This PDF file contains the front matter associated with SPIE Proceedings Volume 7400, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We have measured the entropic elasticity of ds-DNA molecules ranging from 247 to 1298 base pairs in length, using axial optical force-clamp tweezers. We show that entropic end effects and excluded-volume forces become significant for such short molecules. In this geometry, the effective persistence length of the shortest molecules decreases by a factor of two compared to the established value for long molecules, and excluded-volume forces extend the molecules to about one third of their nominal contour lengths in the absence of any external forces. We interpret these results in the framework of a modified wormlike chain model.

One limitation on the performance of optical traps is the noise inherently present in every setup. Therefore, it is the desire of most experimentalists to minimize and possibly eliminate noise from their optical trapping experiments. A step in this direction is to quantify the actual noise in the system and to evaluate how much each particular component contributes to the overall noise. For this purpose we present Allan variance analysis as a straightforward method. In particular, it allows for judging the impact of drift which gives rise to low-frequency noise, which is extremely difficult to pinpoint by other methods. We show how to determine the optimal sampling time for calibration, the optimal number of data points for a desired experiment, and we provide measurements of how much accuracy is gained by acquiring additional data points. Allan variances of both micrometer-sized spheres and asymmetric nanometer-sized rods are considered.

To access the genetic code to be transcribed to RNA, RNA polymerases must first open a "transcription bubble" in the DNA. Structural studies suggest that the minimal model of initiation by T7 bacterophage RNA polymerase (T7 RNAP) consists of two distinct steps: initial binding, in which the T7 RNAP binds to and bends the DNA, and opening, achieved by "scrunching" of the DNA. Since both steps involve mechanical deformation of the DNA, both may be affected by downstream DNA tension. Using an oscillating two-bead optical tweezers assay, we have measured the lifetime of single T7 RNAP-DNA initation complexes under tension. Global maximumlikelihood fitting of force-dependent and non-force-dependent versions of this minimal model shows that there is no conclusively discernible force-dependence of initiation in the measured 0-2 pN DNA tension range.

A double-trap optical tweezers instrument was constructed and its spatial resolution measured. The instrument features real-time control that allows feedback based position- and force-clamping experiments. To study RNA-polymerization by QDE-1, an RNA-dependent RNA-polymerase, we tethered a 7250 nt single-stranded DNA molecule between two optically trapped microspheres. Preliminary constant-force extension trajectories and force-extension curves were collected.

In this work we used a methodology to study chemotaxis of Trypanossoma cruzi (T. Cruzi) in real time using an Optical Tweezers system. Trapped beads were used as a force transducer for measuring forces of the same order of magnitude as typical forces induced by flagellar motion. Optical Tweezers allowed real time measurements of the force vectors, strength and direction, of living parasites under chemical or other kinds of gradients. This seems to be the ideal tool to perform observations of taxis response of cells and microorganisms with high sensitivity to capture instantaneous responses to a given stimulus. We applied this methodology to investigate the T. cruzi under distinct situations: the parasite alone and in the presence of its insect-vector Rhodnius prolixus (R. prolixus).

One essential cause of human ageing is the accumulation of DNA damages during lifetime. Experimental studies require quantitative induction of damages and techniques to visualize the subsequent DNA repair. A new technique, the "immuno fluorescent comet assay", is used to directly visualize DNA damages in the microscope. Using DNA repair proteins fluorescently labeled with green fluorescent protein, it could be shown that the repair of the most dangerous DNA double strand breaks starts with the inaccurate "non homologous end joining" pathway and only after 1 - 1 ½ minutes may switch to the more accurate "homologous recombination repair". One might suggest investigating whether centenarians use "homologous recombination repair" differently from those ageing at earlier years and speculate whether it is possible, for example by nutrition, to shift DNA repair to a better use of the error free pathway and thus promote healthy ageing. As a complementary technique optical tweezers, and particularly its variant "erythrocyte mediated force application", is used to simulate the effects of blood pressure on HUVEC cells representing the inner lining of human blood vessels. Stimulating one cell induces in the whole neighbourhood waves of calcium and nitric oxide, known to relax blood vessels. NIFEDIPINE and AMLODIPINE, both used as drugs in the therapy of high blood pressure, primarily a disease of the elderly, prolong the availability of nitric oxide. This partially explains their mode of action. In contrast, VERAPAMILE, also a blood pressure reducing drug, does not show this effect, indicating that obviously an alternative mechanism must be responsible for vessel relaxation.

In two previous studies we have conducted combined laser subcellular microsurgery and optical trapping on chromosomes in living cells^{1, 2}. In the latter study we used two separate microscopes, one for the trap and one for the laser scissors, thus requiring that we move the cell specimen between microscopes and relocate the irradiated cells. In the former paper we combined the 1064 nm laser trap and the 532 nm laser scissors into one microscope. However, in neither study did we have multiple traps allowing for more flexibility in application of the trapping force. In the present paper we describe a combined laser scissors and tweezers microscope that (1) has two trapping beams (both moveable via rapid scanning mirrors (FSM- 300, Newport Corp.), (2) uses a short pulsed tunable 200 fs 710-990 nm Ti:Sapphire laser for laser microsurgery, and (3) also has the option to use a 337 nm 4 ns UV laser for subcellular surgery. The two laser tweezers and either of the laser ablation beams can be used in a cell surgery experiment. The system is integrated into the robotic-controlled RoboLase system³. Experiments on mitotic chromosomes of rat kangaroo PTK2 cells are described.

It has been known that the shape, the locomotion, and the growth of cells and bacteria are often affected by their interactions with extra cellular matrix (ECM). However, it is difficult to quantify such interactions with conventional biochemical methods. In this paper we report the application of oscillatory optical tweezers to trap and oscillate three types of E. coli, in 0.2% LB agar substrate to quantify the E. coli - substrate interactions in terms of the elasticity modulus G'. The three types of E. coli are BW25113 (wild-type, normal with flagellum), BW25113 (normal with flagellum, but subjected to UV light exposure for 1 hr to deactivate the flagellum), and JW1923 (a null-flagellum mutant of BW25113). Our results indicate that the value of G' for the later two is significantly higher than that for the normal wild-type (WT). We speculated that the interaction with the surrounding is perturbed, and hence reduced, mainly by the motion of the flagellum in the case of the WT.

We present optical and mechanical models for cell's deformation in the optical traps. The morphological deformation of the cell is calculated from the local stress distribution on the cell using the linear theory of thin elastic membrane, which is valid for the cells without internal cytoskeletal structure, such as the red blood cells. This generic approach allows computing for a variety of experimental configurations and predicting cell's deformation. Comparison of the results with numerical calculation tool such as Mie scattering and T-Matrix is made. The coupling coefficient from one fiber to another through the deformed trapped cell in the fiber-optical dual-beam stretcher was calculated for fast monitoring of the cell deformation.

Using both continuous-wave (CW) and high repetition rate femtosecond lasers, we present stable 3-dimensional trapping of 1μm polystyrene microspheres. We also stably trapped 100nm latex nanoparticles using the femtosecond mode-locked laser at a very low average power where the CW lasers cannot trap, demonstrating the significance of the fleeting temporal existence of the femtosecond pulses. Trapping was visualized through dark-field microscopy as well as through a noise free detection using two-photon fluorescence as a diagnostics tool owing to its intrinsic 3- dimensional resolution. Comparison between a Gaussian versus a flat-top Gaussian beam profile demonstrates the importance of laser spatial mode in optical trapping.

We examine the statics and dynamics of charged colloids interacting with periodic optical trap arrays. In particular we study the regime where more than one colloid is confined in each trap, creating effective dimer, trimer, and higher order states called colloidal molecular crystals. The n-mer states have an effective orientational degree of freedom which can be controlled with an external driving field. In general, the external field causes a polarization effect where the orientation of the n-mers aligns with the external field, similar to liquid crystal systems. Additionally, under a rotating external drive the n-mers can rotate with the drive. In some cases a series of structural transitions in the colloidal crystal states occur in the rotating field due to a competition between the ordering of the colloidal molecular crystals and the polarization effect which orients the n-mers in the direction of the drive. We also show that for some parameters, the n-mers continuously rotate with the drive without switching, that depinning transitions can occur where the colloids jump from well to well, and that there are a number of distinct dynamical transitions between the phases. Finally, we illustrate colloidal orderings at fillings of more than four colloids per trap, indicating that it is possible to create higher order colloidal crystal cluster phases.

We present a thorough experimental and theoretical study of micrometer-sized particle dynamics in optical lattices. The spatial parameters of the lattices are set by configuration of the interfering beams with computercontrolled spatial light modulator. The behaviour of particle confined in the interference pattern is monitored by fast CCD camera. Analyses of the particle trajectory and Brownian motion reveal force interaction between the particle and the lattice. We use polymer microspheres as the experimental objects. The experimental results are compared with the theoretical predictions based on the generalized Mie theory.

We consider prolate objects of cylindrical symmetry with radius periodically modulated along the axial direction and we present a theoretical study of optimized objects shapes resulting in up to tenfold enhancement of the axial optical force in comparison with the original unmodulated object shape. We obtain analytical formulas for the axial optical force acting on low refractive index objects where the light scattering by the object is negligible. Numerical results based on the coupled dipole method support the previous simplified analytical conclusions and they are also presented for objects with higher refractive indices. The objects are trapped in a standing wave, that offers many useful advantages in comparison to single beam trapping, especially for submicrometer size particles. It provides axial force stronger by several orders of magnitude, much higher axial trap stiffness, and spatial confinement of particles with higher refractive index.

Laser guidance is the technique that uses a weakly convergent laser beam to trap particles radially in the center of the beam and simultaneously propel them along the beam propagation axis with a travelling distance over millimeters. In this paper, we describe the applications of laser guidance to detect different cell types, including those of phenotypically transformed or gene-modified cells, especially for situations in which fluorescent markers used in flow cytometry for cell detection are not available or their application is contraindicated by clinical restriction. The optical force, which determines the guidance speed of the cell, is dependent on the characteristics, such as size, shape, composition and refractive index, of the cell being guided. Therefore, by measuring the guidance speed of the cell along the laser beam, cells with different properties can be effectively distinguished. We report two experimental results: 1) the laser-guidance system could significantly distinguish the metastatic cancer cell type 4T1 from its non-metastatic counterpart 4T07, which could not be achieved by using a high magnification microscope; 2) The laser-guidance experiment demonstrated that only one gene modification between L-10 and TC-1 cells resulted in ~40% difference in guidance speed. These experimental data indicate that laser guidance can be used to detect subtle differences between sub-cell types.

Optical Chromatography involves the elegant combination of opposing optical and fluid drag forces on colloidal samples within microfluidic environments to both measure analytical differences and fractionate injected samples. Particles that encounter the focused laser beam are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. In our recent devices particles are pushed into a region of lower microfluidic flow, where they can be retained and fractionated. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a sample, differences between samples and thus separations are possible. An optical chromatography beam focused to completely fill a fluid channel is operated as an optically tunable filter for the separation of polymeric/colloidal and biological samples. We demonstrate this technique coupled with an advanced microfluidic platform and show how it can be used as an effective method to fractionate particles in an injected multi-component sample. Our advanced microfluidic design accommodates three lasers simultaneously to effectively create a sequential cascade optical chromatographic separation system.

In our experiments, microscopic polymer and glass spheres in microfluidic devices are manipulated using pressure generated by a high power laser beam. The effect of the laser on the particles and the manipulations are imaged using a microscope lens connect to a CCD camera. Differential forces on particles of varying physical and chemical composition have been measured. The goal is to measure the optical forces on chemically different particles and catalog the associated chemical and physical differences to understand which properties and mechanisms result in force differentials. The aim is to better understand the range of optical separations that may be possible and the extent to which the instrument can differentiate between similar microspheres in terms of size and/or chemical composition.

Optical chromatography is a powerful technique, capable of separating micron-sized particles within a fluid flow, based on their intrinsic properties, including size, shape and refractive index. Briefly, particles in a fluid flow are subject to two forces, the Stokes drag force due to the fluid and then an introduced optical force as supplied by a laser beam, acting in opposite but collinear directions. According to the particle's intrinsic hydrodynamic and optical properties, equilibrium positions may form where the two forces balance, which is highly dependent on the properties of the particle and as a result provides a means for spatial separation in a sample mixture. Optical chromatography is a passive sorting technique, where pre-tagging of the particles of interest is not required, allowing for non-discrete distributions to be evaluated and/or separated. Firstly we review the current stage of optical chromatography. We present a new advance in optical chromatography potentially enabling the unique beam delivery properties of photonic crystal fiber (PCF) to be employed and integrated into microfluidic chips. Also, for the first time a finite element method is applied to the optical field in the theoretical analysis of optical chromatography, which is found to be in excellent agreement with the current ray optics model, even for particles much smaller than the optical wavelength. This will pave the way for the technique to be extended into the nanoparticle regime.

Non-periodic structure of optical pattern can assign distinctive potential landscape to multiple micro particles according to their sizes, and enables optical sorting using only optical gradient force. We have theoretically and experimentally investigated the dependence of success rate of sorting as a function of time on beam power in non fluidic flow system. In the experiment, polystyrene spheres suspended in water were used as samples, and the radii were 0.49 μm, 1.03 μm, and 1.61 μm, respectively. We used multiple optical lines in the experiment, and placed it non-periodically with increasing peak intensities. The experimental result qualitatively agreed with the theoretical one, and the success rate of sorting was more than 90 % with sufficient beam power. We demonstrated our method in the system with flow by placing optical pattern such that optical gradient force acted on particles orthogonal to the flow. In the experiment with flow, triangle optical pattern was used to lift up micro particles onto the top surface where image of optical pattern was formed and to carry the particles to starting position for sorting. Our sorting system can in principle work in broad range of particle velocity with sufficient beam power.

We report on two light-induced droplet actuation mechanisms, floating electrode optoelectronic tweezers (FEOET) and lateral field optoelectrowetting (LOEW), for manipulating aqueous droplets immersed in oil on a featureless photoconductive surface with an open chamber configuration. Droplet functions including transporting, merging, mixing, and splitting, and multi-droplet manipulation have been accomplished. Droplet manipulation in FEOET is based on light-induced dielectrophoretic forces in an electrically insulating medium such as oil. It has been shown that oilimmersed aqueous droplets can be actuated by a light beam with an intensity as low as 400 μW/cm² in FEOET. However, due to the weak force generated by the DEP-based droplet actuation, the droplet moving speed is limited to hundreds of μm/s and performing other droplet manipulation functions such as splitting and injection is challenging on FEOET. On the other hand, LOEW-based droplet actuation is realized by modulating the interfacial surface tension of a droplet on a hydrophobic surface through a light-induced electrowetting effect. Since surface tension provides large forces than DEP on a droplet with a diameter from mm to hundreds of μm, LOEW allows transporting droplets at a speed in the range of cm/s and performing droplet-based functions such as splitting and injection. The open chamber configuration of these platforms provides flexibility in integration with other microfluidic components such as external reservoirs and tubing for broad chemical and biochemical applications.

We utilize a simple parallel electrode setup on which intense holographic optical landscapes are shone. Intense optical illumination creates high local gradients in the dielectric properties of the fluid, which in the presence of an electric field results in a fluid body force. This leads to the creation of toroidal microvortices, which aid the particle concentration process. Fluid drag aiding low frequency AC electrokinetic forces leads to an aggregation of particles on the illuminated regions of the electrode surface. With a fine balance of these forces, we show that such optically driven electrokinetic mechanisms can capture and aggregate nanoparticles (50nm and 100 nm). Particle aggregation is a function of the AC frequency and by using fluorescent particles we characterize the technique as a function of the applied AC frequency. Relatively low optical powers (~20 mW) are utilized in this technique.

We present progress in manipulating silicon nanomembranes using holographic optical tweezers. The holographic optical tweezers technique provides a non-contact means of directly controlling the nanomembranes. Silicon nanomembranes are macroscopic free-standing sheets of single-crystal silicon which can be as thin as 10 nm or less. The thinness of the membranes imparts unique electronic, optical and mechanical properties. This characteristic, combined with the ability to precisely engineer their dimensions, makes silicon nanomembranes ideal candidates for use in new electronic and photonic devices. The nanomembranes utilized for this work have controlled thicknesses of 220 nm and areas reaching up to 200x200 microns. Novel all optically actuated methods for directing membranes in microfluidic flow environments, controllable membrane flexing and as well as vertical reorientation and positioning are outlined.

Electromagnetic waves carry the Abraham momentum, whose density is given by p_EM = S(r,t)/c². Here S(r, t) = E(r, t)×H(r, t) is the Poynting vector at point r in space and instant t in time, E and H are the local electromagnetic fields, and c is the speed of light in vacuum. The above statement is true irrespective of whether the waves reside in vacuum or within a ponderable medium, which medium may or may not be homogeneous, isotropic, transparent, linear, magnetic, etc. When a light pulse enters an absorbing medium, the force experienced by the medium is only partly due to the absorbed Abraham momentum. This absorbed momentum, of course, is manifested as Lorentz force (while the pulse is being extinguished within the absorber), but not all the Lorentz force experienced by the medium is attributable to the absorbed Abraham momentum. We consider an absorptive/ reflective medium having the complex refractive index n₂+i κ₂, submerged in a transparent dielectric of refractive index n₁, through which light must travel to reach the absorber/reflector. Depending on the impedance-mismatch between the two media, which mismatch is dependent on n₁, n₂, κ₂, either more or less light will be coupled into the absorber/reflector. The dependence of this impedance-mismatch on n₁ is entirely responsible for the appearance of the Minkowski momentum in certain radiation pressure experiments that involve submerged objects.

In a recent paper, W. She, J. Yu and R. Feng reported the slight deformations observed upon transmission of a light pulse through a short length of a silica glass nano-filament. Relating the shape and magnitude of these deformations to the momentum of the light pulse inside and outside the filament, these authors concluded that, within the fiber, the photons carry the Abraham momentum. We present an alternative evaluation of force and momentum in a system similar to the experimental setup of She et al. Using precise numerical calculations that take into account not only the electromagnetic momentum inside and outside the filament, but also the Lorentz force exerted by a light pulse in its entire path through the nano-waveguide, we conclude that the net effect should be a pull (rather than a push) force on the end face of the nano-filament.

Two 25 base-pair complementary DNA strands are encapsulated within an optically trapped nano-droplet, and we observe the kinetics of their hybridization in dynamic equilibrium via single molecule fluorescence resonance energy transfer (FRET) as a function of temperature and of the solution's NaCl concentration. We have observed binding and unbinding events between the two freely diffusing DNA strands, and our measurements reveal that the duplex can exist in multiple conformational states at elevated temperatures and low concentrations of NaCl.

We demonstrate the use of holographic optical tweezers to form arrangements of silica beads for trapping and measuring the mechanical properties of micron sized objects, such as oil droplets and yeast cells. This allows us to investigate the mechanical properties of the constrained object, which need not be optically trapped itself (thus preventing radiation damage and allowing objects with a low refractive index to be constrained). By compressing the object with the beads we are able to determine the size of the trapped object and show that there is an elastic coupling between the beads due to the presence of a trapped object. We expect more detailed analysis of the system will allow mechanical and frequency-dependent viscoelastic properties of objects to be investigated.

With the right proportions, a binary suspension of different sized particles may be subject to entropic effects that can generate a depletion-induced attraction between large particles. A manifestation of the induced attraction is the enhanced osmotic compressibility of the larger species in the presence of the smaller species. We conducted an experimental study on how such an enhancement is affected for 190 nm polystyrene spheres in the presence of polyethelyne-oxide in aqueous solutions. Using the gradient force from a tightly focused laser, we can locally concentrate the polystyrene nanoparticles in suspension, and from the changes of local particle density under the known gradient force, we deduce a quantitative measure of the isothermal compressibility of the particles. We report the analysis of these compressibilities and their enhancement by the added polymers for a broad range of particle and polymer concentrations.

In their pioneering work, Burns et al. [Phys. Rev. Lett. 63, 1233 (1989)] discovered a laser-induced optical interaction between dielectric microparticles dispersed in water. This interaction occurred in the plane transversal to the laser beam and, interestingly, induced bound pairs of particles. Accordingly, the observed phenomenon was termed "transverse optical binding" (TOB). Burns et al. argued that TOB arises from coherently induced electric dipoles in the microspheres. Indeed, this explanation verified the experimental observation that the spatial periodicity of the TOB interaction matched the laser wavelength in water. However, relatively little experimental evidence has been provided, to date, for both the strength and functional dependence of this effect on the particle distance. In our study, we used an experimental method which allowed us to directly measure the TOB interaction. As a result, we found that this interaction is surprisingly long-ranged.

Optical binding has been proposed to be responsible for the cluster formation of micron size dielectric spheres in coherent light fields. However, a direct measurement of the forces involved in binding is missing. We report an experimental study of optical binding forces between two optically trapped dielectric spheres. Results for optical forces are presented as a function of three parameters: inter particle separation, particle size, and respective polarizations. A comprehensive calculation based on the generalized Mie scattering theory for the experiment has also been conducted. This paper will present a comparison between experimental and theoretical results.

Evanescent optical landscapes are created by the total internal reflection (TIR) of two counterpropagating laser beams, and used to trap large numbers of submicron particles. Varying the size of dielectric particles and the polarisations of the incident beams results in arrays of different symmetries and lattice spacings. The kinetically formed array is not necessarily the lowest energy structure and spontaneous transitions to alternative packings are sometimes observed. Arrays can build up large amounts of strain energy that can result in the sudden ejection of streams of particles from the array. Nanoparticles of Au show long-range optical binding, but do not form regular arrays.

The phenomenon of optical binding is now experimentally very well established. With a recognition of the facility to collect and organize particles held in an optical trap, the related term 'optical matter' has also been gaining currency, highlighting possibilities for a significant interplay between optically induced inter-particle forces and other interactions such as chemical bonding and dispersion forces. Optical binding itself has a variety of interpretations. With some of these explanations being more prominent than others, and their applicability to some extent depending on the nature of the particles involved, a listing of these has to include the following: collective scattering, laser-dressed Casimir forces, virtual photon coupling, optically induced dipole resonance, and plasmon resonance coupling. It is the purpose of this paper to review and to establish the extent of fundamental linkages between these theoretical descriptions, recognizing the value that each has in relating the phenomenon of optical binding to the broader context of other, closely related physical measurements.

Consideration is given to methods of manipulating optically fabricated particle arrays using broadband radiation and a superposition of optical fields. Specifically, the changes that the optical binding energy experiences, when part of the spectrum of this light is filtered, are analyzed. It is then shown that these optically induced arrays can be reordered by the introduction of additional fields with transverse Poynting vectors. Subsequently, it is shown how pairs of particles can be reordered on a surface by modifying the form of the optical binding interaction. Finally, the effect of particle size on these methods is briefly discussed.

We report our experimental and theoretical progress towards elucidating the nonlinear optical response of nanosuspensions. To date, we have devised a fiber-optic variant of the Z-scan method to accurately measure the nonlinearity of liquid nanosuspensions. Furthermore, we shall show that the optical nonlinearity may be properly accounted theoretically by including both the virial coefficients for the soft-condensed matter system in addition to the exponential term, which does not account for particleparticle interactions, yielding an effective or renormalized Kerr effect in many cases.

We present both theoretical and experimental study of longitudinal optical binding between two nanopatricles or microparticles placed in two counter-propagating Bessel beams. We consider coherent linearly polarized beams and we rotate polarization of one beam so that both beams interfere or do not interfere. We present an experimental demonstration of the remarkable changes in the behaviour of both spheres leading to freezing of the structure if both beams interfere.

Optical Tweezers have become a widespread tool in Cell Biology, microengineering and other fields requiring delicate micromanipulation. But for those sensitive tasks, it remains difficult to handle objects without damaging them. As the precision in position and force measurement increase, the richness of information cannot be fully exploited with simple interfaces such as a mouse or a common joystick. For this reason, we propose a haptic force-feedback optical tweezer command and a force-feedback system controlled by one hand. The system combines accurate force measurement using a fast camera and the coupling of these measured forces with a human operator. The overall transparency allows even the feeling of the Brownian motion.

A Multitouch screen is an obvious choice for a holographic optical tweezers interface, allowing multiple optical traps to be controlled in real-time. In this paper we describe the user interface used for our original multitouch system and demonstrate that, for the user tasks performed, the multitouch performs better than a simple point-and-click interface.

We present a comprehensive laser tweezers software package, comprising the software components for any laser tweezers system. This includes fast hologram generation software, implemented on a graphics card, thereby allowing 100 hundred independently moving traps at video frame rate. The software also includes comprehensive camera suite and image recognition software for multi-particle tracking and analysis. The software is freely available from the authors and online(http://www.physics.gla.ac.uk/Optics/).

We adapt concepts from matched filtering to propose a method for generating rapidly reconfigurable multiple beams. As a phase-only spatially filtering extension of the Generalized Phase Contrast (GPC) technique, the proposed method coined mGPC can yield dynamically reconfigurable optical beam arrays with high light efficiency for optical manipulation, high-speed sorting and other parallel spatial light applications [1].

Due to their immunity to diffraction, Bessel light modes potentially offer advantages in various applications. However, the Bessel beams generated by traditional approaches do exhibit significant intensity variations along their axial propagation length which hampers their applicability. In this paper we present a technique to generate Bessel beams with a tunable axial intensity within the accessible range of spatial frequencies. The beam may be engineered to have a constant intensity along its propagation length.

The year 2007 witnessed the experimental realization of extraordinary laser beams termed Airy and parabolic beams. Surprisingly, these beams are immune to diffraction and in addition exhibit transverse acceleration while propagating. This peculiar property of both Airy and parabolic beams facilitates the clearance of both microparticles and cells from a region in a sample chamber through particle/cell transport along curved trajectories. We term this concept "Optically mediated particle clearing" (OMPC) and, alternatively, "Optical redistribution" (OR) in the presence of a microfluidic environment, where particles and cells are propelled over micrometersized walls. Intuitively, Airy and parabolic beams act as a form of micrometer-sized "snowblower" attracting microparticles or cells at the bottom of a sample chamber to blow them in an arc to another region of the sample. In this work, we discuss the performance and limitations of OMPC and OR which are currently based on a single Airy beam optionally fed by a single parabolic beam. A possible strategy to massively enhance the performance of OMPC and OR is based on large arrays of Airy beams. We demonstrate the first experimental realization of such arrays.

Laser trapping is a widely used technique such as manipulating cells. Recently the trapping technique is used in air, for example, a precision probe for sensing the surface of an object. To expand the applications of the trapping technique in air, more experimental investigations need to be implemented for properties such as trapping forces. We studied the dynamic properties of a micro-sphere (φ8um) optically trapped in air by using a radially or linearly polarized beam. Firstly in order to predict the trapping forces working on a micro-sphere, the forces are analyzed by a ray-tracing method. The results show that an axial force of radial polarization is larger than one of linear polarization. Considering the radial forces, the force of radial polarization is smaller than one of linear polarization. These results can be understood by noting forces generated by p- and s-polarization. Secondly, we examine the trapping efficiency in optical trapping experimentally. Radial trapping efficiency is evaluated by measuring a spring constant. Experimental results and simulated results are in good agreement that the linear polarized beam achieved a 1.25 times higher spring constant than radial polarization. Axial trapping efficiency is examined by measuring minimum trapping laser power. Experimental results are one tenth underestimated although qualitatively they are coincident. Radial polarization is shown to be approximately 2 times higher than linear polarization. Thus, employing radial polarization, the optical trapping of the glass microsphere in air is achieved by using an objective lens with NA0.80.

By using multiple optical traps suitably sized complex bodies can be bound with respect to their positions and orientations. One recent application of this involves the use of an elongated object, equipped with a probe (a "nanotool"), to measure and apply pico-Newton sized forces to, for example, the surface of a cell. This application has been described as an optical atomic force microscope (AFM). Calculations of the mechanical susceptibility of trapped probes, and their hydrodynamic resistance are presented. These quantities are used to assess the subsequent thermal motion of an optically trapped nanotool in the context of the Orstein-Uhlenbeck process. Implications for the resolution and general behavior of the optical AFM referred to above are discussed.

Holographic optical tweezers are used to assemble and control probes made from high aspect-ratio CdS and SiO₂ nanorods and SiO₂ microspheres. Analysis of the probe position allows for the measurement of forces experienced by the tip in a manner analogous to existing scanning probe microscopy (SPM) techniques.

While a variety of different optically-driven micromachines have been demonstrated by a number of groups around the world, there is a striking similarity in the designs used. The typical optically-driven rotor consists of a number of arms attached to a central hub, or elongated stalk in the case of free-floating rotors. This is a consequence of the relationship between the symmetry of a scattering object and the transfer of optical angular momentum from a beam to the object. We use a hybrid discrete-dipole approximation/T-matrix method algorithm to computationally model the scattering by such optically-driven rotors. We systematically explore the effects of the most important parameters of rotors, such as the thickness, length, and width of the arms, in order to maximize the torque efficiency. We show that it is possible to use computational modelling to optimize the design of such devices. We also compare the computational results with experiment.

As an optically trapped micro-object spins in a fluid, there is a consequent flow in the fluid.. Since a free-floating optically-driven microrotor can be moved to a desired position, it can allow the controlled application of a directed flow in a particular location. Here we demonstrate the control and rotation of such a device, an optical paddle-wheel, using a multiple-beam trap. In contrast to the usual situation where rotation is around the beam axis, here we demonstrate rotation normal to this axis.

Engineering semiconductor nanostructures has immense potential for applications pertaining to nanophotonics especially due to their optical properties. Nanostructures can come in various forms i.e. tubes, rods and dots. Each presents themselves as a possible candidate for creating larger photonics structures. In this paper, we describe the optical trapping characteristic of dielectric enhanced nanoparticles. Two techniques of dielectric enhancements are employed: silica coating and microsphere tagging, for the efficient manipulation of nanoparticles.

Optical trapping of nanorods has attracted many researchers due to many potential applications of nanorods in sensor technologies. It is well known that nanorods align with the propagation axis or the polarization direction of a laser beam. However, there are only few studies about the axial rotation of nanorods. In this study, we present a method for the measurement of the rotational frequency of nanorods.

We demonstrate on-demand production and optical manipulation of submicron-sized hydrosomes, water drops in an immiscible medium. We use optical trapping techniques to induce the controlled fusion of multiple drops and study the dynamics of small amounts of reagent encapsulated within the hydrosomes.

Inclined dual-fiber optical tweezers (DFOTs) are investigated both numerically and experimentally. In simulations, the trapping forces of the inclined DFOTs and the single-fiber optical tweezers (SFOTs) are studied along two directions. By comparing the simulation results of the inclined DFOTs and the SFOTs, the inclined DFOTs are found to have more symmetric performance, stronger trapping forces, and more reliable functionality. The spring constant of the DFOTs was calibrated experimentally along one direction. The calibration results agree with those obtained in simulations. Moreover, we created multiple optical traps with an inclined dual-fiber optical tweezers setup. Multiple optical traps were formed at different vertical levels. We demonstrated that this fiber-based trapping system can perform multiple functions, such as particle grouping and stacking. Compared with those formed with objective-based optical tweezers, the multiple traps presented here are small in size and independent of the objective or the substrate, and hence hold the promise to be integrated in microfluidic systems. The inclined DFOTs capable of multiple trapping can be used for on-chip parallel manipulation.

We study a geometrically anisotropic internal nano-layered structure of the shrunken multi-lamellar vesicle(SMLV) by using optical tweezers with a polarized beam. The SMLVs are synthesized from soybean asolectin by using gentle hydration method and has an optically birefringence property, known as a form birefringence. When a laser beam of optical tweezers with an elliptical polarization passes through the material with an optical birefringence, the ordinary and extraordinary components of the laser light experience different phase shift, respectively. Therefore, an optical torque due to the angular momentum conservation can be exerted on the optically birefringence material generating a rotational motion of the system. In this work we analyze the distance between the next bilayers of the membrane structure in SMLV from the experimentally measured data of the rotational motion and propose a simple model in which the SMLV is idealized as a multi-regularly-thin parallel plate structure.

The trap length along the beam axis for an optical trap formed with an upright, oil-immersion microscope was measured. The goals for this effort were twofold. It was deemed useful to understand the depth to which an optical trap can reach for purposes of developing a tool to assist in the fabrication of miniature devices. Additionally, it was desired to know whether the measured trap length favored one or the other of two competing theories to model an optical trap. The approach was to trap a microsphere of known size and mass and raise it from its initial trap position. The microsphere was then dropped by blocking the laser beam for a pre-determined amount of time. Dropping the microsphere in a free-fall mode from various heights relative to the coverslip provides an estimate of how the trapping length changes with depth in water in a sample chamber on a microscope slide. While it was not possible to measure the trap length with sufficient precision to support any particular theory of optical trap formation, it was possible to find regions where the presence of physical boundaries influenced optical traps, and determine that the trap length, for the apparatus studied, is between 6 and 7 micrometers. These results allow more precise control using optical micromanipulation to assemble miniature devices by providing information about the distance over which an optical trap is effective.

In this study we report a detailed description of the trapping forces exerted on an arbitrary oriented micron-sized dielectric spheroid by means of a counterpropagating dual-beam optical trap with a Gaussian transverse field pattern, using a classical optics approximation. Our analysis includes the calculation of the transverse and axial trapping efficiencies as function of the normalized beam waist separation distance, normalized spheroid size, effective index of refraction of the microparticle and ellipticity of the spheroid. The trapping forces produced are compared with those obtained for spheres.