The path that a mesoscopic polarisable particle takes as it flows through a lattice of intensity maxima and minima (optical lattice) depends crucially upon the degree to which it interacts with the lattice. Two particles of dissimilar size, refractive index or even shape will interact in a different manner with such a lattice. Combining this selective interaction with a guiding mechanism has allowed us to achieve lateral separation of particles by all these properties simply by flowing them through an angled optical lattice. We present such particle separation in a variety of three-dimensional optical lattices discussing the importance of parameters such as flow speed, lattice intensity, lattice constant, lattice angle, maxima interconnectivity and flow chamber design. We also present cell sorting with the separation of erythrocytes from lymphocytes and present our flow chamber fabrication methods.

We describe a general way how to calculate optical forces acting on Rayleigh particles or colloids placed into general interference field. In this paper we focus on a configuration with 3 beams laying in one plane and we present an analysis of the particles behaviour. We found that this arrangement can be used for sorting of particles having refractive index higher or lower comparing to the surrounding immersion medium and even for sorting of particles according to their size.

Many proposed applications of microfluidics involve the manipulation of complex fluid mixtures such as blood or bacterial suspensions. To sort and handle the constituent particles within these suspensions, we have developed a miniaturized automated cell sorter using optical traps. This microfluidic cell sorter offers the potential to perform chip-top microbiology more rapidly and with less associated hardware and preparation time than other techniques currently available. To realize the potential of this technology in practical clinical and consumer lab-on-a-chip devices however, microscale control of not only particulates but also the fluid phase must be achieved. To address this, we have developed a mechanical fluid control scheme that integrates well with our optical separations approach. We demonstrate here a combined technique, one that employs both mechanical actuation and optical trapping for the precise control of complex suspensions. This approach enables both cell and particle separations as well as the subsequent fluid control required for the completion of complex analyses.

Laser trapping has been used for the manipulation of a wide variety of microscopic inorganic, metallic, polymeric, and biological particles, including cells, bacteria, and viruses. A more recently developed technique, optical chromatography, uses a lightly focused laser beam introduced into a counter-propagating fluid containing the particles to be trapped. Particle trapping and separation occurs through a balance of fluid drag forces and optical forces. The optical pressure that the laser exerts on a particle depends in part on its size, shape, and refractive index (chemical composition). Particles with a larger refractive index experience greater optical pressure and hence move farther upstream. Particles move against the fluid flow until reaching an equilibrium position where the fluid and optical forces are balanced. This position in relation to the focal point is termed the retention distance. Several important samples of biological origin have been separated and studied, including bacterial cells, spores, and pollens. The utility and prospects for optical chromatography are greatly expanded and the stage is set for many other optical characterization applications.

We use fully dynamic multiple-beam optical manipulators to directly control the activity on miniaturized laboratories that can be operated under a microscope. Scaling down conventional laboratory procedures to micron scale devices facilitates localized synthesis/analysis of chemicals and speeds-up biological assays while maintaining accuracy in the results. The switching mechanism in micro-fabricated fluidic channels and the control of pumps, valves, mixers, filters, separators and the like can be controlled using dynamic light handles. Moreover, continuous liquid flows can be used to transport particles throught the micro-channels and integrate a range of microfluidic components into fully functional devices.

We demonstrate the optical manipulation of microscopic particles within a single optoelectronic device, whose footprint measures 2mm by 3mm, and which is realised entirely in planar technology. The device is fabricated in a GaAs/AlGaAs heterostructure, and consists of two facing banks of lasers that are separated by an etched channel. Particles within this channel experience the simple trapping force of two counter-propagating beams. The lasers operate at a wavelength of 980nm, and each gives up to 10mW of power in a single transverse optical mode. This power is sufficient to deflect, decelerate and hold a variety of micron-scale particles, including fluorescent polymer spheres, and cells in solution. The first results were obtained using planar etched facets, giving highly divergent beams. More elegant beam shapes can be produced by etching curved facets. The main attractions of this technology are its size and self-alignment properties: Many devices can fit into a fraction of the space occupied by a traditional tweezer set-up. Using photo-lithography, the alignment of the lasers is 'perfect', avoiding the difficulties experienced in traditional tweezers. The concept we demonstrate is a truly integrated optical tweezer that is mass-producible and does not require any complex instrumentation to operate.

The compactness of VCSELs (Vertical Cavity Surface Emitting Lasers) provides them the ability to meet the demands of current biochip technologies. In earlier research, optical trapping of live biological cells and microspheres based on VCSEL array has been realized in the form of parallel static traps on a translation stage. In microfluidic systems (lab-on-a-chip devices), the background flow introduces complexity and uncertainty in velocity and force analysis on target microparticles, making the capability of transporting biological objects without moving sample highly desirable. Moreover independently controllable traps offer more flexibility in microparticle manipulation. In this paper, a microscope-integrated VCSEL trapping system capable of independent control and batch processing of microparticles is devised and demonstrated. Optical design considerations for keeping stable trapping performance while multiplexing are addressed. Both a single optical trap and a trap array can be controlled independently by tilting mirror while their relative depth can be adjusted without power loses in optical system. In the micromanipulator, a single VCSEL trap serves as a collector and distributor, while a VCSEL array provides the carrier for synchronous processing and small range shift. Potential improvement based on two independently controlled VCSEL arrays is discussed and related applications are investigated.

To achieve all-optical-lab-on-a-chip systems, it requires optical manipulation tools for both microparticles and microfluids. Optical tweezers have attracted a great deal of interests in manipulating cells or particles. However, it is not effective in handling microfluid. Its high optical power requirement also limits its application in high throughput bio-analysis system. In this paper, we demonstrate two novel mechanisms, optoelectrowetting (OEW) for handling microdroplets, and optoelectronic tweezers (OET) for optical manipulation of microscopic particles with low optical power actuation. Instead of using direct optical force, both mechanisms rely on light induced electrical force for optical manipulation. Optoelectrowetting (OEW) enables control of microfluids in droplet form by optical beams. It is based on light induced electrowetting, which changes surface tension at solid-liquid interface at illuminated area. It is realized by integrating a layer of photoconductive material with electrowetting electrodes. By programming the illumination pattern, we have successfully demonstrated various functions for droplet, such as moving, splitting, and merging. A 100 pico-liter droplet was transported at a speed of 785um/sec by an optical beam with an optical power of 100mW. Optoelectronic tweezers (OET) manipulate cells or particles based on light induced dielectrophoresis (DEP). Trapping or repelling of microscopic particles is achieved with a light intensity of 2W/cm^2, which is 5 orders of magnitudes lower than that required by optical tweezers (10^5~ 10^7 W/cm^2). The liquid containing cells or particles is sandwiched between a photosensitive surface and a transparent ITO glass, with an ac bias between them.. When the laser beam is focused on the photosensitive layer, it creates a virtual electrode on the illuminated area, resulting a non-uniform electric field at the aqueous layer. Cells or particles in the liquid layer are polarized by this non-uniform electric field and driven by the DEP force. The force could be attractive or repulsive, depending on the dielectric properties of the particles and the bias frequency. Using OET, we have demonstrated concentration of polystyrene particles and live E.coli cells using an optical power less than 10uW.

Trapping and manipulation of microparticles using optical tweezers is usually performed within a sample cell formed by two parallel microscope cover slides. In this paper we discuss and demonstrate trapping and manipulation conditions when the cell has more complex configurations like microchannels or capillary tubes. The microchannels are fabricated on the surface of the cover slide by means of lithographic techniques. Experimental results of trapping and micromanipulation for silica microspheres and biological samples immersed in water show the usefulness of our study for microfluidics and biological applications.

We are developing optically based techniques for the manipulation of nano-containers (containers with sub-picoliter volumes) for handling chemicals in order to perform ultra-small volume chemistry. We are currently investigating three systems, liposomes, polymersomes and hydrosomes, for use as nano-containers. Liposomes and polymersomes are closed structures composed of a lipid and polymer membrane, respectively, that acts as a barrier to separate an aqueous interior environment from an aqueous exterior environment. We are typically working with liposomes or polymersomes that are approximately 10 μm in diameter. Hydrosomes are micron-sized, surfactant-stabilized water droplets that reside in a fluorocarbon environment. The optical techniques we are using include optical tweezers, for trapping and remotely moving the nano-containers, and an "optical scalpel" for localized disruption of lipid and polymer membranes in order to induce fusion of liposomes and polymersomes. In all three systems, we are able to bring together two similar nano-container using optical trapping and subsequently fuse them together, which allows their contents to mix. With the liposomes and hydrosomes we have been able to demonstrate their use for performing a controlled, elementary chemical reaction.

Short thin cylinders can be the basic part of mesoscopic actuators powered by laser tweezers. We demonstrate that micro size birefringent colloidal coins can be positioned translationally and orientationally in advance of the prediction of current theoretical modeling. Circularly polarized laser tweezers keep driving the rotation of the microdisk. Viscous flow between a hard wall and a microdisk excites the disk to an oscillation orbit. These disks can be fabricated via solidification of emulsion drops by tuning temperature or solvent concentration so that molecules in the drop assuming a smectic phase.

We report on the observation of a normal streak effect on hollow micron sized spheres when illuminated by a focused Gaussian beam in a conventional optical tweezers setup. The hollow microspheres suspended in water can be optically trapped at the center of the illuminating beam. When the microsphere is illuminated off center, an emerging beam approximately perpendicular respect to the incoming beam is generated. This effect due to total internal reflections has been observed in microspheres with different external diameters, ranging from 5-20 microns. The generated normal beam is used to either pull or push other particles or objects around the microsphere or to remove particles stuck to the sphere due to radiation pressure.

We are engaged in applying the Generalized Phase Contrast (GPC) method which enables lossless light transmission and direct conversion of spatially encoded phase patterns to high contrast intensity distributions. This technique can be used in a number of applications requiring parallel light-beam encoding and we will briefly outline the practical implementations that we have been undertaking as a direct spin out of GPC encoding. These include among others wavefront sensing and generation, advanced user-controlled optical micro manipulation, optical phase-only encryption/decryption and integrated micro-optical implementations.

The use of diffractive optical elements (DOEs) for multiple trapping of dielectric micro-spheres immersed in a fluid has been demonstrated recently. When the DOEs are implemented on a spatial light modulator (SLM), the trapped particles can be independently moved by changing the configuration of the DOE. In this paper we demonstrate phase DOEs implemented on an optical addressable SLM to move an array of trapped particles in a volume of about 20x20x6 μm. Experimental results show the usefulness of this technique for particle micromanipulation in biology.

The micromanipulation of objects into 2-dimensional and 3-dimensional geometries within holographic optical tweezers is carried out using a modified Gerchberg-Saxton algorithm. The modified algorithm calculates phase hologram sequences, used to reconfigure the geometries of optical traps in several planes simultaneously. The hologram sequences are calculated automatically from the initial, intermediate and final trap positions. Manipulation of multiple objects in this way is semi-automated, once the traps in their initial positions are loaded.

A multi-beam, variable footprint, laser beam steering and shaping system is described and used with a microscope to demonstrate multi-particle laser trapping. It is built around a computer-interfaced 512x512 pixel analog phase-only spatial light modulator (SLM) and a 1 W, 1064 nm wavelength laser. Hand sketches on paper made with a digital pen are used to prescribe the footprints, velocities and trajectories of multiple, independently-controlled diffracted spots. Continuous scanning is approximated by automatically designing a sequence of phase-patterns that are run through and diffracted by the SLM. Very complex scanning sequences of dozens of independently controlled spots can be quickly designed and run. The number of beams that we can trap with is necessarily limited due to the low throughput (~23 mW) of the IR light through the microscope optics. Among the trapping experiments done with the system a triangular shaped vortex ring tends to stop single particles at the apexes of the triangle. However, collision with a second particle pushes the first particle past the apex and sets it into motion, leaving the second particle stopped until collision with a third particle. The discrete motion conditioned on collisions is suggestive of a queuing process or a Markov chain.

A method is discussed for using neural networks to control optical tweezers. Neural-net outputs are combined with scaling and tiling to generate 480X480-pixel control patterns for a spatial light modulator (SLM). The SLM can be combined in various ways with a microscope to create movable tweezers traps with controllable profiles. The neural nets are intended to respond to scattered light from carbon and silicon carbide nanotube sensors. The nanotube sensors are to be held by the traps for manipulation and calibration. Scaling and tiling allow the 100X100-pixel maximum resolution of the neural-net software to be applied in stages to exploit the full 480X480-pixel resolution of the SLM. One of these stages is intended to create sensitive null detectors for detecting variations in the scattered light from the nanotube sensors.

We describe a double optical trap for micromanipulation of biomolecules. This set-up allows to bring a DNA molecule grafted between two beads in contact with microelectrodes, in order to perform electrical measurements at the single molecule level in a well-controlled geometry. The use of an opaque surface like silicon compatible with lithography imposes to work in a reflection configuration with the same objective for trapping and imaging the bead. The experimental constraints raised by this optical configuration are detailed. By combining drag force and optical trapping, we are able to trap separately the two beads and to perform force versus extension measurements in this reflection configuration. Finally, deposition of one stretched DNA molecule on Au electrodes fabricated by electron beam lithography on silicon wafers is shown.

In an optical stretcher, infrared laser light is used to exert surface stress on biological cells, causing an elongation of the trapped cell body along the laser beam axis. These optically induced deformations characterize individual cells and cell lines. When integrated within a microfluidic chamber with high throughput, this enables diagnosis of diseases, on a cellular level, that are associated with cytoskeletal processes. Additionally, it allows sorting of cells with high accuracy in a non-contact manner. To determine the surface stress on the cell, ray optics calculations as well as the system transfer operator (T-matrix) approach with an appropriate incident field are used. The latter approach allows a more accurate modeling of the cell in the optical stretcher and reveals a more detailed stress profile acting on the cell surface. Analyzing the deformation behavior of normal and malignantly transformed fibroblasts, significant differences in axial elongation even for sample sizes as low as 30 cells are already measurable on a time scale of 0.1s. Here, malignant transformation of cells is discussed as an example of how any process that affects the cell's optical or mechanical properties allows classification with the optical stretcher.

Laser microbeam and optical tweezers were used for micromanipulation of a heart tissue model consisting of embryonic chicken cardiomyocytes and bibroblasts. Using the laser microbeam a would was created, i.e. a sort of artificial heart infarction was generated. The first steps of wound repair were observed by live cell imaging. A complete filling of teh would primarily by migrating fibroblasts but not by cardiomyocytes was detected 18 hours after wounding. In another set of experiments erythrocyte mediated force application (EMFA) by optical tweezers was applied for optomechanical manipulatoin of cardiomyocytes and fibroblasts. Here we demonstrate induction of dramatic distrubances of calcium waves in a group of synchronously beating cardiomyocytes by an optomechanical input that results in cellular deformation. Surprisingly, it was found that putatively non-excitable fibroblasts respond to this mechanical stress with calcium oscillations. The results reported here indicate that the induction of artificial heart infarction can provide insights into healing processes after mycardial injury. EMFA is capable to examine effects of myocardial overload and to provide important information about processes triggered by mechanical stress on the level of single or very few cells. As a perspective, the preseneted techniques may be used to study the influence of drugs on wound healing and coordination of beating in the heart.

We have investigated the effect of ionic strength on electrically evoked membrane tether force using optical tweezers. Membrane tethers from cochlear outer hair cells (OHCs) and human embryonic kidney (HEK) cells exhibited mechanical response to applied voltage stimuli over a wide frequency range. The electrically evoked variations in the tether force were probed by an optically-trapped microsphere, the image of which was projected on a quadrant photodiode for dynamic measurement of its displacements. Compared to normal saline (140mM NaCl), low ionic-strength solution (10mM NaCl) blocked the electrically evoked tether force for both OHCs and HEK cells. As the Debye length for membrane bilayer was estimated to increase from approximately 0.75nm to 1.88nm, the internal repulsive pressure of the membrane tethers rose consequently, resulting in the enlargement in the equilibrium tether diameter and the decrease in the tether force.

Information on plasma membrane (PM) and cell wall mechanical properties is important for many biophysical applications, especially for those, which involve cells, undergoing significant mechanical stress (red blood cells, outer hair cells, fibrocytes, etc.). Optical tweezers is frequently used to study PM mechanics, particularly by pulling long PM tethers. One of the limitations on using optical tweezers to study cell wall mechanics is associated with transillumination technique of the trapped object position sensing, which prevents accurate mechanical testing in the proximity to the cell. In this work we use an optical tweezers in conjunction with a position-sensing system, which spectrally separates signals from the trapped fluorescent microsphere and imaging background. We have used this setup to study mechanics of the cell wall and PM separated from the underlying cytoskeleton on human embryonic kidney cells. We measured the force exerted by the cell on the trapped microsphere as a function of the cell wall displacement during the process of tether formation, and as a function of time during the process of tether growth and relaxation. Tethering force - cell wall displacement profiles have shown a behavior, implying that tether formation process starts with elastic deformation of the intact cell wall, followed by the plastic deformations and sliding of the PM over the underlying cytoskeleton, and ends with the local separation of a PM. Tethering force - cell wall displacement profiles have been used to estimate tether formation force, stiffness parameter of the cell wall and the works of tether formation, elastic and plastic deformations of the cell wall, related to the mechanical properties of a composite cell wall and cell wall - plasma membrane association strength. Temporal steady-state and relaxation tethering force profiles have been similar to the ones measured using transillumination position sensing, however average force values have been smaller in our case, due to the methodological differences. Our results demonstrate that measurements of cell wall and PM mechanical properties using optically-trapped fluorescent microspheres presents a versatile technology for studying of the cellular mechanics, especially effective in the proximity of the trapped microsphere to the cell.

Vesicles are phospholipid bilayers that form a surface enclosing a volume of water or solution. They are of importance as model systems to study cells, as well as having practical applications such as containers for performing nanochemistry and facilitating drug delivery. Their properties have been studied for decades. Using a holographic laser tweezer array (LTA), which converts a single laser beam into many laser tweezer points, we stretch the vesicles in controlled ways from several points at once, measuring each force applied. Here, we present data on shape deformations of simple, spherical vesicles and on membrane fracture.

We describe a novel method by which it is possible to apply and measure torque directly on particles grabbed in optical tweezers. It can be used to orient particles of micron size or even on single molecules, biopolymers by the use of test particles. The procedure is based on the observation that flat objects are oriented in an optical trap formed by linearly polarized light. The orienting torque originates from the anisotropic scattering of polarized light by the trapped particle. The phenomenon is characterized in detail, the physics is analysed. A tool is developed that exploits this effect to manuipulate biological macromolecules. Microscopic particles are produced by photopolymerisation that exploit this orientation effect and by which the torque is applied upon the biological object. In our system the applied torque can be turned on and off, it is controlled independently of the grabbing force of the tweezers during the manipulation process. The capabilities of the method are demonstrated. The method has great promise for application on DNA, DNA-protein complexes, actin filaments and other biopolymers.

Integrin receptors serve as both mechanical links and signal transduction mediators between the cell and its environment. Experimental evidence demonstrates that conformational changes and lateral clustering of the integrin proteins may affect their binding to ligands and regulate downstream cellular responses; however, experimental links between the structural and functional correlations of the ligand-receptor interactions are not yet elucidated. In the present report, we utilized optical tweezers to measure the dynamic binding between the snake venom rhodostomin, coated on a microparticle and functioned as a ligand, and the membrane receptor integrin alpha(IIb)beta(3) expressed on a Chinese Hamster Ovary (CHO) cell. A progressive increase of total binding affinity was found between the bead and CHO cell in the first 300 sec following optical tweezers-guided contact. Further analysis of the cumulative data revealed the presence of "unit binding force" presumably exerted by a single rhodostomin-integrin pair. Interestingly, two such units were found. Among the measurements of less total binding forces, presumably taken at the early stage of ligand-receptor interactions, a unit of 4.15 pN per molecule pair was derived. This unit force dropped to 2.54 pN per molecule pair toward the later stage of interactions when the total binding forces were relatively large. This stepped change of single molecule pair binding affinity was not found when mutant rhodostomin proteins were used as ligands (a single unit of 1.81 pN per pair was found). These results were interpreted along with the current knowledge about the conformational changes of integrins during the "molecule activation" process.

We have observed a latency time of the order 10 s in the spermidine-induced condensation of YOYO-1-labelled DNA in flow. Higher flow speeds, longer DNA and higher salt all led to an increase in the latency time. We propose that two effects may be relevant to these observations (1) the competition between spermidine and YOYO-1 for DNA binding sites, and (2) the bending of DNA upon spermidine binding, so that flow-induced straightening of the DNA weakens spermidine's affinity for DNA. Moreover, literature data on the mechanical properties of condensed DNA chains suggest that the hydrodynamic drag forces in our experiments should indeed significantly perturb condensation kinetics.

We are investigating the formation of a tissue capsule around a foreign body. This tissue capsule can be used as an autologous graft for the replacement of diseased blood vessels or for bypass surgery. The graft is grown in the peritoneal cavity of the recipient and the formation starts with the adhesion of cells to the foreign body. We identify the cell type and measure the adhesion of these cells to foreign materials using optical tweezers. Cell adhesion to macroscopic samples and microspheres is investigated. No difference in the adhesion force was measurable for polyethylene, silicon and Tygon on a scale accessible to optical tweezers. The density of adherent cells was found to vary strongly, being highest on polyethylene. The mean rupture forces for cell-microsphere adhesion ranged from 24 to 39 pN and changed upon preadsorption of bovine serum albumin. For plain microspheres, the highest mean rupture force was found for PMMA, which also showed the highest adhesion probability for the cell-microsphere interaction.

The ability to controllably rotate, align, or freely spin microparticles in optical tweezers greatly enhances the manipulation possible. A variety of different techniques for achieving alignment or rotation have been suggested and demonstrated. Although these methods are diverse, employing specially shaped particles, birefringent particles, multiple trapping beams, complex beam profiles, vortex modes, plane polarised beams, circularly polarised beams, or other methods, the fundamental principle - that optical torque results from the exchange of electromagnetic angular momentum between the trapping beam and the particle - remains the same. The symmetry of both the particle and the beam play a central role in the transfer of angular momentum. We discuss this in detail, with particular attention paid to the special case of optical torque exerted by an incident beam with zero angular momentum.

Several techniques have been proposed and used for the rotation or alignment of microparticles in optical tweezers. In every case the optical torque results from the exchange of angular momentum between the beam and the particle, and, in principle, can be measured by purely optical means. Measurement of this torque could be useful for quantitative measurements in biological systems and is required to measure properties such as viscosity of liquids in microlitre (or less) volumes. Although elongated particles will align with the plane of polarisation, the torque efficiency is low, typically about 0.05hbar per photon. The use of a beam with an elongated focal spot can increase this torque by a factor of 10-20 times, due to the transfer of orbital angular momentum. We report measurements of the orbital component using an analysing (Laguerre-Gauss) hologram. As a proof of principle experiment, an elliptical beam scattered off a glass rod was simulated on a macroscopic scale. The torque was found to be as much as 0.8hbar per photon. Microscopic elongated objects have been aligned and rotated in optical tweezers and we plan to make measurements of the torques involved.

The Laguerre-Gaussian beams or higher-order Bessel beams, which possess a full-doughnut shaped intensity pattern at the center, repel the low index particles away from the dark central spot due to radiation pressure. Optical beams with fractional helical phase are possible solutions to this problem, as they possess a clear opening slit. By using a spatial light modulator, we demonstrate the control of an opening slit by embedding different fractional helical phase into Bessel beams and Gaussian beams. This dynamic opening and closing of the slit will allow a possible selective process of the micro-particles. This is also a new way to control the guiding of particles in the transverse plane instead of the propagation direction.

Metallic and magnetic nano-particles are generally considered difficult to trap due to strong scattering and absorption forces. In this paper, we propose new methods of trapping metallic and magnetic nano-particles using highly focused cylindrically polarized beams. It is shown that an optical tweezers using cylindrical polarization can stably trap metallic/magnetic particles in 3-dimension, owing to the unique focusing properties of cylindrically polarized beams. Metallic particles can be stably trapped by highly focused radial polarization. The extremely strong axial electrical field from a highly focused radially polarized beam provides a large gradient force. Meanwhile, this strong axial field component does not contribute to the Poynting vector along the optical axis. Consequently, it does not create axial scattering/absorption forces. Owing to such a spatial separation of the peaks of gradient force and scattering/absorption forces, a stable 3-D optical trap for metallic nano-particles can be formed. Similarly, an optical tweezers using azimuthal polarization may provide stable 3-D trapping of magnetic nano-particles due to an extremely strong axial magnetic field.

Gradient radiation forces exerted by strongly focused cylindrical vector beams of radial and azimuthal polarizations on dielectric spheres of different radii and refractive indices were calculated. The effect of longitudinal and transversal components of the focused electrical field on trapping properties was studied. Experiments on optical trapping were performed using low-mode optical fiber excited with Laguerre-Gaussian beam as a source of the trapping beams.

We present a flexible setup for steering of laser tweezers using a high resolution spatial light modulator (SLM). Moving of e.g. trapped cells in the focal plane of the microscope objective is possible without the need for time consuming re-calculation of holograms. Numerous light spots or other modes like the so called "doughnut modes", which carry angular momentum, can be created and controlled independently by "mouse-dragging" the hologram window at the SLM display. In addition, undesired diffraction orders are suppressed using adequately calculated fresnel holograms.

We present experimental results on two colloidal systems showing features of stochastic resonance. The first one is a generic and the most simple case of a single overdamped colloidal particle in a double-well potential created by laser tweezers. For appropriate modulation of the potential stochastic resonance is unambiguously demonstrated. The second system is composed of many coupled particles controlled by laser tweezers constituting a ratchet cellular automaton. The ratchet was used as a transmission line for defects and was operated in a non-deterministic mode. Our results demonstrate that the transmission is optimal at a well defined noise level which is one of the characteristics of stochastic resonance.

Very high frequency oscillations of intense light fields interact with micron-size dielectric objects to exert dc optical forces that allow polarizable particles to levitate, to be trapped and to be bound. Such optical forces are also suitable to arrange cold atoms in optical lattices. Various assemblages of optical traps, including periodic arrays, can be constructed either with independent lasers, or with a single laser beam split into different parts later recombined by interference, as well as through the use of diffractive elements. These optical-well arrays serve as templates for writing and erasing dynamic two-dimensional and three-dimensional "optical crystals", composed of mono-dispersed polystyrene spheres in water. Subsequently, the crystals become diffractive structures themselves. The association of micro-fluidics and optical trapping allows for the formation of optical traps into micro-channels. This leads to perform microchemistry experiments, such as fluorescence detection, on individual bodies attached to trapped particles. Self-trapping due to the optical binding force relates to the interaction between different dielectric objects located in an electromagnetic field; each one reacts not only to the field of the incident beam, but also to the induced fields radiated coherently by all other particles. Optical binding strongly influences the equilibrium state and the behavior of optical crystals. It must have the potential for creating collective effects.

We demonstrate that counter-propagating light fields have the ability to create self-organized one-dimensional optically bound arrays of microscopic particles, where the light fields adapt to the particle locations and vice versa. We are able to create chains of up to 9 particles with only modest laser power. We outline the experimental observation of this phenomenon examining the effect of laser wavelength (780nm and 1064nm) and particle size (1, 2.3 and 3 micron diameter sphere sizes) on the interparticle separation. We develop a theoretical model to describe this situation making use of the beam propagation method to calculate the fields. Using the fields we are able to calculate the gradient and scattering forces experienced by the particles. Equilibrium positions in these forces indicate the predicted positions of the spheres. We find good agreement between the theory and experimental data for two and three particles, if the scattering force is assumed to dominate the axial trapping of the particles. We discuss the limitation of the model when dealing with spheres size of the order of the wavelength of light involved and also the experimental uncertainties relating to the measurement of the laser beam waist separations. The extension of these ideas to two and three dimensional optically bound states is also discussed.

We demonstrate an optical micromanipulation system that emables simultaneous trapping of a plurality of particles and interactive control of their trapping positions in three dimensions (3D). This functionality is achieved through the application of phase-only spatial light modulators (SLM) in a highly efficient generation of advanced counterpropagating-beam or dual-beam traps in an extended array. The array of counterpropagating-beam traps is composed of the s and p -polarized components of a dynamically reconfigurable intensity pattern created via the generalized phase contrast method. A power-preserving scheme using a spatial polarization modulator (SPM) allows us to vary the relative strengths of the two orthogonal polarizations thereby controlling the axial trap positions of the particles. Experimental resulst show the interactive manipulation of poystyrene microspheres in 3D achieving a dynamic range in axial position conrol as large as 20μm.

Bessel beams have a number of properties that make them useful for optical manipulation. These include a central core which does not diffract over a characteristic distance, the reconstruction of the cross sectional intensity around obstacles and the ability of the beams to possess orbital angular momentum. Here we examine the utility of the Bessel beam to quantitatively examine the transfer of spin and orbital angular momentum to particle trapped away from the beam axis. We show the simultaneous transfer of orbital and spin angular momentum to an off-axis particle. We experimentally study how both the spin and orbital angular momentum of light behaves upon passage through microscopic optically trapped particles. Particles trapped with Gaussian and, separately, Bessel light beams in two spatially distinct sample chambers are studied with trapped objects in the first chamber acting as distorting obstacles. We examine the differences between Bessel beams and Gaussian beams in such experiments. We also show how the Bessel beam can be used to simultaneously trap particles that can be as much as 1cm away from each other. We discuss applications of these results and suggest other topics where the Bessel beam may be of use.

Three-body and four-body interactions have been directly measured in a colloidal system comprised of three (or four) charged colloidal particles. Two of the particles have been confined by means of a scanned laser tweezers to a line-shaped optical trap where they diffused due to thermal fluctuations. By means of an additional focused optical trap a third particle has been approached and attractive three-body interactions have been observed. These observations are in qualitative agreement with additionally performed nonlinear Poissson-Boltzmann calculations. Two configurations of four particles have been studied experimentally as well and in both cases a repulsive four-body interaction term has been observed.

We show with computer simulations that a rich variety of static and dynamical colloidal phases can be realized for colloids interacting with two-dimensional periodic substrates. For the static case a new type of colloidal state that we term colloidal molecular crystals occurs when there is an integer number of colloids per substrate minima. Here there is a novel orientational ordering in addition to the positional ordering of the colloids. The colloidal molecular crystals exhibit a multi-step melting in which the orientational ordering is lost first, followed by the positional ordering. This multi-step melting phenomenon agrees well with recent experiments. Additionally we show that at fillings where the number of colloids is an incommensurate fraction of the number of substrate minima, as a function of temperature there is a transition to a state in which local incommensurations become thermally activated. With an applied drive we find that a remarkable number of distinct dynamical phases can be realized, including ordered and disordered flows. We also illustrate flow phases in which the colloidal motion locks to a symmetry direction of the underlying lattice.

The prospect of large astronomical mirrors in space, made of a thin trapped membrane or a mono-layer of sub-micron particles, raises appreciable theoretical and technological problems at the frontiers of physics, chemistry and materials science. We explore various approaches towards testing different materials in the laboratory at temperatures matching those of a sun-shielded structure in interplanetary space (80- 100 K). Trap loading and damping are among the issues considered, as well as the pumping of particles towards a central fringe, using polychromatic light.

We use holographic optical tweezers to trap multiple micron-sized objects and manipulate them in 3-dimensions. Trapping multiple objects allow us to create 3-dimensional structures, examples of which include; simple cubes which can be rotated or scaled, complex crystal structures like the diamond lattice or interactive 3-dimensional control of trapped particles anywhere in the sample volume.

We developed a new type of optical manipulation technique by using a VCSEL array, which we call VCSEL array trapping. Flexible manipulation of small objects is achieved by controlling the spatial and the temporal intensity distribution generated by the VCSEL array sources. We have previously demonstrated parallel translation and non-mechanical translation of microparticles by controlling the emission distribution of the VCSEL array as the light source. Furthermore, we found that a stacked structure when using 2x2 VCSEL beams can be fabricated. In this report, we clarify the stacking process and the conditions for stacking the maximum number microparticles in order to apply this stacking method to three-dimensional structure fabrication. As an experimental result, we confirmed that the stacked structure was lifted up from the sample stage and fabricated without the space between the microparticles. The stacked structure depended on the microparticle size, the beam waist position, and the spot pitch. Furthermore, we investigate the difference of stacking processes when using 2x2 beams and a single beam. As a result, we found that the structure can be fabricated with lower optical power than when using a single beam. This fact indicates that 2x2-beam illumination is particularly effective for stacking micro objects without damage caused by heat.

Recently, the optical trapping technique has been employed to construct three-dimensional microstructures. Such three-dimensional microstructures are created from the fact that optical beams such as, a single focused Gaussian beam, multiple beams from Laguerre-Gaussian (LG) interference patterns, and Bessel beams, are able to stack microparticles on top of the other. Once these microparticles are stacked, the optical forces from the optical beam were able to hold them in place. In this paper, we demonstrate that by lowering the focused point of the LG beam below the cover slip (sample). The LG beam possesses the ability to stack multiple microparticles around its annular intensity rings and thus form a three-dimensional cubic structure. Hence we proposed a new technique of constructing microstructures, which is by creating optical fields with designed optical vortex shape. These microparticles will then be stacked according to the shape of the optical beam. This is an alternative method to obtain a desired three-dimensional crystalline structure, where shaping the optical vortices beam is used instead of using multiple beams.

Sample tracking with a high spatial sensitivity is highly desired in force measurement with optical tweezers. However, the trick that sample tracking via forward scattering pattern detection would provide a higher sensitivity than that via regular image detection has never been investigated. In this paper, we systematically study the influences of the position and the numerical aperture of the condenser on sample tracking via forward scattering pattern detection. In our experiment, a 60X condenser is used to form the forward scattering pattern of a sample bead upon a CCD camera. As the bead is transversely shifted at a step size of 30nm by a PZT XYZ stage, we measure the magnitude of the corresponding shift of the forward scattering pattern when the 60X condenser of different angular apertures is placed at various locations along the optical axis. Our result shows that the most sensitive forward scattering pattern occurs when the condenser collimates the forward scattering light from the sample bead. We also find that the larger the numerical aperture is, the higher the sensitivity of forward scattering pattern detection will be.

Using optical tweezers combined with video based methods we probe non-specific interactions between polystyrene beads and surfaces at different electrolyte concentrations. We present force-distance relationships and using statiscally based maximum likelihood methods, which significantly improves spatial resolutions, the interaction potentials are also found. At low electrolyte concentrations the interaction potentials are in agreement with DLVO theory, but at high electrolyte concentrations the interaction potentials deviate both from DVLE and Lifshitz theories. Also, we probe the specific interaction between biotin and streptavidin. However, these measurements are complicated because the interaction is of very short range and a streptavidin coated polystyrene bead adheres to a biotin coated surface as soon as it encounters the surface during its Brownian motion in the trap.

Typical optical tweezers setups use high numerical aperture oil-immersion objectives to trap particles suspended in an aqueous medium. When trapping deep inside a sample or out of the imaging plane the quality of the trap in such a system deteriorates due to optical aberrations caused by the refractive index mismatch at the glass-water interface. We investigate this effect experimentally by monitoring the two-photon fluorescence of trapped dye-stained polystyrene spheres. We consider the effect of the numerical aperture on the trap quality and also partly corrected the aberrations by optimising the fluorescence signal using an adaptive deformable membrane mirror.

In a typical position detection system for optical tweezers, laser light impinges on a quadrant photodiode, and the signal from the four quadrants of the diode is used to determine the position of a trapped object. A widely used position detection system consists of a Si-PIN photodiode and an infrared laser. In previous work we have demonstrated with two distinct experimental methods how such a system may act as an unintended low-pass filter and we modeled its physical origin mathematically. Here we demonstrate that the general solution to this model can account precisely for the "parasitic" filter's effects up to as large frequencies as we can measure, approximately 100 kHz. Thus we increase the useful bandwidth of tweezers experiments by nearly two decades. This opens for investigations of phenomena in biophysics, soft matter, and polymer science at much higher frequencies than before.

Understanding and controlling neuronal growth are basic objectives in neuroscience, biology, biophysics, and biomedicine, and are vital for the formation of neural circuits in vitro, as well as for nerve regeneration in vivo. All molecular stimuli for neuronal growth eventually address the polymeric cytoskeleton, which advances a neurite's leading edge also known as the growth cone. We have shown that optical forces of a highly focused infrared laser beam influence the motility of a growth cone by biasing the polymerization-driven intracellular machinery. In actively extending growth cones, a laser spot placed at specific areas of the neurite's leading edge affects the growth speed, the direction taken by a growth cone, and the splitting of a growth cone. This novel optical tool manipulates a natural biological process, the cytoskeleton driven morphological changes in growth cones, with potential applications in the formation of neuronal networks and in understanding growth cone motility. The current apparatus combines optical tweezers, phase contrast and fluorescence imaging, and real-time shape detection. Automated and dynamically readjusted irradiation of the growth cone is used to examine and to influence structural and morphological changes of neuronal growth.

The various cell mechanisms, including cell-cell interactions, in native tissue could be better understood by engineering a cell coculture with a micro environment that closely mimics the natural cell arrangement. To this end, we developed a cell micropatterning system that uses a weakly focused laser beam to trap individual cells at the center of the beam and propel them forward onto an appropriate substrate. The optimal methods of introducing different cell types to be patterned into the patterning system and preventing cells from randomly falling onto the pattern were issues to be addressed with this system. Here, we report the development of a multi-chamber, multi-beam laser cell micropatterning system, in which the delivery of specific cells into the beam can be controlled using secondary laser beams. This permits consecutive creation of a pattern involving multiple cell types at specific relative positions. As examples, various patterns of fibroblasts have been created on collagen coated coverslips. In addition, two asynchronously beating clusters of cardiomyocytes were connected with fibroblasts of cardiac origin, yielding a deeper insight into the electrophysiological role of fibroblasts in conduction of the action potentials among cardiomyocytes.

The actin cortex is an adaptive chemo-mechanical polymer network located beneath the cell membrane. A thin, quasi two-dimensional (2D) network, the actin cortex plays a leading role in controlling cellular viscoelasticity, shape, and motility. Regulated by internal and external stimuli, the actin cortex varies its properties with controlled polymerization and depolymerization of actin. For constructing and probing biomimetic actin networks we combined three different techniques to achieve complete spatial, visual and chemical control of the microenvironment: 1) dynamic holographic optical tweezers (HOTs) which produce and independently steer one to hundreds of optical traps, 2) fluorescence microscopy for imaging of actin and 3) a specially-designed microfluidic system, which precisely controls the chemical environment. Using this system, we take two approaches to construct biomimetic 2D actin networks. First HOTs micropattern surfaces with microspheres onto which actin can the be grown. Secondly, HOTs in combination with a multi channel microfluidic system are used to coat optically-trapped microsphere arrays with actin.

Holographic optical tweezers (HOTs) techniques are further developed to study hyaluronan-mediated adhesion of chondrocyte cells. We present a calibration scheme and address fundamental issues concerning the use of HOTs for quantitative force measurements. Influence of SLM pixelation on trap stiffness is observed and can be utilized to design calibrated HOTs more effectively. It is also shown that the HOTs trapping stiffness can vary significantly over short distances. Then we use HOTs cell adhesion assays investigate the viscoelastic and adhesive nature of chondrocytes' pericellular matrix (PCM) at two different time steps (30 minute and 24 hour incubation periods). Surprisingly, no physical influence of the large, presumably gel-like PCM is observed. However, a difference is discerned in the adhesiveness of the two sets of cells. The early-stage cells have reversible adhesion with negatively-charged and fibronectin-coated microspheres even after they are held at the cell surface for 10 seconds. In contrast, late stage cells stick irreversibly to all types of beads: positive, negative, fibronectin and hyaluronan-coated. Additionally, only the late stage cells produce membrane tethers. These observations suggest that the late-stage chondrocytes have less surface-associated hyaluronan and have interesting implications for the role of hyaluronan in the early stages of cell adhesion.

Lambda DNA is used to tether polystyrene microspheres to an optical substrate, thus eliminating the need for high numerical aperture trapping lenses in single beam optical tweezers to provide axial trapping. The tethered particle may be used as a sensor probe to measure local viscosity and forces on the trapped particle such as those induced by fluid flow.

Magnetic tweezers have evolved into an indispensable tool in soft condensed matter and biological physics. They are used to study local forces in biological tissue, to stretch and manipulate DNA, to transport ferrofluids, and to probe protein networks in the cell environment. Magnetic tweezers are stronger and cover a wider range of forces (femto- to nano-Newtons) than optical tweezers (pico-Newtons). However, optical tweezers have proven more versatile than conventional magnetic-tweezers in performing complex local manipulations on the micron scale. Here we use nanomagnets formed by movable domain walls in magnetic films to combine the advantages of conventional, magnetic and optical tweezers. Recently nanomagnetic elements were proposed for micro-manipulation of vortices in superconductors. It was shown that a movable magnetic domain wall can be used to generate, trap, and move vortices due to the wall’s highly inhomogeneous magnetic stray field. Our research builds on recent accomplishments including the application of nanomagnets for the controlled manipulation of paramagnetic colloids, and the measurement of the forces that arrange the colloids in different low-dimensional states of aggregate.

Optical tweezers have become a widely used tool for manipulate microscopic objects. Combined with fast and high sensitive position detection techniques, they are suitable for microrheological measurements of viscous and viscoelastic media. Such measurements require the knowledge of absolute displacements of the trapped particles. Unfortunately position detection device need to be calibrated and several methods have been used and reported in literature. We report an accurate study of the behavior of the conversion factor in the case of the forward light scattering technique, based on an oil-immersion objective lens, which is commonly used in optical tweezers systems. We show how this fundamental parameter is strongly dependent on the distance of the trapped object from the coverslip surface.

We report on an observation of attractive gradient force on dielectric particles suspended in a medium with a higher refractive index. This unexpected phenomenon was observed with micron sized silica spheres (n=1.37) suspended in optically anisotropic nematic liquid crystal (n_o=1.5 and n_e=1.7). The newly discovered interaction has a range an order of magnituded bigger than the laser beam waist diameter. Above transition temperature, where nematic order of a liquid crystal is lost, the gradient force becomes repulsive and it's range is reduced to expected values. We attribute an anomalous gradient force in nematic phase to two phenomena: particle dressing with a liquid crystal molecules resulting in a colloid with a higher effective index of refraction than surroundings and laser field induced distortino of nematically ordered liquid crystal molecules.

Metallic nanoparticles are known to experience enhanced optical trap strengths compared to dielectric particles due to the increased optical volume, or polarizability. In our experience, larger metallic particles (~micron) are not easily trapped because momentum effects due to reflection become significant. Hybrid particles comprised of both metal and dielectric materials can circumvent this limitation while still utilizing a larger polarizability. Heterogeneous nanosystems were fabricated by embedding/coating silica nanoparticles with gold or silver in varying amounts and distributions. These compound particles were manipulated via optical tweezers, and their trapping characteristics quantitatively and qualitatively compared to homogeneous particles of comparable size. The parameters explored include the dependence of the trapping force on the percentage of loading of gold, the size of the embedded colloids, and the distribution of metal within the surrounding matrix or on its surface.

Computational modelling of optical tweezers offers opportunities for the study of a wide range of parameters such as particle shape and composition and beam profile on the performance of the optical trap, both of which are of particular importance when applying this technique to arbitrarily shaped biological entities. In addition, models offer insight into processes that can be difficult to experimentally measure with sufficient accuracy. This can be invaluable for the proper understanding of novel effects within optical tweezers. In general, we can separate methods for computational modelling of optical tweezers into two groups: approximate methods such as geometric optics or Rayleigh scattering, and exact methods, in which the Maxwell equations are solved. We discuss the regimes of applicability of approximate methods, and consider the relative merits of various exact methods. The T-matrix method, in particular, is an attractive technique due to its efficiency for repeated calculations, and the simplicity of determining the optical force and torque. Some example numerical results are presented.

Optical guiding of microscopic particles in femtosecond and continuous wave Bessel light beams is studied and compared. We confirm that optical guiding is an average power effect and observe no difference in the guiding velocities of non-fluorescing polymer spheres. Furthermore, we observe second harmonic generation of guided KTP crystallites in femtosecond Bessel light beams. This observation opens up the prospect of using multi-photon effects in optical manipulation for applications such as optical identification of guided cells for sorting purposes.

We investigate the bound and evanescent fields of the optical whispering gallery modes which are supported by a toroid microcavity and which may be used for a wide range of applications. Results of simulations using finite-difference time domain solutions of Maxwell's equations are compared with semi-analytical solutions based on coupled mode theory. Key parameters such as resonance frequencies, transmittance characteristics, coupling efficiencies, and bending/scattering losses are analyzed as a function of experimental variables such as size, distance, and fabrication roughness. Finally, the feasibility of single-atom detection is discussed.

Optical tweezers are widely used to manipulate individual biomolecules. Conventionally, they hold a molecule at a constant extension while the restoring force of the molecule is measured. Many applications, however, require that the molecule be held under constant force while its extension is measured. We developed two all-optical trapping techniques that provide such constant-force conditions together with a fast extension measurement scheme. Our techniques are based on a scanning-line optical trap, which provides a one-dimensional flat optical potential well. A position-independent lateral force is generated either by a synchronous modulation of the laser intensity during the scan or the use of an asymmetrically shaped beam profile in the back focal plane of the microscope objective. The position of the trapped particle as a measure of the extension of an attached molecule is measured by monitoring the diffraction pattern of the forward-scattered laser light. The performance of these techniques, along with consideration for the design and calibration of the instrument is discussed.

The conformational change of integrin α_IIbβ₃ plays an important role in clot formation. However, the correlation between the structure and the function of integrin α_IIbβ₃ in interacting with its ligand is still not clear. In this report, we focus on the dynamic variation of the binding between integrin α_IIbβ₃ and its ligand, rhodostomin by using a photonic force microscopy (PFM). The PFM is used to trap a rhodostomin-coated bead and, then, shift it to bind a surrounding CHO α_IIbβ₃ cell. Meanwhile, it tracks, with a resolution of 1MHz, the Brownian fluctuations of the trapped bead. Theoretically, the smaller the amplitude of the Brownian fluctuations, the stronger the stiffness of the binding force between the rhodostomin and the CHO α_IIbβ₃ cell. Experimentally, a significant decrease of the Brownian fluctuations was observed during the interval between the 360th seconds and the 400th seconds after the trapped rhodostomin-coated bead contacted an integrin-expressed CHO α_IIbβ₃ cell. This observation reveals that it takes the rhodostomin 360 seconds to seek the correct position to bind to the integrin α_IIbβ₃. After 400 seconds, the rhodostomin has bound rigidly with the integrin α_IIbβ₃. We presume that the integrin α_IIbβ₃ has reached its final stage of conformational change.

The average environmental response of red blood cells (RBCs) is routinely measured in ensemble studies, but in such investigations valuable information on the single cell level is obscured. In order to elucidate this hidden information is is important to enable the selection of single cells with certain properties while subsequent dynamics triggered by environmental stimulation are recorded in real time. It is also desirable to manipulate and control the cells under phsyiological conditions. As shown here, this can be achieved by combining optical tweezers with a confocal Raman set-up equipped with a microfluidic system. A micro-Raman set-up is combined with an optical trap with separate optical paths, lasers and objectives, which enables the acquisition of resonance Raman profils of single RBCs. The microfluidic system, giving full control over the media surrounding the cell, consists of a pattern of channels and reservoirs produced by electron beam lithography and moulded in PDMS. Fresh Hepes buffer or buffer containing sodium dithionite are transported through the channels using electro-osmotic flow, while the direct Raman response of the single optically trapped RBC is registered in another reservoir in the middle of the channel. Thus, it is possible to monitor the oxygenation cycle in a single cell and to study photo-induced chemistry. This experimental set-up has high potential for monitoring the drug response or conformational changes caused by other environmental stimuli for many types of single functional cells since "in vivo" conditions can be created.

We made use of near-field photonic forces in order to manipulate and trap microparticles in an extended area above a solid surface. Structures in evanescent field were created either by imaging a Ronchi ruler (fringe structure) or by focusing five beams (spot structure) at the top of a prism. The surface field couples to microparticles in close proximity, where the near-field wave can be converted to a propagating wave, via photon tunnelling across the gap from prism to microparticles. Due to transverse optical gradients and radiation pressure, microparticles immersed in water were laterally trapped and longitudinally guided along the direction of the evanescent waves. By splitting the laser beam into two equal counterpropagating beams, another evanescent wave was created exactly with the same structure in the opposite direction to the first one. We use this geometry we demonstrate stably trap of thousand of microparticles over an area of about a millimetre squared. Red blood and yeast cells were also individually trapped in an array of potential wells. We believe this is the first demonstration of guiding and, separately, trapped in unison multiple microparticles on a surface.

We report the sub-wavelength patterning of the optical near-field by total internal reflection illumination of a regular array of resonant gold nano-particles. Under appropriate conditions, the in-plane coupling between localized surface plasmon fields gives rise to sub-wavelength light spots between the structures. Measurements performed with an Apertureless Scanning Near-Field Optical Microscope (ASNOM) show good agreement with theoretical predictions based on the Green dyadic method. This concept might offer a convenient way to elaborate extended optical trap landscapes for manipulation of sub-micrometer systems.

The design of a Micro Surface Plasmon Resonance Sensor is proposed. The possibility of implementing the modified Kretschmann-Raether scheme on a chip is investigated. A moving waveguide is used to change the angle of incidence of the beam of light. The main advantages of this sensor are potentially low cost, simplicity of design and very high parallelization. It is possible to have large number of such sensors on one chip, each of them with distinct activated sensing surfaces. The modeling of the sensor, design optimization procedures, and fabrication are also discussed. Computer simulations are used to validate the design.

We present a physical model for the conversion of the evanescent photons into propagating photons detectable by an imaging system. The conversion mechanism consists of two physical processes, near-field Mie scattering enhanced by morphology dependant resonance and vectorial diffraction. For dielectric probe particles, these two processes lead to the formation of an interference-like pattern in the far-field of a collecting objective. The detailed knowledge of the far-field structure of converted evanescent photons is extremely important for designing novel detection systems. The model is also applicable for determination of the near-field force exerted on small particles situated in an evanescent field. This model should find broad applications in near-field imaging, optical nanometry and near-field metrology.

In this paper, we have demonstrated the measurement of the morphology-dependent resonance (MDR) within a laser trapped microsphere excited under two-photon excitation. Both trapping and two-photon excitation are achieved by a single femtosecond pulsed laser beam simultaneously. MDR induced by two-photon excitation has also shown to overcome difficulties in separation of excitation and resonant wavelengths and the inability to confine excitation illumination precisely3. A spectrally resolved image demonstrates that this technique provides an alternative imaging mechanism for near-field imaging and mapping surface tomography.

Laser manipulation technique was applied to the patterning of single nano/microparticles in solution at room temperature. Individual gold nanoparticles were optically manipulated to the surface of a glass substrate in ethylene glycol. An ultraviolet laser beam was focused to the nanoparticle, which led to the transient temperature elevation of the particle, resulting in its photothermal fixation. A set of gold nanoparticles was aligned in the anisotropic optical potential well of a tightly focused laser beam with linear polarization and was adhered onto the substrate through the same photothermal method keeping their alignment. Combination of a microstereolithography with the laser trapping method enabled us to fabricate three-dimensional microstructures of resin and fix microparticles to them.

In this paper, design of diffractive optical element (DOE) for optical bubble creating and controlling with radially polarized incident beam focused by a high numerical aperture (NA) aplanatic lens is proposed and its application in optical trapping is discussed. We use a DOE to modify the phase of the incident radial polarization beam to form different kinds of optical bubbles. Optimization algorithms are used to design the DOE to adjust the bubble size and depth to meet the requirements. The results show that the size of the bubble is inversely proportional to its depth. Owing to the overlapping of the field strengths around the focus, the bubble tends to merge into flattop distribution as it is getting smaller and smaller. With a fixed DOE design, bubbles with smaller size and larger depth can be generally obtained with higher NA, owing to a more confined field strength distribution from the strong longitudinal field component.

The influence of size of the trapped object on its position near the dielectric interface is studied experimentally. The trapping beam is reflected on a surface and creates weak standing wave component in resulting field distribution. This component causes unwanted jumps of the trapped particle, when the beam waist moves axially in the surface vicinity. Particles of different sizes are more and less influenced by the standing wave, respectively. The position of the trapped particle is measured with quadrant photodiode and photomultiplier tube at the same time.

We present and analyse a method that uses an interference of counter-propagating Bessel beams for 3D confinement of high-index micro-particles in array of optical traps. Due to this interference a sort of standing wave (SW) is created with intensity maxima separated by more than half a wavelength of the trapping beam and arranged along propagation axis. Thanks to the non-diffracting nature of this beam the region of SW existence is much longer comparing to the Gaussian beam of the similar beam diameter. Steep axial optical intensity gradients cause axial optical force that enables 3D particle confinement. Moreover, the self-healing property of Bessel beam suppresses the beam modification due to the presence of confined objects.

It is shown that the polarization singularities of vector field are connected with the presence or absence of the angular momentum of electromagnetic field. In the vicinity of C-point the orbital angular momentum is observed. Direction of influence of such orbital angular momentum is defined by the sign of topological charge of vibration phase in this area. Spin angular momentum of a field vanishes on the s-contour. At the same time similar momentum appears if the resulting field is formed by the waves with different wavelengths.

The use of axicons or conical lenses has been implemented and proposed in different optical traps, either to create propagation invariant optical fields like Bessel beams or to generate other kinds of traps like hollow beams. In this work we show that the optical field emerging from a system formed by the combination of two axicons can have very different configurations and propagation properties depending upon a few relevant parameters of the system, such as the internal angles of the axicons, the distance between them and their diameters. In particular, by varying only the distance between the axicons, the emerging field may change from a single Bessel beam to a collimated hollow beam or to an array of optical bottle beams, which corresponds to the interference of two Bessel beams with different parameters. Based on these results, we propose a new dynamically reconfigurable optical trap.

We used laser beams in high-order Hermite-Gaussian modes in an optical tweezer to trap and manipulate irregular micron-sized objects. The orientation of the objects was controlled by the opto-mechanical manipulation of the mode.

We use polarized light to generate light torques for controlling nanoparticles and for producing nanomotors. We report on experiments where we apply light torques to glass nanorods in an optical trap at a known distance from a nearby surface. We tried two different optical traps: (1) a standing wave trap using 20 mW and (2) a single beam trap using 80 mW of light at wavelength 514 nm from an Ar+ laser. The rods studied here are 250-500 nm in diameter and are 1-4 microns long. The motion of the rotating rods is studied and a theoretical model of the motion is presented. The motion can be heavily affected by the presence of a nearby surface. For example, past studies have provided evidence that rotatory motion near a surface can change to rocking motion and vice versa. In this study, we present results of motion free of such surface effects. Studies of the motion of nano-objects are useful in understanding nanorheological phenomena in both biological and inorganic systems.

In this paper, we propose an optical method for sorting micro-particles with holographic optical tweezers. By projecting an optical pattern onto the sample plane of a microscope via its objective, we can separate the sample particles of different sizes flowing to different directions.

Since the discovery that laser radiation could be used to impart momentum to microscopic entities, a wide variety of methods, devices, and applications have followed. The most well known method for harnessing optical pressure, optical trapping, involves the highly convergent diffraction limited focusing of a laser for optical manipulation. A lesser known method, termed optical chromatography, involves mildly focusing a laser propagating directly against a fluid flow. When microscopic particles in the flow path encounter this beam they are trapped axially along the beam and are pushed upstream from the focal point to rest at a point where the optical pressure and fluid forces on the particle are equal. Because the optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, a new level of sensitivity, discrimination, and control is possible. This method has recently been used to separate colloidal particles differing only in refractive index. Flow environments designed and fabricated in poly(dimethylsiloxane) (PDMS) using a modification to the method termed Soft Lithography have been adapted to accommodate a loosely focused laser beam entering through a sidewall of the PDMS microfluidic system. Optical chromatographic separations, active sorting, and other particulate mixture manipulations within these customized PDMS microfluidic channel systems are being investigated. The application of these methods and the design of fluidic and optical environments will allow for the very specific separation, identification, and manipulation of a multitude of microscopic particle suspensions.

A micro total analysis system (μ-TAS) is expected to reduce inspection time or the amount of reagent needed. Because of the small Reynolds number in a microchannel, mixing is performed mainly by diffusion. We have proposed an active method to stir a liquid by an optical mixer to improve efficiency. A fabricated optical mixer (SU-8, 20 μm in diameter) is secured inside a chamber at the focal point of the NA=1.4 objective lens and generates flows around it. We have analyzed the micro-liquid velocity vectors and the flux amount around three kinds of mixers (3, 4 and 5 wings) with an upward-directed YAG laser beam at different depths for both horizontal and vertical rotations.

The stretching and unwinding of polymers under flow is important for understanding the rheological properties of dilute polymer solutions. Scaling theory based on the blob picture of single polymer chains predicts several regimes for the overall shape of a hydrodynamically deformed macromolecule. We studied the shape of a DNA molecule stretched out by steady uniform flow at different velocities using optical trapping of single DNA molecules (tethered on polystyrene beads) and single molecule fluorescence imaging. The results show a gradual transition from a non-draining regime at low velocities to a free-draining regime at high velocities, thus verifying the predictions of the free draining-shell (F-shell) blob model.

Adherence to host cells by a bacterial pathogen is a critical step for establishment of infection. It will contribute greatly to the understanding of bacterial pathogenesis by studying the biological force between a single pair of pathogen and host cell. In our experiment, we use a calibrated optical tweezers system to detach a single Klebsiella pneumoniae, the pathogen, from collagen, the host. By gradually increasing the laser power of the optical tweezers until the Klebsiella pneumoniae is detached from the collagen, we obtain the magnitude of the adhesive force between them. This happens when the adhesive force is barely equal to the trapping force provided by the optical tweezers at that specific laser power. This study is important because Klebsiella pneumoniae is an opportunistic pathogen which causes suppurative lesions, urinary and respiratory tract infections. It has been proved that type 3 fimbrial adhesin (mrkD) is strongly associated with the adherence of Klebsiella pneumoniae. Besides, four polymorphic mrkD alleles: namely, mrkDv1, v2, v3, and v4, are typed by using RFLP. In order to investigate the relationship between the structure and the function for each of these variants, DNA fragments encoding the major fimbrial proteins mrkA, mrkB, mrkC are expressed together with any of the four mrkD adhesins in E. coli JM109. Our study shows that the E. coli strain carrying the mrkDv3 fimbriae has the strongest binding activity. This suggests that mrkDv3 is a key factor that enhances the adherence of Klebsiella Pneumoniae to human body.

In force calibration, when triangular-wave input is exerted on the piezoelectric-driven substage, there is a damped oscillation superposed on the square-wave movement of the trapped bead in our experiments. This will bring about instability for well trapping bead and become the noise source of the bead signal. Consequently, the superposed vibration will influence the precision and the range of force that can be calibrated. In order to analyze derivation of the oscillation and solve it, a model of equivalent oscillator is put forward. Based on this model, we calculate the initial amplitude of oscillation and frequency spectra of the movement of trapped bead, which is well in agreement with the experimental one. We apply sinusoidal-wave input substituted for triangular one to the substage, as a result, oscillation disturbance is avoided effectively, so the trapping stability of optical trap increases and the range of calibrated force has been extended greatly.

Since the first demonstrations of optical trapping, both theoretical and experimental parts of this technique evolved. With all this, the main problem when trapping in the Mie regime is due to the limited numerical aperture a microscope objective has. In literature one finds characterizations of “classical” microscope lens or, at most, water immersed ones. In this paper we are analysing the forces generated in an optical tweezers setup using oil immersed microscope objective and having as entrapped particles water-immersed silica beads. Using such a set-up, we can take advantage of the numerical aperture an oil-immersed objective can reach. This numerical aperture can have a value as high as 1.4. From Roosen's 1 and Ashkin's 2 formulas, we calculated the forces involved in our experiment. We observed that the entrapping range on the optical path axis is larger and asymmetric. This generates the possibility to build optical catapult and optical tweezers in the same time, changing only the distance from the sample to the entrapment point. One of the disadvantages of optical trapping in these conditions is that the focus point and the entrapment one can be different. This fact generates the need of using a second microscope for inspecting the entrapped particle so the optical setup is more complicated. To our belief, this set-up for optical tweezers can have big advantages in the field of optical trapping mainly due to the not so strict trapping spatial conditions.

We present preliminary results from an experimental study of optically-assisted assembly. Interference patterns, formed by the intersection of two coherent laser beams, result in periodic one-dimensional potential wells ("optical trenches"). Polystyrene spheres become trapped in these potential wells and subsequently self-assemble into a two-dimensional periodic structure. The spacing between optical trenches is adjusted dynamically, which offers the freedom to dynamically control the lattice constant, offering a recipe for defect-free assembly which begins with annealing at large lattice constant and subsequent compression into a close-packed structure.

Optical tweezers were used to crystallize colloidal dispersions without manipulating colloids on a single particle level. Using "optical gradient" or "high-frequency dielectrophoretic" forces, we demonstrate control over local particle concentration. This control can be used to induce crystallization and melting in two- and three-dimensional colloidal dispersions using single beam gradient optical tweezers. In our setup, two microscope objectives (one above and one below the sample) allow independent three-dimensional manipulation and imaging of the structure formation inside the sample. We demonstrate crystallization near a wall for a range of particles sizes, refractive index contrasts and numerical apertures. In a colloidal mixture of tracer particles with a high refractive index core and low refractive index host spheres dispersed in a host refractive index matching solvent, control over the tracers can indirectly lead to controlled crystallization of the host particles.

The ability of uropathogenic Escherichia coli (UPEC) to cause urinary tract infections is dependent on their ability to colonize the uroepithelium. Infecting bacteria ascend the urethra to the bladder and then kidneys by attaching to the uroepithelial cells via the differential expression of adhesins. P pili are associated with pyelonephritis, the more severe infection of the kidneys. In order to find means to treat pyelonephritis, it is therefore of interest to investigate the properties P pili. The mechanical behavior of individual P pili of uropathogenic Escherichia coli has recently been investigated using optical tweezers. P pili, whose main part constitutes the PapA rod, composed of ~1000 PapA subunits in a helical arrangement, are distributed over the bacterial surface and mediate adhesion to host cells. We have earlier studied P pili regarding its stretching/elongation properties where we have found and characterized three different elongation regions, of which one constitute an unfolding of the quaternary (helical) structure of the PapA rod. It was shown that this unfolding takes place at an elongation independent force of 27 ± 2 pN. We have also recently performed studies on its folding properties and shown that the unfolding/folding of the PapA rod is completely reversible. Here we present a study of the dynamical properties of the PapA rod. We show, among other things, that the unfolding force increases and that the folding force decreases with the speed of unfolding and folding respectively. Moreover, the PapA rod can be folded-unfolded a significant number of times without loosing its characteristics, a phenomenon that is believed to be important for the bacterium to keep close contact to the host tissue and consequently helps the bacterium to colonize the host tissue.

Using optical tweezers and microfluidics, we stretch either the lipid or polymer membranes of liposomes or polymersomes, respectively, into long nanotubes. The membranes can be grabbed directly with the optical tweezers to produce sub-micron diameter tubes that are several hundred microns in length. We can stretch tubes up to a centimeter in length, limited only by the travel of our microscope stage. We also demonstrate the cross linking of a pulled polymer nanotube.

A microfabricated fluorescence activated cell sorter based on an optical switch enables low-stress sorting of small cell populations (i.e. 1,000 - 100,000 cells), a regime that is not addressed by current cell sorters. The glass microchannels are packaged in a self-contained plastic cartridge that enables "chip-to-world" interconnect for ease of use. The sample is hydrodynamically focused using pneumatic pressure driven flow control and is biased to the waste reservoir by default. A 5 mW, 488 nm semiconductor laser is used for cell detection and fluorescence excitation. A sorting event is triggered based on the fluorescence and light scatter properties of the cell. The optical switch is produced from a 20 W CW 1070 nm Ytterbium fiber laser that is controlled by an acousto-optic modulator to deflect the target cell to the target reservoir. Cells can be recovered after the sort for further manipulations. Sorting performance was evaluated using a stably transfected, GFP expressing HeLa cell line spiked at varying ratios into parental HeLa cells. Throughputs ranged from 20-100 cells/sec depending on initial cell concentrations and flow parameters. Target purities greater than 90% were obtained in most cases with overall recoveries greater than 85%. A 1% subpopulation of GFP-HeLa cells could be enriched by 63-71 fold. Similar results were demonstrated using samples from pancreas and other primary tissues. Optical switching can be scaled to fluidic networks of greater complexity by multiplexing an optical source to multiple locations on a chip, and these locations can be arbitrarily reconfigurable.