This PDF file contains the front matter associated with SPIE Proceedings Volume 8810, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

We demonstrate experimentally the principle of a ”tractor beam” where a microobject is pulled against the photons flow. We show that this geometry and method can be used to delivery several microparticles over a distance of tens of micrometers, to sorting of particles according to their sizes using rotation of beam polarization, and to self-arrangement of microobjects to optically bound microstructures that are pulled againts the beam propagation.

We experimentally demonstrate continuous attraction of macroscopic targets (> 1 cm) towards the source, against a net momentum flux in the system. Use of a simple setup provides an easily understood illustration of the negative radiation pressure concept for tractor beam, and how these are distinct from the gradient force acting in conventional optical tweezers. Here, we map out regimes over which negative radiation forces dominate, and (favorably) compare the thresholds observed to those that emerge from simulations. Theoretical explorations of tractor beam action commonly invoke higher-order Bessel beams, and here we make clear that the reason for this is because of the reduction in axial momentum associated with such hollow-core beams, which allows effects associated with off-axis “skew” momentum to become dominant. Ultimately, there is interest in exploring the language used for describing such effects: radiation pressure versus gradient force (which we suggest might be better described in terms of non‐conservative versus conservative forces), and “orbital” angular momentum (which we suggest might be more appropriately termed “topological” angular momentum).

A new modality for optical micromanipulation is under investigation. Optical wings are shaped refractive objects that experience a force and torque owing to the reflection and transmission of uniform light at the object surface. We present wing designs that provide a restoring torque that returns the wing to a source facing orientation while preserving efficient thrust from radiation pressure. The torsional stiffness and orbital period of a set of optical wing cross-sectional shapes are determined from numerical ray-tracing analyses. These results demonstrate the potential to develop an efficient optomechanical device for applications in microbiology and space flight systems.

Scattering forces on small particles are proportional to the average value of the Poynting vector and to the curl of the spin angular momentum of the light field. In this paper we analyse the relevance of spin non-conservative forces in configurations where scattering forces are different from zero with a null Poynting vector or with a null average value of the Poynting vector.

Diamond anvil cells can be used to study the behavior of materials at high pressure by compressing small samples up to hundreds of GigaPascals. There is no mechanical access to the sample once the cell is pressurized but it is possible to observe the sample through the diamond windows. Optical tweezers can be used to measure the mechanical properties of fluids, such as viscosity, by trapping and monitoring micron sized spheres suspended in the fluid. We use a diamond anvil cell within a modified optical tweezers instrument to measure the viscosity of water as a function of pressure up to 1:3GPa. Development of this technique will allow investigations of the mechanical changes in biological cells and other soft materials placed under high pressure.

Recent developments in space technology like micro-propulsion systems for drag-free control, thermal shielding, ultra-stable laser sources and stable optical cavities set an ideal platform for quantum optomechanical exper- iments with optically trapped dielectric spheres. Here, we will provide an overview of the results of recent studies aiming at the realization of the space mission MAQRO to test the foundations of quantum physics in a parameter regime orders of magnitude beyond existing experiments. In particular, we will discuss DECIDE, which is an experiment to prepare and then study a Schrodinger-cat-type state, where a dielectric nanosphere of around 100 nm radius is prepared in a superposition of being in two clearly distinct positions at the same time. This superposition leads to double-slit-type interference, and the visibility of the interference pattern will be compared to the predictions of quantum theory. This approach allows for testing for possible deviations from quantum theory as our test objects approach macroscopic dimensions. With DECIDE, it will be possible to distinctly test several prominent theoretical models that predict such deviations, for example: the Diósi-Pensrose model, the continuous-spontaneous-localization model of Ghirardi, Rimini, Weber and Pearle, and the model of Károlyházy.

Although many techniques are efficient at measuring optical orbital angular momentum (OAM), they do not allow one to obtain a quantitative measurement for the OAM density across an optical field and instead only measure its global OAM. Numerous publications have demonstrated the transfer of local OAM to trapped particles by illustrating that particles trapped at different radial positions in an optical field rotate at different rotation rates. Measuring these rotation rates to quantitatively extract the OAM density is not only an indirect measurement but also a complicated experiment to execute. In this work we theoretically calculate and experimentally measure the OAM density of light, for both symmetric and non-symmetric optical fields. We outline a simple approach using only a spatial light modulator and a Fourier transforming lens to measure the OAM spectrum of an optical field and we test the approach on superimposed non-diffracting higher-order Bessel beams. We obtain quantitative measurements for the OAM density as a function of the radial position in the optical field for both symmetric and non-symmetric superpositions, illustrating good agreement with the theoretical prediction. The ability to measure the OAM distribution of optical fields has relevance in optical tweezing, and quantum information and processing.

Optical manipulation has allowed for an increase in understanding across scientific fields including biology, chemistry, and atomic physics. Unfortunately, there is still significant debate as to how the dynamics of opticalmatter systems should be modeled. The reason for this is the myriad of formulations for the electromagnetic momentum and force density. While significant advances have been made in recent years to interpret the different formulations of electrodynamics, there remains some confusion. Most formulations include contributions from both matter and field. They may be interpreted as providing the canonical, total, or wave momentum, stress, and force. Examples include the well-known Gordon and Minkowski momenta. Conversely, it is widely accepted that the Abraham momentum of light corresponds to the kinetic subsystem. Changes in the kinetic momentum thus should provide center of mass-energy translations of matter providing a route to deriving equations of motion in optical manipulation experiments. In this paper, the idea of a kinetic formulation of electrodynamics is discussed. Consideration will be given to the three viewpoints which are most commonly argued. First, the kinetic subsystem should be postulated as a fundamental tenant of electrodynamics. Second, the kinetic subsystem of electrodynamics should not be postulated, but rather deduced by experimental and theoretical considerations. Third, a kinetic subsystem of electrodynamics cannot be uniquely defined in matter because the fields inside matter cannot be uniquely measured. Finally, it will be argued that identification of the kinetic subsystem of a particular optical manipulation experiment is not necessary nor preferred in most cases.

The classical theory of electrodynamics is built upon Maxwell’s equations and the concepts of electromagnetic field, force, energy, and momentum, which are intimately tied together by Poynting’s theorem and the Lorentz force law. Whereas Maxwell’s macroscopic equations relate the electric and magnetic fields to their material sources (i.e., charge, current, polarization and magnetization), Poynting’s theorem governs the flow of electromagnetic energy and its exchange between fields and material media, while the Lorentz law regulates the backand- forth transfer of momentum between the media and the fields. As it turns out, an alternative force law, first proposed in 1908 by Einstein and Laub, exists that is consistent with Maxwell’s macroscopic equations and complies with the conservation laws as well as with the requirements of special relativity. While the Lorentz law requires the introduction of hidden energy and hidden momentum in situations where an electric field acts on a magnetic material, the Einstein-Laub formulation of electromagnetic force and torque does not invoke hidden entities under such circumstances. Moreover, the total force and the total torque exerted by electromagnetic fields on any given object turn out to be independent of whether force and torque densities are evaluated using the Lorentz law or in accordance with the Einstein-Laub formulas. Hidden entities aside, the two formulations differ only in their predicted force and torque distributions throughout material media. Such differences in distribution are occasionally measurable, and could serve as a guide in deciding which formulation, if either, corresponds to physical reality.

We have developed a high-aspect ratio optical pipeline aiming to produce a highly collimated stream of micron-size particles in either gaseous or vacuum environments. A hollow, first-order quasi-Bessel beam with variable divergence was generated with a phase-only spatial light modulator (SLM), by superimposing the quadratic phase of a lens and an axicon with a 0.5° base angle. The beam was further re-imaged to form a centimetre-long funnel beam with ~5μm diameter and up to 2000 length-to-diameter aspect ratio. The divergence of the central core of the Bessel beam was controlled by varying the effective lens in the hologram. The SLM-based optical beam was compared to a similar beam composed using a physical axicon. The experimental tests were conducted with 2-μm size polystyrene spherical particles to evaluate the optical force. We present estimated optical forces exerted on the particles in the transverse plane, both depending on the particle size, laser power, and background-gas pressure.

The development of an experimental system in which optical levitation combined with Millikan´s classical oil drop experiment will be presented. The focus of the apparatus is a glass cell (25x72x25 mm³) in which an oil drop is levitated using a vertically aligned laser beam. A laser power of about 0.9 W is needed to capture a drop, whereas typically 0.3 W is sufficient to maintain it in the trap. An alternating electric field is applied vertically across the cell, causing the drop to oscillate in the vertical direction. The amplitude of the oscillations depends on the strength of the electric field and the q/m ratio of the oil drop. The oscillations are observed by imaging scattered laser light onto either a screen or a position sensitive detector. The number of discrete charges on the drop can be reduced by exposing it to either UV-light or a radioactive source. The radius of the drop is measured by detecting the diffraction pattern produced when illuminated with a horizontally aligned He-Ne laser beam. The mass of the drop can then be determined since the density of the oil is known. Hence, absolute measurements of both the mass and the charge of the drop can be obtained. The goal of the experiment is to design a system which can be used to demonstrate several fundamental physical phenomena using the bare eye as the only detector. The experimental set-up will be further developed for studies of light scattering and spectroscopy of liquids and for studies of interactions between liquid drops.

We present dye lasing from optically manipulated glycerol-water aerosols with diameters ranging between 7.7 and 11.0 μm confined in optical tweezers. While being optically trapped near the focal point of an infrared laser, the droplets stained with Rhodamine B were pumped with a Q-switched green laser and their fluorescence emission spectra featuring whispering gallery modes (WGMs) were recorded with a spectrograph. Nonlinear dependence of the intensity of the droplet WGMs on the pump laser fluence indicates dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by adjusting the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium.

The evanescent field from a waveguide can be used to trap and propel a particle. An optical waveguide loop with an intentional gap at the center is used for planar transport and stable trapping of particles. The waveguide acts as a conveyor belt to trap and deliver spheres towards the gap. At the gap, the counter-diverging light fields hold the sphere at a fixed position. Numerical simulation based on the finite element method was performed in three dimensions using a computer cluster. The field distribution and optical forces for rib and strip waveguide designs are compared and discussed. The optical force on a single particle was computed for various positions of the particle in the gap. Simulation predicted stable trapping of particles in the gap. Depending on the gap separation (2-50 μm) a single or multiple spheres and red blood cells were trapped at the gap. Waveguides were made of tantalum pentaoxide material. The waveguides are only 180 nm thick and thus could be integrated with other functions on the chip.

Plasmonic optical traps, or plasmonic “nanotweezers”, have emerged as an attractive alternative for optical manipulation because they circumvent the diffraction limit, producing highly confined and enhanced fields that both relax constraints for microparticle manipulation and offer a route for improving nanoparticle trapping. Here, we present an overview of the use of Au bowtie nanoantenna arrays (BNAs) for plasmonic nanotweezers. We show that optical absorption by the BNAs creates convection currents that resemble a Rayleigh-B´enard pattern and that an absorptive substrate, e.g. Indium-Tin-Oxide, is crucial to achieve large convection velocities. Furthermore, we demonstrate phase-like behavior of trapped particles and that the adhesion layer material and nanostructure orientation strongly affect trapping behavior. In addition, we discuss the use of a femtosecond- pulsed source in plasmonic nanotweezers and demonstrate that the fs pulses (1) augment the near-field optical forces compared to comparable, continuous-wave nanotweezers, and (2) increase the diagnostic capabilities of plasmonic nanotweezers by providing access to the nonlinear optical response of trapped species. Finally, we show for the first time that plasmonic nanoantennas are an effective tool for manipulation objects up to at least 50 μm in diameter. Using low-numerical aperture illumination (0.25-0.6 NA), we show that manipulation of these “macroscopic” objects is facilitated by increasing the number of illuminated nanostructures participating in the trapping event. These results open up a new avenue for the usage of plasmonic nanotweezers and may have applications for manipulating Eukaryotic cells, studying self-organization/aggregation of cells, and micro-scale manufacturing.

Optical tweezers use focused laser to trap microobjects suspended in the medium to the focal point. They are becoming an indispensable tool in microbiology because of its ability to trap tiny biological particles so that single particle analysis is possible. However, it is still very difficult to trap particles such as DNA molecules that are smaller than the diffraction limit. Although trapping of those is possible by increasing the laser power inversely proportional to the cube of the particle diameter, such high power can cause permanent thermal damages. One of the current solutions to this problem is to intensify the local field by the use of the near-field enhancement coming from nanoplasmonic structures illuminated with lasers. Such solution allows one to use low powered laser and still be able to trap them. In this paper, we present the trapping of a single DNA molecule by the use of the strong field enhancement due to a sub-micrometer sized hole drilled on a gold plate by an e-beam milling process and the trapping is verified by the measurement of the scattering signal that comes from the trapped DNA.

Feedback traps can create arbitrary virtual potentials for exploring the dynamics of small Brownian particles. In a feedback trap, the particle position is measured periodically and, after each measurement, one applies the force that would be produced by the gradient of the “virtual potential,” at the particle location. Virtual potentials differ from real ones in that the feedback loop introduces dynamical effects not present in ordinary potentials. These dynamical effects are caused by small time scales associated with the feedback, including the delay between the measurement of a particle’s position and the feedback response, the feedback response that is applied for a finite update time, and the finite camera exposure from integrating motion. Here, we characterize the relevant experimental parameters and compare to theory the observed power spectra and variance for a particle in a virtual harmonic potential. We show that deviations from the dynamics expected of a continuous potential are measured by the ratio of these small time scales to the relaxation time scale of the virtual potential.

Optical traps have been extensively employed to create tailored colloidal crystalline structures where the crystals can have long range order. Here we discuss how colloidal particles on periodic substrates can be used to understand how frustration can produce partially ordered states. We demonstrate how to create artificial spin ice systems using colloidal particles and describe variations on this system that include geometries in which a random loop model can be realized. We also discuss how frustration effects can be used to control grain boundary formation by creating energetic defects in the ground state ordering of these systems.

Einstein’s stochastic description of the random movement of small objects in a fluid, i.e. Brownian motion, reveals to be quite different, when observed on short timescales. The limitations of Einstein’s theory with respect to particle inertia and hydrodynamic memory yield to the apparition of a colored frequency-dependent component in the spectrum of the thermal forces, which is called “the color of Brownian motion”. The knowledge of the characteristic timescales of the motion of a trapped microsphere motion in a Newtonian fluid allowed to develop a high-resolution calibration method for optical interferometry. Well-calibrated correlation quantities, such as the mean square displacement or the velocity autocorrelation function, permit to study the mechanical properties of fluids at high frequencies. These properties are estimated by microrheological calculations based on the theoretical relations between the complex mobility of the beads and the rheological properties of a complex fluid.

Active matter or self driven particle systems include swimming bacteria, crawling cells and artificial swimmers. These systems often exhibit run and tumble dynamics; however, there are also examples of particles that move or swim in circles, such as bacteria near surfaces. Circular swimmers have also been experimentally realized using chiral colloidal particles. Here we examine how a substrate can be used to direct the motion of circle swimmers and separate particles with different swimming chiralities. The combination of the time reversal symmetry breaking by the circular motion as well as the breaking of detailed balance when the particles interact with the barriers leads to the directed motion. We examine this effect for different types of substrate geometries and also consider the effects of temperature. Such substrates could be created using various optical techniques.

Forces play a fundamental role in a wide array of biological processes, regulating enzymatic activity, kinetics of molecular bonds, and molecular motors mechanics. Single molecule force spectroscopy techniques have enabled the investigation of such processes, but they are inadequate to probe short-lived (millisecond and sub-millisecond) molecular complexes. We developed an ultrafast force-clamp spectroscopy technique that uses a dual trap configuration to apply constant loads to a single intermittently interacting biological polymer and a binding protein. Our system displays a delay of only ∼10 μs between formation of the molecular bond and application of the force and is capable of detecting interactions as short as 100 μs. The force-clamp configuration in which our assay operates allows direct measurements of load-dependence of lifetimes of single molecular bonds. Moreover, conformational changes of single proteins and molecular motors can be recorded with sub-nanometer accuracy and few tens of microseconds of temporal resolution. We demonstrate our technique on molecular motors, using myosin II from fast skeletal muscle and on protein-DNA interaction, specifically on Lactose repressor (LacI). The apparatus is stabilized to less than 1 nm with both passive and active stabilization, allowing resolving specific binding regions along the actin filament and DNA molecule. Our technique extends single-molecule force-clamp spectroscopy to molecular complexes that have been inaccessible up to now, opening new perspectives for the investigation of the effects of forces on biological processes.

Single-molecule force spectroscopy is a powerful technique for studying the detailed behaviour of biopolymers such as DNA and proteins: by applying pN-scale forces to individual molecules, properties such as conformations, folding pathways, and intermolecular interaction strengths can be determined. Traditionally these studies have been carried out under static tension. The dynamic response of polymers to a sudden change in force is exper- imentally more challenging as the polymer is often coupled to an external molecular handle, which suppresses important physics at short (∼ms) timescales. Here we use a nanopore to electrically control the application of force to the end of a double-stranded DNA molecule; the other end of the molecule is attached to a bead held in an optical trap. By shutting off the voltage, the fast relaxation dynamics of the free polymer end can be studied. We observe for the first time an enhanced viscous friction which arises from the rapid internal contraction of the DNA, which is fully explained by theory. These studies pave the way for new dynamic force spectroscopy experiments, such as investigations of tension propagation along biomolecules, which has applications for both polymer theory as well as biological systems such as the cytoskeleton where dynamic tension can affect cellular response.

Optical micromanipulation studies have solved a puzzle on DNA damage and repair. Such knowledge is crucial for understanding cancer and ageing. So far it was not understood, why the soft UV component of sunlight, UV-A, causes the dangerous DNA double strand breaks. The energy of UV-A photons is below 4 eV per photon, too low to directly cleave the corresponding chemical bonds in DNA. This is occasionally used to claim that artificial sunbeds, which mainly use UV-A, would not impose a risk on health. UV-A is only sufficient for induction of single strand breaks. The essential new observation is that, when on the opposite strand there is another single strand break at a distance of up to 20 base pairs. These two breaks will be converted into a break of the whole double strand with all its known consequences for cancer and ageing. However, in natural sun the effect is counteracted. Simultaneous red light illumination reduces UV induced DNA damages to 1/3. Since sunlight has a red component, skin tanning with natural sun is not as risky as might appear at a first glance.

Over the last decade, magnetic trapping has emerged as a tool which can complement the optical trap in certain key biophysical assays. Through its low cost, ease of use, stability and scalability, magnetic trapping is an attractive tool for micro-rheological analysis, generation of sorting structures for microfluidics, and also single-molecule experimentation. Here we will compare and contrast optical and magnetic trapping as they pertain to singlemolecule experimentation, and in particular to the study of interactions between single molecules such as proteins and nucleic acids.

Optical trapping is a powerful tool in Life Science research and is becoming common place in many microscopy laboratories and facilities. The force applied by the laser beam on the trapped object can be accurately determined allowing any external forces acting on the trapped object to be deduced. We aim to design a series of experiments that use an optical trap to measure and quantify the interaction force between immune cells. In order to cause minimum perturbation to the sample we plan to directly trap T cells and remove the need to introduce exogenous beads to the sample. This poses a series of challenges and raises questions that need to be answered in order to design a set of effect end-point experiments. A typical cell is large compared to the beads normally trapped and highly non-uniform – can we reliably trap such objects and prevent them from rolling and re-orientating? In this paper we show how a spatial light modulator can produce a triple-spot trap, as opposed to a single-spot trap, giving complete control over the object’s orientation and preventing it from rolling due, for example, to Brownian motion. To use an optical trap as a force transducer to measure an external force you must first have a reliably calibrated system. The optical trapping force is typically measured using either the theory of equipartition and observing the Brownian motion of the trapped object or using an escape force method, e.g. the viscous drag force method. In this paper we examine the relationship between force and displacement, as well as measuring the maximum displacement from equilibrium position before an object falls out of the trap, hence determining the conditions under which the different calibration methods should be applied.

Measurements of optical tweezers forces on biological micro-objects can be used to develop innovative biodiagnostics methods. In the first part of this report, we present a new sensitive method to determine A, B, D types of red blood cells. Target antibodies are coated on glass surfaces. Optical forces needed to pull away RBC from the glass surface increase when RBC antigens interact with their corresponding antibodies. In this work, measurements of stripping optical forces are used to distinguish the major RBC types: group O Rh(+), group A Rh(+) and group B Rh(+). The sensitivity of the method is found to be at least 16-folds higher than the conventional agglutination method. In the second part of this report, we present an original way to measure in real time the wall thickness of bacteria that is one of the most important diagnostic parameters of bacteria drug resistance in hospital diagnostics. The optical tweezers force on a shell bacterium is proportional to its wall thickness. Experimentally, we determine the optical tweezers force applied on each bacteria family by measuring their escape velocity. Then, the wall thickness of shell bacteria can be obtained after calibrating with known bacteria parameters. The method has been successfully applied to indentify, from blind tests, Methicillinresistant Staphylococcus aureus (MRSA), including VSSA (NCTC 10442), VISA (Mu 50), and heto-VISA (Mu 3)

Biomechanics plays a central role in breast epithelial morphogenesis. In this study we have used 3D cultures in which normal breast epithelial cells are able to organize into rounded acini and tubular ducts, the main structures found in the breast tissue. We have identified fiber organization as a main determinant of ductal organization. While bulk rheological properties of the matrix seem to play a negligible role in determining the proportion of acini versus ducts, local changes may be pivotal in shape determination. As such, the ability to make microscale rheology measurements coupled with simultaneous optical imaging in 3D cultures can be critical to assess the biomechanical factors underlying epithelial morphogenesis. This paper describes the inclusion of optical tweezers based microrheology in a microscope that had been designed for nonlinear optical imaging of collagen networks in ECM. We propose two microrheology methods and show preliminary results using a gelatin hydrogel and collagen/Matrigel 3D cultures containing mammary gland epithelial cells.

How the molecular structure of the structural, extracellular matrix protein collagen correlates with its mechanical properties at different hierarchical structural levels is not known. We demonstrate the utility of optical tweezers to probe collagen’s mechanical response throughout its assembly hierarchy, from single molecule force-extension measurements through microrheology measurements on solutions of collagen molecules, collagen fibrillar gels and gelatin. These experiments enable the determination of collagen’s flexibility, mechanics, and timescales and strengths of interaction at different levels of hierarchy, information critical to developing models of how collagen’s physiological function and stability are influenced by its chemical composition. By investigating how the viscoelastic properties of collagen are affected by the presence of telopeptides, protein domains that strongly influence fibril formation, we demonstrate that these play a role in conferring transient elasticity to collagen solutions.

In this report, we review our recent results in the optical micromanipulation of vesicles. Traditionally, vesicle manipulation has been possible by employing photon momentum and optical trapping, giving rise to unique observations of vesicle shape changes and soft matter mechanics. Contrary to these attempts, we employ photon energy rather than momentum, by sensitizing vesicles with an oxidizing moiety. The later converts incident photons to reactive oxygen species, which in turn attack and compromise the stability of the vesicle membrane. Both coherent and incoherent radiation was employed. Polymersome re-organization into smaller diameter vesicles was possible by focusing the excitation beam in the vicinity of the polymersomes. Extended vesicle illumination with a collimated beam lead to their complete destabilization and micelle formation. Single particle analysis revealed that payload release takes place within seconds of illumination in an explosive burst. We will discuss the destabilization and payload release kinetics, as revealed by high resolution microscopy at the single particle level, as well as potential applications in single cell biomodulation.

We present laser trapping behaviors of 200 nm-polystyrene particles in D₂O solution and at its surface using a focused continuous-wave laser beam of 1064 nm. Upon focusing the laser beam into the solution surface, the particles are gathered at the focal spot, and their assembly is expanded to the outside and becomes much larger than the focal volume. The resultant assembly is observed colored under halogen lamp illumination, which is due to a periodic structure like a colloidal crystal. This trapping behavior is much different compared to the laser irradiation into the inside of the solution where a particle-like assembly with a size similar to that of the focal volume is prepared. These findings provide us new insights to consider how radiation pressure of a focused laser beam acts on nanoparticles at a solution surface.

We discuss the design, implementation and performance of a novel platform for the production and optical control of ultra-low interfacial tension droplets in the 1-10 micron regime. A custom-designed, integrated microfluidic system allows the production of oil-in-water emulsion droplets of controllable size. This provides an optimised physical platform in which individual droplets are selected, trapped and shaped by holographic optical tweezers (HOTs) via extended optical landscaping. The 3D structure of the shaped droplet is interrogated by a combination of conventional brightfield imaging and fluorescent structured-illumination sectioning. We detail the problems and limitations of closed-loop holographic control of droplet shape.

We introduce tunable optofluidic microlasers based on optically stretched, dye-doped emulsion droplets confined in a dual-beam optical trap. Optically trapped microdroplets of oil emulsified in water and stained with fluorescent dye act as active ultrahigh-Q optical resonant cavities hosting whispering gallery modes (WGMs) which enable dye lasing with low threshold pump powers. In order to achieve tunable dye lasing, the droplets are pumped with a pulsed green laser beam and simultaneously stretched by light in the dual-beam trap. For a given stretching power, the magnitude of the droplet deformation is dictated by the interfacial tension between the droplet and the host liquid which is adjustable by adding surfactants. Increase of power of the dual-beam trap causes a directly proportional change of the droplet stretching deformation. Subsequently, resonant path lengths of different WGMs propagating in the droplet are modified, leading to shifts in the corresponding microlaser emission wavelenghts. Using this technique, we present all-optical, almost reversible spectral tuning of the lasing WGMs and show that the direction of wavelength tuning depends on the position of the pump beam focus on the droplet, consistent with the deformation of originally spherical droplet towards a prolate spheroid. In addition, we study the effects of changes of the droplet and immersion medium temperature on the spectral position of lasing WGMs and demonstrate that droplet heating leads to red-tuning of the droplet lasing wavelength.

We present a new type of stereo microscopy which can be used for tracking in 3D over an extended depth. The use of Spatial Light Modulators (SLMs) in the Fourier plane of a microscope sample is a common technique in Holographic Optical Tweezers (HOT). This set up is readily transferable from a tweezer system to an imaging system, where the tweezing laser is replaced with a camera. Just as a HOT system can diffract many traps of different types, in the imaging system many different imaging types can be diffracted with the SLM. The type of imaging we have developed is stereo imaging combined with lens correction. This approach has similarities with human vision where each eye has a lens, and it also extends the depth over which we can accurately track particles.

The relative motions of two or more neutral particles, subject to optical trapping forces within a beam, are influenced by intrinsic inter-particle forces. The fundamental character of such forces is well-known and usually derives from dispersion interactions. However, the throughput of moderately intense (off-resonant) laser light can significantly modify the form and magnitude of these intrinsic forces. This optical binding effect is distinct from the optomechanical interactions involved in optical tweezers, and corresponds to a stimulated (pairwise) forward-scattering mechanism. In recent years, attention has begun to focus on optical binding effects at sub-micron and molecular dimensions. At this nanoscale, further manipulation of the interparticle forces is conceivable on the promotion of optically bound molecules to an electronic excited state. It is determined that such excitation may influence the intrinsic dispersion interaction without continued throughput of the laser beam, i.e. independent of any optical binding. Nevertheless, the forwardscattering mechanism is also affected by the initial excitation, so that both the optical binding and dispersion forces can be manipulated on input of the electromagnetic radiation. In addition, the rate of initial excitation of either molecule (or any energy transfer between them) may be influenced by an off-resonant input beam which, thus, acts as an additional actor in the modification of the interparticle force. A possible experimental set-up is proposed to enable the measurement of such changes in the interparticle coupling.

We present a computational model for the simulation of optically interacting nano-structures immersed in a viscous fluid. In this scheme, nanostructures are represented by aggregates of small spheres. All optical and hydrodynamic interactions, including thermal fluctuations, are included. As an example, we consider optical binding of dielectric nanowires in counterpropagating plane waves. In particular, the formation of stable, ladder like structures, is demonstrated. In these arrangements, each nanowire lies parallel to the polarization direction of the beams, with their centres of mass colinear.

When several particles are illuminated by intense laser fields there exist also additional optical forces between the particles induced by scattered fields. This effect is called optical binding and the particles due to their mutual forces induced by light may create optically bound structure. As these forces are of second order (in comparison to forces induced by the incident fields) they are very sensitive to size or shape of the particles. We have previously experimentally studied the optical binding in the configurations employing two counter-propagation Bessel beams. These results were sufficiently supported by our numerical simulations (Coupled dipole method CDM). We study how the difference of sizes of two spherical particles influences their spatial stable separation. Even small changes in the limits of particle-size distribution of used particles may lead to experimentally verifiable differences. The intensities of the two beams must be also tuned to achieve sufficient stability of the whole couple in the experimental sample. By variation of mutual intensities we may also move the couple along the common optical axis. Another results may serve for detection of particles of different composition or shape. Very exciting configuration of ”Optical tractor beam” offers another and simple means of optical sorting. Our numerical results indicate that the behaviour is very sensitive to particles size. Here we present numerical studies how the sizes of individual particles (within the range of the particle-size distribution) influences the particles dynamics in various optical trapping configurations.

Due to the increased complexity in terms of materials and geometries for microsystems new assembling techniques are required. Assembling techniques from the semiconductor industry are often very specific and cannot fulfill all specifications in more complex microsystems. Therefore, holographic optical tweezers are applied to manipulate structures in micrometer range with highest flexibility and precision. As is well known non-spherical assemblies can be trapped and controlled by laser light and assembled with an additional light modulator application, where the incident laser beam is rearranged into flexible light patterns in order to generate multiple spots. The complementary building blocks are generated by a two-photon-polymerization process. The possibilities of manufacturing arbitrary microstructures and the potential of optical tweezers lead to the idea of combining manufacturing techniques with manipulation processes to “microrobotic” processes. This work presents the manipulation of generated complex microstructures with optical tools as well as a storage solution for 2PP assemblies. A sample holder has been developed for the manual feeding of 2PP building blocks. Furthermore, a modular assembling platform has been constructed for an ‘all-in-one’ 2PP manufacturing process as a dedicated storage system. The long-term objective is the automation process of feeding and storage of several different 2PP micro-assemblies to realize an automated assembly process.

By moulding optical fields, holographic optical tweezers are able to generate structured force fields with magni- tudes and length scales of great utility for experiments in soft matter and biological physics. Optically induced force fields are determined not only by the incident optical field, but by the shape and composition of the par- ticles involved. Indeed, there are desirable but simple attributes of a force field, such as rotational control, that cannot be introduced by sculpting optical fields alone. In this work we describe techniques for the fabrication, sample preparation, optical manipulation and position and orientation measurement of non-spherical particles. We demonstrate two potential applications: we show how the motion of a non-spherical optically trapped force probe can be used to infer interactions occurring at its tip, and we also demonstrate a structure designed to be controllably rotated about an axis perpendicular to the optical axis of the beam.

Laser-induced thermal effects in optically trapped microspheres and single cells have been investigated by Luminescence Thermometry. Thermal spectroscopy has revealed a non-localized temperature distribution around the trap that extends over tens of microns, in agreement with previous theoretical models. Solvent absorption has been identified as the key parameter to determine laser-induced heating, which can be reduced by establishing a continuous fluid flow of the sample. Our experimental results of thermal loading at a variety of wavelengths reveal that an optimum trapping wavelength exists for biological applications close to 820 nm. This has been corroborated by a simultaneous analysis of the spectral dependence of cellular heating and damage in human lymphocytes during optical trapping. Minimum intracellular heating, well below the cytotoxic level (43 °C), has been demonstrated to occur for optical trapping with 820 nm laser radiation, thus avoiding cell damage.

We use highly a focused laser beam incident on a carbon coated coverslip to create microcavitation. Full optical control of the radii of the bubbles is attained. Multiple bubbles can also be created and their size changed independently. The dynamics of such multi-bubble systems are studied. These bubble systems generate strong flows such as Marangoni convection and also large thermal gradients. Since the size of the micro-bubbles is highly dependent on the temperature, we anticipate that these systems can be used for precise temperature control of samples. These methods are of use when the knowledge of exact and local temperature profiles are of importance. Furthermore, since bubble expansion can generate orders of magnitude more force than conventional optical tweezers, systems have application in manipulation of particles where large forces are required. We present methods based on optical tweezers for using the generated bubbles as thermal sensors and as opto-mechanical transducers.

We investigate holographic optical trapping combined with step-and-repeat maskless projection stereolithography for fine control of 3D position of living cells within a 3D microstructured hydrogel. C2C12 myoblast cells were chosen as a demonstration platform because their development into multinucleated myotubes requires linear arrangements of myoblasts. C2C12 cells are positioned in the monomer solution with multiple optical traps at 1064 nm and then are encapsulated by photopolymerization of monomer via projection of a 512x512 spatial light modulator (SLM) illuminated at 405 nm. High 405 nm sensitivity and complete insensitivity to 1064 nm is enabled by a lithium acylphosphinate (LAP) salt photoinitiator. Use of a polyethylene glycol dimethacrylate (PEGDMA) based monomer is compared to that of polyethylene glycol (PEG) hydrogels formed by thiol-ene photo-click chemistry for patterning structures with cellular resolution, and for maintaining cell viability. Cells patterned in thiol-ene with RGD are shown to retain viability up to 4 days after the trapping and encapsulation procedure. Further, cells patterned in thiol-ene with RGD and a degradable ester link, are shown to fuse, indicating the initial stages of development of multi-nucleated cells.

Localized surface plasmons (LSPs) have been investigated for applications such as highly sensitive spectroscopies and the enhancement of photochemical reactions. These applications are enabled by the enhancement effect of an incident resonant electromagnetic field (EMF) at the surfaces of noble metallic nanostructures. In particular, the application of LSP has recently attracted much attention for achieving the effective optical trapping of nanoparticles; this is called LSPbased optical trapping (LSP-OT). LSP-OT possesses several advantages; (i) the EMF enhancement effect of LSP enables the incident light intensity to be significantly reduced for stable LSP-OT, (ii) a nano-sized object can be trapped in a nano-space whose volume is much smaller than that of conventional optical tweezers (diffraction limit), (iii) a large and complicated optical set-up is not necessary, and (iv) this technique can potentially be combined with microfluidic devices. That is, plasmonic substrates can work as “double-functional” devices where biomolecules trapped by LSPOT can subsequently be analyzed on the basis of SERS or fluorescence enhancement. Thus, LSP-OT could enable a new technique for manipulating not only nanoparticles, but also smaller molecules such as polymer chains, proteins and DNA. Here, we will present the demonstration of LSP-OT of fluorescent-labeled polystyrene nanospheres. We discuss multiple optical trapping in which a closely packed 2D hexagonal assembly appeared on a metallic nanostructure.

Spatial light modulators (SLMs) are used to diffract light beams for a variety of applications. In particular, optical tweezer trapping has greatly benefited from the advent of phase-mostly and phase-only SLMs to write holograms that produce multiple traps. We are using holographic optical tweezers to trap multiple sensor particles in a lab-on-a-chip measurement platform. As part of this program, we have developed a method for optimizing the diffraction efficiency of a SLM. This general method can be applied in situ and addresses the issues of nonlinear phase modulation and phase modulation less than 2π. The method employs a one-dimensional blazed phase grating written on the SLM. For an ideal SLM, the phase shift is linear and covers 0 - 2π, yielding a first-order diffraction efficiency of unity. For a realistic SLM with nonlinear or reduced phase shift, the efficiency is approximately η = 1 - σ², where σ² is the variance of the phase error from the ideal case. Because each pixel contributes to the phase error independently, this suggests a method to maximize the efficiency by adjusting the phase encoding of the SLM pixel-by-pixel. In practice, we do this by adjusting the gray-scale of each pixel while measuring the first-order diffracted power. The collection of optimal gray values comprises the optimized gray-scale lookup table, which exhibits the nonlinearity required to produce a linear phase grating and the saturated phase encoding that maximizes the efficiency of phase limited SLMs. We have successfully applied this optimization method to two different SLMs.

The maintenance of intact genetic information, as well as the deployment of transcription for specific sets of genes, critically rely on a family of proteins interacting with DNA and recognizing specific sequences or features. The mechanisms by which these proteins search for target DNA are the subject of intense investigations employing a variety of methods in biology. A large interest in these processes stems from the faster-than-diffusion association rates, explained in current models by a combination of 3D and 1D diffusion. Here, we describe the combination of optical tweezers and single molecule fluorescence detection for the study of protein-DNA interaction. The method offers the opportunity of investigating interactions occurring in solution (thus avoiding problems due to closeby surfaces as in other single molecule methods), controlling the DNA extension and tracking interaction dynamics as a function of both mechanical parameters and DNA sequence.

We present a novel model for the prediction of particle trapping, in which different assets are combined to calculate optical forces and torques on free-shaped particles. This model suits well for designing an integrated dual fiber optical trap into a microfluidic chip. The force and torque estimation can easily be integrated in the scheme simulating the surrounding optofluidic chip. The propagation of the trapping laser light going through this micro-optical chip is ensured by the ‘Gaussian Beam Propagation Method’ allowing a realistic propagation of various beam profiles in and near the Rayleigh range. Back-reflected rays into and outside the particle are considered by using a non-sequential ray-tracing model. Our model enables the prediction of circumstances in which particles are trapped and how they behave in the optical trap.

Optical forces can affect the motion of a Brownian particle. For example, optical tweezers use optical forces to trap a particle at a desirable position. Using more complex force fields it is possible to generate more complex configurations. For example, by using two optical traps placed next to each other, it is possible to obtain a bistable potential where a particle can jump between the two potentials with a characteristic time scale. In this proceeding, we discuss a simple finite difference algorithm that can be used to simulate the motion of a Brownian particle in a one-dimensional field of optical forces.

We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution.

Rayleigh scattering correlation microspectroscopy is developed and applied to study diffusion dynamics of some nanospheres in water. It was clearly found that the diffusion constant of gold nanoparticles decreased with increasing excitation laser power at the excitation wavelength of higher absorption cross section. This behavior was explained in terms of a coupling between laser trapping by the scattering excitation laser itself and laser heating of the particle. In the case of non-absorbing nanospheres such as silica and polystyrene, the excitation power dependence can be ascribed only to the laser trapping. Experimental setup is introduced, theoretical formulation is described, and future development of this measurement is considered.

The T-matrix method with the Vector Spherical Wave Function (VSWF) expansions represents some difficulties for computing optical scattering of anisotropic particles. As the divergence of the electric field is nonzero in the anisotropic medium and the VSWFs do not satisfy the anisotropic wave equations one questioned whether the VSWFs are still a suitable basis in the anisotropic medium. We made a systematic and careful review on the vector basis functions and the VSWFs. We found that a field vector in Euclidean space can be decomposed to triplet vectors {L, M, N}, which as non-coplanar. Especially, the vector L is designed to represent non-zero divergence component of the vector solution, so that the VSWF basis is sufficiently general to represent the solutions of the anisotropic wave equation. The mathematical proof can be that when the anisotropic wave equations is solved in the Fourier space, the solution is expanded in the basis of the plan waves with angular spectrum amplitude distributions. The plane waves constitute an orthogonal and complete set for the anisotropic solutions. Furthermore, the plane waves are expanded into the VSWF basis. These two-step expansions are equivalent to the one-step direct expansion of the anisotropic solution to the VSWF basis. We used direct VSWF expansion, along with the point-matching method in the T-matrix, and applied the boundary condition to the normal components displacement field in order to compute the stress and the related forces and torques and to show the mechanism of the optical trap of the anisotropic nano-cylinders.

The work is a study of micromanipulation techniques using DOE-modulated laser beams. Two kinds of crescentshaped and optical bottle beam kinds are covered. The numerical model and experimental results on the trapping and manipulating of yeast cells with modulated laser beams are presented. They show that modulated beams are sometimes more efficient than common Gaussian beam: crescent beams are able to achieve better trap stiffness at the price of altered spatial distribution of that stiffness. Optical bottles are beneficial for their ability to hold the trapped object by its edges therefore decreasing amount of energy absorbed by the microbiological sample.

In previous work we developed methods using optical tweezers to measure protein-mediated formation of loops in DNA structures that can play an important role in regulating gene expression. We previously applied this method to study two-site restriction endonucleases, which were convenient model systems for studying this phenomenon. Here we report preliminary work in which we have applied this method to study p53, a human tumor suppressor protein, and show that we can measure formation of loops. Previous biophysical evidence for loops comes from relatively limited qualitative studies of fixed complexes by electron microscopy4. Our results provide independent corroboration and future opportunities for more quantitative studies investigating structure and mechanics.

Many double-stranded DNA viruses employ a molecular motor to package DNA into preformed capsid shells. Based on structures of phage T4 motor proteins determined by X-ray crystallography and cryo-electron microscopy, Rao, Rossmann and coworkers recently proposed a structural model for motor function. They proposed that DNA is ratcheted by a large conformational change driven by electrostatic interactions between charged residues at an interface between two globular domains of the motor protein. We have conducted experiments to test this model by studying the effect on packaging under applied load of site-directed changes altering these residues. We observe significant impairment of packaging activity including reductions in packaging rate, percent time packaging, and time active under high load. We show that these measured impairments correlate well with alterations in free energies associated with the conformational change predicted by molecular dynamics simulations.

In many viruses molecular motors generate large forces to package DNA to high densities. The dynamics and energetics of this process is a subject of wide debate and is of interest as a model for studying confined polymer physics in general. Here we present preliminary results showing that DNA in bacteriophage phi29 undergoes nonequilibrium conformational dynamics during packaging with a relaxation time >60,000x longer than for free DNA and >3000x longer than reported for DNA confined in nanochannels. Nonequilibrium dynamics significantly increases the load on the motor, causes heterogeneity in the rates of packaging, and causes frequent pausing in motor translocation.

Optical tweezers allow recording mechanical data from single biological molecules such as molecular motors, DNA processing enzymes, nucleic acids. Such data consist of time series that are dominated by thermal noise and such noisy recordings require proper analysis to correctly extract kinetic and mechanical information. Several different analysis approaches have been established in the past years. Here, we propose an analysis method for optical trapping recordings of non-processive motor proteins. The method does not assume any particular interaction kinetics, allows detection of sub-millisecond interactions and quantification of the number of false and lost events. Precise alignment of interaction events and ensemble averaging allow the investigation of the stepping dynamics of non-processive motors with a temporal resolution of few tens of microseconds and a spatial resolution of few angstroms. Our analysis is applied to the study of the motor protein myosin from fast skeletal muscle. Thanks to the high spatio-temporal resolution, we can distinguish three mechanical pathways in the acto-myosin interaction, with several orders of magnitude different kinetics, which contribute in a load-dependent manner to the myosin working stroke.