A century has now passed since the origins of the Abraham-Minkowski controversy pertaining to the correct form of optical momentum in media. Since, the debate has come to reference the general debate over optical momentum, including a number of competing formulations. The pervasive modern view is that the Abraham momentum represents the optical momentum contained within the fields and the Minkowski momentum includes a material component which is coupled with the fields. A recently proposed resolution to the debate identified Abraham’s kinetic momentum as responsible for the overall center-of-mass translations of a medium and Minkowski’s canonical momentum as responsible for local translations of a medium within or with respect to another medium. Still, current literature reveals significant confusion as to how systems of light and matter should be modeled as to deduce the equations of motion when multiple material types are present. For example, the state-of-the-art model for optical dynamics of submerged particles assumes over damped systems such that the mass of the particles is ignored in the equations of motion. In this paper, we apply the subsystem approach to deduce the electrodynamics of such systems. We show that regardless of which electromagnetic momentum continuity law is applied, the equations of motion can be correctly deduced as long as the continuity law is consistent with Maxwells equations and the overall system is closed such that momentum is conserved. Because the closed system includes the material response, the model can be very complex. However, we demonstrate with simple, well-known examples.

The Abraham–Minkowski momentum controversy is the outwardly visible symptom of an inconsistency in the use of the energy–momentum tensor in the case of a plane quasimonochromatic field in a simple linear dielectric. We show that the Gordon form of the electromagnetic momentum is conserved in a thermodynamically closed system. We regard conservation of the components of the four-momentum in a thermodynamically closed system as a fundamental property of the energy–momentum tensor. Then the first row and column of the energy–momentum tensor is populated by the electromagnetic energy density and the Gordon momentum density. We derive new electromagnetic continuity equations for the electromagnetic energy and momentum that are based on the Gordon momentum density. These continuity equations can be represented in the energy–momentum tensor using a material four-divergence operator in which temporal differentiation is performed with respect to ct/n.

Electromagnetic waves carry energy as well as linear and angular momenta. When a light pulse is reflected from, transmitted through, or absorbed by a material medium, energy and momentum (both linear and angular) are generally exchanged, while the total amount of each entity remains intact. The extent of such exchanges between light and matter can be deduced, among other methods, with the aid of the Doppler shift phenomenon. The main focus of the present paper is on the transfer of angular momentum from a monochromatic light pulse to spinning objects such as a mirror, an absorptive dielectric, or a birefringent plate. The fact that individual photons of frequency ω_o carry energy in the amount of ħω_o, where ħ is Planck’s reduced constant, enables one to relate the Doppler shift to the amount of energy exchanged. Under certain circumstances, the knowledge of exchanged energy leads directly to a determination of the momentum transferred from the photon to the material body, or vice versa.

Cavity optomechanics is a rapidly evolving field operating at the intersection of solid-state physics and modern optics. The fundamental process at the heart of this interdisciplinary endeavor is the enhancement of radiation pressure within a high-finesse optical cavity. Isolating this weak interaction, i.e. the momentum transfer of photons onto the cavity boundaries, requires the development of mechanical resonators that simultaneously exhibit high reflectivity (requiring low absorption and scatter loss) and low mechanical dissipation. In a Fabry-Pérot implementation, this is realized by fabricating suspended micrometer-scale mechanical resonators directly from high-reflectivity multilayers. Thus, the properties of the mirror material—particularly the loss angle and optical absorption—drive the ultimate performance of the devices. Interestingly, similar requirements are found in a broad spectrum of applications, ranging from gravitational wave interferometers to stabilized lasers for optical atomic clocks. This overlap leads to an intimate link between advances in the seemingly disparate areas of macroscopic interferometry (e.g. precision measurement and spectroscopy) and micro- and nanoscale optomechanical systems. In this manuscript, I will outline the fascinating implications of cavity optomechanics and present proof-of-concept experiments including MHz-frequency resonators aimed at the demonstration of quantum states of mechanical systems, as well as low-frequency (<1 kHz) devices for the observation of quantum radiation pressure noise. Additionally, I will discuss off-shoot technologies developed in the course of this work, such as a numerical solver for the determination of support-mediated losses in mechanical resonators, as well as a new strategy for the realization of ultra-high-stability optical reference cavities based on transferred crystalline multilayers.

Cavity and Doppler cooling of trapped silica nanospheres and microspheres to their motional ground state is described. Characterisation of the levitation of a range of silica spheres from radius 25 nm to 5 µm in both optical and ion traps in vacuum is reported and prospects for realizing cooling in these systems is discussed.

The optical eigenmode technique offers a global optimisation method delivering the electromagnetic field profile enhancing any linear light-matter interaction. Here, we use this approach to study the optimal beams for trapping, pushing and pulling (tractor beam) a mesoscopic micro-object at resonance with the incident light field.

In this paper we present theory and simulations of an optical spring mirror with emphasis on the incident laser beam configuration and the associated optical trapping forces. We elucidate the physical mechanisms underlying the optical trapping using the example of an incident Gaussian beam and demonstrate that guided-wave trapping shows particular promise for stable trapping in both the translational and rotational degrees of freedom.

We report on experiments studying the Brownian motion of an optically trapped bead in air, and observe for the first time the short-time regime of ballistic motion. Einstein predicted this effect in 1907, but said the experiment would be impossible in practice. Our measurements were enabled by our development of a new detection system that is capable of real-time tracking of the motion of a trapped bead on unprecedented short time scales and correspondingly small length scales. We used the data to measure the average kinetic energy of a Brownian particle, and find good agreement with the energy equipartition theorem of statistical mechanics. Measurement of the instantaneous velocity also allows us to stably trap beads in vacuum, using active feedback to control and cool the center of mass motion to mK temperatures in three dimensions. The system of an optically trapped bead in vacuum can serve as a testing ground for macroscopic quantum superpositions and the role of decoherence. In the opposite extreme, a trapped bead in a fluid can be used to test basic questions in statistical mechanics, and fluid dynamics on the smallest scales.

In this paper, we focus on a particular application of the nanomechanical resonator: mass sensing in all-optical domain. Convectional mass detection is usually based on the electrical environment, where nanomechanical resonator should be suspended between two electrodes above a conducting plate, while a voltage applies on them. However, the heating effect and the energy loss induced by the voltage during the measurement will lead to the imprecision of mass sensing. In order to solve this problem, we propose an ultrasensitive optical scheme to weigh the external particles deposited onto the surface of the nanomechanical resonator via all-optical methods. This optical mass sensing has the potential to break through the limitation of frequency restriction and the sensitivity of mass sensing.

The possibility of measuring microscopic forces down to the femtonewton range has opened new possibilities in fields such as biophysics and nanophotonics. I will review some of the techniques most often employed, namely the photonic force microscope (PFM) and the total internal reflection microscope (TIRM), which are able to measure tiny forces acting on optically trapped particles. I will then discuss several applications of such nanoscopic forces: from plasmonic optical manipulation, to self-propelled microswimmers, to self-organization in large ensembles of particles.

Correlation function analysis combined with optical tweezers technique is proposed for studying of magnetic interaction influence on statistical properties of microparticles Brownian motion in liquid. It is shown that autocorrelation function of Brownian particle displacements from optical trap center contains information about particles rotation frequency in rotating magnetic field. Powerful method based on correlation analysis to detect the interaction between paramagnetic microparticles in a constant magnetic field is suggested. Experimental results show that magnetic interaction changes cross-correlation function of nanometer displacements of two optically trapped particles in a constant external magnetic field.

High precision position measurements often involve the detection of a laser beam that interacts with various components of an experimental setup. In order to achieve the highest precision, instabilities that contribute to a decrease in precision must be identified and quantified. Instabilities include fluctuations in the laser power, fluctuations in the laser pointing and fluctuations in the phase, as well as vibrating mechanical components that are susceptible to excitations and drift. Instabilities lead to unwanted resonances and band structures in the power spectral density of the detector signals. Typically, the most important instabilities are identified by the magnitude and location of resonances or bands in the power spectral density. However, power spectral density plots can be misleading if the width or shape of a resonance or a band are not correctly accounted for. This is especially true for measurements that span a large bandwidth. Here, we discuss Power Spectral Density Integration Analysis as a more intuitive and accurate method for identifying and quantifying instabilities. Resonances and bands are readily identified as step-like features with heights that correctly represent their contribution to the error in the position measurement.

Stem cells are rich in proteins, carbohydrates, deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and various other cellular components which are responsible for a diversity of functions. Mostly the building blocks of these intracellular entities play an active role in absorbing ultra-violet (UV) and visible light sources. Light-matter interactions in biomaterials are a complex situation and subsequent damage may not always amount only from wavelength dependent effects but may also be driven by a wealth of other optical parameters which may lead to a variety photochemical reactions. Previously, literature has reported efficient photo-transfection and differentiation of pluripotent stem cells via near infrared (NIR) femtosecond (fs) laser pulses with minimum compromise to their viability. Therefore, in this study the influence of using different fs laser wavelengths on optical stem cell transfection and differentiation is investigated. A potassium titanyl phosphate (KTP) crystal was employed in frequency doubling a 1064 nm fs laser beam. The newly generated 532 nm fs pulsed beam was then utilized for the first time in transient photo-transfection of ES-E14TG2a mouse embryonic stem (mES) cells. Compared to using 1064 nm fs pulses which non-invasively introduce plasmid DNA and other macromolecules into mES cells, our results showed a significant decline in the photo-transfection efficiency following transfecting with a pulsed fs visible green beam.

The introduction and subsequent expression of external DNA inside single living mammalian cell (transfection) can be achieved by photoporation with femtosecond laser. After photoporation, external DNA can be introduced by trapping and successive insertion of DNA coated nanoparticle in the cell using optical tweezers. To maximize the transfection efficiency, one of the major aspects is that the photoporated cell should not be damaged and cell membrane should heal itself immediately or after sometime while the cells are healed in the CO₂ incubator. Furthermore, the size of hole created as a result of photoporation should be more than the size of DNA coated nanoparticle to be inserted inside the cell. In this paper, an analysis has been done on single cell of important breast cancer cell lines named MCF-7 and MDAMB231. Size of holes created in cell membrane after photoporation has been measured and the required optimum energy with sustained cell life were determined. Using this analysis, most favorable conditions for maximum transfection efficiency can be determined.

We have combined a laser scissors and a laser tweezers to study, (1) the response of nerve fiber growth cones to laser-induced damage on single axons, and (2) localized microfluidic flow generated by laser-driven spinning birefringent particles. In the laser scissors study, sub-axotomy damage elicits a growth cone response whether damage is on the same or an adjacent axon. In laser tweezers study, the axon growth cones turn in response to the optically driven microfluidic flow. In summary, both the laser scissors and the laser tweezers studies elicit growth cone turning responses.o

Double optical tweezers combined with active rheology approach are suggested for dynamic monitoring of the red blood cell elastic properties. Frequency dependence of the phase difference in the forced movement of the erythrocyte opposite edges appeared to be highly dependent on the rigidity of the cellular membrane. Cell relaxation time value is suggested as an effective parameter determining the state of the cell. Photo-induced effects caused by optical trapping are analyzed.

Optically trapped metallic nanoparticles hold great promise as heat transducers in photothermal applications such as drug delivery assays or photothermal therapy. We use the heat dissipated from an optically trapped gold nanosphere to perform a controlled release of a fluorescently labeled vesicle lumen. In the assay, the ambient temperature is kept below the phase transition temperature of the vesicle. When the temperature reaches the phase transition temperature of the lipid, the vesicle becomes leaky and the fluorescently marked lumen diffuses out. We used gel phase vesicles as sensors to quantify the temperature profile around a nanoparticle optically trapped in three dimensions in a similar way as presented in Ref.¹ Trapping of 200 nm gold particles resulted in lower than expected heating, which may be accredited to the displacement of the particle from the optical focus due to high scattering forces experienced by the particle.

Yeast glycolytic oscillations have been studied since the 1950s in cell free extracts and in intact cells. Until recently, sustained oscillations have only been observed in intact cells at the population level. The aim of this study was to investigate sustained glycolytic oscillations in single cells. Optical tweezers were used to position yeast cells in arrays with variable cell density in the junction of a microfluidic flow chamber. The microfluidic flow chambers were fabricated using soft lithography and the flow rates in the different inlet channels were individually controlled by syringe pumps. Due to the low Reynolds number, the solutions mixed by diffusion only. The environment in the junction of the chamber could thus be controlled by changing the flow rates in the inlet channels, with a complete change of environment within 2 s. The optimum position of the cell array was determined by simulations, to ensure complete coverage of the intended solution without any concentration gradients over the cell array. Using a DAPI filter set, the NADH auto fluorescence could be monitored in up to 100 cells simultaneously. Sustained oscillations were successfully induced in individual, isolated cells within specific flow rates and concentrations of glucose and cyanide. By changing the flow rates without changing the surrounding solution, it was found that the cell behavior was dependent on the concentration of chemicals in the medium rather than the flow rates in the range tested. Furthermore, by packing cells tightly, cell-to-cell interaction and synchronization could be studied.

The transverse force profile of a particle in an optical trap is important for the designs of optical trapping-based force transducers. We mapped these force profiles for micron-size polystyrene beads using a pair of overlapping optical traps produced by two highly focused Gaussian beams with unequal intensity; the stronger trap serves as a force transducer to measure the force of the weaker trap in both linear and nonlinear regimes. For particles with size smaller or comparable to the laser wavelength, the force profiles follow closely the gradient of the Gaussian profile, but as the particle size increases, the force profiles deviate from the shape of the gradient of Gaussian for the distance beyond the position of the maximum force. The distance from the center of the trap to the position of the maximum trapping force was found to increase linearly with the particle size. The experimental results are in good agreements with our theoretical model, based on a combination of the Mie theory, vector Debye integral, and Maxwell stress tensor; except that the experimental particle-size dependence of the maximum trapping forces was found to be weaker than that predicted by the theory.

We are using tethered particle motion (TPM) microscopy to observe protein-mediated DNA looping in the lactose repressor system in DNA constructs with varying AT / CG content. We use these data to determine the persistence length of the DNA as a function of its sequence content and compare the data to direct micromechanical measurements with constant-force axial optical tweezers. The data from the TPM experiments show a much smaller sequence effect on the persistence length than the optical tweezers experiments.

Single-molecule studies of the mechanical properties of individual double-stranded DNA have excited interest across many scientific disciplines because of DNA’s fundamental role in biology and DNA’s remarkable overstretching transition at higher forces. Here, we discuss a recent result on the overstretching transition of DNA and on the dynamics of dye molecules intercalating into DNA under tension. Overstretching DNA is mechanical transition whereby DNA’s extension increases by 70% at 65 pN. Notwithstanding more than a decade of experimental and theoretical studies, there remains significant debate on the nature of overstretched DNA. We developed a topologically closed but torsionally unconstrained DNA assay that contains no nicks or free ends. DNA in this assay exhibited the canonical overstretching transition at 65 pN but without hysteresis upon retraction. Controlled introduction of a nick led to hysteresis in the force extension curve. Moreover, the degree of hysteresis increased with the number of nicks. In the second study, we isolated the effects of binding and intercalation of a DNA staining dye, by combining single molecule force spectroscopy with simple buffer exchange. We showed that force-enhanced intercalation can occur from a reservoir of bound dye that was not bis-intercalated, yet remained out of equilibrium with free dye for long periods (<5 min for YOPRO and <2 hr for YOYO). Our work highlights that binding/unbinding and intercalation/de-intercalation are distinct processes that can occur on very different time scales. Taken together, these works highlight ongoing discoveries based on a twenty year old technique, force spectroscopy of single DNA molecules.

Polyamine ions such as spermidine³⁺, along with monovalent and divalent salt ions, screen the negatively charged backbone of dsDNA and thereby facilitate processes in which DNA is confined in small spaces, such as viral DNA packaging. We use optical tweezers to directly manipulate single DNA molecules and have made preliminary measurements of the effect of spermidine on DNA elasticity, condensation, and viral packaging. We determine the concentration of spermidine³⁺ at which dsDNA condenses in the presence of Mg²⁺ and Na⁺ and report a monotonic increase in stretch modulus and decrease in persistence length at incremental spermidine concentrations up to the concentration at which dsDNA condenses. We also discuss the effect of spermidine on DNA packaging in bacteriophage phi29.

The motion of a particle in an optical field is determined by the interplay between the geometry of the incident optical field, and the geometry and composition of the object. There are, therefore, two complementary roots to generating a particular force field. The first, involving sculpting of the optical field with, for example, a spatial light modulator, has been extensively developed. The second approach, which involves sculpting of the particles themselves, has been highlighted recently, but has received much less attention [J. Gluckstad, Nature Photonics, 5, 7–8 (2011)]. However, as modern fabrication methods advance, this avenue becomes increasingly attractive. In the following contribution we show how computational methods may be used to optimize particle geometries so as to reproduce desirable forms of behaviour. In particular, we exhibit a constant force optical spring for use as a passive force clamp in force sensing applications and a high efficiency optical wing.

We demonstrate the use of a two-beam optical trap (an optical stretcher) in a low-cost microfluidic system with the purpose of measuring the mechanical properties of cells and vesicles. Delivery of micrometer-sized particles and cells to the optical stretcher is obtained by acoustophoretic prefocusing. This focusing mechanism aims for target particles to always flow in the correct height relative to the optical stretcher, and is induced by a piezo-electric ultrasound transducer attached underneath the chip and driven at a frequency leading to a vertical standing ultrasound wave in the microchannel. Trapping and manipulation is demonstrated for dielectric beads. In addition, we show trapping, manipulation and stretching of red blood cells and vesicles, whereby we extract the elastic properties of these objects. Our design points towards the construction of a low-cost, high-throughput lab-on-a-chip device for measurement of mechanical properties of cells and vesicles.

We consider the trapping of low refractive index objects, such as ultrasound contrast agent microbubbles, in a dual-beam fibre-optic trap. We confirm numerically that such a configuration results in stable trapping and we present the calculated trapping forces and spring constants. Furthermore we calculate the photonic stress profile over the surface of the trapped microbubble using both ray optics and Mie scattering approaches, and compare the results. We then find the optical stress-induced deformation of the microbubble for both the ray optics and Mie scattering stress profiles using linear elastic membrane theory. We suggest that this method could be a useful tool for quantifying the mechanical properties of the shell material of an ultrasound contrast agent microbubble.

Recently there has been growing interest in what is called active matter, or collections of particles that are self driven rather than driven with an external field. Examples of such systems include swimming bacteria, flocks of birds or fish, and pedestrian flow. There have also been recent experimental realizations of self-driven systems using colloidal particles undergoing self-catalytic interactions. One example of this is light-induced catalysis where the colloids become self-driven in the presence of light. Almost all of these studies have been performed in the absence of a substrate. Here we examine how a substrate can be used to direct the motion of the particles. We demonstrate a self-induced ratchet effect that occurs in the presence of disorder as well as the direction of the particle along symmetry directions of the substrate. The type of substrate we consider may be created using various optical techniques, and studies of this system could lead to insights into the nonequilibrium behavior of active matter as well as to applications such as sorting of different active particle species or of active and non-active particles.

There are many examples of particle assemblies where the particles have competing repulsive and attractive interactions. In solid state systems, it has recently been proposed that exotic vortex states in type-I and type-II superconducting hybrids and type-1.5 superconductors fall into this category. In soft matter systems, competing interactions can arise for charged colloids with short range attraction or with multiple length scale interactions. Systems with competing interactions have been shown to exhibit a wide variety of patterns including stripes, labyrinths, bubbles, and crystalline phases. Although there has been considerable work analyzing these phases for different relative interaction strengths, there is little work on understanding what happens when such systems are driven over a periodic substrate. Such substrates for collective assemblies of particles could be created lithographically or using optical trap arrays and would introduce a new length scale into the system. Here we examine how a system with competing interactions behaves when interacting with a square periodic substrate. We find a novel wetting-dewetting phenomena similar to that of liquids on surfaces. In the presence of a strong substrate, the pattern formation normally found for particles with competing interactions is lost and the particles completely cover the substrate homogeneously. Under an applied drive, such a wetted system undergoes a transition to a partially dewetted state with anisotropic transport and structural features.

Rectification of random motion of particle a motional array of optical traps created by interference of two counter-propagating evanescent waves, i.e. in a so called optical conveyor belt (OCB), leads to the directed and controlled motion of such a particle. Particle clusters are formed if more than one particle enters the OCB. The particles were optically self-arranged into a linear chain with well-separated distances between them. We observed a significant increase in the delivery speed of the whole structure as the number of particles in the chain increased. This could provide faster sample delivery in microfluidic systems. We quantified the contributions to the speed enhancement caused by the optical and hydrodynamical interactions between the particles.

We present an experimental technique allowing size-based all-optical sorting of gold nanoparticles. The technique is based on the red-shift of plasmon resonance, due to retardation effects, with increasing particle size. As a result, smaller gold nanoparticles are influenced strongly by shorter wavelengths whereas larger gold nanoparticles are influenced more strongly by longer wavelengths. We utilise this retardation effect and realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.

Optical trapping is a promising technique which involves holding and manipulating small particles in a non-destructive way. Conventional trapping methods are able to trap dielectric particles with size greater than 100 nm. Using a double-nanohole in a metal film (with sharp tips where the holes meet) has enabled us to trap dielectric particles such as quantum dots and single proteins. This has been achieved even while using low laser power. Since the refractive index of the particle is larger than the surrounding environment, the aperture appears larger when the particle enters the aperture. This allows for more light transmitted through the aperture. The change in transmission changes the light momentum, and by Newton’s third law, there will be a force which will push back the particle to the equilibrium position. The change in light transmission also allows for facile detection of the trapping event. In this work, we use the double-nanohole to trap encapsulated quantum dots. Quantum dots are practically useful for several purposes including computing, biology and electronic devices. The ability to manipulate these particles with precision is critical to development of quantum dots usage in these fields. The CdS quantum dots, which are used in this work, are coated with a polymer shell, with a total size between 20 nm to 22 nm. The trapping and manipulation of quantum dots is promising for nanofabrication technologies that seek to place a quantum dot at a specific location in a plasmonic or nanophotonic structure. The next step in this research will be imaging of quantum dots using their fluorescence while trapping is occurring, so that a clear indication of trapping event will be available.

When irradiated at its resonance frequency, a metallic nanoparticle efficiently converts the absorbed energy into heat which is locally dissipated. This effect can be used in photothermal treatments, e.g., of cancer cells. However, to fully exploit the functionality of metallic nanoparticles as nanoscopic heat transducers, it is essential to know how the photothermal efficiency depends on parameters like size and shape. Here we present the measurements of the temperature profile around single irradiated gold nanorods and nanospheres placed on a biologically relevant matrix, a lipid bilayer. ^[1] We developed a novel assay based on molecular partitioning between two coexisting phases, the gel and fluid phase, within the bilayer. ^{[2, 3]} This assay allows for a direct measurement of local temperature gradients, an assay which does not necessitate any pre-assumptions about this system and is generally applicable to any irradiated nanoparticle system. The nanorods are irradiated with a tightly focused laser beam at a wavelength of 1064 nm where biological matter exhibits a minimum in absorption. By controlling the polarization of the laser light we show that the absorption of light by the nanorod and the corresponding dissipated heat strongly depends on the orientation of the nanorod with respect to the polarization. Finally, by comparing to spherical gold nanoparticles, we demonstrate how a change in shape, from spherical to rod like, leads to a dramatic enhancement of heating when using near infrared light.

In this paper, small plasmonic nanobumps, which consist of metal/dielectric layers are placed on the ring of optical vortex to enhance electric field ampltiude. In this paper, a plasmonic nanobump is placed on the ring of smaller optical vortex. The smaller optical vortex form from the resultant topological phase between the handedness of the incident circular polarized light and the nanoslits spiral. Different designs of plasmonic nanobump are investigated, and tapered nanobump produced higher field enhancement due to higher surface charge density at the tapering end. Higher field intensity at the tip of the plasmonic nanobump produces lower potential, which attract nanoparticle to the region. The optical force increases by the square of the electric field amplitude. This high electric field intensity at the plasmonic nanobump functions as attractive node, which trap molecules inside the optical vortex. Additional plasmonic nanobumps are added onto the other locations of the optical vortex to manipulate the particle trapping positions. This allows the precise control of molecule’s position and movement for imaging, characterization and analysis, which is useful for mobile lab-on-chip devices.

Robotics can use optics feedback in vision-based control of intelligent robotic guidance systems. With light’s miniscule momentum, shrinking robots down to the microscale regime creates opportunities for exploiting optical forces and torques in microrobotic actuation and control. Indeed, the literature on optical trapping and micromanipulation attests to the possibilities for optical microrobotics. This work presents an optical microrobotics perspective on the optical microfabrication and micromanipulation work that we performed. We designed different three-dimensional microstructures and fabricated them by two-photon polymerization. These microstructures were then handled using our biophotonics workstation (BWS) for proof-of-principle demonstrations of optical actuation, akin to 6DOF actuation of robotic micromanipulators. Furthermore, we also show an example of dynamic behavior of the trapped microstructure that can be achieved when using static traps in the BWS. This can be generalized, in the future, towards a structural shaping optimization strategy for optimally controlling microstructures to complement approaches based on lightshaping. We also show that light channeled to microfabricated, free-standing waveguides can be used not only to redirect light for targeted delivery of optical energy but can also for targeted delivery of optical force, which can serve to further extend the manipulation arms in optical robotics. Moreover, light deflection with waveguide also creates a recoil force on the waveguide, which can be exploited for controlling the optical force.

We are presenting so-called Wave-guided Optical Waveguides (WOWs) fabricated by two-photon polymerization and capable of being optically manipulated into any arbitrary orientation. By integrating optical waveguides into the structures we have created freestanding waveguides which can be positioned anywhere in a sample at any orientation using real-time 3D optical micromanipulation with six degrees of freedom. One of the key aspects of our demonstrated WOWs is the change in direction of in-coupled light and the marked increase in numerical aperture of the out-coupled light. Hence, each light propelled WOW can tap from a relatively broad incident beam and generate a much more tightly confined light at its tip. The presentation contains both numerical simulations related to the propagation of light through a WOW and preliminary experimental demonstrations on our BioPhotonics Workstation. In a broader context, this research shows that optically trapped micro-fabricated structures can potentially help bridge the diffraction barrier. This structure-mediated paradigm may be carried forward to open new possibilities for exploiting beams from far-field optics down to the sub-wavelength domain.

Perovskite alkaline niobate (XNbO₃) nanowires are attracting lots of attention having a variety of interesting properties such as significant nonlinear optical response, pronounced birefringence, considerable piezoelectric, pyroelectric, photorefractive, and photocatalytic response, as well as superior mechanical and chemical stability. Their ability to efficiently generate second harmonic signals (SHG) and their birefringence allow the use of these nanostructures as local mechano-optical probes for single molecule detection. To assess which type of nanowires is suitable for specific application, we performed a comparative study on the nonlinear optical response of the different types of chemically synthesized alkaline niobate nanowires: sodium niobate (NaNbO₃), potassium niobate (KNbO₃) and lithium niobate (LiNbO₃) nanowires. An optical trap setup has been used to demonstrate the possibility to steadily trap the nanowires, their ability to generate high second harmonic signals, to waveguide this signal and to be rotated under a highly focused laser beam with changing polarization. Different applications are suggested for the three materials, such as LiNbO₃ nanowires as imaging markers, while KNbO₃ and NaNbO₃ nanowires for trapping and torque experiments and NaNbO₃ nanowires to waveguide SHG light. Functionalization of the XNbO₃ nanowires has been studied and successfully implemented. This is a first crucial step toward their use in biomedical imaging and single molecule applications.

In this proceedings paper we show describe how a microtool can be assembled, and tracked in three dimensions such that its full rotational and translational coordinates, q, are recovered. This allows tracking of the motion of any arbitrary point, d, on the microtool's surface. When the micro-tool is held using multiple optical traps the motion of such a point investigates the inside of an ellipsoidal volume - we term this a `thermal ellipsoid. We demonstrate how the shape of this thermal ellipsoid may be controlled by varying the relative trapping power of the optical traps, and adjusting the angle at which the micro-tool is held relative to the focal plane. Our experimental results follow the trends derived by Simpson and Hanna.

Light forces induced by scattering and absorption in elastic dielectrics lead to strain inducing local density modulations and deformations. These perturbations in turn modify the light propagation and generate an intricate nonlinear response. We generalise an analytic approach where light propagation in one-dimensional media of inhomogeneous density is modelled as a result of multiple scattering between polarizable slices. Using the Maxwell stress tensor formalism we compute the local optical forces and iteratively approach self-consistent density distributions where the elastic back-action balances gradient- and scattering forces. For an optically trapped finite dielectric we derive the nonlinear dependence of trap position, stiffness and total deformation on the object’s size and field configuration. Generally trapping is enhanced by deformation, which exhibits a periodic change between stretching and compression even allowing areas of bistability. This strongly deviates from qualitative expectations based on the change of photon momentum of light crossing the surface of a dielectric.

Holographic control of randomized light opens up new ways for imaging and manipulation. We present a powerful approach towards understanding of light propagation through multimode optical fibres and control of the signal at the fibre output. We introduce an experimental geometry allowing analysis of the light transmission through the multimode fibre and subsequent beam-shaping. We show how to generate arbitrary output optical fields within the accessible field of view and range of spatial frequencies. We also show that this technology has applications in biophotonics. As an example, we demonstrate the manipulation of colloidal microparticles.

We present a method to assembly colloidal particles into a three-dimensional structure utilizing the optical micromanipulation technique. Particles are first assembled into 2-D lattices with optical tweezers. A layer by layer approach is used to form the 3-D structure. The phase correction is applied to eliminate the diffraction from previously assembled layers. The preliminary experimental results for the optimization of the 3-D assembly are reported.

We present a novel method for spatial mapping of the luminescent properties of single optically trapped semiconductor nanowires by combing dynamic optical tweezers with micro-photoluminescence. The technique involves the use of a spatial light modulator (SLM) to control the axial position of the trapping focus relative to the excitation source and collection optics. When a nanowire is held in this arrangement, scanning the axial position of the trapping beam enables different sections of the nanowire axis to be probed. In this context we consider the axial resolution of the luminescence mapping and optimization of the nanowire trapping by spherical aberration correction.

We present an optimized optical tweezers system based upon the conical refraction of circularly polarized light in a biaxial crystal. The described optical arrangement avoids distortions to the Lloyd plane rings that become apparent when working with circularly polarized light in conventional optical tweezers. We demonstrate that the intensity distribution of the conically diffracted light permits optical manipulation of high and low refractive index particles simultaneously. Such trapping is in three dimensions and not limited to the Lloyd plane rings. By removal of a quarter waveplate the system also permits the study of linearly polarized conical refraction. We show that particle position in the Raman plane is determined by beam power, and indicates that true optical tweezing is not taking place in this part of the beam.

Trapping with evanescent fields has become an important tool in many research fields. Evanescent fields allow trapping of particles in close proximity to a surface. However, excitation of these waves may be cumbersome. Recently, trapping with photorefractive electric fields has been demonstrated using dielectric and metallic nano and microparticles. Excitation of these fields is straight forward and, in principle, can be excited with microwatt power level. In this work, we give a comparison of photorefractive and plasmonic trapping emphasizing its advantages and disadvantages. We show that single beam and holographic photorefractive photovoltaic trapping in LiNbO₃ of microparticle in water is possible.

In this paper we describe the first use of Optoelectronic Tweezers (OET), an optically controlled micromanipulation method, to measure the relative stiffness of erythrocytes in mice. Cell stiffness is an important measure of cell health and in the case of erythrocytes, the most elastic cells in the body, an increase in cell stiffness can indicate pathologies such as type II diabetes mellitus or hypertension (high blood pressure). OET uses a photoconductive device to convert an optical pattern into and electrical pattern. The electrical fields will create a dipole within any polarisable particles in the device, such as cells, and non-uniformities of the field can be used to place unequal forces onto each side of the dipole thus moving the particle. In areas of the device where there are no field gradients, areas of constant illumination, the force on each side of the dipole will be equal, keeping the cell stationary, but as there are opposing forces on each side of the cell it will be stretched. The force each cell will experience will differ slightly so the stretching will depend on the cells polarisability as well as its stiffness. Because of this a relative stiffness rather than absolute stiffness is measured. We show that with standard conditions (20Vpp, 1.5MHz, 10mSm-1 medium conductivity) the cell’s diameter changes by around 10% for healthy mouse erythrocytes and we show that due to the low light intensities required for OET, relative to conventional optical tweezers, multiple cells can be measured simultaneously.

Holographic aerosol optical tweezers can be used to trap arrays of aerosol particles allowing detailed studies of particle properties and processes at the single particle level. Recent observations have suggested that secondary organic aerosol may exist as ultra-viscous liquids or glassy states at low relative humidity, potentially a significant factor in influencing their role in the atmosphere and their activation to form cloud droplets. A decrease in relative humidity surrounding a particle leads to an increased concentration of solute in the droplet as the droplet returns to equilibrium and, thus, an increase in the bulk viscosity. We demonstrate that the timescales for condensation and evaporation processes correlate with particle viscosity, showing significant inhibition in mass transfer kinetics using ternary sucrose/sodium chloride/water droplets as a proxy to atmospheric multi-component aerosol. We go on to study the fundamental process of aerosol coagulation in aerosol particle arrays, observing the relaxation of non-spherical composite particles formed on coalescence. We demonstrate the use of bright-field imaging and elastic light scattering to make measurements of the timescale for the process of binary coalescence contrasting the rheological properties of aqueous sucrose and sodium chloride aerosol over a range of relative humidities.

The use of optical tweezers for the analysis of aerosols is valuable for understanding the dynamics of atmospherically relevant particles. However to be able to make accurate measurements that can be directly tied to real-world phenomena it is important that we understand the influence of the optical trap on those processes. One process that is seemingly straightforward to study with these techniques is binary droplet coalescence, either using dual beam traps, or by particle collision with a single trapped droplet. This binary coalescence is also of interest in many other processes that make use of dense aerosol sprays such as spray drying and the use of inhalers for drug delivery in conditions such as asthma or hay fever. In this presentation we discuss the use of high speed (~5000 frames per second) video microscopy to track the dynamics of particles as they approach and interact with a trapped aqueous droplet and develop this analysis further by considering elastic light scattering from droplets as they undergo coalescence. We find that we are able to characterize the re-equilibration time of droplets of the same phase after they interact and that the trajectories taken by airborne particles influenced by an optical trap are often quite complex. We also examine the role of parameters such as the salt concentration of the aqueous solutions used and the influence of laser wavelength.

We present an advanced experimental dual-beam set-up for optical manipulation of airborne particles in air. This system employs an adaptive optical element to control properties of counter-propagating beams overlapping in a sample chamber. Furthermore, it can eliminate optical aberrations in both pathways, online re-align the system remotely from a computer interface, arbitrarily switch in real time between various beams types (vortex, Gauss, Bessel etc.) and their spatial intensity distributions (beam width, vorticity etc.). We demonstrated variety of applications of this system ranging from liquid droplet fusion, multiple beams manipulation and precise airborne particle delivery via optical conveyor belt.

We successfully demonstrate crystallization and crystal rotation of L-alanine in D2O solution using a focused laser beam of 1064 nm with right- or left-handed circularly polarization. Upon focusing each laser beam into a solution/air interface of the solution thin film, one single crystal is generally formed from the focal spot. The necessary time for the crystallization is systematically examined against polarization and power of the trapping laser. The significant difference in the average time is observed between two polarization directions at a relatively high laser power, where the left-handed circularly polarized laser takes 3 times longer than the right-handed one. On the other hand, the prepared crystal is stably trapped and rotated at the focal point by circularly polarized lasers after the crystallization, and the rotation direction is completely controlled by the polarization of the trapping laser. The mechanisms for the crystallization and the crystal rotation are discussed in terms of trapping force and rotation torque of circularly polarized lasers acting on the liquid-like clusters and its bulk crystal, respectively.

Numerical computation of optical tweezers is one path to understanding the subtleties of their underlying mechanism—electromagnetic scattering. Electromagnetic scattering models of optical trapping can be used to find the properties of the optical forces and torques acting on trapped particles. These kinds of calculations can assist in predicting the outcomes of particular trapping configurations. Experimentally, looking at the parameter space is time consuming and in most cases unfruitful. Theoretically, the same limitations exist but are easier to troubleshoot and manage. Towards this end a new more usable optical tweezers toolbox has been written. Understanding of the underlying theory has been improved, as well as the regimes of applicability of the methods available to the toolbox. Here we discus the physical principles and carry out numerical comparisons of performance of the old toolbox with the new one and the reduced (but portable) code.

Whilst the main strength of optical trapping techniques is arguably its precision and dexterity, the complimentary technique of acoustic trapping offers massive scalability and potentially larger forces. Acoustic traps commonly use ultrasonic standing waves to trap particles within the nodes of a pressure field, often over distances upwards of a few cm. Here, an acoustic Bessel beam has been created using a piezoelectric cylinder whereby particles are trapped within the entire 14 mm-diameter of the transducer (1.5 cm² trapping area). In optics, Bessel beams have the ability to trap particles over axial distances of several hundred microns. In this acoustic case, the Bessel function shape of the field is formed within the entire length of the cylinder (10 mm). Polymer spheres ranging from 1 μm to 100 μm in diameter are trapped simultaneously within the nodes of the standing wave field, in this case the concentric rings of a Bessel beam. The smaller particles within this field (< 5m) have also been trapped optically using a single beam optical tweezer, as the acoustic force scales such that it becomes comparable to that of the optical trap. This allows for a large range of particle sizes to be simultaneously trapped in a single device, and for large arrays (hundreds of mm²) to be formed acoustically within which particles can be individually optically trapped. This result demonstrates the complementarity of optical and acoustic trapping which makes it possible to trap large area arrays of particles whilst retaining the dexterity to manipulate individual particles.

We present a micromanipulation system based on dielectrophoretic and acoustophoretic particle tweezing. The combination of non-uniform electrical fields and standing pressure waves is applied to a microfluidic chip and provides essential functions including concentration, focussing, guiding, trapping and sorting of microparticles. Dielectrophoresis is achieved using a photoconductor integrated into a microchannel chip which generates electric fields upon illumination. Acoustophoresis is achieved utilizing a surface acoustic wave device which transmits ultrasound into the chip and forms a pressure standing wave under resonance conditions. The system is characterized in terms of coupling of acoustic energy into the chip as well as dielectrophoretic trapping and guiding efficiency. The individual control of each techniques is demonstrated and applied for sorting of polystyrene beads by dielectrophoretic and acoustophoretic forces.

In cell signaling, different perturbations lead to different responses and using traditional biological techniques that result in averaged data may obscure important cell-to-cell variations. The aim of this study was to develop and evaluate a four-inlet microfluidic system that enables single-cell analysis by investigating the effect on Hog1 localization post a selective Hog1 inhibitor treatment during osmotic stress. Optical tweezers was used to position yeast cells in an array of desired size and density inside the microfluidic system. By changing the flow rates through the inlet channels, controlled and rapid introduction of two different perturbations over the cell array was enabled. The placement of the cells was determined by diffusion rates flow simulations. The system was evaluated by monitoring the subcellular localization of a fluorescently tagged kinase of the yeast “High Osmolarity Glycerol” (HOG) pathway, Hog1-GFP. By sequential treatment of the yeast cells with a selective Hog1 kinase inhibitor and sorbitol, the subcellular localization of Hog1-GFP was analysed on a single-cell level. The results showed impaired Hog1-GFP nuclear localization, providing evidence of a congenial design. The setup made it possible to remove and add an agent within 2 seconds, which is valuable for investigating the dynamic signal transduction pathways and cannot be done using traditional methods. We are confident that the features of the four-inlet microfluidic system will be a valuable tool and hence contribute significantly to unravel the mechanisms of the HOG pathway and similar dynamic signal transduction pathways.

Bacteriophage T4 is a double stranded DNA virus that infects E.coli by injecting the viral genome through the cellular wall of a host cell. The T4 genome must be ejected from the viral capsid with sufficient force to ensure infection. To generate high ejection forces, the genome is packaged to high density within the viral capsid. A DNA translocation motor, in which the protein gp17 hydrolyzes ATP and binds to the DNA, is responsible for translocating the genome into the capsid during viral maturation of T4. This motor generates forces in excess of 60 pN and packages DNA at rates exceeding 2000 base pairs/second (bp/s)¹. Understanding these small yet powerful motors is important, as they have many potential applications. Though much is known about the activity of these motors from bulk and single molecule biophysical techniques, little is known about their detailed molecular mechanism. Recently, two structures of gp17 have been obtained: a high-resolution X-ray crystallographic structure showing a monomeric compacted form of the enzyme, and a cryo-electron microscopic structure of the extended form of gp17 in complex with actively packaging prohead complexes. Comparison of these two structures indicates several key differences, and a model has been proposed to explain the translocation action of the motor². Key to this model are a set of residues forming ion pairs across two domains of the gp17 molecule that are proposed to be involved in force generation by causing the collapse of the extended form of gp17. Using a dual optical trap to measure the rates of DNA packaging and the generated forces, we present preliminary mutational data showing that these several of these ion pairs are important to motor function. We have also performed preliminary free energy calculations on the extended and collapsed state of gp17, to confirm that these interdomain ion pairs have large contributions to the change in free energy that occurs upon the collapse of gp17 during the proposed ratcheting mechanism.

The genomes of many dsDNA viruses are replicated by a mechanism that produces a long concatemer of multiple genomes. These viruses utilize multifunctional molecular motor complexes referred to as "terminases" that can excise a unit genome length of DNA and package it into preformed viral shells. Remarkably, the terminase motor can initiate packaging at the appropriate start point, translocate DNA, sense when a sufficient length has been packaged, and then switch into a mode where it arrests and cleaves the DNA to release a filled virus particle. We have recently developed an improved method to measure single phage lambda DNA packaging using dual-trap optical tweezers and pre-stalled motor-DNA-procapsid complexes. We are applying this method to test proposed mechanisms for the sensor that triggers termination; specifically a velocity-monitor model vs. energy-monitor model vs. capsid-filling monitor model.

The precise cutting of axons in C. elegans using short laser pulses permits the investigation of parameters that may influence axonal regeneration. This study began by building and optimizing a femtosecond laser axotomy setup that we first used to monitor the effect of cutting axons near or far from the cell body of the PLM mechanosensory neurons in C. elegans. To assess regeneration, we developed a scoring system where the angle between the regenerating trajectory and its direct line to the target is measured; we called this measurement the "angle of regeneration". The results indicate that axons cut near the cell body regenerate better than those cut far from the cell body but nearer their target. The role of teneurins, which are transmembrane proteins with a large extracellular domain that are thought to regulate the remodelling of the extracellular matrix, has not yet been explored as a potential contributor to axon regeneration. We cut PLM axons in wild-type or ten-1 mutant worms, and measured the angle of regeneration 48 hours later, and the frequency of reconnection to the target. Our results show that functional ten-1 contributes to successful axon regeneration.

While the behavior of spherical particles confined in light beams is well-studied, the dynamics of confined nonspherical particles may be qualitatively different, but remain largely unexplored. We studied the rotation of microscopic dielectric discs induced by the incident angular momentum of an elliptically polarized Laguerre- Gaussian beam. These flat particles are confined in three dimensions by the beam and are oriented naturally with its long axis along the direction of the propagation of the beam. Due to the rotationally asymmetric shape of the particles, we were able to induce a constant rotation of the particles and control it by changing the vorticity and ellipticity of the beam. We also showed a strong dependence on the induced rotation respect to size of the particles. These results provide a new approach to generate or study flows in the microscopic realm as an alternative to the former techniques based on birefringent, absorbent or chiral particles.

Computational tasks such as the calculation and characterization of the optical force acting on a sphere are relatively straightforward in a Gaussian beam trap. Resulting properties of the trap such as the trap strength, spring constants, and equilibrium position can be easily determined. More complex systems with non-spherical particles or multiple particles add many more degrees of freedom to the problem. Extension of the simple methods used for single spherical particles could result in required computational time of months or years. Thus, alternative methods must be used. One powerful tool is to use dynamic simulation: model the dynamics and motion of a particle or particles within the trap. We demonstrate the use of dynamic simulation for non-spherical particles and multi-particle systems. Using a hybrid discrete dipole approximation (DDA) and T-matrix method, we find plausible equilibrium positions and orientations of cylinders of varying size and aspect ratio. Orientation landscapes revealing different regimes of behaviour for micro-cylinders and nanowires with different refractive indices trapped with beams of differing polarization are also presented. This investigation provides a solid background in both the function and properties of micro-cylinders and nanowires trapped in optical tweezers. This method can also be applied to particles with other shapes. We also investigate multiple-particle trapping, which is quite different from single particle systems, as they can include effects such as optical binding. We show that equilibrium positions, and the strength of interactions between particles can be found in systems of two and more particles.

The use of phase only spatial light modulators for holographic optical trapping results in the appearance of ghost orders, creating unwanted traps with uncontrolled intensity and causing variations in the intensity of the desired traps. By introducing dummy areas in the diffraction plane during the hologram optimization, the intensity in the ghost orders can be significantly reduced. By directing a variable fraction of the light to the dummy area, the optical power in the traps can be controlled independently and kept constant also while moving traps to different arrangements. We present and evaluate an algorithm for hologram generation which utilizes dummy areas and allows arbitrary spot positioning in three dimensions. The method enables the use of holographic optical trapping for applications requiring precise control of the intensity in traps, such as optical force measurement.

The increased application of holographic optical manipulation techniques within the life sciences has sparked the development of accessible interfaces for control of holographic optical tweezers. Of particular interest are those that employ familiar, commercially available technologies. Here we present the use of a low cost games console interface, the Microsoft Kinect for the control of holographic optical tweezers and a study into the effect of using such a system upon the quality of trap generated.

Fast calculation of trapping force provides a more direct way for optimizing designs of optical systems which generate optical traps. In this study, a graphic processing unit (GPU), NVIDIA GTX 275, is used to boost the speed of trapping efficiency calculation under ray optics approximation. The codes of trapping efficiency calculation are implemented in C++. The computing power of GPU is utilized through compute unified architecture device (CUDA) toolkit 4.0. The computing speed is compared with that of central processing unit (CPU), Intel Core 2 Quad Q9550. Over 100x speedup is achieved when single-precision floating-point numbers were used in the calculation.

We present the result of an investigation into the optical trapping of micropaticles using laser beams with a spatially inhomogeneous polarization (cylindrical vector beams). We perform three-dimensional tracking of the Brownian fluctuations in position of a trapped particle and extract the trap spring constants. We characterize the trap geometry by the aspect ratio of spring constants in the directions transverse and parallel to the beam propagation direction and evaluate this figure of merit as a function of polarization angle. We show that the additional degree of freedom present in cylindrical vector beams (CVBs) allows us to control the optical trap strength and geometry by adjusting the polarization of the trapping beam only. Experimental results are compared with a theoretical model of optical trapping using CVBs derived from electromagnetic scattering theory in the T-matrix framework.

We have previously demonstrated that Mie scattered statistically stationary partially coherent electromagnetic fields result in spontaneous creation and nucleation of coherence vortices in the field both inside and outside the scattering particle. In a succeeding study we showed that a regular lattice of coherence vortices can be generated by illuminating a system of three scatterers with partially coherent light. In this paper, we analyze the field scattered by differently arranged systems of scatterers and investigate the coherence variation of the scattered field. We show that different patterns of coherence vortex networks can be realized depending on the spatial arrangement of the scatterers. Different patterns of vortex networks generated in these scattering systems make them suitable candidates for particle manipulation applications at microscopic levels.

We present a comparison of optical trapping of 50-nm-sized silica nanospheres, suspended in water medium, by femtosecond laser pulses and by continuous wave laser beam. With bright field microscopic imaging, we demonstrated that intensity of scattering light at the focal area under fs-pulse mode is much higher than that under cw mode. This result offers a basic interpretation that trapping efficiency of nanometer-sized particles by the ultrashort laser pulses is higher than that by the cw mode at the same laser power. We interpret this finding by means of impulsive peak power of the femtosecond laser pulses.

We propose the use of consumer pico projectors as cost effective spatial light modulators in cell sorting applications. The matched filtering Generalized Phase Contrast (mGPC) beam shaping method is used to produce high intensity optical spots for trapping and catapulting cells. A pico projector’s liquid crystal on silicon (LCoS) chip was used as a binary phase spatial light modulator (SLM) wherein correlation target patterns are addressed. Experiments using the binary LCoS phase SLM with a fabricated Pyrex matched filter demonstrate the generation of intense optical spots that can potentially be used for cell sorting. Numerical studies also show mGPC’s robustness to phase aberrations in the LCoS device, and its ability to outperform a top hat beam with the same power.

In this contribution we focus on optical forces acting upon a metallic particle or a core-shell particle confined in a standing-wave. The considered spheres are composed either by a gold or silver or the consists of of two layers, one of them is metallic (Au, Ag). We present the results of a computational study where we modify the geometrical parameters of the particles and the wavelength of the trapping beams. Except the optical forces we also deal with heating of the particles. This study may suggest optimal particle composition that may be utilized as an optically trapped probe for Surface enhanced Raman spectroscopy of biomolecules.

The derivate of surface plasmon and optical tweezers, so-called plasmonic nano-optical tweezers (PNOT), has attracted much research interest due to its powerful ability for immobilizing nano-objects in the nanoscale, and its potential application in chemo/biosensing and life science. In this work, we use gold nano-rings to construct PNOT, and demonstrate the feasibility to trap metal nanoparticles (Au-NPs) for SERS application from the numerical standpoint. 3D finite-difference time-domain (FDTD) and the Maxwell stress tensor (MST) were used in our simulation study. We show that the interactions of the localized surface plasmon (LSP) excitation and the plasmonic interferences of the nano-ring arrays contribute to a narrow spectral feature around 785 nm, resulting in strong local near-field enhancement and thus intensive field gradient forces. The trapping potential well is as high as 1.31×10^-19 J under a low illuminating power density of 1.0 mW/μm², which makes the trapping events effective enough to overcome Brownian motion of the Au-NPs. Moreover, the existence of multiple potential wells results in a very large active volume of ~106 nm3 for trapping the target particles. The trapped Au-NPs further lead to the formation of nano-gaps that offer a field enhancement of 160 times. Our proposal shows promising applications for sensing and microfluidic integrations.

In this study, a two-dimensional (2D) freeform optical trapping system based on evanescent wave excitation has been developed. The 2D optical trapping system with surface plasmon (SP) enhancement and patterned excitation via digital micromirror device (DMD) which can provide a strong and freeform intensity distribution on the metal surface. Through a gold film with a thickness of 45 nm in the near infrared region, the SP approach with 40-fold electric field enhancement can enhance the intensity distribution on the metal surface. Unlike a fixed gold pattern film fabricated at a glass surface, the freeform SP-enhanced optical trapping technique is more convenient and efficient for biomedical applications.

We present a study of the manipulation of microparticles and the formation of optically bound structures of particles in evanescent wave traps. Two trapping geometries are considered: the first is a surface trap where the evanescent field above a glass prism is formed by the interference of a number of laser beams incident on the prism-water interface; the second uses the evanescent field surrounding a biconical tapered optical fiber that has been stretched to produce a waist of submicron diameter. In the surface trap we observe optical binding of microparticles in to one-dimensional chain structures. In the tapered optical fiber trap we demonstrate both particle transport for long distances along the fiber, and the formation of stable arrays of particles. In both experiments we use video microscopy to track the particle locations and make quantitative measures of the particle dynamics. The experimental studies of particle structures are complemented by light scattering calculations based on Mie theory to infer how the geometries of the observed particle structures are controlled by the underlying incident and scattered optical fields.

Binding between optically co-trapped micro-particles occurs when the scattered optical fields are sufficient to com­ pete with the trapping forces. Such optical binding is seen as being pertinent to large-scale micro-manipulation due to the significant forces present within an optical trapping system comprising multiple micro-particles. One aspect of optical binding involves an inter-particle force relationship that is strongly dependent on optical fre­ quency. In our study we theoretically show that a broadened spectral content associated with frequency doubled broadband ultrashort pulses can result in spatial overlap of contained frequencies. The binding force oscillations are averaged out when sufficient spectral content is present within the pulse.