Yael Roichman

Manipulation and assembly of nanowires with holographic optical traps.

We demonstrate that semiconductor nanowires can be translated, rotated, cut, fused and organized into nontrivial structures using holographic optical traps. The holographic approach to nano-assembly allows for simultaneous independent manipulation of multiple nanowires, including relative translation and relative rotation.

Holographic optical trapping

Holographic optical tweezers use computer-generated holograms to create arbitrary three-dimensional configurations of single-beam optical traps that are useful for capturing, moving, and transforming mesoscopic objects. Through a combination of beam-splitting, mode-forming, and adaptive wavefront correction, holographic traps can exert precisely specified and characterized forces and torques on objects ranging in size from a few nanometers to hundreds of micrometers. Offering nanometer-scale spatial resolution and real-time reconfigurability, holographic optical traps provide unsurpassed access to the microscopic world and have found applications in fundamental research, manufacturing, and materials processing.

Holographic assembly of quasicrystalline photonic heterostructures

Quasicrystals have a higher degree of rotational and point-reflection symmetry than conventional crystals. As a result, quasicrystalline heterostructures fabricated from dielectric materials with micrometer-scale features exhibit interesting and useful optical properties including large photonic bandgaps in two-dimensional systems. We demonstrate the holographic assembly of two-dimensional and three-dimensional dielectric quasicrystalline heterostructures, including structures with specifically engineered defects. The highly uniform quasiperiodic arrays of optical traps used in this process also provide model aperiodic potential energy landscapes for fundamental studies of transport and phase transitions in soft condensed matter systems.

Optical traps with geometric aberrations

We assess the influence of geometric aberrations on the in-plane performance of optical traps by studying the dynamics of trapped colloidal spheres in deliberately distorted holographic optical tweezers. The lateral stiffness of the traps turns out to be insensitive to moderate amounts of coma, astigmatism, and spherical aberration. Moreover holographic aberration correction enables us to compensate inherent shortcomings in the optical train, thereby adaptively improving its performance. We also demonstrate the effects of geometric aberrations on the intensity profiles of optical vortices, whose readily measured deformations suggest a method for rapidly estimating and correcting geometric aberrations in holographic trapping systems.

Tomographic phase microscopy with 180° rotation of live cells in suspension by holographic optical tweezers

We present a new tomographic phase microscopy (TPM) approach that allows capturing the three-dimensional refractive index structure of single cells in suspension without labeling, using 180° rotation of the cells. This is obtained by integrating an external off-axis interferometer for wide-field wave front acquisition with holographic optical tweezers (HOTs) for trapping and micro-rotation of the suspended cells. In contrast to existing TPM approaches for cell imaging, our approach does not require anchoring the sample to a rotating stage, nor is it limited in angular range as is the illumination rotation approach. Thus, it allows noninvasive TPM of suspended live cells in a wide angular range. The proposed technique is experimentally demonstrated by capturing the three-dimensional refractive index map of yeast cells, while collecting interferometric projections at an angular range of 180° with 5° steps. The interferometric projections are processed by both the filtered back-projection method and the diffraction theory method. The experimental system is integrated with a spinning disk confocal fluorescent microscope for validation of the label-free TPM results.

Polyaniline synthesis: influence of powder morphology on conductivity of solution cast blends with polystyrene

Abstract Synthesis of polyaniline (PANI) was performed under different conditions followed by dedoping, redoping with dodecyl benzene sulfonic acid (DBSA) and then blending with PS. The morphologies of the as-polymerized, doped and blended PANI were studied. The main polymerization stages seem to include: PANI oligomers assembling into nuclei, nuclei growing into primary particles (10 nm), primary particles assembling into aggregates (≈0.5 μm) and aggregates assembling into agglomerates (≈10 μm). The morphology of the as-polymerized PANI was found to be strongly related to the rate of oxidant addition, synthesis duration and synthesis temperature. This morphology dominates the effects of DBSA doping and dispersing the resulting PANI–DBSA in the matrix polymer. A fine PANI–DBSA powder with weakly bound aggregates is likely to disperse well in a solvent and hence promote the formation of the desired fine-network morphology and yield a low percolation threshold and high conductivity. Synthesis at a high oxidant addition rate, an excess of oxidant, a relatively high polymerization temperature and a short synthesis duration should diminish the tendency to form dense complex structures. These dense structures prevent efficient DBSA doping, deaggregation and the desired fine-network dispersion of PANI–DBSA in the blends.

Hydrodynamic Pair Attractions between Driven Colloidal Particles

Colloidal spheres driven through water along a circular path by an optical ring trap display unexpected dynamical correlations. We use Stokesian dynamics simulations and a simple analytical model to demonstrate that the paths curvature breaks the symmetry of the two-body hydrodynamic interaction, resulting in particle pairing. The influence of this effective nonequilibrium attraction diminishes as either the temperature or the stiffness of the radial confinement increases. We find a well-defined set of dynamically paired states whose stability relies on hydrodynamic coupling in curving trajectories.

Percolation of electrical conductivity in solution-cast blends containing polyaniline

Abstract The potential offered by intrinsically conductive polymers is limited by their poor mechanical properties. Blending with common thermoplastics can improve processability and mechanical properties and still preserve the electrical conductivity. In such blends, the morphology determines the mechanical and electrical properties. In this research, blends of polyaniline (PANI)-dodecyl benzene sulfonic acid (DBSA) with either polystyrene (PS) in xylene or polyvinylchloride (PVC) in bromobenzene were solution cast. The morphologies of the blends were characterized by optical microscopy (OM), electron microscopy, and small-angle X-ray scattering (SAXS). Electrical conductivity was measured for various compositions. The formation of a continuous network was strongly associated with percolation and conductivity. The morphologies of the two blends are significantly different. This difference arises from the different solvents used and their ability to swell the PAN1 aggregates and to promote their disintegr...

Independent and simultaneous three-dimensional optical trapping and imaging.

Combining imaging and control of multiple micron-scaled objects in three dimensions opens up new experimental possibilities such as the fabrication of colloidal-based photonic devices, as well as high-throughput studies of single cell dynamics. Here we utilize the dual-objectives approach to combine 3D holographic optical tweezers with a spinning-disk confocal microscope. Our setup is capable of trapping multiple different objects in three dimensions with lateral and axial accuracy of 8 nm and 20 nm, and precision of 20 nm and 200 nm respectively, while imaging them in four different fluorescence channels. We demonstrate fabrication of ordered two-component and three dimensional colloidal arrays, as well as trapping of yeast cell arrays. We study the kinetics of the division of yeast cells within optical traps, and find that the timescale for division is not affected by trapping.

Di-alkylated paromomycin derivatives: targeting the membranes of gram positive pathogens that cause skin infections.

A collection of paromomycin-based di-alkylated cationic amphiphiles differing in the lengths of their aliphatic chain residues were designed, synthesized, and evaluated against 14 Gram positive pathogens that are known to cause skin infections. Paromomycin derivatives that were di-alkylated with C7 and C8 linear aliphatic chains had improved antimicrobial activities relative to the parent aminoglycoside as well as to the clinically used membrane-targeting antibiotic gramicidin D; several novel derivatives were at least 16-fold more potent than the parent aminoglycoside paromomycin. Comparison between a di-alkylated and a mono-alkylated paromomycin indicated that the di-alkylation strategy leads to both an improvement in antimicrobial activity and to a dramatic reduction in undesired red blood cell hemolysis caused by many aminoglycoside-based cationic amphiphiles. Scanning electron microscopy provided evidence for cell surface damage by the reported di-alkylated paromomycins.