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Sunday, November 23, 2014

FDTD analysis of optical forces on bowtie antennas for high-precision trapping of nanostructures

Arif E. Cetin

We theoretically investigate the optical forces generated by a high near-field resolution antenna system through finite difference time domain calculations along with the Maxwell stress tensor method. Our antenna choice is bowtie-shaped nanostructures with small gap regions, exploiting propagating waveguide modes as well as localized surface plasmons. Our analysis shows that the antenna system supports large optical forces at the resonance wavelength where the near-field intensities as well as their gradients are the largest within the gap region. We show that the system exhibits much larger optical forces when the incident light polarization is along the bowtie gap as the system can effectively leverage the gap effect, compared to the case when the system is under the polarization normal to the gap. We also investigate the forces on a dielectric bead in the vicinity of the antennas for different positions to show the optical force characteristics of the bowtie-shaped antennas. Finally, the force analysis on different bead radiuses demonstrates the trapping efficiency of our antenna system.

DOI

Photophoretic trapping-Raman spectroscopy for single pollens and fungal spores trapped in air

Chuji Wang, Yong-Le Pan, Steven C. Hill, Brandon Redding

Photophoretic trapping-Raman spectroscopy (PTRS) is a new technique for measuring Raman spectra of particles that are held in air using photophoretic forces. It was initially demonstrated with Raman spectra of strongly-absorbing carbon nanoparticles (Pan et al. [44] (Opt Express 2012)). In the present paper we report the first demonstration of the use of PTRS to measure Raman spectra of absorbing and weakly-absorbing bioaerosol particles (pollens and spores). Raman spectra of three pollens and one smut spore in a size range of 6.2–41.8 µm illuminated at 488 nm are shown. Quality spectra were obtained in the Raman shift range of 1600–3400 cm−1 in this exploratory study. Distinguishable Raman scattering signals with one or a few clear Raman peaks for all four aerosol particles were observed within the wavenumber region 2940–3030 cm−1. Peaks in this region are consistent with previous reports of Raman peaks in the 1600–3400 cm−1 range for pollens and spores excited at 514 nm measured by a conventional Raman spectrometer. Noise in the spectra, the fluorescence background, and the weak Raman signals in most of the 1600–3400 cm−1 region make some of the spectral features barely discernable or not discernable for these bioaerosols except the strong signal within 2940–3030 cm−1. Up to five bands are identified in the three pollens and only two bands appear in the fungal spore, but this may be because the fungal spore is so much smaller than any of the pollens. The fungal spore signal relative to the air-nitrogen Raman band is approximately 10 times smaller than that ratio for the pollens. The five bands are tentatively assigned to the CH2 symmetric stretch at 2948 cm−1, CH2 Fermi resonance stretch at 2970 cm−1, CH3 symmetric stretch at 2990 cm−1, CH3 out-of-plane end asymmetric stretch at 3010 cm−1, and unsaturated =CH stretch at 3028 cm−1. The two dominant bands of the up-to-five Raman bands in the 2940–3030 cm−1 region have a consistent band spacing of 25 cm−1 in all four aerosols. Finally we discuss improvements to the PTRS that should provide a system which can trap a higher fraction of particle types and obtain Raman spectra over a larger range (e.g., 200–3600 cm−1) than those achieved here.

DOI

Collective flow dynamics across a bacterial carpet: Understanding the forces generated

Yi-Teng Hsiao, Jing-Hui Wang, Kuan-Ting Wu, Jengjan Tsai, Cheng-Hung Chang and Wei-Yen Woon

Bacterial carpets consist of randomly anchored uni-polar-flagellated sodium-motive bacterial matrix are prepared by flow deposition. Collective flow dynamics across the bacterial carpets are probed with optical tweezers-microsphere assay. Around the center of a uniform bacterial cluster, collective forces that pull microsphere towards carpet surface are detected at a distance of 10 μm away from carpets. At sodium-motive driving over a critical value, the force magnitudes increase abruptly, suggesting a threshold-like transition of hydrodynamic synchronization across bacterial carpet. The abrupt force increase is explained in term of bifurcation to phase synchronization in a noisy non-linearly coupled rotor array mediated by hydrodynamic interactions.

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Characterizing conical refraction optical tweezers

C. McDonald, C. McDougall, E. Rafailov, and D. McGloin

Conical refraction occurs when a beam of light travels through an appropriately cut biaxial crystal. By focusing the conically refracted beam through a high numerical aperture microscope objective, conical refraction optical tweezers can be created, allowing for particle manipulation in both Raman spots, and in the Lloyd/Poggendorff rings. We present a thorough quantification of the trapping properties of such a beam, focusing on the trap stiffness, and how this varies with trap power and trapped particle location. We show that the lower Raman spot can be thought of as a single-beam optical gradient force trap, while radiation pressure dominates in the upper Raman spot, leading to optical levitation rather than trapping. Particles in the Lloyd/Poggendorff rings experience a lower trap stiffness than particles in the lower Raman spot, but benefit from rotational control.

DOI

Friday, November 21, 2014

Template stripped double nanohole in a gold film for nano-optical tweezers

Ana Zehtabi-Oskuie, Aurora A Zinck, Ryan M Gelfand and Reuven Gordon

Double nanohole (DNH) laser tweezers can optically trap and manipulate objects such as
proteins, nanospheres, and other nanoparticles; however, precise fabrication of those DNHs has been expensive with low throughput. In this work, template stripping was used to pattern DNHs with gaps as small as 7 nm, in optically thick Au films. These DNHs were used to trap streptavidin as proof of operation. The structures were processed multiple times from the same template to demonstrate reusability. Template stripping is a promising method for high-throughput, reproducible, and cost efficient fabrication of DNH apertures for optical trapping.

DOI

Cleaved fiber optic double nanohole optical tweezers for trapping nanoparticles

Ryan M. Gelfand, Skylar Wheaton, and Reuven Gordon
We demonstrate the trapping of single 20 and 40 nm polystyrene spheres at the cleaved end of a fiber optic with a double nanohole aperture in gold and without any microscope optics. An optical transmission increase of 15% indicates a trapping event for the 40 nm particle, and the jump is 2% for the 20 nm particle. This modular technique can be used to replace those used with current optical trapping setups that require complicated free space optics and frequent calibration, with one that is modular and requires no free space optics. This simple arrangement with the potential for fiber translation is of interest for future biosensor and optical nano-pipette devices.

DOI

Optical sorting of nonspherical and living microobjects in moving interference structures

Petr Jákl, Alejandro V. Arzola, Martin Šiler, Lukáš Chvátal, Karen Volke-Sepúlveda, and Pavel Zemánek

Contactless, sterile and nondestructive separation of microobjects or living cells is demanded in many areas of biology and analytical chemistry, as well as in physics or engineering. Here we demonstrate advanced sorting methods based on the optical forces exerted by travelling interference fringes with tunable periodicity controlled by a spatial light modulator. Besides the sorting of spherical particles we also demonstrate separation of algal cells of different sizes and particles of different shapes. The three presented methods offer simultaneous sorting of more objects in static suspension placed in a Petri dish or on a microscope slide.

DOI

Thursday, November 20, 2014

An integrated centrifugo-opto-microfluidic platform for arraying, analysis, identification and manipulation of individual cells

R. Burger, D. Kurzbuch, R. Gorkin, G. Kijanka, M. Glynn, C. McDonagh and J. Ducrée

In this work we present a centrifugal microfluidic system enabling highly efficient collective trapping and alignment of particles such as microbeads and cells, their multi-colour fluorescent detection and subsequent manipulation by optical tweezers. We demonstrate array-based capture and imaging followed by “cherry-picking” of individual particles, first for fluorescently labelled polystyrene (PS) beads and then for cells. Different cell lines are discriminated based on intracellular as well as surface-based markers.

DOI

Birefringence of a normal human red blood cell and related optomechanics in an optical trap

Belavadi Venkatakrishnaiah Nagesh; Yogesha; Ramarao Pratibha; Praveen Parthasarathi; Shruthi Subhash Iyengar; Sarbari Bhattacharya; Sharath Ananthamurthy

A normal human red blood cell (RBC) when trapped with a linearly polarized laser, reorients about the electric polarization direction and then remains rotationally bound to this direction. This behavior is expected for a birefringent object. We have measured the birefringence of distortion-free RBCs in an isotonic medium using a polarizing microscope. The birefringence is confined to the cell’s dimple region and the slow axis is along a diameter. We report an average retardation of 3.5±1.5  nm for linearly polarized green light (λ=546  nm). We also estimate a retardation of 1.87±0.09  nm from the optomechanical response of the RBC in an optical trap. We reason that the birefringence is a property of the cell membrane and propose a simple model attributing the origin of birefringence to the phospholipid molecules in the lipid bilayer and the variation to the membrane curvature. We observe that RBCs reconstituted in shape subsequent to crenation show diminished birefringence along with a sluggish optomechanical response in a trap. As the arrangement of phospholipid molecules in the cell membrane is disrupted on crenation, this lends credence to our conjecture on the origin of birefringence. Dependence of the birefringence on membrane contours is further illustrated through studies on chicken RBCs.

DOI

Structural Rearrangements in CHO Cells After Disruption of Individual Cytoskeletal Elements and Plasma Membrane

Špela Zemljič Jokhadar, Jure Derganc

Cellular structural integrity is provided primarily by the cytoskeleton, which comprises microtubules, actin filaments, and intermediate filaments. The plasma membrane has been also recognized as a mediator of physical forces, yet its contribution to the structural integrity of the cell as a whole is less clear. In order to investigate the relationship between the plasma membrane and the cytoskeleton, we selectively disrupted the plasma membrane and each of the cytoskeletal elements in Chinese hamster ovary cells and assessed subsequent changes in cellular structural integrity. Confocal microscopy was used to visualize cytoskeletal rearrangements, and optical tweezers were utilized to quantify membrane tether extraction. We found that cholesterol depletion from the plasma membrane resulted in rearrangements of all cytoskeletal elements. Conversely, the state of the plasma membrane, as assessed by tether extraction, was affected by disruption of any of the cytoskeletal elements, including microtubules and intermediate filaments, which are located mainly in the cell interior. The results demonstrate that, besides the cytoskeleton, the plasma membrane is an important contributor to cellular integrity, possibly by acting as an essential framework for cytoskeletal anchoring. In agreement with the tensegrity model of cell mechanics, our results support the notion of the cell as a prestressed structure.

DOI