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Friday, June 30, 2017

Computational study of optical force between two nanodistant plasmonic submicrowires

Masoud Rezvani Jalal and Saba Fathollahi

In this paper, the optical force between two circular plasmonic wires of submicrometer diameter (0.3 μm) with nanometer surface-to-surface distances (3–30 nm) interacting with radiation of a complex point source (𝜆≈0.2λ≈0.2 μm) is numerically studied. Calculations (which are based on the Müller integral equations and the Maxwell stress tensor) show that an attractive optical force with a number of distinct peaks is created in distances 3–10 nm. However, for plasmonic–dielectric and plasmonic–reflector double-wires, the optical force has no such peaks. Comparisons reveal that the peaks are originated from the excitation of coupled surface plasmon polaritons in the gap region between the plasmonic wires.

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In situ self-assembly and photopolymerization for hetero-phase synthesis and patterning of conducting materials using soft oxometalates in thermo-optical tweezers

Subhrokoli Ghosh, Santu Das, Shuvojit Paul, Preethi Thomas, Basudev Roy, Partha Mitra, Soumyajit Roy and Ayan Banerjee

We demonstrate a novel method of simultaneous photoassisted hetero-phase synthesis, doping, and micro-scale patterning of conductive materials. The patterning is performed by controlled self-assembly mediated by a micro-bubble induced in an optical tweezers configuration. The high temperature generated due to the light field of the tweezers also drives diverse chemical reactions that lead to the in situ formation of conducting metal-oxides and polymers due to a charge transfer mechanism with soft oxometalates (SOMs). We synthesize two conducting polymers – polypyrrole and polyaniline – doped by the metal oxides Mo–O2 and Mo–O3, from dispersions of the respective organic compounds with the SOMs, and form permanent patterns out of them by continuous self-assembly arising from manipulation of the micro-bubble using Marangoni flows generated by the tweezers. The electrically conducting patterns of width varying between around 4–50 μm, are written in the form of simple Hall-bar geometries, and a four-probe measurement technique yields conductivities on the order of ∼450–600 Siemens cm−1 – which is much higher than that reported for both polypyrrole and polyaniline in earlier work. This technique can easily be used in patterning complicated electrical circuits in mesoscopic length scales, and can also be extended to solution processed electronic device development by green chemical routes.

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Myosin Va molecular motors manoeuvre liposome cargo through suspended actin filament intersections in vitro

Andrew T. Lombardo, Shane R. Nelson, M. Yusuf Ali, Guy G. Kennedy, Kathleen M. Trybus, Sam Walcott & David M. Warshaw

Intracellular cargo transport relies on myosin Va molecular motor ensembles to travel along the cell’s three-dimensional (3D) highway of actin filaments. At actin filament intersections, the intersecting filament is a structural barrier to and an alternate track for directed cargo transport. Here we use 3D super-resolution fluorescence imaging to determine the directional outcome (that is, continues straight, turns or terminates) for an ∼10 motor ensemble transporting a 350 nm lipid-bound cargo that encounters a suspended 3D actin filament intersection in vitro. Motor–cargo complexes that interact with the intersecting filament go straight through the intersection 62% of the time, nearly twice that for turning. To explain this, we develop an in silico model, supported by optical trapping data, suggesting that the motors’ diffusive movements on the vesicle surface and the extent of their engagement with the two intersecting actin tracks biases the motor–cargo complex on average to go straight through the intersection.

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Improvement of Sensing and Trapping Efficiency of Double Nanohole Apertures via Enhancing the Wedge Plasmon Polariton Modes with Tapered Cusps

Mostafa Ghorbanzadeh, Steven Jones, Mohammad Kazem Moravvej-Farshi, and Reuven Gordon

In the past few years, double nanohole (DNH) apertures in a gold film have been used extensively to trap and sense biological and artificial dielectric nanoparticles. Using numerical simulations we show that the conical shape of a DNH, milled by a focused ion beam into a thin gold layer, which is an inherent property of the fabrication process, plays a critical role in the sensitivity of the DNHs, and is beneficial to the optical sensing and trapping applications. The slope of the metallic wedges in an appropriately designed DNH leads to 2D nanofocusing of gap surface plasmons (GSPs) and couples them to the wedge plasmon polaritons (WPPs), creating “hot spots” required for trapping. The transmission variations due to the trapping polystyrene nanoparticles of radii 11 ± 1 nm by particularly designed DNHs, measured at the wavelength near the corresponding wedge mode resonance, are shown to be in good agreements with numerical results using conically modeled DNHs. This observation highlights the extreme sensitivity of aperture assisted trapping, specifically with regard to the DNH structure. These findings open up new routes toward the design and optimization of efficient aperture structures for trapping and sensing applications.

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Low-loss nanowire and nanotube plasmonic waveguide with deep subwavelength light confinement and enhanced optical trapping forces

Xiaogang Chen, Qijing Lu, Xiang Wu, Hongqin Yang and Shusen Xie

With the rapid development of the micro/nano fabrication technology, the semiconductor nanowires and nanotubes with size and dimensions controllable realize wide applications in nanophotonics. In this talk, we propose two kinds of hybrid plasmonics waveguides, one is consisting of nanowires, another is consisting of nanotubes. By employing the simulating with different geometric parameters, the basic waveguiding properties, including the effective mode area, the propagation length, the mode character and the optical trapping forces can be achieved. Compared with previous plasmonic waveguide with plane metal substrate, current plasmonics waveguides with ease of fabrication have the advantage of long propagation length and effectively optical trapping of nanoparticles with deep subwavelength light confinement, which may be very useful for nanophotonic integrated circuits, nanolasers and biosensing.

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Plasmonic Chiral Nanostructures: Chiroptical Effects and Applications

Yang Luo, Cheng Chi, Meiling Jiang, Ruipeng Li, Shuai Zu, Yu Li, Zheyu Fang

The plasmonic chiroptical effect has been used to manipulate chiral states of light, where the strong field enhancement and light localization in metallic nanostructures can amplify the chiroptical response. Moreover, in metamaterials, the chiroptical effect leads to circular dichroism (CD), circular birefringence (CB), and asymmetric transmission. Potential applications enabled by chiral plasmonics have been realized in various areas of nanoscience and nanotechnology. In this review, both basic theories and state-of-the-art studies on plasmonic chiroptical effects are summarized. Molecular chiroptical effects are drastically enhanced by metallic nanostructures that can generate a “superchiral” field, which arises from the strong electromagnetic interactions. Both intrinsic and extrinsic plasmonic chiral metamaterials formed by the periodic arrangement of metallic nanostructured units show high levels of CB, CD, and asymmetric transmission. Consequent applications including photo detection, molecular sensing, and chirality tuning are discussed, and a perspective of emerging concepts such as Pancharatnam−Berry (PB) phase in this booming research field is presented.

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Wednesday, June 28, 2017

Temperature-induced Coalescence of Droplets Manipulated by Optical Trapping in an Oil-in-Water Emulsion

Manami MITSUNOBU, Sakurako KOBAYASHI, Nobuyuki TAKEYASU, Takashi KANETA

Coalescence of oil droplets in an oil-in-water (O/W) emulsion was achieved with heating and optical trapping. Three types of O/W emulsions were prepared by adding a mixture of butanol and n-decane to an aqueous solution containing a cationic surfactant (cetyltrimethylammonium bromide, CTAB), an anionic surfactant (sodium dodecyl sulfate, SDS), or a neutral hydrophilic polymer (polyethylene glycol, PEG) as an emulsifier. Two oil droplets in the emulsions were randomly trapped in a square capillary tube by two laser beams in order to induce coalescence. Coalescence of the droplets could not be achieved at room temperature (25°C) regardless of the type of emulsifier. Conversely, the droplets prepared with PEG coalesced at a temperature higher than 30°C, although the droplets with ionic surfactants CTAB and SDS did not coalesce even at the elevated temperature due to their electrostatic repulsion. The size of the resultant coalesced droplet was consistent with that calculated from the size of the two droplets of oil, which indicated successful coalescence of the two droplets. We also found that the time required for the coalescence could be correlated with the temperature using an Arrhenius plot.

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Vector assembly of colloids on monolayer substrates

Lingxiang Jiang, Shenyu Yang, Boyce Tsang, Mei Tu & Steve Granick

The key to spontaneous and directed assembly is to encode the desired assembly information to building blocks in a programmable and efficient way. In computer graphics, raster graphics encodes images on a single-pixel level, conferring fine details at the expense of large file sizes, whereas vector graphics encrypts shape information into vectors that allow small file sizes and operational transformations. Here, we adapt this raster/vector concept to a 2D colloidal system and realize ‘vector assembly’ by manipulating particles on a colloidal monolayer substrate with optical tweezers. In contrast to raster assembly that assigns optical tweezers to each particle, vector assembly requires a minimal number of optical tweezers that allow operations like chain elongation and shortening. This vector approach enables simple uniform particles to form a vast collection of colloidal arenes and colloidenes, the spontaneous dissociation of which is achieved with precision and stage-by-stage complexity by simply removing the optical tweezers.

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Propagation properties and radiation forces of the Airy Gaussian vortex beams in a harmonic potential

Zihao Pang and Dongmei Deng

We investigate the propagation properties and the radiation forces of Airy Gaussian vortex (AiGV) beams in a harmonic potential analytically and numerically in this paper. Obtaining the propagation expression of AiGV beams by solving the dimensionless linear (2+1) D Schrödinger equation in a harmonic potential, we perform the track, the intensity and phase distributions, the propagation shapes, the energy flow and the angular momentum of AiGV beams in a harmonic potential with the method of numerical simulations. The trajectory acting like a cosine curve is shown. Periodic inversion and phase oscillation are demonstrated during propagation. The influence of the distribution factor and the vortex factor on the propagation of AiGV beams in a harmonic potential are discussed. Likewise, the motion of the Poynting vector and the angular momentum is elucidated respectively. As for the radiation forces, we explore the gradient and scattering forces on Rayleigh dielectric particles induced by AiGV beams. In particular, it’s found that the value of the scattering force is approximately seven orders of magnitude larger than that of the gradient force during the propagation in a harmonic potential.
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Plasmonic optical trapping of nanometer-sized J- /H- dye aggregates as explored by fluorescence microspectroscopy

Ayaka Mototsuji, Tatsuya Shoji, Yumi Wakisaka, Kei Murakoshi, Hiroshi Yao, and Yasuyuki Tsuboi

In the present study, we explored plasmonic optical trapping (POT) of nanometer-sized organic crystals, carbocyanine dye aggregates (JC-1). JC-1 dye forms both J- and H- aggregates in aqueous solution. POT behavior was analyzed using fluorescence microspectroscopy. POT of JC-1 aggregates was realized in an increase in their fluorescence intensity from the focus area upon plasmon excitation. Repeating on-and-off plasmonic excitation resulted in POT of JC-1 aggregates in a trap-and-release mode. Such POT of nanometer-sized dye aggregates lying in a Rayleigh scattering regime (< 100 nm) is important toward molecular manipulation. Furthermore, interestingly, we found that the J-aggregates were preferentially trapped than H-aggregates. It possibly indicates semi-selective optical trapping of nanoparticles on the basis of molecular alignments.

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