Monday, November 30, 2015

Influence of arbitray mode in cluster formation in optical tweezers

R. Kumar

Three dimension multiparticle optical trapping and dynamic manipulation in a pre-defined fashion is a routine affair often realized by spatio-temporal modulation of fundamental Gaussian beam either using lens or diffractive optical elements (DOE). We present multi-particle trapping which is achieved due to cumulative effects of both the weak focusing of arbitrary beam and constructive interference arising from scrambling outwardly from trapped polystyrene spheres. The arbitrary mode is generated from diode pumped solid state source due to cavity imperfections per-se. The prime motivation of this article is to establish the usability of arbitrary mode in arranging as external optics in OTs for achieving colloidal clusters of microspheres from which forwardly scattered light can be utilized as feedback in shaping the wavefronts to be focused in highly turbid media. In general, it offers overall cost-cutting of experimental set-up and particularly suitable for fundamental demonstration of trapping to undergraduates only.


Transient dynamics of a colloidal particle driven through a viscoelastic fluid

Juan Ruben Gomez-Solano and Clemens Bechinger
We study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.


Femtosecond Nanostructuring of Glass with Optically Trapped Microspheres and Chemical Etching

Aleksander Shakhov, Artyom Astafiev, Alexander Gulin, and Victor A. Nadtochenko

Laser processing with optically trapped microspheres is a promising tool for nanopatterning at sub-diffraction limited resolution in a wide range of technological and biomedical applications. In this paper, we investigate sub-diffraction limited structuring of borosilicate glass with femtosecond pulses in the near-field of optically trapped microspheres combined with chemical post-processing. Glass surface was processed by single laser pulses at 780 nm focused by silica microspheres and then subjected to selective etching in KOH, which produced pits in the laser affected zones (LAZs). Chemical post-processing allowed obtaining structures with better resolution and reproducibility. We demonstrate production of reproducible pits with diameter as small as 70 nm (λ/11). Complex 2-Dimensional structures with 100 nm (λ/8) resolution were written on the glass surface point by point with microspheres manipulated by optical tweezers. Furthermore, the mechanism of laser modification underlying selective etching was investigated with mass-spectrum analysis. We propose that increased etching rate of laser-treated glass result from change in its chemical composition and oxygen deficiency.


Versatile microsphere attachment of GFP-labeled motors and other tagged proteins with preserved functionality

Michael Bugiel, Horatiu Fantana, Volker Bormuth, Anastasiya Trushko, Frederic Schiemann, Jonathon Howard, Erik Schäffer, Anita Jannasch

Microspheres are often used as handles for protein purification or force spectroscopy. For example, optical tweezers apply forces on trapped particles to which motor proteins are attached. However, even though many attachment strategies exist, procedures are often limited to a particular biomolecule and prone to non-specific protein or surface attachment. Such interactions may lead to loss of protein functionality or microsphere clustering. Here, we describe a versatile coupling procedure for GFP-tagged proteins via a polyethylene glycol linker preserving the functionality of the coupled proteins. The procedure combines well-established protocols, is highly reproducible, reliable, and can be used for a large variety of proteins. The coupling is efficient and can be tuned to the desired microsphere-to-protein ratio. Moreover, microspheres hardly cluster or adhere to surfaces. Furthermore, the procedure can be adapted to different tags providing flexibility and a promising attachment strategy for any tagged protein.


Thursday, November 26, 2015

Holographic Raman tweezers controlled by multi-modal natural user interface

Zoltán Tomori, Peter Keša, Matej Nikorovič, Jan Kaňka, Petr Jákl, Mojmír Šerý, Silvie Bernatová, Eva Valušová, Marián Antalík and Pavel Zemánek

Holographic optical tweezers provide a contactless way to trap and manipulate several microobjects independently in space using focused laser beams. Although the methods of fast and efficient generation of optical traps are well developed, their user friendly control still lags behind. Even though several attempts have appeared recently to exploit touch tablets, 2D cameras, or Kinect game consoles, they have not yet reached the level of natural human interface. Here we demonstrate a multi-modal 'natural user interface' approach that combines finger and gaze tracking with gesture and speech recognition. This allows us to select objects with an operator's gaze and voice, to trap the objects and control their positions via tracking of finger movement in space and to run semi-automatic procedures such as acquisition of Raman spectra from preselected objects. This approach takes advantage of the power of human processing of images together with smooth control of human fingertips and downscales these skills to control remotely the motion of microobjects at microscale in a natural way for the human operator.


Optical trapping and manipulation of nanoparticles using a meta plasmonic structure

Rehab Kotb, Mahmoud El Maklizi, Yehea Ismail and Mohamed A Swillam
In this paper, a novel structure of nano optical tweezers using triple-slit plasmonic grating structure is introduced and analyzed. The tweezers have deep potential wells that can trap sub-10 nm dielectric particle stably and efficiently. The resultant 50 KT potential well provides tight 2D trapping to the particle. The plasmonic device allows for steering the particle by simply changing the angle of the incident plane. This simple control allows efficient manipulation of the trapped particle over wide range of angles.


Probing the Red Blood Cells Aggregating Force With Optical Tweezers

Kisung Lee, Danilina, A.V. ; Kinnunen, M. ; Priezzhev, A.V. ; Meglinski, I.

The red blood cells (RBC) aggregation is of current basic science and clinical interest, as a determinant of blood microcirculation. Thus, the measurement and assessment of the RBC aggregation property (aggregability) and aggregation state at different physiologic conditions of a human individual or laboratory animal are an important issue. In this paper, in order to assess the dynamics of RBC interaction, optical tweezers were used to probe the forces during the RBC doublet formation or disruption. We show that in autologous plasma, RBC aggregating and disaggregating forces have different absolute values, ca 2-4 pN and dozens of piconewton, correspondingly. We speculate that in plasma, RBC aggregation and disaggregation processes have different driving forces.


Probing the Casimir force with optical tweezers

D. S. Ether jr., L. B. Pires, S. Umrath, D. Martinez, Y. Ayala, B. Pontes, G. R. de S. Araújo, S. Frases, G.-L. Ingold, F. S. S. Rosa

We propose to use optical tweezers to probe the Casimir interaction between microspheres inside a liquid medium for geometric aspect ratios far beyond the validity of the widely employed proximity force approximation. This setup has the potential for revealing unprecedented features associated to the non-trivial role of the spherical curvatures. For a proof of concept, we measure femtonewton double-layer forces between polystyrene microspheres at distances above 400 nm by employing very soft optical tweezers, with stiffness of the order of fractions of a fN/nm. As a future application, we propose to tune the Casimir interaction between a metallic and a polystyrene microsphere in saline solution from attraction to repulsion by varying the salt concentration. With those materials, the screened Casimir interaction may have a larger magnitude than the unscreened one. This line of investigation has the potential for bringing together different fields including classical and quantum optics, statistical physics and colloid science, while paving the way for novel quantitative applications of optical tweezers in cell and molecular biology.


Wednesday, November 25, 2015

Dimensionality constraints of light-induced rotation

László Oroszi, András Búzás, Péter Galajda, Lóránd Kelemen, Anna Mathesz, Tamás Vicsek, Gaszton Vizsnyiczai and Pál Ormos

We have studied the conditions of rotation induced by collimated light carrying no angular momentum. Objects of different shapes and optical properties were examined in the nontrivial case where the rotation axis is perpendicular to the direction of light propagation. This geometry offers important advantages for application as it fundamentally broadens the possible practical arrangements to be realised. We found that collimated light cannot drive permanent rotation of 2D or prism-like 3D objects (i.e., fixed cross-sectional profile along the rotation axis) in the case of fully reflective or fully transparent materials. Based on both geometrical optics simulations and theoretical analysis, we derived a general condition for rotation induced by collimated light carrying no angular momentum valid for any arrangement: Permanent rotation is not possible if the scattering interaction is two-dimensional and lossless. In contrast, light induced rotation can be sustained if partial absorption is present or the object has specific true 3D geometry. We designed, simulated, fabricated, and experimentally tested a microscopic rotor capable of rotation around an axis perpendicular to the illuminating light.


Direct observation of processive exoribonuclease motion using optical tweezers

Furqan M. Fazal, Daniel J. Koslover, Ben F. Luisi, and Steven M. Block

Bacterial RNases catalyze the turnover of RNA and are essential for gene expression and quality surveillance of transcripts. In Escherichia coli, the exoribonucleases RNase R and polynucleotide phosphorylase (PNPase) play critical roles in degrading RNA. Here, we developed an optical-trapping assay to monitor the translocation of individual enzymes along RNA-based substrates. Single-molecule records of motion reveal RNase R to be highly processive: one molecule can unwind over 500 bp of a structured substrate. However, enzyme progress is interrupted by pausing and stalling events that can slow degradation in a sequence-dependent fashion. We found that the distance traveled by PNPase through structured RNA is dependent on the A+U content of the substrate and that removal of its KH and S1 RNA-binding domains can reduce enzyme processivity without affecting the velocity. By a periodogram analysis of single-molecule records, we establish that PNPase takes discrete steps of six or seven nucleotides. These findings, in combination with previous structural and biochemical data, support an asymmetric inchworm mechanism for PNPase motion. The assay developed here for RNase R and PNPase is well suited to studies of other exonucleases and helicases.


Laser refrigeration of hydrothermal nanocrystals in physiological media

Paden B. Roder, Bennett E. Smith, Xuezhe Zhou, Matthew J. Crane, and Peter J. Pauzauskie

Coherent laser radiation has enabled many scientific and technological breakthroughs including Bose–Einstein condensates, ultrafast spectroscopy, superresolution optical microscopy, photothermal therapy, and long-distance telecommunications. However, it has remained a challenge to refrigerate liquid media (including physiological buffers) during laser illumination due to significant background solvent absorption and the rapid (∼ps) nonradiative vibrational relaxation of molecular electronic excited states. Here we demonstrate that single-beam laser trapping can be used to induce and quantify the local refrigeration of physiological media by >10 °C following the emission of photoluminescence from upconverting yttrium lithium fluoride (YLF) nanocrystals. A simple, low-cost hydrothermal approach is used to synthesize polycrystalline particles with sizes ranging from <200 nm to >1 μm. A tunable, near-infrared continuous-wave laser is used to optically trap individual YLF crystals with an irradiance on the order of 1 MW/cm2. Heat is transported out of the crystal lattice (across the solid–liquid interface) by anti-Stokes (blue-shifted) photons following upconversion of Yb3+ electronic excited states mediated by the absorption of optical phonons. Temperatures are quantified through analysis of the cold Brownian dynamics of individual nanocrystals in an inhomogeneous temperature field via forward light scattering in the back focal plane. The cold Brownian motion (CBM) analysis of individual YLF crystals indicates local cooling by >21 °C below ambient conditions in D2O, suggesting a range of potential future applications including single-molecule biophysics and integrated photonic, electronic, and microfluidic devices.


Lateral forces on circularly polarizable particles near a surface

Francisco J. Rodríguez-Fortuño, Nader Engheta, Alejandro Martínez & Anatoly V. Zayats

Optical forces allow manipulation of small particles and control of nanophotonic structures with light beams. While some techniques rely on structured light to move particles using field intensity gradients, acting locally, other optical forces can ‘push’ particles on a wide area of illumination but only in the direction of light propagation. Here we show that spin–orbit coupling, when the spin of the incident circularly polarized light is converted into lateral electromagnetic momentum, leads to a lateral optical force acting on particles placed above a substrate, associated with a recoil mechanical force. This counterintuitive force acts in a direction in which the illumination has neither a field gradient nor propagation. The force direction is switchable with the polarization of uniform, plane wave illumination, and its magnitude is comparable to other optical forces.


Tuesday, November 24, 2015

Plasmon-mediated binding forces on gold or silver homodimer and heterodimer

Jiunn-Woei Liaw, Ting-Yu Kuo, Mao-Kuen Kuo

This study theoretically investigates plasmon-mediated optical binding forces, which are exerted on metal homo or heterodimers, induced by the normal illumination of a linearly polarized plane wave or Gaussian beam. Using the multiple multipole method, we analyzed the optical force in terms of Maxwell׳s stress tensor for various interparticle distance at some specific wavelengths. Numerical results show that for a given wavelength there are several stable equilibrium distances between NPs of a homodimer, which are slightly shorter than some integer multiples of the wavelength in medium, such that metal dimer acts as bonded together. At these specific interparticle distances, the optical force between dimer is null and serves a restoring force, which is repulsive and attractive, respectively, as the two NPs are moving closer to and away from each other. The spring constant of the restoring force at the first stable equilibrium is always the largest, indicating that the first stable equilibrium distance is the most stable one. Moreover, the central line (orientation) of a dimer tends to be perpendicular to the polarization of light. For the cases of heterodimers, the phenomenon of stable equilibrium interparticle distance still exists, except there is an extra net photophoretic force drifting the heterodimer as one. Moreover, gradient force provided by a Gaussian beam may reduce the stability of these equilibriums, so larger NPs are preferred to stabilize a dimer under illumination of Gaussian beam. The finding may pave the way for using optical manipulation on the gold or silver colloidal self-assembly.


Optical torque reversal and spin-orbit rotational Doppler shift experiments

Davit Hakobyan and Etienne Brasselet

We report on optical rotational Doppler frequency shift experiments in the context of a counter-intuitive optomechanical phenomenon that is the angular analog of so-called negative optical radiation forces, which involves spin-orbit scattering of light. In practice, spin-orbit opto-mechanical effects arising from the interaction between polarized light and azimuthally varying birefringent optical elements are retrieved from mechano-optical experiments that involve spatial of the medium. Two kinds of experiments (single-beam and two-beam geometries) are performed and both approaches are discussed in the framework of previous dynamical geometric phase and rotational Doppler shift experiments based on spin and/or orbital angular momentum of light.


Droplet Manipulations in Two Phase Flow Microfluidics

Arjen M. Pit, Michèl H. G. Duits and Frieder Mugele

Even though droplet microfluidics has been developed since the early 1980s, the number of applications that have resulted in commercial products is still relatively small. This is partly due to an ongoing maturation and integration of existing methods, but possibly also because of the emergence of new techniques, whose potential has not been fully realized. This review summarizes the currently existing techniques for manipulating droplets in two-phase flow microfluidics. Specifically, very recent developments like the use of acoustic waves, magnetic fields, surface energy wells, and electrostatic traps and rails are discussed. The physical principles are explained, and (potential) advantages and drawbacks of different methods in the sense of versatility, flexibility, tunability and durability are discussed, where possible, per technique and per droplet operation: generation, transport, sorting, coalescence and splitting.


MD Simulation of Brownian Motion of Buckminsterfullerene Trapping in Nano-Optical Tweezers

M. Y. Abdollahzadeh Jamalabadi

Optical tweezers are a relatively new technique for non-invasive manipulation tool in biology and physics for studying single molecules. Brownian motion of a trapped particle poses a challenge to develop the Optical tweezers. Standard methods to analyze the optical tweezers data rely on using power spectrum of the Brownian motion of a dielectric bead trapped in the tweezers for macro scales. In this study the well-known MD code, GROMACS, is modified to find the variation of position and velocity of all atoms in the system of buckminsterfullerene solved in water. By applying the statistical methods our molecular dynamics simulations reveals the diffusion coefficient of the motion and the standard deviation of the Brownian motion. The simulation of system performs for the variety of trap constant and a model for estimation of the diffusion coefficient and the standard deviation of the Brownian motion is presented. Finally, experimental results are discussed based on the proposed model.


Monday, November 23, 2015

Microfluidic Sample Preparation for Single Cell Analysis

Sanjin Hosic, Shashi K. Murthy, and Abigail N Koppes

Single cell analysis is the measurement of transcription, translation, regulatory, and signaling events within individual cells at the molecular level. The goal is to analyze and synthesize information from single cells in order to holistically understand the cell population. This reductionist approach allows researchers to unravel how molecular events within a single cell link to the behavior of tissues, organs, and eventually whole organisms. Single cell analysis has gained significant traction over the past decade, as evidenced by the number of recent reviews.1-3 The field continues to expand exponentially and necessitates a review of developments that have occurred over the past three years. The transition from bulk to single cell analyses has been fueled in part by studies highlighting single cell heterogeneity and stochasticity relative to whole cell populations.4-5 The random variability in these cell populations is likely due to intrinsic noise. Intrinsic noise refers to cell-to-cell variation in transcription and translation products such as ions, mRNA, and proteins. These components are governed by phenomena such as reaction rates and molecular collisions. Given the flexible and dynamic nature of the cell membrane, reactions and molecular collisions will occur stochastically. Thus, it is unreasonable to assume that all cells within a population are equal at any given moment, and only a large number of single cell measurements will reveal this heterogeneity and provide the statistical power to model it. Modeling approaches are necessary for interpreting the massive amount of data generated with single cell analyses such as whole genome sequencing. Furthermore, these models may ultimately guide the optimum operation of a bioprocess such as the production of valuable biotherapeutics via cell culture or deterministic stem cell reprogramming for regenerative medicine.6 Such findings have driven the development of new analytical systems to probe biology at the resolution of a single cell. In order to study single cells accurately and efficiently, systems with high sensitivity and throughput are needed. The small dimensions of microfluidic systems enable single cell and reagent manipulation with minimal dilution,8 resulting in high sensitivity assays. Furthermore, microfluidic systems offer several key advantages toward the study of single cells including facile automation, parallelization, and reagent reduction.8 Early researchers found that sample preparation such as cell manipulation, compartmentalization, and lysis was significantly more difficult to implement at the single cell scale compared to in bulk. However, sample preparation preceding molecular analysis has also been miniaturized, allowing facile sample processing. As such, microfluidic systems have been developed and applied toward the study of single cells extensively.9-10 Given microfluidics’ instrumental role in single cell analysis up to this point, we can expect continued innovations in microfluidics to better enable single cell biology. In this review, novel microfluidic techniques currently used toward sample preparation and subsequent single cell analysis are highlighted. Techniques are discussed in terms of discrete sample preparation steps that may be necessary for characterizing single cells; tissue dissociation into cell suspensions, sorting heterogeneous cell populations into homogenous populations, isolating, and lysing single cells (Figure 1). With each discrete step, conventional approaches are discussed first and then microfluidic based strategies are reviewed. Finally, the future direction for developing microfluidic single cell analysis technology is discussed.


Photodynamic assembly of nanoparticles towards designable patterning

Huan Wang, Yong-lai Zhang, Hong Xia, qidai chen, Kwang-Sup Lee and Hongbo Sun

Recent advancements in nanotechnology continue to stimulate the development of functional devices based on nanomaterials. However, controllable assembly of these tiny nanomaterials into functional structures is still a big challenge for practical applications, nowhere is this more obvious than in the fields of nanodevices. Currently, despite the fact that self-assembly technologies have revealed great potential to reach this end, serious problems with respect to morphology control, designable assembly and even flexible patterning set huge obstacle to the fabrication of functional devices. Nowadays, in addition to self-assembly technologies that make use of relative weak interaction force, photodynamic assembly (PDA) technology has emerged as a promising route to architect functional materials in a controlled manner. In this review, we summarize the recent development in PDA technology for flexible patterning of NPs. Basic fundamentals of PDA that resort to optical trapping (OT) and typical examples regarding to far-field/near-field OT for PDA of various nanoparticles (NPs) have been reviewed. Particularly, femtosecond laser induced photodynamic assembly (FsL-PDA) that enable designable patterning of NPs through a direct writing manner has been introduced. Finally, current challenges and future prospects of this dynamic field are discussed based on our own opinion.


Determining the 3D Orientation of Optically Trapped Upconverting Nanorods by in situ Single-particle Polarized Spectroscopy

Paloma Rodriguez Sevilla, Lucia Labrador, Dominika Wawrzynczyk, Marcin Nyk, Marek Samoc, Ajoy K. Kar, Mark Mackenzie, Lynn Paterson, Daniel Jaque García and Patricia Haro

An approach to unequivocally determine the three dimensional orientation of optically manipulated NaYF4:Er3+,Yb3+ upconverting nanorods (UCNRs) is demonstrated. Long-term immobilization of individual UCNRs inside single and multiple resonant optical traps allow for stable single UCNR spectroscopy studies. Based on the strongly polarization dependent upconverted luminescence of UCNRs it is possible to unequivocally determine, in real time, their three dimensional orientation when optically trapped. In single-beam traps, polarized single particle spectroscopy has concluded that UCNRs orientate parallel to the propagation axis of the trapping beam. On the other hand, when multiple-beam optical tweezers are used, single particle polarization spectroscopy demonstrated how full spatial control over UCNR orientation can be achieved by changing the trap-to-trap distance as well as on the relative orientation between optical traps. All these results show the possibility of real time three dimensional manipulation and tracking of anisotropic nanoparticles with wide potential application in modern nanobiophotonics.


Mass-manufacturable polymer microfluidic device for dual fiber optical trapping

Diane De Coster, Heidi Ottevaere, Michael Vervaeke, Jürgen Van Erps, Manly Callewaert, Pieter Wuytens, Stephen H. Simpson, Simon Hanna, Wim De Malsche, and Hugo Thienpont

We present a microfluidic chip in Polymethyl methacrylate (PMMA) for optical trapping of particles in an 80µm wide microchannel using two counterpropagating single-mode beams. The trapping fibers are separated from the sample fluid by 70µm thick polymer walls. We calculate the optical forces that act on particles flowing in the microchannel using wave optics in combination with non-sequential ray-tracing and further mathematical processing. Our results are compared with a theoretical model and the Mie theory. We use a novel fabrication process that consists of a premilling step and ultraprecision diamond tooling for the manufacturing of the molds and double-sided hot embossing for replication, resulting in a robust microfluidic chip for optical trapping. In a proof-of-concept demonstration, we show the trapping capabilities of the hot embossed chip by trapping spherical beads with a diameter of 6µm, 8µm and 10µm and use the power spectrum analysis of the trapped particle displacements to characterize the trap strength.


Thursday, November 19, 2015

Trapping of classical particles by an electromagnetic potential well deepening over time

A. Ch. Izmailov
On the basis of fundamental relations of classical mechanics, we established a mechanism of trapping and localization of sufficiently slow particles by an electromagnetic potential well that becomes deeper over time (up to a certain limit). It is assumed that these particles are contained in high vacuum, and acting upon them forces are not dissipative. Such potential wells can be created by means of an electromagnetic field (nonresonance radiation, in particular) with fixed spatial distribution and nondecreasing over time electric field strength. Trapping and localization of particles in such electromagnetic traps, which takes place due to gradient forces, is analyzed for laser beams with typical intensity distribution. The obtained results can be used in high-resolution spectroscopy of different particles, including, in some cases, atoms and molecules.


Plasmon-Exciton Interactions Probed Using Spatial Co-Entrapment of Nanoparticles by Topological Singularities

Paul J. Ackerman, Haridas Mundoor, Ivan I. Smalyukh, and Jao van de Lagemaat

We study plasmon-exciton interaction by using topological singularities to spatially confine, selectively deliver, co-trap and optically probe colloidal semiconductor and plasmonic nanoparticles. The interaction is monitored in a single quantum system in the bulk of a liquid crystal medium where nanoparticles are manipulated and nanoconfined far from dielectric interfaces using laser tweezers and topological configurations containing singularities. When quantum dot-in-a-rod particles are spatially co-located with a plasmonic gold nanoburst particle in a topological singularity core, its fluorescence increases because blinking is significantly suppressed and the radiative decay rate increases by nearly an order of magnitude owing to the Purcell effect. We argue that the blinking suppression is the result of the radiative rate change that mitigates Auger recombination and quantum dot ionization, consequently reducing nonradiative recombination. Our work demonstrates that topological singularities are an effective platform for studying and controlling plasmon-exciton interactions.


Optical forces in nanorod metamaterial

Andrey A. Bogdanov, Alexander S. Shalin & Pavel Ginzburg

Optomechanical manipulation of micro and nano-scale objects with laser beams finds use in a large span of multidisciplinary applications. Auxiliary nanostructuring could substantially improve performances of classical optical tweezers by means of spatial localization of objects and intensity required for trapping. Here we investigate a three-dimensional nanorod metamaterial platform, serving as an auxiliary tool for the optical manipulation, able to support and control near-field interactions and generate both steep and flat optical potential profiles. It was shown that the ‘topological transition’ from the elliptic to hyperbolic dispersion regime of the metamaterial, usually having a significant impact on various light-matter interaction processes, does not strongly affect the distribution of optical forces in the metamaterial. This effect is explained by the predominant near-fields contributions of the nanostructure to optomechanical interactions. Semi-analytical model, approximating the finite size nanoparticle by a point dipole and neglecting the mutual re-scattering between the particle and nanorod array, was found to be in a good agreement with full-wave numerical simulation. In-plane (perpendicular to the rods) trapping regime, saddle equilibrium points and optical puling forces (directed along the rods towards the light source), acting on a particle situated inside or at the nearby the metamaterial, were found.


Wednesday, November 18, 2015

Gold Nanorod Rotary Motors Driven by Resonant Light Scattering

Lei Shao, Zhong-Jian Yang, Daniel Andrén, Peter Johansson, and Mikael Käll

Efficient and robust artificial nanomotors could provide a variety of exciting possibilities for applications in physics, biology and chemistry, including nanoelectromechanical systems, biochemical sensing, and drug delivery. However, the application of current man-made nanomotors is limited by their sophisticated fabrication techniques, low mechanical output power and severe environmental requirements, making their performance far below that of natural biomotors. Here we show that single-crystal gold nanorods can be rotated extremely fast in aqueous solutions through optical torques dominated by plasmonic resonant scattering of circularly polarized laser light with power as low as a few mW. The nanorods are trapped in 2D against a glass surface, and their rotational dynamics is highly dependent on their surface plasmon resonance properties. They can be kept continuously rotating for hours with limited photothermal side effects and they can be applied for detection of molecular binding with high sensitivity. Because of their biocompatibility, mechanical and thermal stability, and record rotation speeds reaching up to 42 kHz (2.5 million revolutions per minute), these rotary nanomotors could advance technologies to meet a wide range of future nanomechanical and biomedical needs in fields such as nanorobotics, nanosurgery, DNA manipulation and nano/microfluidic flow control.


Dynamic chromatin regulation from a single molecule perspective

Beat Fierz
Chromatin regulatory processes, like all biological reactions, are dynamic and stochastic in nature, but can give rise to stable and inheritable changes in gene expression patterns. A molecular understanding of those processes is key for fundamental biological insight into gene regulation, epigenetic inheritance, lineage determination and therapeutic intervention in the case of disease. In recent years great progress has been made in identifying important molecular players involved in key chromatin regulatory pathways. Conversely, we are only beginning to understand the dynamic interplay between protein effectors, transcription factors and the chromatin substrate itself. Single-molecule approaches employing both highly defined chromatin substrates in vitro, as well as direct observation of complex regulatory processes in vivo open new avenues for a molecular view of chromatin regulation. This review highlights recent applications of single-molecule methods and related techniques to investigate fundamental chromatin regulatory processes.


Lateral optical force on paired chiral nanoparticles in linearly polarized plane waves

Huajin Chen, Yikun Jiang, Neng Wang, Wanli Lu, Shiyang Liu, and Zhifang Lin

We demonstrate that a lateral optical force (LOF) can be induced on paired chiral nanoparticles with opposite handedness under the illumination of a linearly polarized plane wave. The LOFs on both chiral particles are equal and thus can move the pair sideways, with the direction depending on the separation between two particles, as well as the handedness of particle chirality. Analytical theory reveals that the LOF comes largely from the optical potential gradient established by the multiple scattering of light between the paired particles with asymmetric chirality. In addition, it is weakly dependent on the material loss of a particle, a feature of gradient force, while heavily dependent on the magnitude and handedness of particle chirality. The effect is expected to find applications in sorting and separating chiral dimers of different handedness.


Tuesday, November 17, 2015

In situ calibrating optical tweezers with sinusoidal-wave drag force method

Li Di, Zhou Jin-Hua, Hu Xin-Yao, Zhong Min-Cheng, Gong Lei, Wang Zi-Qiang, Wang Hao-Wei and Li Yin-Mei

We introduce a corrected sinusoidal-wave drag force method (SDFM) into optical tweezers to calibrate the trapping stiffness of the optical trap and conversion factor (CF) of photodetectors. First, the theoretical analysis and experimental result demonstrate that the correction of SDFM is necessary, especially the error of no correction is up to 11.25% for a bead of 5 μm in diameter. Second, the simulation results demonstrate that the SDFM has a better performance in the calibration of optical tweezers than the triangular-wave drag force method (TDFM) and power spectrum density method (PSDM) at the same signal-to-noise ratio or trapping stiffness. Third, in experiments, the experimental standard deviations of calibration of trapping stiffness and CF with the SDFM are about less than 50% of TDFM and PSDM especially at low laser power. Finally, the experiments of stretching DNA verify that the in situ calibration with the SDFM improves the measurement stability and accuracy.


Spectrally reconfigurable integrated multi-spot particle trap

Kaelyn D. Leake, Michael A. B. Olson, Damla Ozcelik, Aaron R. Hawkins, and Holger Schmidt

Optical manipulation of small particles in the form of trapping, pushing, or sorting has developed into a vast field with applications in the life sciences, biophysics, and atomic physics. Recently, there has been increasing effort toward integration of particle manipulation techniques with integrated photonic structures on self-contained optofluidic chips. Here, we use the wavelength dependence of multi-spot pattern formation in multimode interference (MMI) waveguides to create a new type of reconfigurable, integrated optical particle trap. Interfering lateral MMI modes create multiple trapping spots in an intersecting fluidic channel. The number of trapping spots can be dynamically controlled by altering the trapping wavelength. This novel, spectral reconfigurability is utilized to deterministically move single and multiple particles between different trapping locations along the channel. This fully integrated multi-particle trap can form the basis of high throughput biophotonic assays on a chip.


High-throughput linear optical stretcher for mechanical characterization of blood cells

Kevin B. Roth, Keith B. Neeves, Jeff Squier and David W. M. Marr

This study describes a linear optical stretcher as a high-throughput mechanical property cytometer. Custom, inexpensive, and scalable optics image a linear diode bar source into a microfluidic channel, where cells are hydrodynamically focused into the optical stretcher. Upon entering the stretching region, antipodal optical forces generated by the refraction of tightly focused laser light at the cell membrane deform each cell in flow. Each cell relaxes as it flows out of the trap and is compared to the stretched state to determine deformation. The deformation response of untreated red blood cells and neutrophils were compared to chemically treated cells. Statistically significant differences were observed between normal, diamide-treated, and glutaraldehyde-treated red blood cells, as well as between normal and cytochalasin D-treated neutrophils. Based on the behavior of the pure, untreated populations of red cells and neutrophils, a mixed population of these cells was tested and the discrete populations were identified by deformability.


Monday, November 16, 2015

Diffusion and Reactivity in Ultraviscous Aerosol and the Correlation with Particle Viscosity

Frances H. Marshall, Rachael E.H. Miles, Young-Chul Song, Peter B. Ohm, Rory M. Power, Jonathan P Reid and Cari S. Dutcher

The slow transport of water, organic species and oxidants in viscous aerosol can lead to aerosol existing in transient states that are not solely governed by thermodynamic principles but by the kinetics of gas-particle partitioning. The relationship between molecular diffusion constants and particle viscosity (for example, as reflected in the Stokes-Einstein equation) is frequently considered to provide an approximate guide to relate the kinetics of aerosol transformation with a material property of the aerosol. We report direct studies of both molecular diffusion and viscosity in the aerosol phase for the ternary system water/maleic acid/sucrose, considering the relationship between the hygroscopic response associated with the change in water partitioning, the volatilisation of maleic acid, the ozonolysis kinetics of maleic acid and the particle viscosity. Although water clearly acts as a plasticiser, the addition of minor fractions of other organic moieties can similarly lead to significant changes in the viscosity from that expected for the dominant component forming the organic matrix (sucrose). Here we highlight that the Stokes-Einstein relationship between the diffusion constant of water and the viscosity of the particle may be more than an order of magnitude in error, even at viscosities as low as 1 Pa s. We show that the thermodynamic relationships of hygroscopic response that underpin such kinetic determinations must be accurately known to retrieve accurate values for diffusion constants; such data are often not available. Further, we show that scaling of the diffusion constants of organic molecules of similar size to those forming the matrix with particle viscosity may be well represented by the Stokes-Einstein equation, suppressing the apparent volatility of semi-volatile components. Finally, the variation in uptake coefficients and diffusion constants for oxidants and small weakly interacting molecules may be much less dependent on viscosity than the diffusion constants of more strongly interacting molecules such as water.


Spatial Filtering of a Diode Laser Beam for Confocal Raman Microscopy

Kitt, Jay P.; Bryce, David A.; Harris, Joel M.

With the development of single-longitudinal mode diode lasers, there has been an increase in using these sources for Raman spectroscopy. This is largely due to the cost-effectiveness of diode lasers, which offer savings not only in initial capital cost, but also electrical, cooling, and replacement costs over time, when compared with ion lasers. The use of diode-lasers in confocal Raman microscopy has remained a challenge, however, due to poor transverse beam quality. In this work, we present the design and implementation of a simple spatial filter capable of adapting a single-mode diode laser source to confocal Raman microscopy, yielding comparable spatial resolution as a gas-ion laser beam for profiling and optical-trapping applications. For profiling applications, spatial filtering improved x,y resolution of the beam by a factor 10, which in turn increased optical-trapping forces by ∼90 times and yielded sevenfold greater Raman scattering signal intensity from an optically trapped phospholipid vesicle.


Friday, November 13, 2015

Nanoscale volume confinement and fluorescence enhancement with double nanohole aperture

Raju Regmi, Ahmed A. Al Balushi, Hervé Rigneault, Reuven Gordon & Jérôme Wenger
Diffraction ultimately limits the fluorescence collected from a single molecule, and sets an upper limit to the maximum concentration to isolate a single molecule in the detection volume. To overcome these limitations, we introduce here the use of a double nanohole structure with 25 nm gap, and report enhanced detection of single fluorescent molecules in concentrated solutions exceeding 20 micromolar. The nanometer gap concentrates the light into an apex volume down to 70 zeptoliter (10−21 L), 7000-fold below the diffraction-limited confocal volume. Using fluorescence correlation spectroscopy and time-correlated photon counting, we measure fluorescence enhancement up to 100-fold, together with local density of optical states (LDOS) enhancement of 30-fold. The distinctive features of double nanoholes combining high local field enhancement, efficient background screening and relative nanofabrication simplicity offer new strategies for real time investigation of biochemical events with single molecule resolution at high concentrations.


Enhanced Optical Manipulation of Cells Using Antireflection Coated Microparticles

Derek Craig, Alison McDonald, Michael Mazilu, Helen Rendall, Frank Gunn-Moore, and Kishan Dholakia

We demonstrate the use of antireflection (AR) coated microparticles for the enhanced optical manipulation of cells. Specifically, we incubate CHO-K1, HL60, and NMuMG cell lines with AR-coated titania microparticles and subsequently performed drag force measurements using optical trapping. Direct comparisons were performed between native, polystyrene microparticle and AR microparticle tagged cells. The optical trapping efficiency was recorded by measuring the Q value in a drag force experiment. CHO-K1 cells incubated with AR microparticles show an increase in the Q value of nearly 220% versus native cells. With the inclusion of AR microparticles, cell velocities exceeding 50 μm/s were recorded for only 33 mW of laser trapping power. Cell viability was confirmed with fluorescent dyes and cells expressing a fluorescent ubiquitination-based cell cycle protein (FUCCI), which verified no disruption to the cell cycle in the presence of AR microparticles.


Photothermal Superheating of Water with Ion-Implanted Silicon Nanowires

Paden B. Roder, Sandeep Manandhar, Bennett E. Smith, Xuezhe Zhou, Vaithiyalingam S. Shutthanandan and Peter J. Pauzauskie

Elucidating the temperatures of particles trapped in laser tweezers is of great importance and impacts studies and applications in various areas of science and engineering. A unique method is demonstrated for extracting the local temperature of trapped ion-implanted silicon nanowires undergoing hot Brownian motion in superheated water, and their efficacy as agents for photothermal cancer therapy is investigated.


Thursday, November 12, 2015

Automated analysis of single cells using Laser Tweezers Raman Spectroscopy

Stephen Casabella, Peter Gardner, Patricia Scully and N J Goddard

In recent years, significant progress has been made into the label-free detection and discrimination of individual cancer cells using Laser Tweezers Raman Spectroscopy (LTRS). However, the majority of examples reported have involved manual trapping of cells, which is time consuming and may lead to different cell lines being analysed in discrete batches. A simple, low-cost microfluidic flow chamber is introduced which allows single cells to be optically trapped and analysed in an automated fashion, greatly reducing the level of operator input required. Two implementations of the flow chamber are discussed here; a basic single-channel device in which the fluid velocity is controlled manually, and a dual-channel device which permits the automated capture and analysis of multiple cell lines with no operator input. Results are presented for the discrimination of live epithelial prostate cells and lymphocytes, together with a consideration of the consequences of traditional ‘batch analysis’ typically used for LTRS of live cells.


Recent advances in holographic 3D particle tracking

Pasquale Memmolo, Lisa Miccio, Melania Paturzo, Giuseppe Di Caprio, Giuseppe Coppola, Paolo A. Netti, and Pietro Ferraro

Particle tracking is a fundamental technique for investigating a variety of biophysical processes, from intracellular dynamics to the characterization of cell motility and migration. However, observing three-dimensional (3D) trajectories of particles is in general a challenging task in classical microscopy owing to the limited imaging depth of field of commercial optical microscopes, which represents a serious drawback for the analysis of time-lapse microscopy image data. Therefore, numerous automated particle-tracking approaches have been developed by many research groups around the world. Recently, digital holography (DH) in microscopy has rapidly gained credit as one of the elective techniques for these applications, mainly due to the uniqueness of the DH to provide a posteriori quantitative multiple refocusing capability and phase-contrast imaging. Starting from this paradigm, a huge amount of 3D holographic tracking approaches have been conceived and investigated for applications in various branches of science, including optofluids, microfluidics, biomedical microscopy, cell mechano-trasduction, and cell migration. Since a wider community of readers could be interested in such a review, i.e., not only scientists working in the fields of optics and photonics but also users of particle-tracking tools, it should be very beneficial to provide a complete review of state-of-the-art holographic 3D particle-tracking methods and their applications in bio-microfluidics.


Wednesday, November 11, 2015

Developing a video tracking method to study interactions between close pairs of optically trapped particles in three dimensions

Allan Raudsepp, Marjorie Griffiths, Andrew J. Sutherland-Smith, and Martin A. K. Williams

We develop a video tracking method that utilizes an interpolation-based normalized cross-correlation approach to track the position of microscopic spherical particles in three dimensions. Subnanometer resolution is demonstrated. The method does not assume that the particle’s image is radially symmetric, making it useful for determining the position when particles are close and their images overlap. This is demonstrated in a study of the electrostatic and hydrodynamic interactions between a pair of beads in dual laser tweezers traps.


On artifacts in single-molecule force spectroscopy

Pilar Cossioa, Gerhard Hummera, and Attila Szabo

In typical force spectroscopy experiments, a small biomolecule is attached to a soft polymer linker that is pulled with a relatively large bead or cantilever. At constant force, the total extension stochastically changes between two (or more) values, indicating that the biomolecule undergoes transitions between two (or several) conformational states. In this paper, we consider the influence of the dynamics of the linker and mesoscopic pulling device on the force-dependent rate of the conformational transition extracted from the time dependence of the total extension, and the distribution of rupture forces in force-clamp and force-ramp experiments, respectively. For these different experiments, we derive analytic expressions for the observables that account for the mechanical response and dynamics of the pulling device and linker. Possible artifacts arise when the characteristic times of the pulling device and linker become comparable to, or slower than, the lifetimes of the metastable conformational states, and when the highly anharmonic regime of stretched linkers is probed at high forces. We also revisit the problem of relating force-clamp and force-ramp experiments, and identify a linker and loading rate-dependent correction to the rates extracted from the latter. The theory provides a framework for both the design and the quantitative analysis of force spectroscopy experiments by highlighting, and correcting for, factors that complicate their interpretation.


Trapping and Manipulation of Copper Nanoparticles in Rayleigh Regime

E. Thanikaivelan, R. Jothilakshmi, P. Murugakoothan

The copper nanoparticles were synthesized by green technology using leaf extracts of Ocimum sanctum. The synthesized copper nanoparticles were confirmed by the change of colour after the addition of leaf extract into the copper sulfate solution. The synthesized copper was characterized by X-ray Diffraction (XRD), transmission electron microscopy (TEM) and UV-vis absorption spectroscopy. The copper nanoparticles are crystallized with FCC structure. The synthesized copper nanoparticles exhibit spherical morphology with average particle size of 20 nm. The copper nanoparticle exhibits absorption broad band between 550 nm – 575 nm. The optical trapping effect of Gaussian beam acting on a copper nanoparticle in Rayleigh regime was studied. The optical scattering and optical gradient forces were calculated for 20 nm copper particle.


Silver Nanoparticles - Trapping and Manipulation

E. Thanikaivalan, R. Jothilakshmi, P. Murugakoothan

Silver nanoparticles with different radii were synthesized using silver nitrate solution added with sodium borohydride solution at different ratios. The synthesized silver nanoparticles of radii 25.3 nm, 31 nm, 33.6 nm and 37.1 nm were characterized by transmission electron microscopy (TEM) and UV-vis absorption spectroscopy. The synthesized silver nanoparticles exhibit spherical morphology for all radii. The silver nanoparticles exhibit the plasmon resonance band between 390 nm – 400 nm. The optical trapping effect of Gaussian beam acting on a silver nanoparticle in Rayleigh regime was studied. The optical scattering and optical gradient forces were calculated for silver nanoparticles of different radii.


Transverse Chiral Optical Forces by Chiral Surface Plasmon Polaritons

M.H. Alizadeh and Bjoern M. Reinhard

Recently the concepts of transverse spin angular momentum and Belinfante spin momentum of evanescent waves have drawn considerable attention. Here, we investigate these novel physical properties of electromagnetic fields in the context of chiral surface plasmon polaritons. We demonstrate, both analytically and numerically, that chiral surface plasmon polaritons possess transverse spin angular momentum and Belinfante momentum with rich and non-trivial characteristics. We also show that the transverse spin angular momentum of chiral surface plasmon polaritons leads to the emergence of transverse optical forces in opposite directions for chiral objects of different handedness. The magnitude of this chiral transverse optical force on a chiral particle is comparable to the magnitude of the achiral optical forces on the particle, namely the gradient force which arises from the intensity gradient and the scattering force which is the result of the linear momentum transfer of the photon to the particle. This finding may pave the way for realization of optical separation of chiral biomolecules.


Tuesday, November 10, 2015

Single-molecule perspectives on helicase mechanisms and functions

Bo Sun & Michelle D. Wang

Helicases are a diverse group of molecular motors that utilize energy derived from the hydrolysis of nucleoside triphosphates (NTPs) to unwind and translocate along nucleic acids. These enzymes play critical roles in nearly all aspects of nucleic acid metabolism, and consequently, a detailed understanding of helicase mechanisms at the molecular level is essential. Over the past few decades, single-molecule techniques, such as optical tweezers, magnetic tweezers, laminar flow, fluorescence resonance energy transfer (FRET), and DNA curtains, have proved to be powerful tools to investigate the functional properties of both DNA and RNA helicases. These approaches allow researchers to manipulate single helicase molecules, perturb their free energy landscape to probe the chemo-mechanical activities of these motors, and to detect the conformational changes of helicases during unwinding. Furthermore, these techniques also provide the capability to distinguish helicase heterogeneity and monitor helicase motion at nanometer spatial and millisecond temporal resolutions, ultimately providing new insights into the mechanisms that could not be resolved by ensemble assays. This review outlines the single-molecule techniques that have been utilized for measurements of helicase activities and discusses helicase mechanisms with a focus on functional and mechanistic insights revealed through single-molecule investigations in the past five years.


An electrodynamics-Langevin dynamics (ED-LD) approach to simulate metal nanoparticle interactions and motion

N. Sule, S. A. Rice, S. K. Gray, and N. F. Scherer
Understanding the formation of electrodynamically interacting assemblies of metal nanoparticles requires accurate computational methods for determining the forces and propagating trajectories. However, since computation of electromagnetic forces occurs on attosecond to femtosecond timescales, simulating the motion of colloidal nanoparticles on milliseconds to seconds timescales is a challenging multi-scale computational problem. Here, we present a computational technique for performing accurate simulations of laser-illuminated metal nanoparticles. In the simulation, we self-consistently combine the finite-difference time-domain method for electrodynamics (ED) with Langevin dynamics (LD) for the particle motions. We demonstrate the ED-LD method by calculating the 3D trajectories of a single 100-nm-diameter Ag nanoparticle and optical trapping and optical binding of two and three 150-nm-diameter Ag nanoparticles in simulated optical tweezers. We show that surface charge on the colloidal metal nanoparticles plays an important role in their optically driven self-organization. In fact, these simulations provide a more complete understanding of the assembly of different structures of two and three Ag nanoparticles that have been observed experimentally, demonstrating that the ED-LD method will be a very useful tool for understanding the self-organization of optical matter.


Application of femtosecond laser scalpel and optical tweezers for noncontact biopsy of late preimplantation embryos

I. V. Ilina , Yu. V. Khramova, M. A. Filatov, M. L. Semenova, D. S. Sitnikov

We demonstrate that one of the key steps of preimplantation genetic diagnosis called embryo biopsy can be successfully performed in noncontact mode by means of femtosecond laser scalpel and optical tweezers. Embryo biopsy was carried out in late-stage mouse preimplantation embryos. Femtosecond laser pulses were applied to detach the desired amount of trophectoderm cells from the blastocyst, while the optical tweezers trapped the cells and moved them out of the embryo. The parameters of laser radiation were opti-mized so as to efficiently perform embryo biopsy and preserve the viability of the treated embryos.


Activity-driven fluctuations in living cells

É. Fodor, M. Guo, N. S. Gov, P. Visco, D. A. Weitz and F. van Wijland

We propose a model for the dynamics of a probe embedded in a living cell, where both thermal fluctuations and nonequilibrium activity coexist. The model is based on a confining harmonic potential describing the elastic cytoskeletal matrix, which undergoes random active hops as a result of the nonequilibrium rearrangements within the cell. We describe the probe's statistics and we bring forth quantities affected by the nonequilibrium activity. We find an excellent agreement between the predictions of our model and experimental results for tracers inside living cells. Finally, we exploit our model to arrive at quantitative predictions for the parameters characterizing nonequilibrium activity, such as the typical time scale of the activity and the amplitude of the active fluctuations.


Monday, November 9, 2015

Improvements for Manipulating DNA with Optical Tweezers

Zhu Chun-Li and Li Jing

Recently, numerous biological macromolecular experiments have been conducted with optical tweezers. For the single molecular stretching experiment with optical tweezers, three ways to determine the initial adhesion point of DNA on the coverslip are described in this work. In addition, a new method through analyzing the displacement variance of the trapped particle to obtain the trap height is introduced. Using our proposed methods, the obtained force-extension curve for the operated dsDNA agrees well with the worm-like chain model. These improved methods are also applicable to other related biological macromolecular experiments requiring high precision.


Chiral route to pulling optical forces and left-handed optical torques

Antoine Canaguier-Durand and Cyriaque Genet

We analyze how chirality can generate pulling optical forces and left-handed torques by cross-coupling linear-to-angular momenta between the light field and the chiral object. In the dipolar regime, we reveal that such effects can emerge from a competition between nonchiral and chiral contributions to dissipative optical forces and torques, a competition balanced by the strength of chirality of the object. We extend the analysis to large chiral spheres where the interplay between chirality and multipolar resonances can give rise to a break of symmetry that flips the signs of both optical forces and torques.


Enzyme-Powered Hollow Mesoporous Janus Nanomotors

Xing Ma, Anita Jannasch, Urban-Raphael Albrecht, Kersten Hahn, Albert Miguel-López, Erik Schäffer, and Samuel Sánchez

The development of synthetic nanomotors for technological applications in particular for life science and nanomedicine is a key focus of current basic research. However, it has been challenging to make active nanosystems based on biocompatible materials consuming nontoxic fuels for providing self-propulsion. Here, we fabricate self-propelled Janus nanomotors based on hollow mesoporous silica nanoparticles (HMSNPs), which are powered by biocatalytic reactions of three different enzymes: catalase, urease, and glucose oxidase (GOx). The active motion is characterized by a mean-square displacement (MSD) analysis of optical video recordings and confirmed by dynamic light scattering (DLS) measurements. We found that the apparent diffusion coefficient was enhanced by up to 83%. In addition, using optical tweezers, we directly measured a holding force of 64 ± 16 fN, which was necessary to counteract the effective self-propulsion force generated by a single nanomotor. The successful demonstration of biocompatible enzyme-powered active nanomotors using biologically benign fuels has a great potential for future biomedical applications.


Simulations of radiation pressure experiments narrow down the energy and momentum of light in matter

Max Bethune-Waddell and Kenneth J Chau

Consensus on a single electrodynamic theory has yet to be reached. Discord was seeded over a century ago when Abraham and Minkowski proposed different forms of electromagnetic momentum density and has since expanded in scope with the gradual introduction of other forms of momentum and force densities. Although degenerate sets of electrodynamic postulates can be fashioned to comply with global energy and momentum conservation, hope remains to isolate a single theory based on detailed comparison between force density predictions and radiation pressure experiments. This comparison is two-fold challenging because there are just a handful of quantitative radiation pressure measurements over the past century and the solutions developed from different postulates, which consist of approximate expressions and inferential deductions, are scattered throughout the literature. For these reasons, it is appropriate to conduct a consolidated and comprehensive re-analysis of past experiments under the assumption that the momentum and energy of light in matter are degenerate. We create a combined electrodynamic/fluid dynamic simulation testbed that uses five historically significant sets of electrodynamic postulates, including those by Abraham and Minkowski, to model radiation pressure under diverse configurations with minimal assumptions. This leads to new interpretations of landmark investigations of light momentum, including the Balazs thought experiment, the Jones–Richards and Jones–Leslie measurements of radiation pressure on submerged mirrors, observations of laser-deformed fluid surfaces, and experiments on optical trapping and tractor beaming of dielectric particles. We discuss the merits and demerits of each set of postulates when compared to available experimental evidence and fundamental conservation laws. Of the five sets of postulates, the Abraham and Einstein–Laub postulates provide the greatest consistency with observations and the most physically plausible descriptions of electrodynamic interactions. Force density predictions made by these two postulates are unique under many conditions and their experimental isolation is potentially within reach.


Wednesday, November 4, 2015

Seriate microfluidic droplet coalescence under optical forces in a channel flow

Hyunjun Cho, Jin Ho Jung, Hyung Jin Sung

The microfluidic holding and coalescence behaviors of droplets subjected to an optical trap within a channel flow were numerically studied by using the lattice Boltzmann method and the dynamic ray tracing. A tightly focused Gaussian laser beam positioned laterally with respect to the channel flow direction was used as the optical trap. In such a system, seriate droplet coalescence was observed between an optically trapped droplet and the subsequent droplet. The numerically predicted droplet coalescence behavior agreed well with the experimental results. A sufficiently high laser power was required to capture an incoming droplet and induce coalescence with the subsequent droplet. The effects of various flow and optical parameters on the coalescence behavior were investigated.


In vivo quantification of peroxisome tethering to chloroplasts in tobacco epidermal cells using optical tweezers

Hongbo Gao, Jeremy Metz, Nick A Teanby, Andy D Ward, Stanley W Botchway, Benjamin Coles, Mark Pollard and Imogen Sparkes

Peroxisomes are highly motile organelles that display a range of motions within a short time frame. In static snapshots they can be juxtaposed to chloroplasts which has led to the hypothesis that they are physically interacting. Here, using optical tweezers we have tested the dynamic physical interaction in vivo. Using near-infrared optical tweezers, combined with TIRF microscopy, we were able to trap peroxisomes and approximate the forces involved in chloroplast association in vivo, and observed weaker tethering to additional unknown structures within the cell. We show that chloroplasts and peroxisomes are physically tethered through peroxules, a poorly described structure in plant cells. We suggest peroxules have a novel role in maintaining peroxisome-organelle interactions in the dynamic environment. This could be important for fatty acid mobilisation and photorespiration through interaction with oil bodies and chloroplasts, highlighting a fundamentally important role for organelle interactions for essential biochemistry and physiological processes.


Red blood cells in retinal vascular disorders

Rupesh Agrawal, Joseph Sherwood, Jay Chhablani, Ashutosh Ricchariya, Sangho Kim, Philip H. Jones, Stavroula Balabani, David Shima

Microvascular circulation plays a vital role in regulating physiological functions, such as vascular resistance, and maintaining organ health. Pathologies such as hypertension, diabetes, or hematologic diseases affect the microcirculation posing a significant risk to human health. The retinal vasculature provides a unique window for non-invasive visualisation of the human circulation in the vivo and retinal vascular image analysis has been established to predict the development of both clinical and subclinical cardiovascular, metabolic, renal and retinal disease in epidemiologic studies.
Blood viscosity which was otherwise thought to play a negligible role in determining blood flow based on Poiseuille's law till 1970s has now been shown to play equally if not more important role in controlling microcirculation and quantifying blood flow. Understanding the hemodynamics/rheology of the microcirculation and its changes in diseased states remains a challenging task; this is due to the particulate nature of blood, the mechanical properties of the cells (such as deformability and aggregability) and the complex architecture of the microvasculature.
In our review, we have tried to postulate possible role of red blood cell (RBC) biomechanical properties and laid down future framework for research related to hemorrheological aspects of blood in patients with retinal vascular disorders.


Developments in spontaneous and coherent Raman scattering microscopic imaging for biomedical applications

C. Krafft, I. W. Schie, T. Meyer, M. Schmitt and J. Popp

First, the potential role of Raman-based techniques in biomedicine is introduced. Second, an overview about the instrumentation for spontaneous and coherent Raman scattering microscopic imaging is given with a focus of recent developments. Third, imaging strategies are summarized including sequential registration with laser scanning microscopes, line imaging and global or wide-field imaging. Finally, examples of biomedical applications are presented in the context of single cells, laser tweezers, tissue sections, biopsies and whole animals.


Tuesday, November 3, 2015

Scaling of optical forces on Au–PEG core–shell nanoparticles

Donatella Spadaro, Maria A. Iatì, Maria G. Donato, Pietro G. Gucciardi, Rosalba Saija, Anurag R. Cherlakola, Stefano Scaramuzza, Vincenzo Amendola and Onofrio M. Maragò

Optical trapping of hybrid core–shell gold–polymer particles is studied. Optical forces are measured for different gold core size and polymer shell thickness, revealing how a polymer shell increases the trapping efficiency with respect to the bare gold nanoparticles. Data are in agreement with calculations of optical trapping based on electromagnetic scattering theory in the T-matrix approach. The scaling behaviour of optical forces with respect to the ratio between polymer layer thickness and the whole particle radius is found and discussed.


A self-stabilized coherent phonon source driven by optical forces

D. Navarro-Urrios, N. E. Capuj, J. Gomis-Bresco, F. Alzina, A. Pitanti, A. Griol, A. Martínez & C. M. Sotomayor Torres

We report a novel injection scheme that allows for “phonon lasing” in a one-dimensional opto-mechanical photonic crystal, in a sideband unresolved regime and with cooperativity values as low as 10−2. It extracts energy from a cw infrared laser source and is based on the triggering of a thermo-optical/free-carrier-dispersion self-pulsing limit-cycle, which anharmonically modulates the radiation pressure force. The large amplitude of the coherent mechanical motion acts as a feedback that stabilizes and entrains the self-pulsing oscillations to simple fractions of the mechanical frequency. A manifold of frequency-entrained regions with two different mechanical modes (at 54 and 122 MHz) are observed as a result of the wide tuneability of the natural frequency of the self-pulsing. The system operates at ambient conditions of pressure and temperature in a silicon platform, which enables its exploitation in sensing, intra-chip metrology or time-keeping applications.


Plasmonic optical tweezers: A long arm and a tight grip

Yasuyuki Tsuboi
By taking advantage of the thermal gradient that is generated in plasmonic systems and by using an a.c. field, plasmonic tweezers can have a large radius of action and can trap and manipulate single nano-objects.


On radiation forces acting on a transparent nanoparticle in the field of a focused laser beam

A A Afanas'ev, L S Gaida, D V Guzatov, A N Rubinov and A Ch Svistun

Radiation forces acting on a transparent spherical nanoparticle in the field of a focused Gaussian laser beam are studied theoretically in the Rayleigh scattering regime. Expressions are derived for the scattering force and Cartesian components of the gradient force. The resultant force acting on a nanoparticle located in the centre of a laser beam is found. The parameters of the focused beam and optical properties of the nanoparticle for which the longitudinal component of the gradient force exceeds the scattering force are determined. Characteristics of the transverse gradient force are discussed.


Optical separation and controllable delivery of cells from particle and cell mixture

Yuchao Li / Hongbao Xin / Chang Cheng / Yao Zhang / Baojun Li

Cell separation and delivery have recently gained significant attention in biological and biochemical studies. In thiswork, an optical method for separation and controllable delivery of cells by using an abruptly tapered fiber probe is reported. By launching a laser beam at the wavelength of 980 nm into the fiber, a mixture of cells with sizes of ~5 and ~3 μm and poly(methyl methacrylate) particles with size of 5 μm are separated into three chains along the direction of propagation of light. The cell and particle chains are delivered in three dimensions over 600 μm distance. Experimental results are interpreted by numerical simulations. Optical forces and forward migration velocities of different particles and cells are calculated and discussed.