate of the object and the OAM of the scattered photon. In this work we measure the rotational Doppler shift from micron-sized calcite particles spinning in an optical trap at tens of Hz. In this case the signal is complicated by the geometry of the rotating particle, and the effect of Brownian motion. By careful consideration of these influences, we show how the signal is robust to both, representing a new technique with which to probe the rotational motion of micro-scale particles.

Vortex beams with different topological charge usually have different profiles and radii of peak intensity. This introduces a degree of complexity the fair study of the nature of optical OAM (orbital angular momentum). To avoid this, we introduced a new approach by creating a perfect vortex beam using an annular illuminating beam with a fixed intensity profile on an SLM that imposes a chosen topological charge. The radial intensity profile of such an experimentally created perfect vortex beam is independent to any given integer value of its topological charge. The well-defined OAM density in such a perfect vortex beam is probed by trapping microscope particles. The rotation rate of a trapped necklace of particles is measured for both integer and non-integer topological charge. Experimental results agree with the theoretical prediction. With the flexibility of our approach, local OAM density can be corrected in situ to overcome the problem of trapping the particle in the intensity hotspots. The correction of local OAM density in the perfect vortex beam therefore enables a single trapped particle to move along the vortex ring at a constant angular velocity that is independent of the azimuthal position. Due to its particular nature, the perfect vortex beam may be applied to other studies in optical trapping of particles, atoms or quantum gases.

We investigated the behavior of an oblate spheroidal polystyrene microparticle trapped in a focused vortex beam when the beam vorticity and polarization were modified. We demonstrated that such particles can be trapped in three dimensions, spin in a circularly polarized beam and an optical vortex beam around the axis parallel to the beam propagation. We compared the immediate frequencies and showed that contribution from the circularly polarized beam is one order of magnitude weaker comparing to the beam angular orbital momentum. Using a phase-only spatial light modulator we generated several vortex beam traps with well-defined parameters. Measuring the rotations of trapped spheroids we observed hydrodynamic phase and frequency locking for certain sets of beam parameters.

The spin orbit interaction (SOI) of light leading to the evolution of trajectory dependent geometric phase and associated spin Hall shift (SHS) in circularly polarized light has led to several fascinating manifestations in scattering, tight focusing, and imaging processes. However, most of these observations are at the sub-wavelength level, with somewhat limited applications of a general nature. We investigate the SOI in an optical trap for a linearly polarized trapping beam where the both the trajectory dependent geometric phase as well as the SHS are magnified significantly due to a stratified medium. The stratified medium is created using an index mismatched cover slip that modifies the radial intensity distribution near the focal plane of the trap due to diffraction effects. The modified intensity distribution causes trapping of polystyrene beads in ring-like patterns, while the tight focusing in the stratified medium also leads to a large spin redirection geometric phase that creates intensity side lobes in the azimuthal direction near the focal plane. Single trapped asymmetric particles can be trapped in the side lobes and translated along the ring by changing the polarization angle of the input beam. A 3D analysis of polarization reveals the generation of polarization vortices as well as spatially separated regions of opposite circular polarizations near the focal plane leading to controlled rotation of trapped particles, again by a linearly polarized input beam. The study can have several interesting consequences in the manipulation of mesoscopic particles in an optical trap.

To exploit the angular momentum degree of freedom of the light to control the mechanical effects that results from its linear momentum is an intriguing challenge that may open several new routes towards enhanced optical trapping, manipulation and sorting of microscopic entities. This issue can be addressed by exploiting the interplay between the chirality of matter and the chirality of optical fields. Here we will report on our recent progresses on helicity-dependent optomechanics of chiral microparticles.

Micro-sized colloidal particles dispersed in nematic liquid crystal become topological defects in uniform orientation of liquid crystal. Since they increase the elastic energy of the liquid crystal, a long-ranged anisotropic interaction is induced between them. In this study, we reported the interparticle force measured by various methods utilizing optical tweezers. The interparticle force depends on the type of particle-defect pair and its dependence on the interparticle distance is in agreement with the theoretical prediction using electrostatic analogy. This anisotropic force enables us to construct characteristic clusters, which cannot be realized in conventional water-based colloidal dispersions. We made some novel colloidal assemblies in two dimensions by utilizing optical tweezers to demonstrate the availability of the anisotropic force in nematic colloids

We analyzed the temporal responses of biological cells in the jumping and vibrating optical tweezers for tugging, wiggling and stretching the cells with the finite element method. Some new concepts were established, which might be investigated in the future experiments, such as the jumping of local stress and local strain, independently on the recovery time of the viscoelastic material and on the jumping frequency, the energy dissipation in the hysteresis cycles, the cytoplasm fluid field and its interaction with the cell membrane. The cell was modeled with full 3D structure and viscoelastic continuum materials.

During development and in the adult brain, neurons continuously explore the environment searching for guidance cues, leading to the appropriate connections. Elucidating these mechanisms represents a gold goal in neurobiology. Here, I discuss our recent achievements developing new approaches to locally probe the growth cones and stimulate neuronal cell compartments with high spatial and temporal resolution. Optical tweezers force spectroscopy applied in conjunction with metabolic inhibitors reveals new properties of the cytoskeleton dynamics. On the other hand, using optically manipulated microvectors as functionalized beads or filled liposomes, we demonstrate focal stimulation of neurons by small number of signaling molecules.

The force exerted by optical tweezers can be measured by tracking the momentum changes of the trapping beam, a method which is more general and powerful than traditional calibration techniques as it is based on first principles, but which has not been brought to its full potential yet, probably due to practical difficulties when combined with high-NA optical traps, such as the necessity to capture a large fraction of the scattered light. We show that it is possible to measure forces on arbitrary biological objects inside cells without an in situ calibration, using this approach. The instrument can be calibrated by measuring three scaling parameters that are exclusively determined by the design of the system, thus obtaining a conversion factor from volts to piconewtons that is theoretically independent of the physical properties of the sample and its environment. We prove that this factor keeps valid inside cells as it shows good agreement with other calibration methods developed in recent years for viscoelastic media. Finally, we apply the method to measuring the stall forces of kinesin and dynein in living A549 cells.

Microscopy incorporating spatial light modulators (SLMs) enables three dimensional (3D) excitation and monitoring of the activity of neuronal ensembles, enabling studies of neuronal circuit activity both in vitro and in vivo. In this paper we present a portable (22 cm x 42.5 cm x 30 cm), SLM-based epi-fluorescence upright microscope (“Pocketscope”) that enables 3D calcium imaging and photoactivation of neurons in brain slices. Here we describe the implementation of the instrument; quantify the volume over which neural activity can be excited; and demonstrate the use of the system for mapping neural circuits in brain slices.

Stereo-microscopy is a technique that enables a sample to be imaged from two directions simultaneously, allowing the tracking of microscopic objects in three dimensions. This is achieved by illuminating the sample from different directions, each illumination direction producing an individual image. These images are superimposed in the image plane but can be easily separated using a diffractive optical element in the Fourier plane of the imaging arm. Therefore this enables 3-dimensional coordinates to be reconstructed using simple 2-dimensional image tracking and parallax. This is a powerful technique when combined with holographic optical tweezers (HOT), where multiple objects can be trapped and tracked simultaneously in three dimensions. In this work, we extend this concept to four different illumination directions: quad stereo-microscopy. This allows us to measure the accuracy of tracking in three dimensions, and to optimise the system.

Dual and multiple beam optical tweezers allow for advanced trapping geometries beyond single traps, however, these increased manipulation capabilities, usually complicate the detection of position and force. The accuracy of position and force measurements is often compromised by crosstalk between the detected signals, this crosstalk leading to a systematic error on the measured forces and distances. In dual-beam optical trapping setups, the two traps are typically orthogonal polarized and crosstalk can be minimized by inserting polarization optics in front of the detector, however, this method is not perfect because of the de-polarization of the trapping beam introduced by the required high numerical aperture optics. Moreover, the restriction to two orthogonal polarisation states limits the number of detectable traps to two. Here, we present an easy-to-implement simple method to efficiently eliminate cross-talk in dual beam setups.1 The technique is based on spatial filtering and is highly compatible with standard back-focal-plane photodiode based detection. The reported method significantly improves the accuracy of force-distance measurements, e.g., of single molecules, hence providing much more scientific value for the experimental efforts. Furthermore, it opens the possibility for fast and simultaneous photodiode based detection of multiple holographically generated optical traps.

In this work, we present and discuss several developments implemented in an instrument that uses the detection of the light momentum change for measuring forces in an optical trap. A system based on this principle provides a direct determination of this magnitude regardless of the positional response of the sample under the effect of an external force, and it is therefore to be preferred when in situ calibrations of the trap stiffness are not attainable or are difficult to achieve. The possibility to obtain this information without relying upon a harmonic model of the force is more general and can be used in a wider range of situations. Forces can be measured on non-spherical samples or non-Gaussian beams, on complex and changing environments, such as the interior of cells, or on samples with unknown properties (size, viscosity, etc.). However, the practical implementation of the method entails some difficulties due to the strict conditions in the design and operation of an instrument based on this method. We have focused on some particularly conflicting points. We developed a process and a mechanism to determine and systematically set the correct axial position of the device. We further analyzed and corrected the non-uniform transmittance of the optical system and we finally compensated for the variations in the sensor responsivity with temperature. With all these improvements, we obtained an accuracy of ~5% in force measurements for samples of different kinds.

With suitable calibration, optical tweezers can be used to measure forces. If the maximum force that can be exerted is of interest, calibration can be performed using viscous drag to remove a particle from the trap, typically by moving the stage. The stage velocity required to remove the particle then gives the escape force. However, the escape force can vary by up to 30% or more, depending on the particle trajectory. This can have significant quantitative impact on measurements. We describe the variation of escape force and escape trajectory, using both experimental measurements and simulations, and discuss implications for experimental measurement of forces.

Asymmetric particles, such as biological cells, often experience torque under optical tweezers due to birefringence or unbalanced scattering forces, which makes precise determination of the torque crucial for calibration and control of the particles. The estimate of torque relies on the accurate measurement of rotational motion, which has been achieved by various techniques such as measuring the intensity fluctuations of the forward scattered light, or the polarization component orthogonal to the trapping light polarization in plasmonic nanoparticles and vaterite crystals. Here we present a simple yet high sensitive technique to measure rotation of such an asymmetric trapped particle by detecting the light backscattered onto a quadrant photodiode, and subtracting the signals along the two diagonals of the quadrants. This automatically suppresses the common mode translational signal obtained by taking the difference signal of the adjacent quadrants, while amplifying the rotational signal. Using this technique, we obtain a S/N of 200 for angular displacement of a trapped micro-rod by 5 degrees, which implies a sensitivity of 50 mdeg with S/N of 2. The technique is thus independent of birefringence and polarization properties of the asymmetric particle and depends only on the scattering cross-section.

We present a new method for calibrating an optical-tweezer setup that is based on Bayesian inference¹. This method employs an algorithm previously used to analyze the confined trajectories of receptors within lipid rafts^2,3. The main advantages of this method are that it does not require input parameters and is insensitive to systematic errors like the drift of the setup. Additionally, it exploits a much larger amount of the information stored in the recorded bead trajectory than standard calibration approaches. The additional information can be used to detect deviations from the perfect harmonic potential or detect environmental influences on the bead. The algorithm infers the diffusion coefficient and the potential felt by a trapped bead, and only requires the bead trajectory as input. We demonstrate that this method outperforms the equipartition method and the power-spectrum method in input information required (bead radius and trajectory length) and in output accuracy. Furthermore, by inferring a higher order potential our method can reveal deviations from the assumed second-order potential. More generally, this method can also be used for magnetic-tweezer calibration.

Dense coverage of DNA by proteins is a ubiquitous feature of cellular processes such as DNA organization, replication and repair. We present a single-molecule approach capable of visualizing individual DNA-binding proteins on densely covered DNA and in the presence of high protein concentrations. Our approach combines optical tweezers with multicolor confocal and stimulated emission depletion (STED) fluorescence microscopy. Proteins on DNA are visualized at a resolution of 50 nm, a sixfold resolution improvement over that of confocal microscopy. High temporal resolution (<50 ms) is ensured by fast one-dimensional beam scanning. Individual trajectories of proteins translocating on DNA can thus be distinguished and tracked with high precision. We demonstrate our multimodal approach by visualizing the assembly of dense nucleoprotein filaments with unprecedented spatial resolution in real time. Experimental access to the force-dependent kinetics and motility of DNA-associating proteins at biologically relevant protein densities is essential for linking idealized in vitro experiments with the in vivo situation.

Nanoaperture based trapping has developed as a significant tool among the various optical tweezer systems for trapping of very small particles down to the single nanometer range. The double nanohole aperture based trap provides a method for efficient, highly-sensitive, label-free, low-cost, free-solution single molecule trapping and detection. We use the double nanohole tweezer to understand biomolecular phenomena like protein unfolding, binding, structural conformation of DNA, protein-DNA interactions, and protein small molecule interactions.

I will describe a new technique to trap matter at the nanometer scale in fluids. Rather than apply external fields to the object of interest, our approach relies on spatial tailoring of the interaction between an object and its neighbouring surfaces in order to create spatial potential minima in three dimensions. We demonstrate how the strong and long-ranged electrostatic interaction can be modulated by tailoring substrate geometry to achieve stable spatial trapping of charged objects, as small as single proteins in solution.

We induced forced and auto oscillations in one-dimensional photonic crystals with localized defects when light impinges transversally to the defect layer. The photonic structure consists of a microcavity like structure formed of two onedimensional photonic crystals made of free-standing porous silicon, separated by variable air gap and the working wavelength is 633 nm. The force generation is made evident by driving a laser light by means of a chopper; the light hits the photonic structure and induces a vibration and the vibration is characterized by using a very sensitive vibrometer. Moreover we measured peak displacements and velocities ranging from 2 up to 35 microns and 0.4 up to 2.1 mm/s with a power of 13 mW. Recent evidence showed that giant resonant light forces could induce average velocity values of 0.45 mm/s in microspheres embedded in water with 43 mW light power.

Recent advances in optical manipulation have made it an ideal tool to create one, two, and three dimensional periodic optical potential. Such periodic potentials have found interesting technological and fundamental applications such as micro particle sorting and optical fractionation. Plasmon enhanced optical trapping techniques using metallic nanostructures can overcome the diffraction limits of far-field optical trap techniques and therefore permit trapping of nanoparticle with deep sub wavelength dimensions. Here we report the trapping of nanoparticles for a plasmon-enhanced two dimensional optical lattice integrated with microfluidic chip. We observe the trapping of nanoparticles over such an optical lattice. Such an integrated device allows the directional control of nano particles and provides a suitable platform for stochastic transport experiment such as nanoscale optical sorting.

The amplitude of optical forces on flowing dielectric microparticles can be actuated by coating them partially with metallic nanospheres and exposing them to laser light within the surface plasmon resonance. Here, optical forces on both pure silica particles and silica-gold raspberries are characterized within an optical chromatography setup by measuring the Stokes drag versus laser beam power. Results are compared to Mie theory predictions for both core dielectric particles and core-shell ones where the shell is described by a continuous dielectricmetal composite of dielectric constant determined from the Maxwell Garnett approach. The nice observed quantitative agreement demonstrates that radiation pressure forces are directly related to the metal concentration present at the microparticle surface and that nano-metallic objects increase the magnitude of optical forces compared to pure dielectric particles of the same overall size, even at very low metal concentration. Behaving as “micro-sized nanoparticles", the benefit of microparticles coated with metallic nanospheres is thus twofold: (i) to enhance optofluidic manipulation and transport at the microscale and (ii) to increase sensing capabilities at the nanoscale, compared to separated pure dielectric particles and single metallic nanosystems.

We present a study of the trapping properties of Au nanorods of different aspect ratios in an optical tweezers and comparison with other characterization techniques like transmission electron microscope (TEM) imaging and dynamic light scattering (DLS). This study provides information on the dynamics and orientation of Au nanorods inside an optical trap based on a time study of their localised surface plasmon resonance (LSPR) features. The results indicate that the orientation of the Au nanorods trapped in our optical tweezers varies with time and LSPR spectra can provide information on the angle of the nanorod with respect to the direction of propagation of the trapping laser.

In the following study we investigate the dynamics of high aspect ratio nanowires held in a single gradient force optical trap in an overdamped environment. Power spectrum analysis performed on the stochastic trajectory of the optically trapped nanowires indicate that the motion of these nanowires shows characteristics of underdamped motion, where a broad resonance peak is present in the power spectrum of amplitude fluctuations under certain conditions. The resonance occurs when the nanowires are trapped at a height of 50 μm from the cover slip of the sample chamber. The emergence of a resonance peak in the power spectrum could be attributed to the non-conservative motion of nanowires being nonspherical, thus creating a bias towards cyclic motion as examined theoretically by Simpson and Hanna [12].

Plasmonic nanoantennas make effective optical tweezers, owing to their characteristic field enhancement and confinement properties which produce large near-field intensity gradients. The trapping dynamics of plasmonic nanotweezers are strongly affected by their resonant optical absorption, which can produce significant heating and induce rapid convective flows in the surrounding fluid medium. We here consider a new class of plasmonic nanotweezers based on an array of elevated bowtie nanoantennas (BNA), whereby BNAs are suspended on optically transparent, 500-nm tall silica pillars. We discuss how the plasmonic properties of these pillar-BNAs (pBNAs) can be manipulated in large areas of 80 × 80-micron using low-input power densities. This modification in local plasmonic properties is expected to result in a much more complex optical trapping landscape. We also find that the temperature increase in the pBNAs is more than 10× higher than in comparable substrate-bound structures (for the same input intensity), in which the substrate acts as a heat sink that mitigates temperature increase, and we investigate the role of this enhanced thermo plasmonic heating on plasmonic trapping dynamics.

Colloids interacting with complex landscapes created by optical means exhibit a remarkable variety of novel orderings and equilibrium states. It is also possible to study nonequilibrium properties for colloids driven over optical traps when there is an additional external electric field or some other form of external driving. Recently a new type of colloidal system has been realized in which the colloids are self-driven or self-motile and undergo a persistent random walk. Self motile particle systems fall into the broader class of self-driven systems called active matter. For the case of externally driven colloidal particles moving over random or periodic arrangements of traps, various types of pinning or jamming effects can arise. Far less is known about the mobility of active matter particles in the presence or random or periodic substrates. For example, it is not known whether increasing the activity of the particles would reduce the jamming effects caused by effective friction between particles. Here we show by varying the activity and the density of active particles that various types of motion can arise. In some cases, increasing the self-driving leads to a reduction in the net flow of particles through the system.

Microparticles such as colloids and microorganisms moving in viscous liquid interact with each other via hydrodynamic interaction and often exhibit complex collective behaviors. In this study, we observed the collective motion of many colloids moving along the same circular path in water by utilizing optical vortex. The characteristic collective motion including clustering and dissociation of the particles was observed and their dynamic patterns depend on the number of the particles on the path. By addition of different sized particles, the specific clusters can be selectively induced. Those experimental findings are reproduced by numerical simulation which takes into account the hydrodynamic interaction with Oseen approximation and the radial optical trapping force.

Coordinated motion at low Reynolds number is widely observed in biological micro-systems, but the underlying mechanisms are often unclear. A holographic optical tweezers system is used to experimentally study this phenomenon, by employing optical forces to drive a pair of coplanar microspheres in circular orbits with a constant tangential force. In this system synchronisation is caused by hydrodynamic forces arising from the motion of the two spheres. The timescales of their synchronisation from large initial phase differences are explored and found to be dependent on how stiffly the microspheres are confined to their circular orbits. These measured timescales show good agreement with numerical simulations.

Predicted by Einstein in his 1905 paper on Brownian motion, colloidal particles in suspension reach osmotic equilibrium under gravity. The idea was demonstrated by J.B. Perrin to win Nobel Prize in Physics in 1926. We show Einstein’s equation for osmotic equilibrium can be applied to colloids in a conservative force field generated by optical gradient forces. We measure the osmotic equation of state of 100nm Polystyrene latex particles in the presence of KCl salt and PEG polymer. We also obtain the osmotic compressibility, which is important for determining colloidal stability and the internal chemical potential, which is useful for predicting the phase transition of colloidal systems. This generalization allows for the use of any conservative force fields for systems ranging from colloidal systems to macromolecular solutions.

Using optical tweezers for micro-rheological investigations of a surrounding fluid has been routinely demonstrated. In this work, we will demonstrate that rheological measurements of the bulk and surface properties of aerosol particles can be made directly using optical tweezers, providing important insights into the phase behavior of materials in confined environments and the rate of molecular diffusion in viscous phases. The use of holographic optical tweezers to manipulate aerosol particles has become standard practice in recent years, providing an invaluable tool to investigate particle dynamics, including evaporation/ condensation kinetics, chemical aging and phase transformation. When combined with non-linear Raman spectroscopy, the size and refractive index of a particle can be determined with unprecedented accuracy <+/- 0.05%). Active control of the relative positions of pairs of particles can allow studies of the coalescence of particles, providing a unique opportunity to investigate the bulk and surface properties that govern the hydrodynamic relaxation in particle shape. In particular, we will show how the viscosity and surface tension of particles can be measured directly in the under-damped regime at low viscosity. In the over-damped regime, we will show that viscosity measurements can extend close to the glass transition, allowing measurements over an impressive dynamic range of 12 orders of magnitude in relaxation timescale and viscosity. Indeed, prior to the coalescence event, we will show how the Brownian trajectories of trapped particles can yield important and unique insights into the interactions of aerosol particles.

A counter-propagating optical trap has been used to study thin organic films on the surface of solid particles levitated in air. Micron sized silica spheres have been trapped in air between opposed 1064 nm laser beams, and illuminated with a broadband white LED. Backscattered light from the trapped particle was collected to obtain a Mie spectrum over the 495-670 nm wavelength range and this was used to determine particle radius and wavelength dependent refractive index (Jones et al., 2013). The trapped particle was coated using a flow of organic vapour and the resultant thin film analysed using a coated sphere model. Resonance positions in the Mie spectrum were monitored with time in order to determine film formation, thickness and refractive index. Whilst thin films are believed to form naturally on atmospheric aerosols (Tervahattu et al., 2002), a debate remains as to whether the organic component completely coats the aerosol surface or partially engulfs it. Such films are readily oxidised in the atmosphere causing a change in aerosol properties and knowledge of aerosol properties is required to understand their effect on the climate. The use of optical trapping combined with Mie spectra acquisition to study and characterise coated solid particles is therefore an important step in atmospheric science.

To resolve some of the significant uncertainties in the impact of aerosols on global climate, new tools are required to probe light scattering and absorption by aerosol particles. Ideally, such tools should allow direct measurements on individual particles over extended periods of time, providing data to better constrain the optical properties of aerosol, how they depend on the environmental conditions (relative humidity and temperature) and how they change with time. Here, we present a new technique using a combination of a Bessel beam to manipulate individual particles and cavity ringdown spectroscopy for ultrasensitive measurements of the optical extinction. We show that particles can be spatially separated along the propagation direction of a Bessel beam according to their size and refractive index when confined by a Bessel beam core and a counter-propagating gas flow, referred to as optical chromatography. The time-dependent position of a particle is shown to be a consequence of the differing size dependencies of the forces arising from Stokes drag and radiation pressure. We also show that particles captured in a Bessel beam can be moved in and out of an optical cavity formed by two highly reflective mirrors. The time constant for the ringdown in light coupled within the cavity can then be used to measure the optical cross-section of the individual particle with high accuracy. An individual particle can be captured indefinitely and its change in optical cross-section measured with change in environmental conditions.

There are many examples of interacting particles that have both repulsive and attractive interaction terms. Assemblies of such particles can form clumps, gel type states, labyrinths and other patterns, while two-dimensional systems with purely repulsive interactions typically form hexagonal crystals. Here we examine the two-dimensional pattern formation of colloids with competing interactions in the presence of a quasi-one dimensional periodic substrate. For soft matter systems, such substrates can be created by various optical means. We show that the substrate can induce various patterns including commensurate bubble phases as well as modulated stripes aligned with the substrate. Beyond colloids, these results should also be general to other systems that can be modeled as particles with competing interactions moving over a surface with a quasi-one dimensional periodic substrate or modulation.

We report the combined use of optical sorting and acoustic levitation to give particle sorting. Differing sizes of microparticles are sorted optically both with and without the aid of acoustic levitation, and the results compared to show that the use of acoustic trapping can increase sorting efficiency. The use of a transparent ultrasonic transducer is also shown to streamline the integration of optics and acoustics. We also demonstrate the balance of optical radiation pressure and acoustic levitation to achieve vertical sorting.

Reduced blood deformability is clinically linked to several diseases. It is important to develop sensitive tools to measure the loss of blood deformability. The evanescent field of an optical waveguide can trap and propel red blood cells along the waveguide. Here we propose to use the evanescent field from a narrow optical waveguide to trap and deform red blood cells. We demonstrate that the intensity gradient of the evanescent field at the edge of narrow waveguides (1-3 μm) can be used to squeeze a blood cell. The RBCs are squeezed to a size comparable to the waveguide width. When the laser is switched on the cell is attracted towards the waveguide and is held in place. Subsequently, the part of the cell not on the waveguide is pulled in across the waveguide. The result is a cell (7-8 μm in diameter) squeezed down to a significantly smaller width (typically 3 μm). The cell regains its original shape when laser is switched-off.

In this paper we show how the electrical lysis of cells can be controlled through the use of an optoelectronic device and describe how the lysis is affected by the size and shape of the cell as well as the geometry of the device and the light patterns used. By selectively illuminating a photoconductor “virtual electrodes” are created allowing the precise control of electrical fields onto the cell within the device, electrically rupturing their membranes and allowing access to intracellular contents. We examine this optically controlled electrical lysis method and discuss its advantages as a sample preparation technique.

We present a Pulsed Laser Activated Cell Sorter (PLACS) realized by exciting laser induced cavitation bubbles in a PDMS microfluidic channel to create high speed liquid jets to deflect detected fluorescent samples for high speed sorting. Pulse laser triggered cavitation bubbles can expand in few microseconds and provide a pressure higher than tens of MPa for fluid perturbation near the focused spot. This ultrafast switching mechanism has a complete on-off cycle less than 20 μsec. Two approaches have been utilized to achieve 3D sample focusing in PLACS. One is relying on multilayer PDMS channels to provide 3D hydrodynamic sheath flows. It offers accurate timing control of fast (2 m sec^-1) passing particles so that synchronization with laser bubble excitation is possible, an critically important factor for high purity and high throughput sorting. PLACS with 3D hydrodynamic focusing is capable of sorting at 11,000 cells/sec with >95% purity, and 45,000 cells/sec with 45% purity using a single channel in a single step. We have also demonstrated 3D focusing using inertial flows in PLACS. This sheathless focusing approach requires 10 times lower initial cell concentration than that in sheath-based focusing and avoids severe sample dilution from high volume sheath flows. Inertia PLACS is capable of sorting at 10,000 particles sec^-1 with >90% sort purity.

We present and discuss a set of experiments based on the application of the nonlinear properties of colloidal nanosuspensions to induce waveguides with a high‐power CW laser beam (wavelength 532nm) and its use for controlling an additional probe beam. The probe is a CW laser of a different wavelength (632nm), whose power is well below the critical value to induce nonlinear effects in the colloidal medium. We also discuss a technique for the characterization of the induced waveguides.

Manipulation and transport of microparticles and even fluorescent molecules by thermally induced gradient of the order parameter is demonstrated in the nematic liquid crystal. IR light absorption of a focused beam of the laser tweezers is used to heat locally a thin layer of the nematic liquid crystal by several degrees, thus creating a spatial gradient of temperature of the nematic liquid crystal over tens of micrometers. It is observed that a colloidal particle with dipolar symmetry of the director configuration is attracted into the hot spot of the tweezers. The strength of trapping potential increases linearly with particle radius, which indicates that the trapping is due to elastic energy of the distorted nematic liquid crystal around the particle. By using fluorescent molecules instead of colloidal particles, we observed that this thermal trapping of colloidal particles is efficient down to the nanoscale, as fluorescent molecules are also attracted to the hotter regions of the liquid crystal. This effect is absent in the isotropic phase.

We show how optocapillary stresses and optical radiation pressure effects in two-phase liquids open the way for investigating the difficult problem of liquid thread breakup at small scales when surfactants are present at the interface or when the roughness of the interface becomes significant. Using thermocapillary stresses driven by light to pinch a surfactant-laden microjet, we observe deviations from the expected visco-capillary law governed by a balance between viscosity and interfacial tension. We suggest that these deviations are due to time varying interfacial tension resulting from the surfactant depletion at the neck pinching location, and we experimentally confirm this mechanism. The second case is representative of the physics of nanojets. Considering a near critical liquid-liquid interface, where the roughness of the interfaces may be tuned, we use the radiation pressure of a laser wave to produce stable fluctuating liquid columns and study their breakup. We show how pinching crosses over from the visco-capillary to a fluctuation dominated regime and describe this new regime. These experiments exemplify how optofluidics can reveal new physics of fluids.

Pumping and mixing of small volumes of liquid samples are basic processes in microfluidic applications. Among the number of different principles for active transportation of the fluids microrotors have been investigated from the beginning. The main challenge in microrotors, however, has been the driving principle. In this work a new approach for a very simple magnetic driving principle has been realized. More precisely, we take advantage of optical grippers to fabricate various microrotors and introduce an optical force method to characterize the fluid flow generated by rotating the structures through magnetic actuation. The microrotors are built of silica and magnetic microspheres which are initially coated with Streptavidin or Biotin molecules. Holographic optical tweezers (HOT) are used to trap, to position, and to assemble the microspheres with the chemical interaction of the biomolecules leading to a stable binding. Using this technique, complex designs of microrotors can be realized. The magnetic response of the magnetic microspheres enables the rotation and control of the structures through an external magnetic field. The generated fluid flow around the microrotor is measured optically by inserting a probe particle next to the rotor. While the probe particle is trapped by optical forces the flow force leads to a displacement of the particle from the trapping position. This displacement is directly related to the flow velocity and can be measured and calibrated. Variations of the microrotor design and rotating speed lead to characteristic flow fields.

We have estimated the mitotic forces exerted on individual isolated mammalian chromosomes using optical trapping. The chromosomes were trapped by an optical tweezers system created by a continuous wave ytterbium laser at 1064 nm. Individual chromosomes were trapped at different in situ powers in the range of ≈20-50 mW. The corresponding trapping forces were determined by a viscous drag method. In the range of laser powers used, the preliminary data show a linear relationship between the chromosome trapping forces and in situ powers. We have calculated the dimensionless trapping efficiency coefficient (Q) of the chromosomes at 1064 nm and the corresponding effects of trapping power on Q. The value of Q in our experiments was determined to be ≈0.01. The results of this study validate optical tweezers as a non-invasive and precise technique to determine intracellular forces in general, and specifically, the spindle forces exerted on the chromosomes during cell division.

We studied experimentally self-arrangement of sub-micrometer size particles creating a one-dimensional colloidal waveguide of length several tens of micrometers. We investigated positions and behavior of particles in the chain. We also focused on 2D colloidal waveguides created in elliptical Gaussian beams. We employed geometry of counter-propagating beams enhanced with spatial light modulator to shape the intensity profile of counter-propagating beams.

This work reports a new type of optical fiber tweezers based on polymeric micro-lenses. The lenses are achieved by means of an economical and fast fabrication process, using an in-fiber photo-polymerization technique. The polymerization radiation is guided towards the fiber tip creating a polymeric waveguide. The method allows tailoring the geometry of the tip by adjusting the fabrication parameters. Furthermore, more complex shapes can be fabricated by exploring modal effects at the polymerization/trapping wavelengths, which can be used for different applications such as trapping, beam shaping and patterned illumination.

Focusing the continuous laser radiation on the water with absorbing particles results in the emergence of shock waves and medium blooming periodic in time. The illuminating beam diameter growth at the constant laser power results in the decrease of the signals’ modulation frequency, improving their stability and increasing their amplitudes. The decrease of signal’s modulation frequency is caused by the growth of time, which is needed for heating the medium to the critical temperature. Improving the stability and the increase of optical and acoustic signals’ amplitudes take place due to the growth of the number of particles participating in cavitation.

Microrheology, the study of the behavior of fluids on the microscopic scale, has been and continues to be one of the most important subjects that can be applied to characterize the behavior of biological fluids. It is extremely difficult to make rapid measurement of the viscoelastic properties of the interior of living cells. Liposomes are widely used as model system for studying different aspects of cell biology. We propose to develop a microrheometer, based on real-time control of optical tweezers, in order to investigate the viscoelastic properties of the fluid inside liposomes. This will give greater understanding of the viscoelastic properties of the fluids inside cells. In our experiment, the liposomes are prepared by different methods to find out both a better way to make GUVs and achieve efficient encapsulation of particle. By rotating the vaterite inside a liposome via spin angular momentum, the optical torque can be measured by measuring the change of polarization of the transmitted light, which allows the direct measurement of viscous drag torque since the optical torque is balanced by the viscous drag. We present an initial feasibility demonstration of trapping and manipulation of a microscopic vaterite inside the liposome. The applied method is simple and can be extended to sensing within the living cells.

Holographic Raman tweezers (HRT) manipulates with microobjects by controlling the positions of multiple optical traps via the mouse or joystick. Several attempts have appeared recently to exploit touch tablets, 2D cameras or Kinect game console instead. We proposed a multimodal “Natural User Interface” (NUI) approach integrating hands tracking, gestures recognition, eye tracking and speech recognition. For this purpose we exploited “Leap Motion” and “MyGaze” low-cost sensors and a simple speech recognition program “Tazti”. We developed own NUI software which processes signals from the sensors and sends the control commands to HRT which subsequently controls the positions of trapping beams, micropositioning stage and the acquisition system of Raman spectra. System allows various modes of operation proper for specific tasks. Virtual tools (called “pin” and “tweezers”) serving for the manipulation with particles are displayed on the transparent “overlay” window above the live camera image. Eye tracker identifies the position of the observed particle and uses it for the autofocus. Laser trap manipulation navigated by the dominant hand can be combined with the gestures recognition of the secondary hand. Speech commands recognition is useful if both hands are busy. Proposed methods make manual control of HRT more efficient and they are also a good platform for its future semi-automated and fully automated work.

Optical forces can affect the motion of a Brownian particle. For example, optical tweezers use optical forces to trap a particle at a desirable position. Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion in a confined potential can provide insight into out-of-equilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion in an optical potential using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations.

We have developed a multiplexed holographic optical tweezers system with an imaging spectrometer to manipulate multiple optically trapped nanosensors and detect multiple fluorescence spectra. The system uses a spatial light modulator (SLM) to control the positions of infrared optical traps in the sample so that multiple nanosensors can be positioned into regions of interest. Spectra of multiple nanosensors are detected simultaneously with the application of an imaging spectrometer. Nanosensors are capable of detecting changes in their environment such as pH, ion concentration, temperature, and voltage by monitoring changes in the nanosensors' emitted fluorescence spectra. We use streptavidin labeled quantum dots bound to the surface of biotin labeled polystyrene microspheres to measure temperature changes by observing a corresponding shift in the wavelength of the spectral peak. The fluorescence is excited at 532 nm with a wide field source.

This document presents a first approach to study the behavior of a static fluid radiated by infrared light (980nm,100mW) transmitted by a single-mode optical fiber, for this simulation temperature and radiation pressure are calculated based on the intensity delivered by a laser diode. The Computing Fluid Dynamics (CFD) results were based on a mesh Tet/Hybrid, TGrid for a Silica micro-particle and a mesh Hex/Wedge, Cooper for the beam. The results show that as the particle moves along the axis, temperature and pressure decreases, having the points of mayor temperature and pressure around the axis. The conclusion of this work is that it is possible to simulate the interactions between the beam, the micro-particle and the surrounding medium in terms of temperature, velocity and pressure using the energy and viscous model.

In this work, we present numerical and experimental generation of reconfigurable vector beams employing synthetic phase holograms (SPH) that provide the optimum diffraction efficiency and high quality of reconstruction. The vector beams with spatially variable polarization are generated by the linear recombination of two orthogonally homogeneous polarized scalar modes. The scalar modes are generated with a SPH displayed in a phase-only Spatial Light Modulator (SLM) and superimposed using a common-path interferometer which consists of a 4-f system. We demonstrate the generation of high order TE and TM vector Bessel, Mathieu and Weber beams which could be used for optical trapping applications.

The significant increase in the air pollution, and the impact on climate change due to the burning of fossil fuel has led to the research of alternative energies. Bio-ethanol obtained from a variety of feedstocks can provide a feasible solution. Mixing bio-ethanol with gasoline leads to a reduction in CO emission and in NOx emissions compared with the use of gasoline alone. However, adding ethanol leads to a change in the fuel evaporation. Here we present a preliminary investigation of evaporation times of single ethanol-gasoline droplets. In particular, we investigated the different evaporation rate of the droplets depending on the variation in the percentage of ethanol inside them. Two different techniques have been used to trap the droplets. One makes use of a 532nm optical tweezers set up, the other of an electrodynamics balance (EDB). The droplets decreasing size was measured using video analysis and elastic light scattering respectively. In the first case measurements were conducted at 293.15 K and ambient humidity. In the second case at 280.5 K and a controlled environment has been preserved by flowing nitrogen into the chamber. Binary phase droplets with a higher percentage of ethanol resulted in longer droplet lifetimes. Our work also highlights the advantages and disadvantages of each technique for such studies. In particular it is challenging to trap droplets with low ethanol content (such as pure gasoline) by the use of EDB. Conversely such droplets are trivial to trap using optical tweezers.

Precise control of particle positioning is desirable in many optical propulsion and sorting applications. Here, we develop an integrated platform for particle manipulation consisting of a combined optical nanofiber and optical tweezers system. Individual silica microspheres were introduced to the nanofiber at arbitrary points using the optical tweezers, thereby producing pronounced dips in the fiber transmission. We show that such consistent and reversible transmission modulations depend on both particle and fiber diameter, and may be used as a reference point for in-situ nanofiber or particle size measurement. Therefore we combine SEM size measurements with nanofiber transmission data to provide calibration for particle-based fiber assessment. We also demonstrate how the optical tweezers can be used to create a ‘particle jet’ to feed a supply of microspheres to the nanofiber surface, forming a particle conveyor belt. This integrated optical platform provides a method for selective evanescent field manipulation of micron-sized particles and facilitates studies of optical binding and light-particle interaction dynamics.

Double nanohole apertures in metal films have proven to be efficient plasmonic devices for trapping nanoparticles as small as single proteins.¹ To date, this technique has relied on weak transmission far beyond the wavelength cutoff, ignoring the prospect of plasmon field enhancement. In this work we present details on the design and fabrication of arrays of nanoring apertures on gold films. These devices feature efficient light localization in small gaps similar to double nanohole apertures, but additionally benefit from surface plasmon resonances in the near infrared spectrum. We perform polarization-resolved spectrometry on the arrays and discuss their potential for nanoparticle trapping.

We present a dual-modality microbubble trapping system that incorporates the fine spatial resolution of optical tweezers, with the long range, high force manipulation of acoustic tweezers, in a single microfluidic system. We demonstrate aggregation of polymer microbubbles in the node of an acoustic field, and subsequent selection and separation of a single microbubble using holographic optical tweezers. We further characterize the optical tweezers by measuring the transverse spring constant, and use the calibrated trap to determine the acoustic force on the bubble for varying parameters of optical trap diameter and power, and acoustic frequency and driving voltage. Further development of the system to include acoustic emission measurement is presented, with the goal of having a multi-purpose mechanical and cavitation detection set-up combined into a single system.

We present the stable trapping of luminescent 300-nm cerium-doped YAG particles in aqueous suspension using a dual fiber tip optical tweezers. The particles were elaborated using a specific glycothermal synthesis route together with an original protected annealing step. We obtained harmonic trap potentials in the direction transverse to the optical fiber axes. In the longitudinal direction, the potential shows some sub-structure revealed by two peaks in the distribution statistics with a distance of about half the wavelength of the trapping laser. We calculated intensity normalized trapping stiffness of 36 pN•μm^-1W^-1. These results are compared to previous work of microparticle trapping and discussed thanks to numerical simulations based on finite element method.

Based on an ongoing trend in miniaturization and due to the increased complexity in MEMS-technology new methods of assembly need to be developed. Recent developments show that particularly optical forces are suitable to meet the requirements. The unique advantages of optical tweezers (OT) are attractive due to their contactless and precise manipulation forces. Spherical as well as non-spherical shaped pre-forms can already be assembled arbitrarily by using appropriate beam profiles generated by a spatial light modulator (SLM), resulting in a so called holographic optical tweezer (HOT) setup. For the fabrication of shape-complementary pre-forms, a two-photon-polymerization (2PP) process is implemented. The purpose of the process combination of 2PP and HOT is the development of an optical microprocessing platform for assembling arbitrary building blocks. Here, the optimization of the 2PP and HOT processes is described in order to allow the fabrication and 3D assembling of interlocking components. Results include the analysis of the dependence of low and high qualities of 2PP microstructures and their manufacturing accuracy for further HOT assembling processes. Besides, the applied detachable interlocking connections of the 2PP building blocks are visualized by an application example. In the long-term a full optical assembly method without applying any mechanical forces can thus be realized.

We have built a complex apparatus for optical trapping, stretching, heating and concurrent whispering gallery mode (WGM) lasing excitation of liquid crystal (LC) emulsion micro-droplets doped with various fluorescent dyes. We have explored the changes of WGM lasing wavelength when the LC droplets were optically stretched or electrically heated beyond the transition to the isotropic phase. We have found that the range of lasing wavelengths was in some cases considerably higher than when we optically stretched ordinary fluorescent oil droplets in our previous experiments.

Retinal pigment epithelial (RPE) cells play an integral role in the renewal of photoreceptor disk membranes. As rod and cone cells shed their outer segments, they are phagocytosed and degraded by the RPE, and a failure in this process can result in retinal degeneration. We have studied the role of myosin VI in nonspecific phagocytosis in a human RPE primary cell line (ARPE-19), testing the hypothesis that this motor generates the forces required to traffic phagosomes in these cells. Experiments were conducted in the presence of forces through the use of in vivo optical trapping. Our results support a role for myosin VI in phagosome trafficking and demonstrate that applied forces modulate rates of phagosome trafficking.

Super-paramagnetic particles are used extensively in diagnostics and other research applications for the purification of cells, biomolecules and assays. Here, we demonstrate full 3D optical manipulation of such sub-micrometer sized particle using optical tweezers. We report three types of anomalous behavior of such a trapped particle. Firstly, the analysis of particle motion recorded by the quadrant photodiode reveals spikes in the total detected intensity that are formed by a set of very fast oscillations. Secondly, the dependence of trap stiffness on the laser power deviates from the expected linear increase for higher trapping powers. And further, such a particle cannot be optically trapped above certain trapping power threshold. We speculate that these effects are caused by particle internal structure and by its heating.

It has been shown that a spatial soliton can be created when a CW laser travels through a suspension of dielectric nanoparticles, provided its power is above a critical value [Opt. Lett. Vol. 7: 276 (1982)]. Recently, it was demonstrated that these soliton-like beams can be used as waveguides for controlling an additional low-power laser (probe beam) [Opt. Lett. Vol. 38: 5284 (2013)]. Here we present an experimental study of the interaction between two solitons propagating through a nanocolloid and we analyze their use to create a beam splitter for a probe beam.

We demonstrate a technique for generation of hollow optical beams (HOB’s), the cross-section intensity distribution of which has a predetermined shape. This technique is based on using multimodal Bessel beams. We can generate Bessellike non-diffracting beams by varying parameters of individual beams and adjusting their individual energy contribution to the generated light distribution. This technique allows designing transmission functions for elements that shape both non-rotating and rotating beams. Such laser beams can be used for controlled manipulation of non-spherical objects in liquids or air.

Optical manipulation of small particles has long been challenging mainly due to reduced gradient force. Rotation of particles by light is even more difficult since that requires the particle to be absorbing or to exhibit large polarizability and optical anisotropy. Otherwise, the optical field has to carry orbital angular momentum. Recently surface-plasmonenhanced optical near field has been used to effectively trap small particles. However, rotation and spinning of isotropic dielectric particles by light has not been demonstrated, not to mention a single device capable of multiple functions. Here, we report the first demonstration of selective trapping or rotation of isotropic dielectric micro-particles using one single plasmonic device, a plasmonic Archimedes spiral. Such functionality is of great interest and may find applications in various fields, such as protein folding analysis and local mixing in microfluidic channels.

We show that trapping and manipulation of microparticles can be achieved by Rayleigh convection currents using a low power lasers. Light absorbed by thin film of amorphous silicon (a:Si) creates the convection currents. In contrast to previous works, we show that multiple trapping can be achieved using solid microparticles without the creation of vapor bubbles. For low power (~1 mW), particles are trapped at the center of the beam, however at higher powers (~3 mW) particles are trapped on a ring around the beam due to two competing forces: Stokes and thermophoretic forces. Numerical simulations confirm that thermal gradients are responsible for the trapping mechanism.

The authors present a technique for modulation laser beams into beams with special properties, capable of trapping and rotating objects of micro-size. There are two approaches covered: the first one utilizes zero-order Bessel beams superposition for light tubes shaping, and the second one forms Bessel beams superpositions with diffractional optical elements with spatially partitioned topological charge or, alternatively, with vortex axicones. The authors also cover micro-mechanical object manufacturing, optimized for rotating in vortex Bessel beams superposition. Results of numerical simulations and experimental data are presented and then discussed.