This PDF file contains the front matter associated with SPIE Proceedings Volume 7038, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

This paper reports the measurement of elastic constants between two DNA strands that are simultaneously hybridized by a third target-DNA linker. Two probe-DNA strands that are immobilized on fluorescent beads and a target-DNA linker formed a hybridized assembly through the Watson-Crick based pairing. Elastic constants of the resulting assembly were measured using a force calibrated dual optical trap. This study can be used to detect the existence of a target-DNA linker with a specified nucleotide sequence, indicating its potential use in DNA biosensors.

Optical tweezers measurements were employed to directly observe viral DNA packaging in wild type and packaging mutants of bacteriophage lambda. Several key findings are reported here: DNA packaging by purified wild type lambda motors was measured for the first time, showing nearly identical behavior in packaging DNA to crude extracts of terminase components. A slow packaging lambda mutant, T194M, was found to package DNA at ~10× slower velocity than wild type. Meanwhile another packaging mutant Y46F was found to package DNA slower than the wild type (60-70% the velocity of the wild type velocity) as well as slipping >10x more frequently (per length of DNA) than wild type. Another mutant (K84A) showed slower packaging (60-70% the velocity of wildtype), but displayed slipping and pausing behavior similar to wild type. Finally the pausing and slipping dependence on length of DNA packaged of the various terminases studied was discovered, suggesting further structural defects of the mutants that are detrimental to translocation. These studies confirm the location of an ATPase center in the N-terminal portion of gpA which is responsible for translocation of dsDNA.

The hardware-based Anti-Brownian ELectrokinetic trap (ABEL trap) features a feedback latency as short as 25 μs, suitable for trapping single protein molecules in aqueous solution. The performance of the feedback control loop is analyzed to extract estimates of the position variance for various controller designs. Preliminary data are presented in which the trap is applied to the problem of determining the distribution of numbers of ATP bound for single chaperonin multi-subunit enzymes.

We used one-dimensional oscillatory optical tweezers in a discrete scanning (or jumping) mode to trap and stretch individual mice erythrocytes and measured their deformation as a function of the jumping distance of the oscillatory trapping beam. In general, we observed that the length of the long axis of red blood cells (RBCs) decreased slightly at small jumping distance and then increased after the jumping distance exceeded a threshold value on the order of a few microns, which is consistent with a recent theoretical prediction. The deformability of three types of mice RBCs, namely the wide-type mice (serving as the control group), old mice, and gene knockout mice were measured and compared. Statistical analysis of their deformability reveals that the RBCs of old mice can be distinguished from those of knockout mice even though these two types of mice exhibit many similar aging-like features.

We are exploring a biological application of optical tweezers with fluorescence imaging for microrheometry. Measurement of the power spectrum of Brownian motion of a trapped probe particle or vesicle provides information on the viscoelastic properties of the surrounding medium which can change in response to cellular processes or the effect of drugs.

This paper reports an experimental study of the interparticle interactions present in a model colloid system composed of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. By independently measuring the fluorescence intensity as a function of particle number density, we were able to determine the relationship between the radiation pressure generated by the optical trap and the resulting number density increase, yielding the calculation of the isothermal compressibility of the colloid system. Optical trapping was made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linearly with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio required to detect the changes in local particle density induced by optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from the diffraction-limited focal region of the two laser beams.

In suspensions containing microspheres and a sub-micron species, such as nanoparticles or a polymer, an attractive force can result between the microspheres. This attraction arises due to an entropic interaction, often referred to as a depletion force. In this work we demonstrate an application of the depletion force to the controlled assembly of crystalline templates for the production of photonic band gap (PBG) materials. The method makes use of holographic optical tweezers to assemble crystalline arrays of silica or polystyrene microspheres, in which depletion interactions are used to stabilise the structures being built. In addition, we use the holographic optical tweezers to characterise the attraction between pairs of microspheres in the system.

The Fluctuation Theorems (FTs) of Evans & Searles and of Crooks are fundamental theorems of modern thermodynamics that have been suggested to be of practical use to scientists and engineers. Non-equilibrium processes with energy fluctuations on the order of thermal energy, κ_BT, are described by the FTs; examples include the stretching of a DNA molecule, the localisation of a colloidal particle in an optical trap of changing strength, and translation of an optically trapped colloidal particle. If the path or process is traversed over long times or the system is sufficiently large that it can be considered in the classical, thermodynamic limit, then, in principle, there is only one value of the energy characterising the path. However, for small systems, there exists a distribution of energy values and this distribution is associated with non-equilibrium fluctuations of the system that do not average out over short time. The FT of Evans & Searles, as well as the FT of Crooks (from which the Jarzynski relation is derived), describe the symmetry of this energy distribution about zero. This distribution is inherent to the dynamics of small systems, such as nano-machines and single molecular motors. In this paper we present the FTs in a single unified language, considering that the work done on the system is either purely dissipative, achieves a change in thermodynamic state of the system, or a combination of these. We demonstrate this with a single colloidal particle in an optical trap and a single DNA molecule stretched in an OT experiment.

We explore theoretically and experimentally the first creation of extended longitudinally optically bound chains of microparticles in one dimension. We use the geometry of two counter-propagating "non-diffracting" light fields, so termed Bessel beams. Such beams suppressed the influence of the axial intensity profiles of the illuminating beams on the self-organisation process which then depended critically upon the inter-beads interactions. Beam homogeneity and extended propagation allowed the creation of 200μm long chains of organised micro-particles and the first observation of multi-stability: short range multistability within a single chain and a long-range multi-stability between several distinct chains. Our observations are supported by theoretical results of the coupled dipole method.

It has been conjectured for some time that colloidal suspensions can act as artificial self-guiding media and support solitary beam-like solutions. The optical forces, along a diverging Gaussian beam, act to pull and retain the diffusing nanoparticles into its beam path. Consequently, the nanoparticle suspension acts to guide the diverging Gaussian beam and maintain the beam waist over a distance longer than its Rayleigh range. In this paper, we present a detailed analysis of beam propagation within nanoparticle suspensions. Using a recently developed theory by El-Ganainy et. al. (1), we seek to understand the beam dynamics by monitoring the scattered light from the particles along the propagation of the beam. An initial comparison of the theoretical and the experimental results shows interesting deviations due to the exponential nature of the optical nonlinearity.

In the area of optical micro-manipulations Bessel beams are well known for their unique properties such as non-diffracting propagation over a large area or their ability to reconstruct themselves after passing a disturbing obstacle. In this paper we demonstrate how the spatial spectrum phase modulation of such Bessel beam can be used for its precise three-dimensional position control or even splitting it in several parallel Bessel beams. Applying these features to a simple computer-driven interactive setup enabled us to guide selected particles between remote planes demonstrating the possibility of active sorting of micro-objects. In the case of two axially shifted co-axial Bessel beams a system of counter-propagating Bessel beams can be obtained using a mirror. The interference of such counter-propagating beams provide a standing-wave axial modulation of the field intensity. The position of this standing wave peaks can be controlled altering phase of one of the beams leading to the concept of an 'Optical conveyor belt' for transport of micro-objects. However, using a time-sharing between the two beams causes that the interference is suppressed, but their correct axial overlap assures a stable position for object confinement. This geometry can be used then for real-time interactive three-dimensional position control of several objects. Such light fields have broader applications, for example in two-photon processes in biophysics such as photoporation of living cells providing transport of modified DNA from surrounding medium inside the cell volume and consequent synthesis of fluorescent protein.

Optical binding is a phenomenon that is exhibited by micro-and nano-particles systems, suitably irradiated with off-resonance laser light. When several particles are present, the effect commonly results in the formation of particle assemblies. In the optically induced potential energy surfaces responsible for such assembly formation, the location and intensity of local energy maxima and minima depend on the particle configurations with respect to the input beam polarization and Poynting vector. This paper reports the results of recent quantum electrodynamical studies on the energy landscapes for systems of three and more particles; the analysis of local minima allows determination of the energetically most favorable positions, and it shows how the addition of further particles subtly modifies each energy landscape. The analysis includes the identification and characterization of potential points of stability, as well as the forces and torques that the particles experience as a consequence of the throughput electromagnetic radiation. As such, the development of theory represents a rigorous and general formulation paving the way towards a fuller comprehension of nanoparticle assembly based on optical binding.

CdS nanowires and silica microspheres are manipulated using optical traps into 1D and 2D structures. The bonding occurs through the use of biotin and streptavidin.

Techniques to measure the trapping force in an optical tweezers without any prior assumptions about the trap shape have been developed. The response of a trapped micro or nanoparticle to a step input is measured and then used to calculate the trapping force experienced by the particle as a function of it's position in the trap. This method will provide new insight into the trapping behavior of nanoparticles, which are more weakly bound than microparticles and thereby explore larger regions of the trapping potential due to Brownian motion. Langevin dynamics simulations are presented to model the system and are used to demonstrate this technique. Preliminary experimental results are then presented to validate the simulations. Finally, the measured trapping forces, from simulations and laboratory experiments, are integrated to recover the trapping potential.

In live sciences, the observation and analysis of moving living cells, molecular motors or motion of micro- and nano-objects is a current field of research. At the same time, microfluidic innovations are needed for biological and medical applications on a micro- and nano-scale. Conventional microscopy techniques are reaching considerable limits with respect to these issues. A promising approach for this challenge is nonlinear dynamic phase contrast microscopy. It is an alternative full field approach that allows to detect motion as well as phase changes of living unstained micro-objects in real-time, thereby being marker free, without contact and non destructive, i.e. fully biocompatible. The generality of this system allows it to be combined with several other microscope techniques such as conventional bright field or fluorescence microscopy. In this article we will present the dynamic phase contrast technique and its applications in analysis of micro organismic dynamics, micro flow velocimetry and micro-mixing analysis.

We demonstrate stable three-dimensional optical trapping of 780nm silica particles using a dispersion-compensated 12.9fs infrared pulsed laser and a trapping microscope system with 1.40NA objective. To achieve these pulse durations we use the Multiphoton Intrapulse Inteference Phase Scan (MIIPS) method to compensate for the significant temporal dispersion introduced by the trapping system. We demonstrate orders of magnitude reduction in pulse duration at the sample, and a dramatic increase in the efficiency of multiphoton excitation at the sample. The use of dispersion-compensated ultrashort pulses will therefore be a valuable tool for enhancing non-linear processes in optically trapped particles. In addition, ultrashort pulses can allow the use of pulse shaping to control these nonlinear processes, yielding the possibility of advanced applications using coherent control of trapped particles.

One of the fundamental goals in biology is to understand the interplay between biomolecules of different cells. This happen, for example, in the first moments of the infection of a vector by a parasite that results in the adherence to the cell walls. To observe this kind of event we used an integrated Optical Tweezers and Confocal Microscopy tool. This tool allow us to use the Optical Tweezers to trigger the adhesion of the Trypanosoma cruzi and Trypanosoma rangeli parasite to the intestine wall cells and salivary gland of the Rhodnius prolixus vector and to, subsequently observe the sequence of events by confocal fluorescence microscopy under optical forces stresses. We kept the microorganism and vector cells alive using CdSe quantum dot staining. Besides the fact that Quantum Dots are bright vital fluorescent markers, the absence of photobleaching allow us to follow the events in time for an extended period. By zooming to the region of interested we have been able to acquire confocal images at the 2 to 3 frames per second rate.

We develop the analysis technology for evaluating the effect of medication using living cells. This proposed system consists of two elemental technologies. One is the 3-D component-distribution measurement, another one is the micro-probe operation for local dosing. In this measurement technology, the cell is rotated by light pressure for obtaining the 2-D images from multiple directions. And the 3-D component distribution is reconstructed from these 2-D images. The micro-probe operation that is actuated by light pressure can control the position and attitude of micro-ample. The opposed type beams illuminate the cell at the proximal two points. In this case, the light pressure can act as the rotating torque. And, we can successfully obtain the 3-D phase-contrast images. Furthermore, by operating the distance between two focal points, we can control the attitude of bacillary micro-probe. In addition, we describe the separate measurement of viscous force and elastic force for the cell palpation. This method focuses attention on the surface effect that acts as the dominant factor rather than the volume effect for microscope. Thus, we construct the dynamical model that considers about the viscous-force by the biological cells and the viscous force by the nutrient medium. And these forces are separately calculated from the measured vibration amplitude.

Janus particles are composed of two fused hemispheres of different substances. Such an anisotropic structure results in different trapping characteristics under a focused laser beam. In this paper, we show the axial and transversal trapping forces of Janus particles under a focused laser beam and discuss the conditions that would result in stable trapping. We also show the performed experiments and the applied numerical simulations using the Finite Difference Time Domain method.

Optical forces and torques acting on microscopic objects trapped in focussed laser beams promise flexible methods of driving micromachines through a microscope cover slip or even a cell wall. We are endeavouring to engineer special purpose micro-objects for a range of tasks. Colloidal self assembly of calcium carbonate provides birefringent spheres which can exert considerable torque, while two photon polymerisation allows us to fabricate objects of arbitrary shape that can be designed to exchange both spin and orbital angular momentum. Numerical calculations of forces and torques can allow an optimal design, and optical measurements provide us with certain knowledge of the forces and torques which are actually exerted.

Two-photon polymerization of optically curing resins is a powerful method to fabricate micron sized objects which can be used as tools to measure properties at small scales. These microdevices can be driven by means of externally applied focused laser beams (optical tweezers) through angular momentum exchange, giving rise to a net torque. The advantage of the optical drive is that no contact is required, therefore making the microdevices suited to non-invasive biological applications. The fabrication method is versatile and allows building objects of any 3D shape. We discuss the design and modelling of various optically driven rotors. In particular, we consider fabrication of microspheres with an internal shape birefringence in order to obtain rotation in an optical trap. The reason for fabricating this type of object is that they are well-suited for studies of mechanical properties of single biomolecules such as the torsional stiffness of DNA or torque generated by molecular motors. The microspheres fabricated are able to transduce torques of 2000 pNnm with optical powers of 500 mW and could be rotated with frequencies up to 40 Hz in circularly polarized light.

Optical manipulation of nano- and micro-scale particles via optical tweezers and optical landscapes continues to be of great interest in several fields, reflected by the myriad experimental pursuits suggesting selective and parallel control over particles of anisotropic shape (blood cells, nanorods, etc.). Our work here approaches the goal of a complete model of these phenomena by means of optical scattering principles and, specifically, the T-matrix method. Here we describe the salient features of our model, which tends toward a complete and consistent modeling scheme for determining the behavior of dielectric, polarizable, mesoscale particles of anisotropic shape in arbitrary intensity gradients. We explore forces and torques caused by periodic optical landscapes as well as torques induced by the polarization orientation of the electric field.

We describe and analyze experiments, where optical manipulation of small colloidal particles in the nematic liquid crystal (NLC) was used to create artificial colloidal structures, such as 1D chains and 2D colloidal crystals, and superstructures of different types of colloids. In all cases, the colloidal particles are strongly bound to each other, with a typical pair interaction energy of several 1000 k_BT per 1μm size particle. There are two distinct mechanisms of colloidal binding in a spatially homogeneous NLC: (i) binding via spatially localized topological (point) defects, and (ii) binding via entangled topological defects, where the defect line winds around and wraps several colloidal particles.

We present a parallel calibration technique especially designed to evaluate the potential well spatial shape of multiple holographic optical tweezers. The basic principle is to extend the conventional unidirectional drag force method to a multi-axes one, associated with image processing. We then focus on the characterization of anisotropic distorted optical potential wells of holographic optical tweezers, where the distortion comes from optical aberrations and/or effects due to nearby ghost or regular traps. We also address the dependence of the diffraction efficiency with the trap position in a given traps pattern.

We report the realization of a new iterative Fourier-transform algorithm for creating holograms that can diffract light into an arbitrary two-dimensional intensity profile. We show that the predicted intensity distributions are smooth with a fractional error from the target distribution at the percent level. We demonstrate that this new algorithm outperforms the most frequently used alternatives typically by one and two orders of magnitude in accuracy and roughness, respectively. The techniques described in this paper outline a path to creating arbitrary holographic atom traps in which the only remaining hurdle is physical implementation.

Holographic optical tweezers permit the simultaneous control of multiple optical traps. In this paper we examine the use of such systems for the purposes of micromanipulation and assembly of microstructured materials. To this end, optically induced forces and torques on a variety of objects are evaluated using numerical and semi-analytical methods. In the following paper we describe implementations of these methods (the finite difference time domain and T-matrix methods respectively) and present some salient results before concentrating on a particular application that involves the use of entropic forces to promote aggregation between microspheres.

The generation of optical landscapes may be regarded as a communication system: A user sends information to a designated region in space where the information is represented using optical parameters like intensity. Information capacity has been used for understanding superresolution in optical systems and we adapt this concept to gain useful insights for characterizing techniques for generating optical landscapes. In particular, we investigate the information capacity of the generalized phase contrast method and computer-generated holography. We also consider the invariance of information capacity and discuss its implications for the generation of dynamic optical landscapes.

Many types of optical tweezers arrays have been proposed and developed for use in conjunction with microfluidics for bio-chemical essays. Trap arrays rely on different methods allowing various degrees of flexibility and relative trapping efficiencies. Among the different techniques currently employed, it is not simple to distinguish which ones produce adequate performances for a given task in bio-chemistry. Experimental results for trapping efficiently diverse biological specimens allow distinguishing between the properties of optical trap arrays based on techniques as different as interferometry, holography, Fresnel or Fraunhoffer diffraction of diffractive structures, generalized phase contrast, microlens assemblies, micro-mirrors matrices, and also clusters of individual tweezers. The bulkiness of those systems is another important factor in the design of labs-on-a-chip; in particular the use of cumbersome microscope objectives can be detrimental to chip optimization. Arrangements of tweezers produced with different concepts should be compared in terms of efficiency, ease of use, and number of traps simultaneously exploitable

Optical tweezers are a relatively new technique for non-invasive manipulation of micro particles. It has applications in different scientific and technological areas, such as: cell and molecule sorting, nano device assembly, and analysis of bio motors. Standard methods to create optical tweezers rely on using either the manipulation of a single beam or a spatial light modulator. Here a new optical tweezer design is presented that employs concepts from the realm of adaptive optics. The prototype system employs a deformable mirror to control the 3D position of the traps. A specific closed-loop control algorithm, adapted from adaptive optics technology allows a real-time precise monitoring of the deformable mirror shape and provides exquisite accuracy in trap placement. The first results obtained with the prototype design are presented.

Large deformations can easily be introduced in liquid microdroplets by applying relatively small external forces or controlling the evaporation/condensation kinetics. This makes liquid microdroplets attractive to serve as the building blocks of largely tunable optical switches or filters that are essential in optical communication systems based on wavelength division multiplexing. Solid optical microcavities have not found large use in these applications, mainly due to their rigid nature. The fact that liquid microdroplets are low-cost and disposable can also prove to be important in mass production of these photonic devices. Here, we show that local heating with an infrared laser can be used to largely tune the whispering gallery modes (WGMs) of water/glycerol or salty water microdroplets standing on a superhydrophobic surface. In the scheme presented, a liquid microdroplet kept in a humidity chamber is stabilized on a superhydrophobic surface, and an infrared laser beam is focused near the center of the microdroplet. As a result of the local heating, the temperature of the liquid microdroplet increases, and the water content in the liquid microdroplet evaporates until a new equilibrium is reached. At the new equilibrium state, the non-volatile component (i.e. glycerol or salt) attains a higher concentration in the liquid microdroplet. We report tunability over large spectral ranges up to 30 nm at around 590 nm. For salty water microdroplets the reported spectral tuning mechanism is almost fully reversible, while for the case of glycerol/water microdroplets the spectral tuning mechanism can be made highly reversible when the chamber is saturated with glycerol vapor and the relative water humidity approaches unity.

Droplet microfluidics is an emerging area in miniaturisation of chemical and biological assays, or "lab-on-a-chip" devices. Normally consisting of droplets flowing in rigid microfluidic channels they offer many advantages over conventional microfluidic design but lack any form of active control over the droplets. We present work, using holographic beam shaping, that allows the real time reconfigurability of microfluidic channels allowing us to redirect, slow, stop, and merge droplets with diameters of approximately 200 microns. A single beam is be sufficient to perform simple tasks on the droplets but by using holographic beam shaping we can produce multiple foci or continuous patterns of light that enable a far more versatile tool.

Optoelectronic Tweezers (OET) creates patterned electrical fields by selectively illuminating a photoconductive layer sandwiched between two electrodes. The resulting electrical gradients are used to manipulate microscopic particles, including biological cells, using the dielectrophoresis (DEP) force. Previously it has been shown that up to 15,000 traps can be created with just 1 mW of optical power¹, and that OET traps are 470 times stiffer than traps created with optical tweezers of the same power². In this paper we explore the use of OET for trapping HeLa cells. First, experiments are performed using glass beads as a model particle, and the results are compared with numerical simulations to confirm our ability to model the electrical field gradients in the OET device. We then track trapped HeLa cells in different sizes of traps, showing maximum cell velocities of 60 μm s^-1 using an illumination intensity of just 2.5 W cm^-2. We measure the electrical properties of the cell's membrane by analyzing the cell's DEP frequency response and use this information to model the forces on the cell. We find that it is possible to create a trap with a stiffness of 3×10^-6 N m^-1 that does not vary with position within the trap.

Optical trapping is a flexible and noninvasive technique that allows for the manipulation of single dielectric particles. Conventional single and multiple beam laser traps however, are limited by the amount of trapping sites that can be embedded in their wave field with a sufficiently high intensity gradient. We make use the interference of multiple beams and total internal reflection to couple an extended evanescent optical field to a large number of particles in a 2-D periodical landscape. The particles are confined and manipulated by modifying the spatial parameters of the landscape. We ultimately intend to use this technique for the parallel fusion of multiple pairs of microscopic droplets to investigate the dynamics of micro-reactions.

This study has observed microscopic level cavitation processes in shelled second generation ultrasound contrast agent microbubbles. The spatial and temporal resolutions required for this undertaking have been achieved via a unique hybridisation of optical trapping with ultra high speed microphotography. Upon insonation with ultrasound in the region of 0.5-4MPa, microjets were observed to develop within, and subsequently issue from, cavitating bubbles. Jet impact into target substrates, including monolayers of biological cells, was observed. These observations provide direct evidence for the involvement of microjetting events during ultrasound exposure on live cells, a process that may have future potential as a novel non-invasive route to drug- and gene-based therapies.

Aerosols play a crucial role in many areas of science, ranging from atmospheric chemistry and physics, to pharmaceutical aerosols and drug delivery to the lungs, to combustion science and spray drying. The development of new methods for characterising the properties and dynamics of aerosol particles is of crucial importance if the complex role that particles play is to be more fully understood. Optical tweezers provide a valuable new tool to address fundamental questions in aerosol science. Single or multiple particles 1-15 μm in diameter can be manipulated for indefinite timescales. Linear and non-linear Raman and fluorescence spectroscopies can be used to probe particle composition, phase, component mixing state, and size. In particular, size can be determined with nanometre accuracy, allowing accurate measurements of the thermodynamic properties of aerosols, the kinetics of particle transformation and of light absorption. Further, the simultaneous manipulation of multiple particles in parallel optical traps provides a method for performing comparative measurements on particles of different composition. We will present some latest work in which optical tweezers are used to characterise aerosol dynamics, demonstrating that optical tweezers can find application in studies of hygroscopicity, the mixing state of different chemical components, including the phase separation of immiscible phases, and the kinetics of chemical transformation.

We present results describing the behavior of optically trapped airborne particles, both solid and liquid. Using back focal plane interferometry we measure characteristic power spectra describing the position fluctuations within the trap. We show it is easy to transfer between an over and under damped regime by either varying the trapping power or the distance into the medium the beam is focused. The results assist in the understanding of airborne tweezers and it is hoped having under damped systems could lead to exploring analogies in many areas of fundamental physics.

Aerosol tweezing with a super-continuum laser source has been successfully demonstrated. Salt-water droplets in the range between 3 and 7 microns in diameters are trapped with a 300nm-wide super-continuum spectrum. As the spectrum covers a few Mie resonances, the optical force is averaged and the trapping efficiency varies smoothly with the square of the radius as in the case of the ray optics approximation. On-axis elastically back-scattered spectrum allows a direct and precise determination of the trapped droplet. Evaporation of a single droplet is precisely followed using this method. Alternative spectroscopic droplet sizing techniques are proposed and discussed.

With the recent development of microfabrication technology, the measurement technology to evaluate geometric quantities is demanded to assure their accuracy. In order to measure the 3D shape of these microcomponents, a novel nano-CMM system has been developed based on an oscillated probing technique, which uses an optically trapped particle. The particle as a probe is trapped by focused laser light using an objective in the air. The trapped particle is laterally oscillated or circularly at the focal plane of the objective using AOD (acousto-optical deflector). The motion of the trapped particle is induced by a trapping force toward a focal spot and damped by the viscosity of the surrounding atmosphere. The frequency response of the oscillated particle typically agrees with the spring-mass-damper model. On the other hand the response disagrees with the theoretical curve of the model at high frequency range, i.e. 4.6% at 4000 Hz. It is considered the difference is caused from the numerical error for the fluid effect, which is given by the stokes formula 6πηr In this report, we construct a fluid simulation using SMAC method that calculates fluid resistance against an oscillating sphere in noninertial frame of reference. The fluid effect is investigated in order to improve the model of the sphere motion. 2D simulation indicates the same tendency in frequency response of the oscillating sphere with amplitudes of 500 nm in 100-4000 Hz frequency range. 3D simulation could improve the measurement accuracy of nano-CMM system as compared with 2D simulation.

The correct form of electromagnetic wave momentum in matter has been debated in the literature for over a century, with the main candidates being the Minkowski and the Abraham formulations. Recently, a third momentum expression has been proposed, averaging the previous two, and has been argued to be the resolution of the debate. In this paper, we revisit the various formulations and show that the debate has been carried out on a wrong platform: the question is not which momentum expression is right and which is wrong, but which is measurable and which is not. In that regard, experiments have been overwhelmingly in favor of the Minkowski form, which, however, does not discredit the Abraham form as a theoretical concept. The third form of momentum, averaging the previous two, is here argued to be merely the result of a mathematical exercise, which physical assumptions need to be revisited.

The macroscopic equations of Maxwell combined with a generalized form of the Lorentz law are a complete and consistent set; not only are these five equations fully compatible with the special theory of relativity, they also conform with the conservation laws of energy, momentum, and angular momentum. The linear momentum density associated with the electromagnetic field is Ρ_EM(r,t)=E(r,t)×H(r,t)/c², whether the field is in vacuum or in a ponderable medium. [Homogeneous, linear, isotropic media are typically specified by their electric and magnetic permeabilities ε_ο ε(ω) and μ_ομ(ω).] The electromagnetic momentum residing in a ponderable medium is often referred to as Abraham momentum. When an electromagnetic wave enters a medium, say, from the free space, it brings in Abraham momentum at a rate determined by the density distribution Ρ_EM(r,t), which spreads within the medium with the light's group velocity. The balance of the incident, reflected, and transmitted (electromagnetic) momenta is subsequently transferred to the medium as mechanical force in accordance with Newton's second law. The mechanical force of the radiation field on the medium may also be calculated by a straightforward application of the generalized form of the Lorentz law. The fact that these two methods of force calculation yield identical results is the basis of our claim that the equations of electrodynamics (Maxwell + Lorentz) comply with the momentum conservation law. When applying the Lorentz law, one must take care to properly account for the effects of material dispersion and absorption, discontinuities at material boundaries, and finite beam dimensions. This paper demonstrates some of the issues involved in such calculations of the electromagnetic force in magnetic dielectric media.

It is demonstrated that waveguide eigenmodes with a rotating phase may exert a longitudinal force, positive or negative, as well as a torque on the guiding structure or part of it. A general formulation of the linear and angular momentum currents flowing in the waveguide is given. Several examples are considered, including a lossy dielectric cylinder bounded inside a hollow waveguide, and a lossy dielectric fiber. The results of this study may be used for a novel type of light driven rotating machines.

Singular optical beams have been studied for many years after the pioneering work where the wave function of the laser radiation is presented as a steady-state solution of the wave equation for a harmonic oscillator. A major step in understanding the nature of singular beams has been made by introducing the concept of the angular momentum of light and analyzing local energy transfer in a vortex beam. It is now well accepted that the orbital angular momentum of light is an intrinsic feature of the optical vortex. However, the orbital angular momentum was always analyzed for travelling modes and the important issue of the orbital angular momentum associated with standing waves still remains open. The main motivation of our work is to reveal the structure of the orbital angular momentum in a standing wave formed by the counter-propagating optical vortices and study its suitability for an optical trapping and guiding. In this work we show that a superposition of two (or more) vortex beams generates a field structure which has a form of a standing wave in both the radial and longitudinal directions, but it is rotating simultaneously along the tangential direction. We demonstrate that then field of this optical vortex structure could be used as an optical trap and simultaneously transfer the angular momentum of the electromagnetic wave to an object inside the area of vortex localisation. We believe this study provides a basis for developing a novel concept of three-dimensional optical traps where vortices could be created in a local volume by a direct transfer of the angular orbital momentum of the electromagnetic wave to trapped objects.

We have developed a magneto-optic tweezers that offer new experimental possibilities when laser tweezers were traditionaly used. The magneto-optic tweezers combine a multi-trap optical tweezers based on acousto optic deflectors and homogeneous magnetic field which direction and magnitude can be time modulated in arbitrary fashion. Superparamagnetic beads that are readily available from several commercial sources are used as trap handles. They can be manipulated using optical tweezers in a well known way. By applying magnetic field additional repulsive or attractive interaction between the particles can be induced, giving rise to new micromanipulation possibilities. Several examples of how magneto-optic traps can be used in colloidal physics reasearch and potential applications in biophysics and microfluidic systems are presented.

Using a Fresnel zone plate, we demonstrate optical trapping with a larger numerical aperture than is commonly available with commercial objective lenses. The zone plate is fabricated onto the inner wall of the fluidic cell and, consequently, focusing is free from on axis aberrations due to an absence of dielectric interfaces. Using zone plates with extremely large focusing angles, we observe an enhanced ellipticity in the trapping volume. For a zone plate with a numerical aperture of 0.986n_water (1.308), we observe a trapping stiffness that is more than four times stiffer perpendicular to the polarization than parallel to the polarization. By rotating the incident linear polarization state, the trapping stiffness along a given direction can be modulated by a factor of four. The ellipticity in the focal volume is due to the presence of an axial field component whose magnitude is proportional to the sine of the focusing angle of the lens.

Feedback enhanced optical tweezers, based on Proportional and Integral (PI) control, are routinely used for increasing the stiffness of optical traps. Digital implementation of PI controller, using DSP or FPGA, enables easy maneuverability of feedback gains. In this paper, we report occurrence of a peak in the thermal noise power spectrum of the trapped bead as the proportional gain is cranked up, which imposes a limit on how stiff a trap can be made using position feedback. We explain the reasons for the deviant behavior in the power spectrum and present a mathematical formula to account for the anomaly, which is in very good agreement with the experimental observations. Further, we present a new method to do the closed loop system identification of feedback enhanced optical tweezers by applying a frequency chirp. The system model thus obtained greatly predicts the closed loop behavior of our feedback based optical tweezers system.

Optical tweezers constitute an obvious choice as the experimental technique for manipulation and trapping of organelles in living cells. For quantitative determination of the forces exerted in such in vivo systems, however, tools for reliable calibration of the optical tweezers are required. This is complicated by the fact that the viscoelastic properties of the cytoplasm are a priori unknown. We elaborate on a previously reported theoretical calibration procedure and verify its authenticity experimentally. With this approach, we may at the same time determine the trapping characteristics of the optical tweezers and the viscoelastic properties of the cytoplasm. The method employs the fluctuation-dissipation theorem (FDT) which is assumed valid for the situations considered. This allows for extracting the requested properties from two types of measurements that we denote as passive and active. In the passive part, the Brownian motion of a particle inside the trap is observed. In the active part, the system is slightly perturbed and the response of the trapped particle is tracked. Gently oscillating the stage on which the sample is mounted allows the delay between the position of the stage and the response of the trapped bead, using a quadrant photodiode, to be quantified. No assumptions about the particle radius or geometry or about the frequency-dependent friction coefficient are needed. The paper contains the theoretical background of the method in terms of convenient formulations of the fluctuation-dissipation theorem and application of the method in two types of experiments. Further we discuss experimental concerns which are i) the choice of driving characteristics in the active part of the calibration procedure and ii) statistical errors.

We present an experimental demonstration of a method using optical tweezers proposed by Fischer and Berg-Sorensen for measuring viscoelasticity using optical tweezers. It is based on a sinusoidal oscillation of the liquid in combination with force measurements using optical tweezers. We verify the method by applying it to measurements in water, glycerol and polyethylene oxide (PEO).

We propose a technique for the characterization of a 1D-periodic optical potential by studying the dynamics of non-brownian microscopic particles immerse in water (negligible thermal noise). It has been demonstrated that in the Mie regime, a periodic light pattern applied to a particle acts as an effective potential that depends on the size of the particle respect to the period of the optical landscape [I. Ricardez-Vargas, et.al. Appl. Phys. Lett. 88, 121116 (2006)]. We verify this fact by studying the dynamics of a particle moving within the pattern due to the effect of a known constant external force. The periodic light pattern is generated with interference techniques whereas the external force is applied by means of a controlled inclination of the sample cell. We fit the experimental results for the ensemble average of particle position against time with a theoretical model of the physical situation. In this way we obtain a curve for the optical force as a function of particle's position for different periods.

Optical trapping, mixing, and sorting of micro- and nano-scale particles of arbitrary shape (e.g., blood cells and nanorods) are but a few of the burgeoning applications of optical interference landscapes. Due to their non-invasive, non-contact manipulation potential, biologists and nanotechnologists alike are showing increased interest in this area and experimental results continue to be promising. A complete and reliable theoretical description of the particles' response within these fields will allow us to accurately predict their behavior and motion. We develop an electrostatic model of the optical force and torque on anisotropic particles in optical intensity gradients. The complete optical field is defined and a Maxwell stress tensor approach is taken to realize the force and torque induced by the electric field due to the polarizability of the particle. We utilize the properties of real dielectrics and steady state optical fields to extend this approach to the electrodynamic case inherent in optical trapping. We then compare our results against our recently reported form factor approach and use the differences to try to determine the importance of polarizability in optical trapping.

The use of spatial light modulators to generate arbitrary optical field distributions has been extensively used to trap and manipulate dynamically a large number of particles. Here we show that by using phase computer generated holograms displayed on a spatial light modulator (SLM) sorting of microparticles can be achieved at relatively low power. The algorithm used for the generation of the PCGH is based on iterative Fourier transform algorithm which generate a spots array in the Fourier plane, then controlling some parameters as: the spot separation, the direction and velocity of the pattern displacement, optical sorting of micron-sized particles can be achieved.

The dual beam fiber trap is an important tool in the field of optical micromanipulation. The characteristics of these traps are governed by the fibers used. Photonic crystal fibers have emerged in recent years and may be engineered to have vastly different properties to conventional fibers. In particular, endlessly single-mode photonic crystal fiber (ESM-PCF) will guide any wavelength of light in a single-mode and is commercially available in core diameters up to 35 μm. By utilising these unique properties, we show that it is possible to create novel dual beam ESM-PCF traps for micron size particles. Firstly, we characterise an ESM-PCF trap when using a near-infrared laser coupled into a 25 μm core fiber when trapping a sample in a square capillary. We calculate the trap stiffness for polymer micro-spheres and show that aside from the expected confining potential, it is possible to create line and repulsive potentials. Interference effects due to the capillary are observed. Secondly, we create a dual wavelength standing wave trap which can selectively move two sizes of particles in an optical conveyor belt. Finally, we use a supercontinuum source to create the first white light dual beam fiber trap and show that the low coherence length of the source results in interference free potentials. Overall PCF has great promise for future studies.

The positioning accuracy when a phase-only one dimensional spatial light modulator (SLM) is used for beam steering is limited by the number of pixels and their quantized phase modulation. Optimizing the setting of the SLM pixels individually can lead to the inaccuracy being a significant fraction of the diffraction limited spot size. This anomalous behaviour was simulated numerically, and experiments showed the same phenomena with very good agreement. However, by including an extra degree of freedom in the optimization of the SLM setting, we show that the accuracy can be improved by a factor proportional to the number of pixels in the SLM.

In an optical trap, micron-sized dielectric particles in aqueous solutions can be held by a tightly focused laser beam. The optical force on the particle is composed of an attractive gradient force and a destabilizing scattering force. To optimize the trapping potential, we reduced the scattering force by using coated microspheres. The shell of the particle was designed such that it acts as an anti-reflection coating. We made and characterized such particles and found that in comparison with the uniform microspheres of the same diameter a more than two-fold stiffening of the trap. Compared to larger spheres, we achieved an increase in trap stiffness of up to 10-fold. These results quantitatively agree with our calculations based on the generalized Lorenz-Mie theory. By improving the trapping potential higher overall forces can be achieved with the same laser power, or vice versa the same force can be reached by using less laser power. A higher maximal force increases the range of possible experiments, and a reduced laser intensity leads to less photo-toxic interactions or laser heating relevant for biological applications.