The majority of mechanisms that can be deployed for optical micromanipulation are not especially amenable for extension into the nanoscale. At the molecular level, the rich variety of schemes that have been proposed to achieve mechanical effect using light commonly exploit specific chemical structures; familiar examples are compounds that can fold by cis-trans isomerization, or the mechanically interlocked architectures of rotaxanes. However, such systems are synthetically highly challenging, and few of them can realistically form the basis for a true molecular motor. Developing the basis for a very different strategy based on programmed electronic excitation, this paper explores the possibility of producing controlled mechanical motion through optically induced modifications of intermolecular force fields, not involving the limitations associated with using photochemical change, nor the high intensities required to produce and manipulate optical binding forces between molecules. Calculations reveal that significant, rapidly responsive effects can be achieved in relatively simple systems. By the use of suitable laser pulse sequences, the possibilities include the generation of continuous rotary motion, the ultimate aim of molecular motor design.

The implementation of high instantaneous peak power of a femtosecond laser pulse at moderate time-averaged power (~10 mW) to trap latex nanoparticles, which is otherwise impossible with continuous wave illumination at similar power level, has recently been shown [De, A. K., Roy, D., Dutta, A. and Goswami, D. "Stable optical trapping of latex nanoparticles with ultrashort pulsed illumination", Appd. Opt., 48, G33 (2009)]. However, direct measurement of the instantaneous trapping force/stiffness due to a single pulse has been unsuccessful due to the fleeting existence (~100 fs) of the laser pulse compared with the much slower time scale associated with the available trapping force/stiffness calibration techniques, as discussed in this proceeding article. We also demonstrate trapping of quantum dots having dimension similar to macromolecules.

Optical tweezers are well-known tools for the mechanical manipulation of single molecules in aqueous solutions. Here I will discuss a new development - the combination of optical tweezers with solid-state nanopores. Nanopores are holes in thin membranes usually a few 10s of nm in diameter or even with single nm diameters. In aqueous solutions an ionic current can be driven through a nanopore and thus the translocation of a single molecule detected. Although this information can be used to characterize the length and charge of the molecules, there is no information about the force or position during this process. I will discuss how optical tweezers can be used to mechanically control the translocation process, what we learned so far and where we are going with the technology. In particular, I will show that the optical tweezers/nanopore combination proved to be of exceptional value in unraveling the coupling between electrokinetic and hydrodynamic effects during voltage-driven translocation. This has implication for a wide range of applications ranging from gel electrophoresis to DNA manipulation for lab-on-the chip technology.

Friction limits the operation of macroscopic machines. Using optical tweezers, we showed that friction also limits the operation of molecular machines by measuring the friction between single yeast kinesin-8, Kip3p, and its microtubule track. The protein friction arises from the force necessary to break the adhesive bonds that Kip3p forms with discretely, 8-nm spaced binding sites on its track. A model based on bond rupture dynamics with a single energy barrier described the data. A uctuation analysis confirmed Kip3p stepping during diffusion. Here, we validate our experimental results and data analysis by a Monte Carlo simulation. Our data have implications for other molecular machines or actively driven proteins, and give further insight into diffusion of proteins along polymers such as microtubules or DNA.

Forces on the order of a hundred femtonewtons can drastically prevent the formation of protein-mediated DNA loops, which are a common regulatory component of cellular function and control. To investigate how such an acutely sensitive mechanism might operate within a noisy environment, as might typically be experienced within a cell, we have studied the response of DNA loop formation under an optically induced, fluctuating, mechanical tension. We show that mechanical noise strongly enhances the rate of loop formation. Moreover, the sensitivity of the loop formation rate to mechanical fluctuations is relatively independent of the baseline tension. This suggests that tension along the DNA molecule could act as a robust means of regulating transcription in a noisy in vivo environment.

Mechanical drift between an atomic force microscope (AFM) tip and sample is a longstanding problem that limits tipsample stability, registration, and the signal-to-noise ratio during imaging. We demonstrate a robust solution to drift that enables novel precision measurements, especially of biological macromolecules in physiologically relevant conditions. Our strategy - inspired by precision optical trapping microscopy - is to actively stabilize both the tip and the sample using locally generated optical signals. In particular, we scatter a laser off the apex of commercial AFM tips and use the scattered light to locally measure and thereby actively control the tip's three-dimensional position above a sample surface with atomic precision in ambient conditions. With this enhanced stability, we overcome the traditional need to scan rapidly while imaging and achieve a 5-fold increase in the image signal-to-noise ratio. Finally, we demonstrate atomic-scale (~ 100 pm) tip-sample stability and registration over tens of minutes with a series of AFM images. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip-sample interaction. This work extends atomic-scale tip-sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.

Three-dimensional position of optically trapped dielectric particles can be detected by measuring the back-focal plane interference pattern of incident and scattered fields. Time-domain surface current based near zone to far zone transformation was implemented to compute the interference pattern by a spherical scatterer under a focused Gaussian beam. Computed results are compared with experimental data for validations.

Optical traps are nowadays quite ubiquitous in biophysical and biological studies. The term is often used synonymously with optical tweezers, one particular incarnation of optical traps. However, there is another kind of optical trap consisting of two non-focused, counter-propagating laser beams. This dual-beam trap predates optical tweezers by almost two decades and currently experiences a renaissance. The advantages of dual-beam traps include lower intensities on the trapped object, decoupling from imaging optics, and the possibility to trap cells and cell clusters up to 100 microns in diameter. When used for deforming cells this trap is referred to as an optical stretcher. I will review several applications of such traps in biology and medicine for the detection of cancer cells, sorting stem cells, testing light guiding properties of retinal cells and the controlled rotation of cells for single cell tomography.

We have used the Maxwell stress tensor method to calculate the optical forces acting upon a dielectric nanosphere in the proximity of gold nanoantenna structure optically excited by a plane wave. We have explored the dependence of optical forces for the full range of excitation angles with the conclusion that the maximum force occurs for the excitation at critical angle. The large force at this angle is, however, at the expense of greatly increased intensity in the volume of the particle from which we conclude that the important measure for the trapping efficiency in the case of plasmonic nanostructures is not the incident intensity of the plane wave, but rather the local intensity averaged over the volume of the particle. Our calculations further show multiple trapping sites with similar trapping properties, which leads to uncertainty in the trapping position. Furthermore, our calculations show that the heating effects might play a significant role in the experimentally observed trapping.

We present direct observation of particle transfer and assembling upon laser irradiation under a microscope. We employed gold nanoparticles (60 nm) dispersed in water as optical markers and studied laser trapping and accompanying phenomenon by wide-field Rayleigh scattering microscopy. At the focal spot of the near IR laser, laser trapping of gold was observed. Simultaneously, we observed that the particle migration toward the focal spot from all the directions within several tens micrometer. We consider that thermocapillary effect due to laser heating can assist the particle migration from far away, resulting in concentration increase not only at the focal point but also near the surrounding area.

Plasmonic nanoparticles, typically gold and silver colloids, can be trapped by a highly focused Gaussian beam. The behavior of the particles in an optical trap, such as the alignment, stability and interaction between particles, depends on their plasmonic nature, determined by the correlation between the size, shape and material of the particles, and the wavelength and polarization of the trapping laser. For instance, an elongated nanoparticle aligns parallel to the polarization of a NIR trapping laser to minimize the optical potential energy. However, nanowires tend to align perpendicular to the polarization. A dimer of two isotropic nanoparticles in principle acts similar to a nanorod with its "long axis" (dimer axis) parallel to the laser polarization. These results are evidenced by dark-field scattering imaging and spectra, and agree well with discrete dipole approximation simulations of the near-fields around different nanostructures. Elongated nanoparticles, dimers and nanowires all rotate when the laser polarization is rotated. Irradiated under a circularly polarized laser, trapped objects spin spontaneously due to the transfer of angular momentum from the incident photons. The interaction between two gold nanoparticles in a dimer is complex because it involves the optical potential and the DLVO potential. The latter can be probed to some extent using dark-field scattering spectroscopy.

Optical trapping and manipulation of Au submicron-particles using a holographic tweezer (multiple vortex tweezer) based on optical multiple vortex involving several phase singularities in a wavefront was presented.

Individual colloidal quantum dots can be optically trapped and manipulated by a single infrared laser beam operated at low laser powers.^{1, 2} If the absorption spectrum and the emission wavelength of the trapping laser are appropriately chosen, the trapping laser light can act as a source for two-photon excitation of the trapped quantum dot. This eliminates the need for an additional excitation laser in experiments where individual quantum dots are used both as force transducers and for visualization of the system. To use quantum dots as handles for quantitative optical force transduction, it is crucial to perform a precise force calibration. Here, we present an Allan variance analysis³ of individual optically trapped quantum dots and show that the optimal measurement time for experiments involving individual quantum dots is on the order of 0.3 seconds. Due to their small size and strong illumination, quantum dots are optimal for single molecule assays where, optimally, the presence of the tracer particle should not dominate the dynamics of the system. As an example, we investigated the thermal fluctuations of a DNA tether using an individual colloidal quantum dot as marker, this being the smallest tracer for tethered particle method reported.

We report on the dynamics of micro-photoluminescence of single InP semiconductor nanowires trapped in a gradient force optical tweezers. Nanowires studied were of zinc blende, wurtzite or mixed phase crystal poly-types and ranged in length from one to ten micrometers. Our results show that the band-edge emission from trapped nanowires exhibits a quenching of the initial intensity with a characteristic time scale of a few seconds and an associated spectral red shift is also observed in the mixed phase nanowires.

We present a versatile technique that enhances the axial stability and range in counter-propagating (CP) beam-geometry optical traps. It is based on computer vision to track objects in unison with software implementation of feedback to stabilize particles. In this paper, we experimentally demonstrate the application of this technique by real-time rapid repositioning coupled with a strongly enhanced axial trapping for a plurality of particles of varying sizes. Also exhibited is an interesting feature of this approach in its ability to automatically adapt and trap objects of varying dimensions which simulates biosamples. By working on differences rather than absolute values, this feedback based technique makes CPtrapping nullify many of the commonly encountered pertubations such as fluctuations in the laser power, vibrations due to mechanical instabilities and other distortions emphasizing its experimental versatility.

The counter-propagating geometry opens an extra degree of freedom for shaping light while subsuming single-sided illumination as a special case (i.e., one beam set turned off). In its conventional operation, our BioPhotonics Workstation (BWS) uses symmetric, co-axial counter-propagating beams for stable three-dimensional manipulation of multiple particles. In this work, we analyze counter-propagating shaped-beam traps that depart from this conventional geometry. We show that projecting shaped beams with separation distances previously considered axially unstable can, in fact, enhance the trap by improving axial and transverse trapping stiffness. We also show interesting results of trapping and micromanipulation experiments that combine optical forces with fluidic forces. These results hint about the rich potential of using patterned counter-propagating beams for optical trapping and manipulation, which still remains to be fully tapped.

Two counter-propagating Bessel beams are used to create an optical trap to confine polydisperse aerosol droplets. A single arm can be used to optically guide droplets over macroscopic distances. Two opposing beams create a trapping region to optically confine particles over distances of 4mm. Droplets are optically trapped in the surrounding rings and the central core and are characterised using light scattering techniques. The elastically scattered fringe spacing from the 532nm trapping beam and from a 633nm probe beam are used to independently size droplets using Mie theory, as well as assessing the size from glare spots.

Laser separation of particles is achieved using forces resulting from the momentum exchange between particles and photons constituting the laser radiation. Particles can experience different optical forces depending on their size and/or optical properties, such as refractive index. Thus, particles can move at different speeds in the presence of an optical force, leading to spatial separations. Several studies for aqueous suspension of particles have been reported in the past. In this paper, we present extensive analysis for optical forces on non-absorbing aerosol particles. We used a loosely focused Gaussian 1064 nm laser to simultaneously hold and deflect particles entrained in flow perpendicular to their direction of travel. The gradient force is used to hold the particles against the viscous drag for a short period of time. The scattering force simultaneously pushes the particles during this period. Theoretical calculations are used to simulate particle trajectories and to determine the net deflection: a measure of the ability to separate. We invented a novel method for aerosol generation and delivery to the flow cell. Particle motion was imaged using a high speed camera working at 3000+ frames per second with a viewing area up to a few millimeters. An 8W near-infrared 1064 nm laser was used to provide the optical force to the particles. Theoretical predictions were corroborated with measurements using polystyrene latex particles of 20 micron diameter. We measured particle deflections up to about 1500 microns. Such large deflections represent a new milestone for optical chromatography in the gas phase.

A variation on the typical optical chromatography system was used to measure optical force differentials of complex micro-particles that have been assembled or fabricated using bead chemistries, bio-molecule tethers, or biological bead coatings. A number of bio-inspired particle types have been created to help elucidate the origin of optical force differentials that are known or suspected in biological systems such as bacterial cells / spores, and mammalian cells. A number of optical force measurements will be presented for a variety of micro-fabricated particles and the results and capabilities discussed.

We demonstrate optical manipulation and sorting of micrometer-sized dielectric particles using one-dimensional periodic interference pattern created by interference of two beams in a sample space. These beams are generated by a combined phase grating applied on the spatial light modulator which allows to set dynamically the position and spatial period of the interference pattern. If a microparticle of fixed size is placed into such pattern, the optical forces acting upon it vary according to the spatial period of this optical lattice. We show how to use this property for sorting of mixtures of particles by moving either the interference pattern or the sample chamber. The mechanism is examined both theoretically and experimentally.

We present a theoretical model and the experimental demonstration of the rocking ratchet effect in the deterministic regime using an optical trapping device. Our system consists of a dielectric spherical particle in a 1D optical potential created by means of an interference pattern of asymmetric fringes. In order to achieve the asymmetry of the fringes, three light beams are interfered by pairs by controlling their relative polarization states, intensities and phases. A periodic time-dependent external force of zero average is introduced by moving the sample with respect to the optical pattern, for which the translation stage is driven sideways. The drag force acting on the particle due to this relative motion has the effect of tilting the optical potential periodically in opposite directions, providing the "rocking" mechanism. We show that an inversion of the asymmetry in the effective optical potential occurs as the size of the particle is varied, and therefore, we can observe opposite motion of different particles within the same optical pattern. The dynamics of the system is studied in terms of the different control parameters, such as the size of the particles, the period and asymmetry of the fringes, the amplitude and frequency of the rocking mechanism, and the power level in the sample.

Active contactless optical sorting of microobjects represents very useful technique in many areas of biology, chemistry, and medicine. We suggest here a configuration that combines optical sorting, trapping, excitation, and detection paths and provides efficient sorting of biological samples according to their various parameters (fluorescence, Raman spectrum, CCD image, motion etc.). This approach is based on the shape of the laser beam and we succeeded in sorting of several types of living microorganisms.

We studied experimentally and theoretically the formation of one-dimensional optically bound structures of polystyrene particles placed near the surface. These structures were created as the result of the illumination of a colloidal suspension by relatively wide Gaussian beam (beam waist 20 μm and wavelength 532 nm) that was reflected backwards with a tiny tilt with respect to the incident beam. We have measured quantitatively the binding forces between individual particles and compared the experimental results with the theoretical simulations based on the coupled dipole method (CDM).

In this paper, we detail two techniques for standing wave evanescent field optical trapping utilizing total internal reflection at a prism-water interface. Firstly, we describe an actively-locked cavity enhancement technique that generates circulating powers in excess of 10 W over an area of 150 μm x 75 μm on the prism surface using a 400 mW source, as well as providing control over the shape of the underlying transverse cavity mode. Secondly, we have combined an inverted optical tweezers with a counter-propagating evanescent wave trapping experiment, providing a useful platform for exploring light scattering interactions between small ensembles of particles. The resulting structures are compared to our theoretical model based upon Generalised Lorentz-Mie Theory.

We address the problem of the stochastic particle transitions between stable positions in a one-dimensional periodic potential profile. With respect to the experimental realization such stable positions are represented by the optical traps formed in a standing wave. The behavior of sub-micrometer sized particles in this "optical potential energy landscape" is analyzed theoretically and experimentally and the stress is put on the particles jumps between the neighboring optical traps. Our theoretical model assumes over-damped stochastic motion of a particle in a finite-depth potential well. Subsequently, Mean First Passage Time is utilized to express the new quantity called the Mean Optical Trap Escape Time (MOTET) that describes the mean time of the particle escape to a neighboring stable position (optical trap).

We developed a freely available interactive simulation of optical traps and their biological applications (phet.colorado.edu). The target audience is undergraduate majors as well as more advanced researchers. The simulation has three panels: optical traps, manipulating DNA, and measuring molecular motors. Each panel has options that allow students to interactively explore key physical ideas. For instance, viscosity can be turned off to see the critical aspect of dissipation, or time can be slowed down to see the oscillating electric field and the induced charge separation. An overview of the simulation and specific exercises suitable for an undergraduate class are discussed.

Optically trapped Brownian particles move under the effect of both the random thermal motion and the deterministic optical forces. They can, therefore, be a very powerful tool to study statistical physics phenomena, relying both on the presence of a natural noisy background and on a finely controllable deterministic force field. Here we will take a closer look to a few of these phenomena and to the insights that optical manipulations techniques have permitted us to gain.

The force field experienced by a sphere, trapped in a tightly focused Gaussian beam, is approximately conservative for small displacements. For lower symmetry systems, this is not generally the case. Even when very tightly trapped, a particle in such a system displays the effects of the non-conservative force field to which it is exposed. It does not come to thermal equilibrium, but reaches a steady state in which its stochastic motion is subject to a deterministic, cyclic bias. Here, we examine the dynamics of such a system, and show that the non-conservative nature of the force field manifests itself in both the covariance and the spectral density of the generalized coordinates of the particle. In addition, we show that the coupling between different types of thermal motion of such particles, i.e. rotational and translational, is asymmetric, which leads to the periodic bias to the motion. These points are illustrated through computational simulations of the Brownian dynamics of a trapped silica disk.

We computed the optical field scattered by a nanorod with the T-matrix approach and then the optical force and torque by the Maxwell stress tensor. Surface stress integration over the nanorod surface, including the side, top and bottom ends of the nanorod, has been performed to obtain the stresses on each section of the nanorod in order to understand the trap mechanism. The torque caused by beam orbital momentum has also been analyzed. The trap stability against the shift in position and orientation of the nanorod due to the natural Brownian motion are studied via computing the gradients of the optical force as a function of the size, length and tilt angle of the nanorod and the beam numerical aperture.

Having three distinct radii, ellipsoidal particles can be rigidly bound in Gaussian traps. The elongated intensity profile of the beam exerts forces that both confine, and orient the particle whilst the polarization of the beam provides a further orientational constraint. Consequently, the longest axis of the ellipsoid tends to align itself with the beam axis and the next longest with the polarization direction. In this article we examine the optical force fields experienced by ellipsoidal particles in Gaussian beams. The relationship between the general properties of these traps, especially their stability and stiffness, with particle shape is investigated.

Since a light beam can carry angular momentum (AM) it is possible to use optical tweezers to exert torques to twist or rotate microscopic objects. The alignment torque exerted on an elongated particle in a polarized light field represents a possible torque mechanism. In this situation, although some exchange of orbital angular momentum occurs, scattering calculations show that spin dominates, and polarization measurements allow the torque to be measured with good accuracy. This phenomenon can be explained by considering shape birefringence with an induced polarizability tensor. Another example of a shape birefringent object is a microsphere with a cylindrical cavity. Its design is based on the fact that due to its symmetry a sphere does not rotate in an optical trap, but one could break the symmetry by designing an object with a spherical outer shape with a non spherical cavity inside. The production of such a structure can be achieved using a two photon photo-polymerization technique. We show that using this technique, hollow spheres with varying sizes of the cavity can be successfully constructed. We have been able to demonstrate rotation of these spheres with cylindrical cavities when they are trapped in a laser beam carrying spin angular momentum. The torque efficiency achievable in this system can be quantified as a function of a cylinder diameter. Because they are biocompatible and easily functionalized, these structures could be very useful in work involving manipulation, control and probing of individual biological molecules and molecular motors.

Thermocavitation is a mechanism induced by a focused CW laser beam into a high absorbing solution. As a result an overheated region is created followed by explosive phase transition and consequently the formation of an expanding bubble. Once the bubble reaches a cooler region it collapses very rapidly crating a shock wave. Thermocavitation can be a useful tool for the generation of ultrasonic waves and controlled ablation with the important difference compared with pulsed lasers that low power lasers are required. In particular, the above mentioned pressure waves may be capable of producing damage to substrates, for example, in metallic and dielectric thin films. In this work, we present an application of the thermocavitation phenomena which consist in the formation of micro-holes on thin films of titanium and Indium Tin Oxide (ITO) deposited on glass substrate. The micro holes can be employed as a micrometer light sources or spatial filters.

Ultrasonic fields can be used to trap and manipulate micron-scale particles and second-phase fluids, utilising energy densities that do not impair cell viability. The technology can be seen as complementary to optical trapping as the size of the potential wells generated can be relatively large, making ultrasound suitable for the formation and manipulation of cell agglomerates, but less suitable for the manipulation of individual cells. This paper discusses physical phenomena associated with ultrasonic manipulation, including radiation forces, cavitation, and acoustic streaming. The technology is well suited to integration within "Lab on a Chip" devices and can involve excitation by plane, focussed, flexural, or surface acoustic waves. Example applications of resonators are discussed including particle filtration and concentration, cell washing, and biosensor enhancement. A recently developed device that uses both ultrasonic and magnetic forces to enhance the detection of tuberculosis bacteria using magnetic beads is discussed in detail. This approach uses ultrasonic levitation forces to overcome some of the issues associated with purely magnetic trapping. The technology has been implemented in a device in which the main fluidic components are disposable to allow for low production costs and improved control of biohazards.

A dual-trap optical tweezers is used for deforming the red blood cell (RBC) in suspension and studying its elasticity. The radiation force is applied directly to the cell without physical contact. The 3D radiation stress distribution was computed by ray tracing, the generalized Lorentz-Mie theory with the T-matrix and the FDTD via the Maxwell stress tensor. The 3D deformation of the cells was computed with the elastic membrane theory. The calculated deformation can fit to experimental data resulting in cell's elasticity coefficient. The static approach is valid only for small deformation (5- 10%). For a large deformation such as that of the RBC, we consider re-distribution of the radiation stress on the morphologically deformed cell. This stress re-distribution in turn induces subsequent deformation of the deformed cell and new stress re-distribution. The recursive process continues until a final equilibrium state is achieved. This iterative computation was implemented with the finite element method using the COMSOL^TM multi-physics models. The deformation results can fit to the experimental data for cell's deformation up to 20%.

Force measurements made with a translating holographic optical trap (HOT) of a viscous and a viscoelastic medium are investigated. In purely viscous media, Stokes drag cannot be measured with a translating HOT with established methods. In the viscoelastic system of the pericellular coat, the standard force curves generated by a fixed optical trap coupled with a moving stage can reliably be reproduced by translating HOT experiments. The viscoelastic cell coat provides an example where slow relaxation dynamics makes force measurements relatively insensitive to differences between measurements. These preliminary studies suggest that when the relaxation time scale of a system is much slower than the time scale of the HOT updates, translating HOTs can be reliably used to make force measurements on a viscoelastic, non-equilibrium system.

The composition of the cell membrane and the surrounding physiological factors determine the nature and dynamics of membrane-cytoskeleton coupling. Mechanical strength of a cell is mainly derived from such coupling. In this article, we investigate the effect of extra cellular cholesterol on the membrane-cytoskelaton connectivity of single cell endothelium and consequent remodeling of its mechanical properties. Using optical tweezers as a force probe, we have measured membrane stiffness (k_m), membrane microviscosity (η_eff ) and the two-dimensional shear modulus (G′(f)) as a function of extracellular cholesterol in the range of 0.1mM to 6mM. We find that membrane stiffness and shear modulus are dependent on cholesterol-induced membrane-cytoskeletal organization. Further, by disrupting the membranecytoskeletal connectivity with Cytochalasin D, an actin delpolymerizing molecule, we recover pure membrane behaviour devoid of any cytoskeleton attachment. However, behaviour of η_eff was found to be unaffected by disruption of membrane-cytoskeleton organization. We infer that cholesterol is playing a distinct role in modulating membrane organization and membrane-cytoskeleton connectivity independently. We further discuss implications of our approach in characterizing cellular mechanics.

Intracellular stresses generated by molecular motors can actively modify cytoskeletal network, which causes changes in intracellular mechanical properties. We study the out-of-equilibrium microrheology in living cells. This paper reports measurements of the intracellular mechanical properties using passive and optical tweezers-based active microrheology approaches and endogenous organelle particles as probes. Using the fluctuation-dissipation theorem, we compared the two approaches measurements and distinguished thermal and non-thermal fluctuations of mechanical properties in living cells.

Microbial biofilms are present on biotic and abiotic surfaces and have a significant impact on many fields in industry, health care and technology. Thus, a better understanding of processes that lead to development of biofilms and their chemical and mechanical properties is needed. In the following paper we report the results of active laser tweezers microrheology study of optically inhomogeneous extracellular matrix secreted by Visbrio sp. bacteria. One particle and two particle active microrheology were used in experiments. Both methods exhibited high enough sensitivity to detect viscosity changes at early stages of bacterial growth. We also showed that both methods can be used in mature samples where optical inhomogeneity becomes significant.

We demonstrate a multi-functional system capable of multiple-site two-photon excitation of photo-sensitive compounds as well as transfer of optical mechanical properties on an array of mesoscopic particles. We use holographic projection of a single Ti:Sapphire laser operating in femtosecond pulse mode to show that the projected three-dimensional light patterns have sufficient spatiotemporal photon density for multi-site two-photon excitation of biological fluorescent markers and caged neurotransmitters. Using the same laser operating in continuous-wave mode, we can use the same light patterns for non-invasive transfer of both linear and orbital angular momentum on a variety of mesoscopic particles. The system also incorporates high-speed scanning using acousto-optic modulators to rapidly render 3D images of neuron samples via two-photon microscopy.

We have developed a method that employs nanocapsules, optical trapping, and single-pulse laser photolysis for delivering bioactive molecules to cells with both high spatial and temporal resolutions. This method is particularly suitable for a cell-culture setting, in which a single nanocapsule can be optically trapped and positioned at a pre-defined location next to the cell, followed by single-pulse laser photolysis to release the contents of the nanocapsule onto the cell. To parallelize this method such that a large array of nanocapsules can be manipulated, positioned, and photolyzed simultaneously, we have turned to the use of spatial light modulators and holographic beam shaping techniques. This paper outlines the progress we have made so far and details the issues we had to address in order to achieve efficient parallel optical manipulations of nanocapsules and particles.

Forces experienced by colloidal particles in an AC electric field such as dielectrophoresis (DEP) and AC electro-osmosis (ACEO) have been widely investigated for their application in microfluidic devices. In order to provide a more complete theoretical basis for such AC electrokinetic mechanisms, we propose a method to quantify the two forces upon one individual particle using optical tweezers as a force transducer and lock-in phase sensitive detection technique to allow high selectivity. Using this method, we isolated the ACEO force from the DEP force for charged polystyrene sphere in deionized (DI) water. ACEO free DEP crossover frequencies and a comprehensive 2D-mapping of the frequency dependent ACEO forces are presented in this paper.

A novel optoelectrofulidic system integrated optical image concentration and alignment system, dielectrophoresis phenomenon, microfluidic and friendly real-time control interface is first reported in this article. A new application of photoconductive material oxotitanium phthalocyanine (TiOPc) for microparticle applying has been first described and demonstrated by our research group. Basis on the special character of the photoconductive material, a TiOPc-based optoelectronic tweezers (Ti-OET) is utilized for single and massive cells/particles manipulation. The objects wanted to be manipulated are defined with different behaviors (e.g., press, release, drag and move) using Flash® software when the cursor acts on them. It also reveals the application for biological application to form the cells trapping with three sorts of cells, HMEC-1, HepG2 and HEK293t. Another application of our optoelectrofulidic system is to fabricate a TiOPc-based flow cytometry chip which can be used for sorting the 15μm diameter particles with 105 μm/s velocity. When the 10Vp.p. voltage and 45 kHz AC frequency apply on the top and button ITO electrode, the illuminated light pattern will become a spatially virtual switch inside the microchannel. The dielectrophoresis force between top ITO glass and button photoconductive layer controlled by the friendly interface will concentrate the cells/particles as a straight line and individually direct each one in different paths. In summary, we have established an optoelectronfulidic-based chip and spatially virtual switch system which are applied for cell pattern and particles sorting. In the future, this easy manipulation approach can place the full power of optoelectronfulidic chip into the biological operators' hands.

This paper reports on-chip based optical detection with three-dimensional spatial resolution by integration of an optofluidic microscope (OFM) in a microfluidic pinched flow fractionation (PFF) separation device. This setup also enables on-chip particle image velocimetry (PIV). The position in the plane perpendicular to the flow direction and the velocity along the flow direction of separated fluorescent labeled polystyrene microspheres with diameters of 1 μm, 2.1 μm, 3 μm and 4μm is measured using the OFM readout. These results are bench marked against those obtained with a PFF device using a conventional fluorescence microscope as readout. The size separated microspheres are detected by OFM with an accuracy of ≤ 0.92μm. The position in the height of the channel and the velocity of the separated microspheres are detected with an accuracy of 1.4 μm and 0.08mm/s respectively. Throughout the measurements of the height and velocity distribution, the microspheres are observed to move towards the center of the channel in regard to its height.

In this paper, we introduce a novel sensor scheme which merges nano-photonics and nano-fluidics on a single platform through the use of free-standing photonic crystals (PhCs). PhCs offer great freedom to manipulate the spatial extent and the spectral characteristics of the electromagnetic fields. Also, nanoholes in PhCs provide a natural platform to transport solutions. By harnessing these nano-scale openings, we theoretically and experimentally demonstrate that both fluidics and light can be manipulated at sub-wavelength scales. In this scheme, the free standing PhCs are sealed in a chamber such that only the nano-scale hole arrays enable the flow between the top and the bottom channels. The nanohole arrays are used as sensing structures as well as nanofluidic channels. Compared to the conventional fluidic channels, we can actively steer the convective flow through the nanohole openings for effective delivery of the analytes to the sensor surface. This scheme also helps to overcome the surface tension of highly viscous solution and guarantees that the sensor can be totally immersed in solution. We apply this method to detect refractive index changes in aqueous solutions. Bulk measurements indicate that active delivery of the convective flow results in better performance. The sensitivity of the sensor reaches 510 nm/RIU for resonance located around 850 nm with a line-width of ~10 nm in solution. Experimental results are matched very well with numerical simulations. We also show that cross-polarization measurements can be employed to further improve the detection limit by increasing the signal-to-noise ratio.

The dynamic response characteristics of a liquid crystal (LC) device are dependent upon its viscosity coefficients. Local shear viscosity coefficients, or Miesowicz viscosity coefficients, η_i, are of particular importance for backflow effects and their optimisation allows for faster LC device response times. With such a wide range of LC materials available, information regarding their viscous properties is often incomplete. Micromanipulation with laser tweezers offers an alternative method for determining shear viscosity coefficients. Micron sized dielectric particles are dispersed in homeotropically and planarly aligned nematic LC, sandwiched between two coverslips. The microfluidic behaviour of the LC is investigated using a computer controlled laser tweezer system where particle tracking is performed using a high speed CMOS camera to record bead displacement for power spectral density analysis. We investigate the effective viscosity coefficients parallel and perpendicular to the director n, η^II_eff and η⊥_eff respectively. These are directly related to the Miesowicz viscosity coefficients for homeotropic alignment η1, and homogenous alignment η2 and η3. The results infer practically pertinent details about the viscoelastic properties of liquid crystals, and particles in LC systems.

Surfaces -defined as the interfaces between solids and liquids- have attracted much attention in optics and biology, such as total internal reflection imaging (TIRF) and DNA microarrays. Within the context of optofluidics however, surfaces have received little attention. In this paper, we describe how surfaces can define or enhance optofluidic function. More specifically we discuss chemical interfaces that control the orientation of liquid crystals and the stretching of individual nucleic acids, diffractive and plasmonic nanostructures for lasing and opto-thermal control, as well as microstructures that read pressure and form chemical patterns.

We report a novel method, the optical bottle that was used to directly measure the osmotic bulk modulus for a colloid suspension. We determined the bulk modulus by optically trapping multiple nanoparticles and considered a mechanical balance between the compressive laser gradient force pressure and the resulting resistive osmotic pressure. Osmotic bulk moduli results measured with the optical bottle are presented for aqueous suspensions of latex particles as a function of solution ionic strength; and are compared to results from identical samples measured using turbidity spectra.

Spherical colloids with asymmetric surface properties, e.g., 'Janus' particles with two unique faces, are an emerging class of materials that can provide mechanisms for controlling colloidal particle dynamics. Several reports in the literature detail the fabrication of Janus particles as well as their behavior under the influence of external electric, magnetic and optical fields. Here we present an in depth study of the magnetic and optical properties of 10 μm spherical metal-coated Janus particles, and we demonstrate new mechanisms to control their assembly, transport, and achieve total positional and orientational control at the single particle level. Through the application of external magnetic fields Janus particles formed kinked-chain assemblies. Janus particles can also be transported in rotating magnetic field via hydrodynamic surface effects. Optical fields can control the rotation and clustering of Janus particles at low laser power, but not at higher powers due to the formation of cavitation bubbles and large scattering forces. The unique magnetic and optical properties of Janus particles were leveraged to engineer 'dot' Janus particles that can be utilized to achieve near holonomic control of a single colloid in an optomagnetic trap.

We discuss a powerful technique for removing optical aberrations from optical systems with a spatial light modulator. In optical trapping systems this technique enables compensating for wavefront and amplitude deviations directly at the sample chamber thus bringing significant enhancement of optical trapping performance.

Fluorescence correlation spectroscopy is one of the most sensitive methods for biomolecule and nanoparticle studies. By fluorescently labeling the target particles, statistical analysis of the fluorescence fluctuation inside the focal volume of a laser beam yield information of the concentration and diffusion of the target particles in the illuminated volume. However, the application of FCS is limited by its detection range of 10¹⁰-10¹⁴ particles/mL. To overcome this sensitivity threshold on the low concentration end, we designed a hybrid system that augments FCS with optical trapping. By using the optical gradient force from a second laser focused to the same illuminated volume, we were able to show that the local concentration of particles can be enriched significantly, thus extending the useful range of FCS. In this work, we describe this novel hybrid optical method for nanoparticle detection by first considering freely diffusing particles about the illuminating volume, and then compare the results to nanoparticles under the influence of the optical trapping laser. Analysis of trapped particle number permits measurement of trapping energy as well as determine ambient concentration out of the trap. Furthermore, the hindered diffusion of trapped particles due to optical forces will be discussed.

A micro object with lower refractive index than surrounding liquid is subjected its motion to an optical repulsive force. The optical repulsive force is generated with a focused beam array, which is dynamically formed by a computer-generated hologram displayed on a liquid crystal spatial light modulator. We demonstrate manipulations of hollow glass spheres in water and water droplets in organic solvent using the optical repulsive forces.

Zeolite crystals have a wide use as model systems for artificial light harvesting systems, as nano-containers for supramolecular organization or as building blocks for 1D and 2D assemblies of several crystals. In particular the assembly of zeolite L crystals with the aim to bridge the gap between the nano- and the macroscopic world has been a focus of research during the last years. However, almost all available approaches to order, assemble and pattern Zeolite L are restricted to large amounts of crystals. Although these approaches have proven to be powerful for many applications, but they have only limited control over positioning or orientation of single crystals and are lacking if patterns or structures are required which are composed of a few or up to a few hundred individual crystals. We demonstrate here that holographic optical tweezers are a powerful and versatile instrument to control zeolite L on the single crystal level. It is shown that full three-dimensional positioning, including rotational control, of any zeolite L crystal can be achieved. Finally, we demonstrate fully reversible, dynamic patterning of a multitude of individually controlled zeolite L crystals.

Time-division multiplexing in the proposed holographic optical tweezers has been used to quasi-simultaneously generate two different intensity patterns, a carrier beam spot and a beam array, by alternately sending the corresponding hologram patterns to a spatial light modulator. Since the switching of the input holograms degrades the spatial stability of a Brownian particle trapped within the generated intensity spot area, it is necessary to numerically investigate the conditions in the time-division multiplexing for a particle to be stably trapped in a focused Gaussian beam. A potential field generated by the beam spot is analytically calculated by the generalized Lorenz-Mie theory model, and the spatiotemporal stability of the particle trapped within the potential field is numerically investigated by the Smoluchowski equation. The simulation based on the explicit method reveals the spatiotemporal stability of the trapped particle related to the particle size, the switching rate, and the focused laser beam power. Finally, the validity of the numerical analysis in this work is confirmed by experiments.

The 0-th order diffraction light in an intensity pattern produced by a hologram disturbs optical manipulation of micro-objects in dynamic holographic optical tweezers (HOT). The purpose of this study is to investigate polarization characteristics of amplitude and phase modulations in the HOT to reduce the influence of the 0-th order beam spot. Numerical simulations are conducted using a Jones matrix of the system, whose the validity is experimentally confirmed. The optimum conditions to reduce the influence of the 0-th order beam spot can be estimated on the bases of the numerical results and its effectiveness in performance of the HOT system is experimentally demonstrated by the optical manipulation of polystyrene particles.

We present a new approach of combining Lab-on-a-chip technologies with optical manipulation technique for accurate investigations in the field of cell biology. A general concept was to develop and combine different methods to perform advanced electrophysiological investigations of an individual living cell under optimal control of the surrounding environment. The conventional patch clamp technique was customized by modifying the open system with a gas-tight multifunctional microfluidics system and optical trapping technique (optical tweezers). The system offers possibilities to measure the electrical signaling and activity of the neuron under optimum conditions of hypoxia and anoxia while the oxygenation state is controlled optically by means of a spectroscopic technique. A cellbased microfluidics system with an integrated patch clamp pipette was developed successfully. Selectively, an individual neuron is manipulated within the microchannels of the microfluidic system under a sufficient control of the environment. Experiments were performed to manipulate single yeast cell and red blood cell (RBC) optically through the microfluidics system toward an integrated patch clamp pipette. An absorption spectrum of a single RCB was recorded which showed that laser light did not impinge on the spectroscopic spectrum of light. This is promising for further development of a complete lab-on-a-chip system for patch clamp measurements.

Macrophages are members of the leukocyte family. Tissue damage causes inflammation and release of vasoactive and chemotactic factors, which trigger a local increase in blood flow and capillary permeability. Then, leukocytes accumulate quickly to the infection site. The leukocyte extravasation process takes place according to a sequence of events that involve tethering, activation by a chemoattractant stimulus, adhesion by integrin binding, and migrating to the infection site. The leukocyte extravasation process reveals that adhesion is an important part of the immune system. Optical tweezers have become a useful tool with broad applications in biology and physics. In force measurement, the trapped bead as a probe usually uses a polystyrene bead of 1 μm diameter to measure adhesive force between the trapped beads and cell by optical tweezers. In this paper, using the ray-optics model calculated trapping stiffness and defined the linear displacement ranges. By the theoretical values of stiffness and linear displacement ranges, this study attempted to obtain a proper trapped particle size in measuring adhesive force. Finally, this work investigates real-time adhesion force measurements between human macrophages and trapped beads coated with lipopolysaccharides using optical tweezers with backscattered detection.

This work describes our image-analysis software, CellStress, which has been developed in Matlab and is issued under a GPL license. CellStress was developed in order to analyze migration of fluorescent proteins inside single cells during changing environmental conditions. CellStress can also be used to score information regarding protein aggregation in single cells over time, which is especially useful when monitoring cell signaling pathways involved in e.g. Alzheimer's or Huntington's disease. Parallel single-cell analysis of large numbers of cells is an important part of the research conducted in systems biology and quantitative biology in order to mathematically describe cellular processes. To quantify properties for single cells, large amounts of data acquired during extended time periods are needed. Manual analyses of such data involve huge efforts and could also include a bias, which complicates the use and comparison of data for further simulations or modeling. Therefore, it is necessary to have an automated and unbiased image analysis procedure, which is the aim of CellStress. CellStress utilizes cell contours detected by CellStat (developed at Fraunhofer-Chalmers Centre), which identifies cell boundaries using bright field images, and thus reduces the fluorescent labeling needed.

In an optical tweezers system, the force measurement with a resolution less than pico-Newton can be achieved by precise measurement and analysis of the trapped particle trajectory. Typically, this single particle tracking technique is realized by a quadrant position sensor which detects the scattering lights of the trapping laser beam from the trapped particle. However, as the radius of the trapped particle is larger than the wavelength of the trapped laser, the scattering pattern becomes complicated, and it limits the tracking region and the signal sensitivity on the trapped particle. To solve this issue, an extra probing laser with optimized focal offset according to the trapping laser is applied to improve the flexibility and performance of our particle tracking system for each particle size. A rule of thumb between the optimized focal offsets and particle size is also concluded from the experimental results and theoretical simulations.

Optical chromatography involves loosely focusing a laser beam into a fluid flowing opposite to the direction of laser propagation. When microscopic particles in the flow path encounter this beam they are optically trapped along the beam and are pushed upstream by the radiation pressure from the laser focal point to rest at a position where the optical and fluid drag forces on the particle balance. Because optical and fluid forces are sensitive to differences in the physical and chemical properties of a particle, fine separations are possible. A laser beam which completely fills a fluid channel has been operated as an optically tunable filter for the separation of polymeric/colloidal and biological samples. We demonstrate here how this technique coupled with an advanced microfluidic platform can be used as both a coarse and fine method to fractionate particles in an injected sample. The microfluidic network allows for a monodisperse mixed particle sample of polystyrene and poly(methyl methacrylate) to be injected, hydrodynamically focused and completely separated. To test the limit of separation, a mixed polystyrene sample containing two particles varying in size by less than 0.2% was run in the system. The analysis of the resulting separation sets the framework for continued work to perform ultra-fine separations.

We proposed a method to guide micro-particles within a millimeter region. A cylindrical mirror is used to create an optical line segment for guiding particles. In order to increase the numerical aperture, Polydimethylsiloxane (PDMS) is poured on the cylindrical mirror. At the top of the PDMS layer, a fluidic channel is fabricated. As a collimated laser beam is incident on the cylindrical mirror, the laser beam is tightly focused and is transformed into a line-shaped pattern in the fluidic channel. In this way, a simple and cost-effective optical guiding system can be achieved.

Approximate methods such a Rayleigh scattering and geometric optics have been widely used for the calculation of forces in optical tweezers. We investigate their applicability and usefulness, comparing results using these approximate methods with exact calculations.

We investigate experimentally and theoretically plasmon-enhanced optical trapping of metal nanoparticles. We calculate the optical forces on gold and silver nanospheres through a procedure based on the Maxwell stress tensor in the transition T-matrix formalism. We compare our calculations with experimental results finding excellent agreement. We also demonstrate how light-driven rotations can be generated and detected in non-symmetric nanorods aggregates. Analyzing the motion correlations of the trapped nanostructures, we measure with high accuracy both the optical trapping parameters, and the rotation frequency induced by the radiation pressure.

In optical tweezers, it can be comprehended that the larger the inclination angle between the condensed laser beam and the optical axis contributes more to axial trapping force, while the central part of laser beam with smaller inclination angle contributes more transverse trapping force. Therefore donut-shaped beam is used to improve the problem of lessaxial trapping for common optical tweezers. Some research reports have shown that the efficiency of a trapping force can be enhanced by using a donut-shaped beam. In this paper we present the dependence of the axial and the transverse components of a trapping force on the configuration of a focused donut-shaped beam. The simulation result will provide a simple and easy guide for optical tweezers users to adjust the configuration of a focused donut-shaped beam for optimal trapping performance.

Microassembly has been identified as one of critical techniques in innovating the promising era of micro/nano technology. Several works have been investigated to fabricate various micro-devices such as micro-sensors and microactuators. Assembly plays an important role for fabricating micro-devices. However, there are only few studies in the assembly of microparts. In this paper, we present manipulation and assembly of three-dimensional microparts produced by two-photon polymerization where optical trapping technique was used to manipulate microparts. We show exemplary microassembly formed by assembling two microparts, a movable female part and a male part fixed on a glass substrate.

A fiber-optic probe based on a hollow optical fiber was developed for highly-sensitive remote-Raman measurements of particles in solution. A lens mounted at the distal end of the fiber was optimally designed to suppress the spherical aberration and maximize the numerical aperture by the ray-optics method, and fabricated by polishing a SrTiO₃ ball of 1 mm diameter. Polystyrene particles of 60 μm diameter dispersed in NaCl aqueous solution were three-dimensionally trapped by the prototype probe. The Raman spectrum of the polystyrene particle was clearly observed when the particle was optically trapped at the beam focus.

A single dense liquid droplet of urea is formed by irradiating a focused continuous wave near-infrared laser beam to a glass/solution interface of a thin film of the unsaturated D₂O solution though its dynamic deformation. Conversely, in the supersaturated solution, neither droplet formation nor large solution deformation is observed. This can be explained on the basis of its high viscosity. In addition, crystal growth and dissolution are demonstrated by focusing the laser beam close to the crystal generated in the solution. All results are here discussed in view of local temperature elevation, mass transfer due to convection, and laser trapping of the clusters due to photon pressure, by comparing with experimental results for glycine.

Dielectrophoretic (DEP) force is a result of a non-uniform electric field and the relative polarizability of a neutral particle and the fluid in which it is suspended. Measuring DEP force provides information concerning the electrical properties of the particle and thus provides one way of identifying or distinguishing one particle from another. In this study, a microfluidic DEP platform with hyperbolic quadrupole electrode geometry was implemented for particle characterization purposes. The platform was used to measure the conductivity and permittivity of polystyrene microparticles in a carrier fluid. A useful feature of the hyperpolic electrode geometry is the linearity of the electric field gradient that it produces. The linear field gradient provides a straightforward way to measure the DEP force, and consequently the electrical properties of the particle, simply by measuring the particle movement within the field. According to the simulations good linearity is achieved within the full circular area between the electrode tips of the geometry. Besides DEP force a particle may undergo many other forces during such an experiment and thus may move not only laterally between the electrodes but also wander above the electrodes. Therefore unlike previous studies the electrodes of the implemented platform were made of indium-tin-oxide (ITO) to achieve full transparency and in consequence better view of the particle motion when using common transluminescence microscopy. Electrode transparency revealed that particles have motion also in the depth direction, especially above the electrodes, and that accurate mobility measurements may require particle observation in three dimensions. The electrical properties of polystyrene microparticles were determined by measuring their mobility in a linearly increasing electric field produced by the hyperbolic ITO electrode geometry with an active region of 65 μm in radius. The experiments were done using a transluminescence video microscope and 2 μm polystyrene particles in 0.1 mM KCL dilution of 1.42 mS/m conductivity. The mobility of the particle was determined as an average of the particle's lateral displacements in consecutive video frames. Based on their mobility the polystyrene particles showed a conductivity of 3.3 mS/m and permittivity of 54 0 ε₀ within the frequency range of 0.1-15 MHz.

A novel technique for the fabrication of micro-structures on Ni-alloy by DPSS laser ablation was studied and reported in this paper. Using a q-switched Nd:YVO4 laser, a Ni alloy was micro-machined without lithography-based technologies. The effects of various process parameters such as working power, laser frequency, scan speed and number of scan were examined during laser processing. The removal of debris during ablation was also studied, and performed under vacuum conditions. The obtained prototype was tested by optical microscopy, Scanning Electron Microscopy, EDX and 3D microprofilometer. The obtained structured nickel alloy can be used as master for imprinting on glass substrates for lab-on-chip applications.

Optical tweezers have become an important tool to measure forces in biology. The trapped particle displacements acquired from the position detection system are applied to calibrate trapping stiffness using power spectrum method. The near infrared light is typically used as a laser source to reduce the damage to a cell or cellular organelles and the biological objects can be held and moved by exerting piconewton forces. In force measurement, optical force strength is calculated by multiplying trapping stiffness and trapped bead displacement. Optical tweezers perform a wider range of experiments through the integration of a quadrant photodiode for position detection. Both forward-scattered detection and backward-scattered detection are the typical position detection. This study discussed both backward-scattered detection and forward-scattered detection that add a probing beam and their linear detection ranges that describe the precise position of the trapped bead. This work also discussed their linear detection ranges related to the distance between the two laser system focuses, confirming the optimum positions of the two focuses. The result indicated that the linear detection range of backward-scattered detection is longer than the forward-scattered detection. Hence, backwardscattered detection measures the longer displacement of the trapped bead in optical force measurement.

An experimental characterization of the 3D forces, acting on a trapped polystyrene bead in a counter-propagating beam geometry, is reported. Using a single optical trap with a large working distance (in the BioPhotonics Workstation), we simultaneously measure the transverse and longitudinal trapping force constants. Two different methods were used: The Drag force method and the Equipartition method. We show that the counterpropagating beams traps are simple harmonic for small displacements. The force constants reveal a transverse asymmetry as κ_- = 9.7 pN/μm and κ₊ = 11.3 pN/μm (at a total laser power of 2x35 mW) for displacements in opposite directions. The Equipartition method is limited by mechanical noise and is shown to be applicable only when the total laser power in a single 10 μm counter-propagating trap is below 2x20 mW.

Optical tweezers have been widely used to study DNA properties including time dependent changes in conformation; however, such studies have emphasized direct fluorescent observation of the conformations of dyed DNA molecules. In this work we explore DNA conformations that allow undyed DNA to link to spatially separated surfaces. In one set of experiments, we used optical tweezers to hold a polystyrene bead at a fixed distance from the sample capillary wall and measured the probability of the binding as a function of the separation between the polystyrene bead and the capillary, where the beads were fully confined in liquid. In a separate magnetic crystal experiment, we used magnetic forces to control the separation between magnetic beads in a hexagonal lattice at an air-water interface and measured the probability of linking to beads in the crystal. In both types of experiments peak binding occurs at a surface separation several times longer than the radius of gyration of the DNA. These experiments provide fundamental information on elusive, but significant DNA conformations, as well as technologically useful information on the probability of the DNA binding that will link two surfaces.

The delivery of extra cellular molecules into cells is essential for cell manipulation. For this purpose genetic materials (DNA/RNA) or proteins have to overcome the impermeable cell membrane. To increase the delivery efficiency and cell viability of common methods different nano- and micro material based approaches were applied. To manipulate the cells, the membrane is in contact with the biocompatible material. Due to a field enhancement of the laser light at the material and the resulting effect the cell membrane gets perforated and extracellular molecules can diffuse into the cytoplasm. Membrane impermeable dyes, fluorescent labelled siRNA, as well as plasmid vectors encoded for GFP expression were used as an indicator for successful perforation or transfection, respectively. Dependent on the used material, perforation efficiencies over 90 % with a cell viability of about 80 % can be achieved. Additionally, we observed similar efficiencies for siRNA transfection. Due to the larger molecule size and the essential transport of the DNA into the nucleus cells are more difficult to transfect with GFP plasmid vectors. Proof of principle experiments show promising and adequate efficiencies by applying micro materials for plasmid vector transfection. For all methods a weakly focused fs laser beam is used to enable a high manipulation throughput for adherent and suspension cells. Furthermore, with these alternative optical manipulation methods it is possible to perforate the membrane of sensitive cell types such as primary and stem cells with a high viability.

Plasmonic metal nanoparticles have recently generated significant interest in both fundamental and applied nanoscience. An emerging area of interest within plasmonics is the study of optical forces on metal nanoparticles. These forces can be used to manipulate and assemble particles into useful geometries. In this work Au nanoparticles are optically trapped and deposited onto surfaces using both focused beam (gradient) as well as total internal reflection (TIR) based optical trapping. In the case of focused beam trapping, single spherical Au nanoparticles can be rapidly deposited to arbitrary locations on a surface with high spatial precision (~100 nm). By controlling both the particle stability and the surface chemistry, large areas (10's of μm²) can be patterned with Au nanoparticles. For TIR-based trapping, dense arrays of high-aspect ratio Au bipyramids with spot sizes ~10 μm² are deposited on surfaces. Au bipyramids are deposited via a plasmon-selective photothermal heating mechanism. Both of these methods are fast (patterning large areas in minutes) and require no lithography or scanning probes.