Search

Optical trapping is having ever-increasing impact in science $-$ particularly biophysics, photonics and most recently in quantum optomechanics $-$ owing to its superior capability for manipulating nanoscale structures and materials. However, essentially all experimental optical trapping studies in the optical dipole regime have, to date, been dominated by the interaction between a material's electric polarizability, $\alpha_{e}$, and the electric part of the incident electromagnetic field, and therefore described by electric field intensity gradient forces. Optical trapping based on optical magnetic light-matter interactions has not been experimentally addressed despite it's immediate extension of the boundaries of optical trapping research and applications. This paper addresses this long-standing deficiency through the realization of optical magnetic trapping of large index of refraction (i.e., Si) nanoparticles and also presents a formalism for quantitative understanding of the experimental findings. Our experimental optical trapping results require including optical magnetic polarizability, $\alpha_{m}$, and electric-magnetic scattering forces associated with the Photonic Hall effect that are qualitatively and quantitatively validated by Maxwell stress tensor calculations. Our findings bring new opportunities for nanoparticle manipulation, potentially relax the limitations Ashkin claimed based on the optical Earnshaw's theorem, motivate optical matter formation by optical magnetic interactions, and suggest new N-body effects and symmetry breaking to drive dynamics of optical matter systems.

It is well-known that the interaction between passivated nanoparticles can be tuned by their complete immersion in a chosen solvent, such as water. What remains unclear on a molecular level is how nanoparticle interactions may be altered in the presence of solvent vapor where complete immersion is not achieved. In this paper, we report an all-atom molecular dynamics simulation study of the change in pair potential of mean force between dodecane thiol ligated gold nanoparticles (AuNPs) when exposed to water vapor. With the equilibrium vapor pressure of water at 25 \degree C, there is very rapid condensation of water molecules onto the surface of the AuNPs in the form of mobile clusters of 100-2000 molecules that eventually coalesce into a few large clusters. When the distance between two AuNPs decreases, a water cluster bridging them provides an adhesive force that increases the depth and alters the shape of the pair-potential of mean force. That change of shape includes a decreased curvature near the minimum, consistent with experimental data showing that cyclic exposure to water vapor and its removal reversibly decreases and increases the Young's modulus of a freely suspended self-assembled monolayer of these AuNPs., Comment: 26 pages, 11 figures. Accepted for publication in JCP

The search for local structures within a disordered medium has led to proposals of several methods for probing transient short-range symmetry in a homogeneous mono-atomic liquid. We offer a comparison of different characterizations of such local structure in an in-homogeneous liquid. We simulate the interfaces between a Lennard-Jones liquid and (i) a flat Lennard-Jones wall, (ii) a wall of face-centered cubic-packed Lennard-Jones particles, and (iii) a Lennard-Jones vapor. The interface density distributions in cases (i) and (ii) have oscillations extending several particle diameters into the bulk liquid, while that in case (iii) the interface density distribution monotonically decays from the bulk liquid to the vapor. We search for transient ordered fluctuations, which we identify with local ordered structures, using the Aperture Cross Correlation Function of the system. We find that only particles in the FCC-liquid interface that are in the two layers closest to the FCC surface exhibit preferred four-fold symmetry. Furthermore, the absence of three- or five-fold symmetry peaks in the Aperture Cross Correlation Function of the liquid-vapor interface yields a different picture of that interface than derived from the high density of bi-pyramidal clusters found in the Topological Cluster Classification study (Molecular Physics, 109 (7-10), 1393-1402 (2011)). While the Aperture Cross Correlation Function results need not be viewed as contradictory with the Topological Cluster Classification results, we argue the advantages of using the former to characterize transient ordered fluctuations as being both experimentally accessible in favorable cases and not reliant on a list of structures.

We report studies of the phase diagrams of two two-dimensional systems composed of particles with repulsive pair potentials, each of which supports a high-density Kagome lattice phase. The commonalities in the phase diagrams of the systems we have studied and other 2D systems suggests the existence of a universal mechanism driving all to favor a similar series of packing arrangements as the density is increased, but the simulations considered show that the only such general rule proposed, namely the S\"uto theorem, is not a necessary condition for the support of multiple distinct lattice structures by a particular pair potential. We also find that close to the liquid-to-Kagome phase transition the transient structured fluctuations in the liquid have Kagome symmetry whereas deeper in the liquid phase the fluctuations have hexagonal symmetry. As the deviation of the liquid density from the transition density decreases fluctuations with hexagonal symmetry are replaced with those with Kagome symmetry. When the transition is string phase-to-Kagome phase the transient structured fluctuations in the string phase near the transition have both six-fold and other than six-fold symmetries, with stronger preference for six-fold symmetry in the Truskett system than in the Torquato system. The path of the string-to-Kagome transition in the Truskett system involves intermediate honeycomb configurations that subsequently buckle to form a Kagome lattice. The path of the string-to-Kagome transition in the Torquato system suggests that the Kagome phase is formed by coiled strings merging together; increasing density generates a Kagome phase with imperfections such as 8-particle rings.

We report the results of molecular dynamics simulations of the properties of a pseudo-atom model of dodecane thiol ligated 5-nm diameter gold nanoparticles (AuNP) in vacuum as a function of ligand coverage and particle separation in three state of aggregation: the isolated AuNP, an isolated pair of AuNPs and a square assembly of AuNPs. Our calculations show that for all values of the coverage the ligand density along a radius emanating from the core of an isolated AuNP oscillates along the chain up to the fourth pseudo-atom, then smoothly decays to zero. We examine the ligand envelope as a function of the coverage and demonstrate that the deformation of that envelope generated by interaction between the NPs is coverage-dependent, so that the shape, depth and position of the minimum of the potential of mean force displays a systematic dependence on the coverage. We propose an accurate analytical description of the calculated potential of mean force with parameters that scale linearly with the ligand coverage. We define and calculate an effective pair potential of mean force for a square configuration of particles; our definition contains, implicitly, both the three- and four-particle contributions to deviation from additivity. We find that the effective pair potential of mean force in this configuration has a different minimum and a different well depth than the isolated pair potential of mean force. Previous work has found that the three-particle contribution to deviation from additivity is monotone repulsive, whereas we find that the combined three- and four-particle contributions have an attractive well, implying that the three- and four-particle contributions are of comparable magnitude but opposite sign, thereby suggesting that even higher order correction terms likely play a significant role in the behavior of assemblies of many nanoparticles., Comment: 28 pages, 10 figures

From Newtons third law, the principle of actio et reactio, we expect the forces between interacting particles to be equal and opposite. However, non-reciprocal forces can arise. Specifically, this has recently been shown theoretically in the interaction between dissimilar optically trapped particles mediated by an external field. As a result, despite the incident external field not having a transverse component of momentum, the particle pair experiences a force in a direction transverse to the light propagation direction. In this letter, we directly measure the net non-reciprocal force in electrodynamically interacting nanoparticle dimers illuminated by plane waves and confined to pseudo one-dimensional geometries. We show by electrodynamic theory and simulations that inter-particle interactions cause asymmetric scattering from heterodimers and therefore, the non-reciprocal forces are a consequence of momentum conservation. Finally, we demonstrate experimentally that non-reciprocal dynamics occur generally for illuminated asymmetric scatterers., Comment: 15 pages, 4 figures. Supporting information containing 24 pages and 12 figures

A major impediment to a more complete understanding of barrier crossing and other single-molecule processes is the inability to directly visualize the trajectories and dynamics of atoms and molecules in reactions. Rather, the kinetics are inferred from ensemble measurements or the position of a transducer (e.g. an AFM cantilever) as a surrogate variable. Direct visualization is highly desirable. Here, we achieve the direct measurement of barrier crossing trajectories by using optical microscopy to observe position and orientation changes of pairs of Ag nanoparticles in an optical ring trap, i.e. passing events. A two-step mechanism similar to a bimolecular exchange reaction is revealed by analysis that combines detailed knowledge of each trajectory, a statistically significant number of repetitions of the passing events, and the driving force-dependence of the process. We find that while the total event rate increases with driving force, this increase is only due to increased rate of encounters. There is no drive force-dependence on the rate of barrier crossing because the key motion for the process involves a radial motion of one particle as a thermal (random) fluctuation allowing the other to pass. This simple experiment can readily be extended to study more complex barrier crossing processes by replacing the spherical metal nanoparticles with anisotropic ones or through more intricate optical trapping potentials.

We report the structure of transient fluctuations in the liquid phase of a two-dimensional system that exhibits several ordered phases with different symmetries. The density-temperature phase diagram of the system studied, composed of particles with a repulsive shouldered soft-core pair interaction, has regions with stable liquid and hexatic phases, a square solid phase, two separate hexagonal solid phases and a quasi-crystalline phase with 12-fold symmetry. The structure of a fluctuation was determined by computing the same-time aperture cross correlation function of particle configurations. Along a low-temperature isotherm that intersects all the ordered phases we find that the liquid phase exhibits structured fluctuation with hexagonal symmetry near both liquid-hexatic transition lines. Along the same isotherm and in the stable liquid between the lower density hexatic-to-liquid and the higher density liquid-to-square solid transitions, we find that transient hexagonal ordered fluctuations dominate the liquid region near the hexatic-to-liquid transition and square ordered fluctuations dominate the liquid region near the liquid-to square solid transition, but that both structured fluctuations occur at all densities between these transition lines. At a higher temperature, at phase points in the liquid above but close to the density maximum of an underlying transition, there are ordered fluctuations that can be correlated with the structure of the lower temperature phase., Comment: 35 pages, 16 figures

Experimental studies of the variation of the mean square displacement (MSD) of a particle in a confined colloid suspension that exhibits density variations on the scale length of the particle diameter are not in agreement with the prediction that the spatial variation in MSD should mimic the spatial variation in density. The predicted behavior is derived from the expectation that the MSD of a particle depends on the system density and the assumption that the force acting on a particle is a point function of position. The experimental data come from studies of the MSDs of particles in narrow ribbon channels and between narrowly spaced parallel plates, and from new data, reported herein, of the radial and azimuthal MSDs of a colloid particle in a dense colloid suspension confined to a small circular cavity. In each of these geometries a dense colloid suspension exhibits pronounced density oscillations with spacing of a particle diameter. We remove the discrepancy between prediction and experiment using the Fisher-Methfessel interpretation of how local equilibrium in an inhomogeneous system is maintained to argue that the force acting on a particle is delocalized over a volume with radius equal to a particle diameter. Our interpretation has relevance to the relationship between the scale of inhomogeneity and the utility of translation of the particle MSD into a position dependent diffusion coefficient, and to the use of a spatially dependent diffusion coefficient to describe mass transport in a heterogeneous system., Comment: 16 pages, 7 figures

The properties of a classical simple liquid can be strongly affected by application of an external potential that supports inhomogeneity. To understand the nature of these property changes the equilibrium particle distribution functions of the liquid have, typically, been evaluated individually as functions of system control parameters, such as the packing fraction of a hard sphere liquid. In this study we focus attention on two distribution functions that characterize the inhomogeneous liquid: the pair direct correlation function $c(\mathbf{r}_1,\mathbf{r}_2)$ and the pair correlation function $g(\mathbf{r}_1,\mathbf{r}_2)$. We solve the Ornstein-Zernike equation for the inhomogeneous liquid to obtain $c(\mathbf{r}_1,\mathbf{r}_2)$ as an explicit function of $g(\mathbf{r}_1,\mathbf{r}_2)$, with the latter considered to be an experimental observable, using information about the well studied and resolved $g^0(\mathbf{r}_1-\mathbf{r}_2)$ and $c^0(\mathbf{r}_1-\mathbf{r}_2)$ for the parent homogeneous ($^0$) system. The result obtained with our formulation is tested against the exact solutions for the correlation and distribution functions of a one-dimensional inhomogeneous hard rod liquid. Following the success of that test the formalism is extended to obtain $c(\mathbf{r}_1,\mathbf{r}_2)$ as an explicit function of $g(\mathbf{r}_1,\mathbf{r}_2)$ in a three dimensional liquid., Comment: 18 pages, 3 figures

We consider combined stimulated Raman adiabatic passage (STIRAP) and fast forward field (FFF) control of selective vibrational population transfer in a polyatomic molecule. The motivation for using this combination control scheme is twofold: (i) to overcome transfer inefficiency that occurs when the STIRAP fields and pulse durations must be restricted to avoid excitation of population transfers that compete with the targeted transfer and (ii) to overcome transfer inefficiency resulting from embedding of the actively driven subset of states in a large manifold of states. We show that, in a subset of states that is coupled to background states, a combination of STIRAP and FFFs that do not individually generate processes that are competitive with the desired population transfer can generate greater population transfer efficiency than can ordinary STIRAP with similar field strength and/or pulse duration. The vehicle for our considerations is enhancing the yield of HNC in the driven ground state-to-ground state nonrotating HCN/HNC isomerization reaction and selective population of one of a pair of near degenerate states in non rotating SCCl$_2$., Comment: arXiv admin note: text overlap with arXiv:1409.0784

We report studies of state-to-state vibrational energy transfer in an isolated polyatomic molecule driven by combined stimulated Raman adiabatic passage (STIRAP) and counter-diabatic fields (CDF), using as vehicles selective population of one of a pair of near degenerate states in SCCl$_2$ and enhancing the yield of HNC in the HCN/HNC isomerization reaction. The efficiency of the population transfer within a subset of states embedded in a dense manifold of states is examined for the cases that the transition dipole moments between the initial and target states are much smaller than and comparable to the transition dipole moments between background states, and for the case that the subset states have large transition dipole moments with the background states. We show that, in a subset of states that is coupled to background states, a combination of STIRAP fields and CDFs that do not individually generate processes that are competitive with the desired population transfer can generate greater population transfer efficiency than can ordinary STIRAP with field strength and/or pulse duration similarly restricted. We also show that the exact CDF for an isolated three level system is a useful approximation to the exact CDF for three and five state systems embedded in background states, and we report a study of the stability of the STIRAP + approximate CDF control protocol to variation of the CDF.

We derive the driving potential that accelerates adiabatic population transfer from an initial state to a target state in a lattice system without unwanted excitation of other states by extending to discrete systems the fast-forward theory of adiabatic transfer. As an example we apply the theory to a model that describes a Bose-Einstein condensate in a quasi one-dimensional optical lattice, and show that modulation of the tilting of the lattice potential can transfer the population of the Bose-Einstein condensate from site to site with high fidelity and without unwanted excitations.

Hard spheres in Newtonian fluids serve as paradigms for Non-Newtonian materials phenomena exhibited by colloidal suspensions. A recent experimental study (Cheng et al. 2011 Science, 333, 1276) showed that upon application of shear to such a system, the particles form string-like structures aligned in the vorticity direction. We explore the mechanism underlying this out-of-equilibrium organization with Steered Transition Path Sampling, which allows us to bias the Brownian contribution to rotations of close pairs of particles and alter the dynamics of the suspension in a controlled fashion. Our results show a strong correlation between the string structures and the rotation dynamics. Specifically, the simulations show that accelerating the rotations of close pairs of particles, not increasing their frequency, favors formation of the strings. This insight delineates the roles of hydrodynamics, Brownian motion, and particle packing, and, in turn, informs design strategies for controlling the assembly of large-scale particle structures.

We report the results of experimental studies of the short time-long wavelength behavior of collective particle displacements in quasi-one-dimensional and quasi-two-dimensional colloid suspensions. Our results are represented by the behavior of the hydrodynamic function H(q) that relates the effective collective diffusion coefficient, D_e(q) with the static structure factor S(q) and the self-diffusion coefficient of isolated particles D_0: H(q)=D_e(q)S(q)/D_0. We find an apparent divergence of H(q) as q->0 with the form H(q) proportional to q^-gamma, 1.70 we infer that D_e(q) does. We provide evidence that this divergence arises from the interplay of boundary conditions on the flow of the carrier liquid and many-body hydrodynamic interactions between colloid particles that affect the long wavelength behavior of the particle collective diffusion coefficient in the suspension. We speculate that in the q1D and q2D systems studied the divergence of H(q) might be associated with a q-dependent partial slip boundary condition, specifically an effective slip length that increases with decreasing q. We also verify, using data from the work of Lin, Rice and Weitz (J. Chem. Phys. 99, 9585 (1993)), the prediction by Bleibel et al (arXiv:1305.3715), that D_e(q) for a monolayer of colloid particles constrained to lie in the interface between two fluids diverges as 1/q as q->0. The verification of that prediction, which is based on an analysis that allows two-dimensional colloid motion embedded in three-dimensional suspending fluid motion, supports the contention that the boundary conditions that define a q2D system play a very important role in determining the long wavelength behavior of the collective diffusion coefficient.

We investigate the response of a dense monodisperse quasi-two-dimensional (q2D) colloid suspension when a particle is dragged by a constant velocity optical trap. Consistent with microrheological studies of other geometries, the perturbation induces a leading density wave and trailing wake, and we use Stokesian Dynamics (SD) simulations to parse direct colloid-colloid and hydrodynamic interactions. We go on to analyze the underlying individual particle-particle collisions in the experimental images. The displacements of particles form chains reminiscent of stress propagation in sheared granular materials. From these data, we can reconstruct steady-state dipolar flow patterns that were predicted for dilute suspensions and previously observed in granular analogs to our system. The decay of this field differs, however, from point Stokeslet calculations, indicating that the finite size of the colloids is important. Moreover, there is a pronounced angular dependence that corresponds to the surrounding colloid structure, which evolves in response to the perturbation. Put together, our results show that the response of the complex fluid is highly anisotropic owing to the fact that the effects of the perturbation propagate through the structured medium via chains of colloid-colloid collisions.

We report calculations of the density of maximally random jamming (aka random close packing) of one-component and binary hard disc fluids. The theoretical structure used provides a common framework for description of the hard disc liquid to hexatic, the liquid to hexagonal crystal and the liquid-to-maximally random jammed state transitions. Our analysis is based on locating a particular bifurcation of the solutions of the integral equation for the inhomogeneous single particle density at the transition between different spatial structures. The bifurcation of solutions we study is initiated from the dense metastable fluid, and we associate it with the limit of stability of the fluid, which we identify with the transition from the metastable fluid to a maximally random jammed state. For the one-component hard disc fluid the predicted packing fraction at which the metastable fluid to maximally random jammed state transition occurs is 0.84, in excellent agreement with the experimental value 0.84 \pm 0.02. The corresponding analysis of the limit of stability of a binary hard disc fluid with specified disc diameter ratio and disc composition requires extra approximations in the representations of the direct correlation function, the equation of state, and the number of order parameters accounted for. Keeping only the order parameter identified with the largest peak in the structure factor of the highest density regular lattice with the same disc diameter ratio and disc composition as the binary fluid, the predicted density of maximally random jamming is found to be 0.84 to 0.87, depending on the equation of state used, and very weakly dependent on the ratio of disc diameters and the fluid composition, in agreement with both experimental data and computer simulation data.

We present the results of a computer simulation study of the liquid density distribution normal to the interface between liquid Hg and the reconstructed (0001) face of sapphire. The simulations are based on an extension of the self-consistent quantum Monte Carlo scheme previously used to study the structure of the liquid metal-vapor interface. The calculated density distribution is in very good agreement with that inferred from the recent experimental data of Tamam et al (J. Phys. Chem. Lett. 1, 1041-1045 (2010)). We conclude that, to account for the difference in structure between the liquid Hg-vapor and liquid-Hg-reconstructed (0001) Al2O3 interfaces, it is not necessary assume there is charge transfer from the Hg to the Al2O3. Rather, the available experimental data are adequately reproduced when the van der Waals interactions of the Al and O atoms with Hg atoms and the exclusion of electron density from Al2O3 via repulsion of the electrons from the closed shells of the ions in the solid are accounted for., Comment: 26 pages, 11 figures

We report the results of an experimental study of a colloid fluid confined to a quasi-one dimensional (q1D) ribbon channel as a function of channel width and colloid density. Our findings confirm the principal predictions of previous theoretical studies of such systems. These are (1) that the density distribution of the liquid transverse to the ribbon channel exhibits stratification; (2) that even at the highest density the order along the strata, as measured by the longitudinal pair correlation function, is characteristic of a liquid; and (3) the q1D pair correlation functions in different strata exhibits anisotropic behavior, resembling that found in a Monte Carlo simulation for the in-plane pair correlation function of a hard sphere fluid in a planar slit., Comment: 33 pages, 12 figures, accepted to Physical Review E

A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand., Comment: 9 pages, 5 figures, to appear in J. Chem. Phys

Two advantageous roles of the influence of measurement on a system subject to coherent control are exposed using a five-level model system. In particular, a continuous measurement of the population in a branch state in the Kobrak-Rice extended stimulated Raman adiabatic passage scheme is shown to provide a powerful means for controlling the population transfer branching ratio between two degenerate target states. It is demonstrated that a measurement with a large strength may be used to completely shut off the yield of one target state and that the same measurement with a weak strength can dramatically enhance the robustness of the controlled branching ratio against dephasing., Comment: 5 pages, 3 figures, to appear in J. Chem. Phys

A simple theoretical solution to the design of a control field that generates complete population transfer from an initial state, via $N$ nondegenerate intermediate states, to one arbitrary member of $M$ ($M\leq N$) degenerate states is constructed. The full control field exploits an $(M+N-1)$-node null adiabatic state, created by designing the relative phases and amplitudes of the component fields that together make up the full field. The solution found is universal in the sense that it does not depend on the exact number of the unwanted degenerate states or their properties. The results obtained suggest that a class of multi-level quantum systems with degenerate states can be completely controllable, even under extremely strong constraints, e.g., never populating a Hilbert subspace that is only a few dimensions smaller than the whole Hilbert space., Comment: 12 pages, 5 figures, submitted to Phys. Rev. A

We have studied the correlated Brownian motion of micron-sized particles suspended in water and confined between two plates. The hydrodynamic interaction between the particles exhibits three anomalies. (i) The transverse coupling is negative, i.e., particles exert "anti-drag" on one another when moving perpendicular to their connecting line. (ii) The interaction decays with inter-particle distance r as 1/r^2, faster than in unconfined suspensions but slower than near a single wall. (iii) At large distances the pair interaction is independent of concentration within the experimental accuracy. The confined suspension thus provides an unusual example of long-range, yet essentially pairwise correlations even at high concentration. These effects are shown to arise from the two-dimensional dipolar form of the flow induced by single-particle motion., Comment: 4 pages, 4 figures