Friday, April 29, 2016

Light-induced optomechanical forces in graphene waveguides

Brahim Guizal and Mauro Antezza

We show that the electromagnetic forces generated by the excitations of a mode in graphene-based optomechanical systems are highly tunable by varying the graphene chemical potential, and orders of magnitude stronger than usual non-graphene-based devices, in both attractive and repulsive regimes. We analyze coupled waveguides made of two parallel graphene sheets, either suspended or supported by dielectric slabs, and study the interplay between the light-induced force and the Casimir-Lifshitz interaction. These findings pave the way to advanced possibilities of control and fast modulation for optomechanical devices and sensors at the nano- and microscales.


Direct observation of intermediate states in model membrane fusion

Andrea Keidel, Tobias F. Bartsch & Ernst-Ludwig Florin

We introduce a novel assay for membrane fusion of solid supported membranes on silica beads and on coverslips. Fusion of the lipid bilayers is induced by bringing an optically trapped bead in contact with the coverslip surface while observing the bead’s thermal motion with microsecond temporal and nanometer spatial resolution using a three-dimensional position detector. The probability of fusion is controlled by the membrane tension on the particle. We show that the progression of fusion can be monitored by changes in the three-dimensional position histograms of the bead and in its rate of diffusion. We were able to observe all fusion intermediates including transient fusion, formation of a stalk, hemifusion and the completion of a fusion pore. Fusion intermediates are characterized by axial but not lateral confinement of the motion of the bead and independently by the change of its rate of diffusion due to the additional drag from the stalk-like connection between the two membranes. The detailed information provided by this assay makes it ideally suited for studies of early events in pure lipid bilayer fusion or fusion assisted by fusogenic molecules.


Mechanical Resonators for Quantum Optomechanics Experiments at Room Temperature

R. A. Norte, J. P. Moura, and S. Gröblacher

All quantum optomechanics experiments to date operate at cryogenic temperatures, imposing severe technical challenges and fundamental constraints. Here, we present a novel design of on-chip mechanical resonators which exhibit fundamental modes with frequencies f and mechanical quality factors Qm sufficient to enter the optomechanical quantum regime at room temperature. We overcome previous limitations by designing ultrathin, high-stress silicon nitride (Si3N4) membranes, with tensile stress in the resonators’ clamps close to the ultimate yield strength of the material. By patterning a photonic crystal on the SiN membranes, we observe reflectivities greater than 99%. These on-chip resonators have remarkably low mechanical dissipation, with Qm∼108, while at the same time exhibiting large reflectivities. This makes them a unique platform for experiments towards the observation of massive quantum behavior at room temperature.


Ultralow-Noise SiN Trampoline Resonators for Sensing and Optomechanics

Christoph Reinhardt, Tina Müller, Alexandre Bourassa, and Jack C. Sankey

In force sensing, optomechanics, and quantum motion experiments, it is typically advantageous to create lightweight, compliant mechanical elements with the lowest possible force noise. Here, we report the fabrication and characterization of high-aspect-ratio, nanogram-scale Si3N4 “trampolines” having quality factors above 4×107 and ringdown times exceeding 5 min (mHz linewidth). These devices exhibit thermally limited force noise sensitivities below 20  aN/Hz1/2 at room temperature, which is the lowest among solid-state mechanical sensors. We also characterize the suitability of these devices for high-finesse cavity readout and optomechanics applications, finding no evidence of surface or bulk optical losses from the processed nitride in a cavity achieving finesse 40,000. These parameters provide access to a single-photon cooperativity C0∼8 in the resolved-sideband limit, wherein a variety of outstanding optomechanics goals become feasible.


Raman activated cell sorting

Yizhi Song, Huabing Yin, Wei E Huang

Single cell Raman spectra (SCRS) are intrinsic biochemical profiles and ‘chemical images’ of single cells which can be used to characterise phenotypic changes, physiological states and functions of cells. On the base of SCRS, Raman activated cell sorting (RACS) provides a label-free cell sorting approach, which can link single cells to their chemical or phenotypic profiles. Overcoming naturally weak Raman signals, establishing Raman biomarker as sorting criteria to RACS and improving specific sorting technology are three challenges of developing RACS. Advances on Raman spectroscopy such as stimulated Raman scattering (SRS) and pre-screening helped to increase RACS sorting speed. Entire SCRS can be characterised using pattern recognition methods, and specific Raman bands can be extracted as biomarkers for RACS. Recent advances on cell sorting technologies based on microfluidic device and surface-ejection enable accurate and reliable single cell sorting from complex samples. A high throughput RACS will be achievable in near future by integrating fast Raman detection system such as SRS with microfluidic RACS and Raman activated cell ejection (RACE).


Tuesday, April 26, 2016

Universal spin-momentum locked optical forces

Farid Kalhor, Thomas Thundat and Zubin Jacob

Evanescent electromagnetic waves possess spin-momentum locking, where the direction of propagation (momentum) is locked to the inherent polarization of the wave (transverse spin). We study the optical forces arising from this universal phenomenon and show that the fundamental origin of recently reported non-trivial optical chiral forces is spin-momentum locking. For evanescent waves, we show that the direction of energy flow, the direction of decay, and the direction of spin follow a right hand rule for three different cases of total internal reflection, surface plasmon polaritons, and HE11 mode of an optical fiber. Furthermore, we explain how the recently reported phenomena of lateral optical force on chiral and achiral particles are caused by the transverse spin of the evanescent field and the spin-momentum locking phenomenon. Finally, we propose an experiment to identify the unique lateral forces arising from the transverse spin in the optical fiber and point to fundamental differences of the spin density from the well-known orbital angular momentum of light. Our work presents a unified view on spin-momentum locking and how it affects optical forces on chiral and achiral particles.


Graphene-based plasmonic force switch

M. Ghorbanzadeh, S. Darbari and M. K. Moravvej-Farshi

We take advantage of a Kretschmann configuration to design a plasmonic force switch. It consists of a prism/Au/SiO2 stack topped by a gated graphene sheet, as an electrically active optofluidic particle sorting system. We show that using a small gate voltage, one can switch the plasmon-wave induced force on a target particle, and hence its velocity. Simulations show that by electrical tuning of the grapheneelectrochemical potential in a narrow range of ∼65 meV—i.e., equivalent to an applied gate voltage of ∼4.3 V—the graphenesurface plasmons can absorb the Ausurface plasmons, switching off the plasmonic force exerted on the target particle with an ON/OFF ratio of more than 20. Numerical results also show that the maximum sensitivity of the particle's velocity to the graphene electrochemical potential is ∼1136 μm/eV-s. The proposed electrically active plasmonic force switch offers opportunities in developing tunable on-chip optical micromanipulations with multiple parallel functionalities and low power consumption.


Motion of Optically Heated Spheres at the Water–Air Interface

A. Girot, N. Danné, A. Würger, T. Bickel, F. Ren, J. C. Loudet, and B. Pouligny

A micrometer-sized spherical particle classically equilibrates at the water–air interface in partial wetting configuration, causing about no deformation to the interface. In condition of thermal equilibrium, the particle just undergoes faint Brownian motion, well visible under a microscope. We report experimental observations when the particle is made of a light-absorbing material and is heated up by a vertical laser beam. We show that, at small laser power, the particle is trapped in on-axis configuration, similarly to 2-dimensional trapping of a transparent sphere by optical forces. Conversely, on-axis trapping becomes unstable at higher power. The particle escapes off the laser axis and starts orbiting around the axis. We show that the laser-heated particle behaves as a microswimmer with velocities on the order of several 100 μm/s with just a few milliwatts of laser power.


Monday, April 25, 2016

Ignition and combustion characteristics of jet fuel liquid film containing graphene powders at meso-scale

Xuefeng Huang, Shengji Li

At meso-scale, ignition and combustion characteristics of jet fuel liquid film containing graphene powders were investigated. Jet fuel/graphene suspensions were prepared, and sprayed to produce the liquid film. Liquid film was ignited by optical tweezers, five distinctive stages including graphene trap, ignition and combustion of graphene, bubble formation, jet fuel vaporization and bubble growth, bubble rupture and combustion of liquid film were identified. Ignition of graphene is prior to jet fuel. The combustion heat of graphene serves a heat source to accelerate the vaporization of jet fuel. The graphene serves as a nucleation point to form a bubble. Expansion of both combustion products and jet fuel vapor result in the bubble growth. The thickness of bubble boundary layer depends on graphene concentration. As the bubble escaped, liquid film ruptured and micro-explosion occurred. Jet fuel was then ignited, and combusted sustainably till burnt out. During combustion, the flame front fluctuated slightly, indicating good flame stability. Finally, a schematic physical model was presented to analyze the inductive mechanism of graphene for ignition and combustion of jet fuel liquid film by optical tweezers.


Optical Trapping-Formed Colloidal Assembly with Horns Extended to the Outside of a Focus through Light Propagation

Tetsuhiro Kudo, Shun-Fa Wang, Ken-ichi Yuyama, and Hiroshi Masuhara

We report optical trapping and assembling of colloidal particles at a glass/solution interface with a tightly focused laser beam of high intensity. It is generally believed that the particles are gathered only in an irradiated area where optical force is exerted on the particles by laser beam. Here we demonstrate that, the propagation of trapping laser from the focus to the outside of the formed assembly leads to expansion of the assembly much larger than the irradiated area with sticking out rows of linearly aligned particles like horns. The shape of the assembly, its structure, and the number of horns can be controlled by laser polarization. Optical trapping study utilizing the light propagation will open a new avenue for assembling and crystallizing quantum dots, metal nanoparticles, molecular clusters, proteins, and DNA.


Effect of crystallization kinetics on the properties of spray dried microparticles

Alberto Baldelli, Rory M. Power, Rachael E. H. Miles, Jonathan P. Reid & Reinhard Vehring

A droplet chain technique was used to study the influence of the crystallization process on the morphology of spray dried microparticles. A piezoceramic dispenser produced a chain of monodisperse solution droplets with an initial diameter in the range of 60 to 80 µm. Aqueous solutions of sodium nitrate were prepared in concentrations ranging from 5 mg/ml to 5⋅10−5 mg/ml. The solution droplets were injected into a laminar flow with gas temperatures varying from 25 to 150°C, affecting the droplet temperature and the evaporation rate, accordingly. Dried particles with diameters between 0.3 and 18 µm were collected. The properties of the collected microparticles were studied and correlated with a particle formation model which predicted the onset of saturation and crystallization. The model accounted for the dependence of the diffusion coefficient of sodium nitrate in water on droplet viscosity. The viscosity trend for sodium nitrate solutions was determined by studying the relaxation time observed during coalescence of two aqueous sodium nitrate droplets levitated in optical tweezers. The combination of theoretical derivations and experimental results showed that longer time available for crystallization correlates with larger crystal size and higher degrees of crystallinity in the final microparticles.


Photon and physical phenomena responsible for its momentum

V.P. Torchigin, A.V. Torchigin

We derive a magnitude of the momentum of light in matter by means of matching two irrefutable not contradictory thought experiments where no preliminary assumptions about kinds of optically induced forces responsible for a change of the momentum of light in matter are made. The total momentum increases in the matter by n times due to the Coulomb kind of force in a dielectric investigated by Maxwell. There are two different component of the total momentum. These are the mechanical component arising due to a motion of conventional material objects, mass of whose is non-zero and the electromagnetic component produced by a travelling electromagnetic wave, mass of which is equal to zero. The following types of optically induced forces provide a redistribution of the total momentum between these components. These are the kind of the Abraham-like force produced in matter by an electromagnetic wave, intensity of which is changed in time and the Helmholtz-like force arising in a field of an electromagnetic wave due to an inhomogeneity of the electrostriction pressure produced by the light wave. The mechanical component of the momentum of the light is negative and the electromagnetic component is greater than the total momentum.


Coupled electrostatic and material surface stresses yield anomalous particle interactions and deformation

B. A. Kemp, I. Nikolayev and C. J. Sheppard

Like-charges repel, and opposite charges attract. This fundamental tenet is a result of Coulomb's law. However, the electrostatic interactions between dielectric particles remain topical due to observations of like-charged particle attraction and the self-assembly of colloidal systems. Here, we show, using both an approximate description and an exact solution of Maxwell's equations, that nonlinear charged particle forces result even for linear material systems and can be responsible for anomalous electrostatic interactions such as like-charged particle attraction and oppositely charged particle repulsion. Furthermore, these electrostatic interactions and the deformation of such particles have fundamental implications for our understanding of macroscopic electrodynamics.


Friday, April 22, 2016

Detecting Bacterial Surface Organelles on Single Cells using Optical Tweezers

Johan Zakrisson, Bhupender Singh, Pontus Svenmarker, Krister Wiklund, Hanqing Zhang, Shoghik Hakobyan, Madeleine Ramstedt, and Magnus Andersson

Bacterial cells display a diverse array of surface organelles that are important for a range of processes such as: intercellular communication, motility and adhesion leading to biofilm formation, infections and bacterial spread. More specifically, attachment to host cells by Gram-negative bacteria are mediated by adhesion pili, which are nm wide and µm long fibrous organelles. Since these pili are significantly thinner than the wavelength of visible light, they cannot be detected using standard light microscopy techniques. At present, there is no fast and simple method available to investigate if a single cell expresses pili while keeping the cell alive for further studies. In this study, we present a method to determine the presence of pili on a single bacterium. The protocol involves imaging the bacterium to measure its size, followed by predicting the fluid drag based on its size using an analytical model, and thereafter oscillating the sample while a single bacterium is trapped by an optical tweezer to measure its effective fluid drag. Comparison between the predicted and the measured fluid drag thereby indicate the presence of pili. Herein, we verify the method using polymer coated silica microspheres and Escherichia coli bacteria expressing adhesion pili. Our protocol, can in real time and within seconds assist single cell studies by distinguishing between piliated and non-piliated bacteria.


Design of a high-performance optical tweezer for nanoparticle trapping

D. Conteduca, F. Dell’Olio, C. Ciminelli , T. F. Krauss, M. N. Armenise

Integrated optical nanotweezers offer a novel paradigm for optical trapping, as their ability to confine light at the nanoscale leads to extremely high gradient forces. To date, nanotweezers have been realized either as photonic crystal or as plasmonic nanocavities. Here, we propose a nanotweezer device based on a hybrid photonic/plasmonic cavity with the goal of achieving a very high quality factor-to-mode volume (Q/V) ratio. The structure includes a 1D photonic crystal dielectric cavity vertically coupled to a bowtie nanoantenna. A very high Q/V ~ 107 (λ/n)−3 with a resonance transmission T = 29 % at λ R = 1381.1 nm has been calculated by 3D finite element method, affording strong light–matter interaction and making the hybrid cavity suitable for optical trapping. A maximum optical force F = −4.4 pN, high values of stability S = 30 and optical stiffness k = 90 pN/nm W have been obtained with an input power P in = 1 mW, for a polystyrene nanoparticle with a diameter of 40 nm. This performance confirms the high efficiency of the optical nanotweezer and its potential for trapping living matter at the nanoscale, such as viruses, proteins and small bacteria.


Near-infrared Laser-induced Temperature Elevation in Optically-trapped Aqueous Droplets in Air

Shoji ISHIZAKA, Jiang MA, Terufumi FUJIWARA, Kunihiro YAMAUCHI, Noboru KITAMURA

Near-infrared laser-induced temperature elevation in single aqueous ammonium sulfate droplets levitated in air were evaluated by means of laser trapping and Raman spectroscopy. Since the vapor pressure in an aqueous solution droplet should be thermodynamically in equilibrium with that of water in air, the equilibrium size of the droplet varies sensitively through evaporation/condensation of water in accordance with the temperature change of the droplet. In this study, we demonstrated that the changes in the size of an optically levitated aqueous ammonium sulfate droplet were induced by irradiation of a 1064-nm laser beam as a heat source under an optical microscope. Temperature elevation in the droplet was evaluated successfully by means of Raman spectroscopy, and the values determined were shown to be in good agreement with those by the theoretical calculations based on the absorption coefficient of water at 1064-nm and the thermal conductivity of air. To the best of our knowledge, this is the first experimental demonstration showing that the absorption coefficient evaluated from changes in the size of optically-trapped aqueous droplets is consistent with that of pure water.


Flow cytometric and near-infrared Raman spectroscopic investigation of quality in stained, sorted, and frozen-thawed buffalo sperm

Xiao-xia Li, Meng Wang, Huan-hua Chen, Qing-yang Li, Huan Yang, Hui-yan Xu, Yang-qing Lu, Ming Zhang, Xiao-gan Yang, Sheng-sheng Lu, Ke-huan Lu

Flow cytometry and Laser Tweezers Raman spectroscopy have been used to investigate Nili-Ravi buffalo (Bubalus bubalis) sperm from different samples (fresh, stained, sorted and frozen-thawed) of the flow-sorting process to optimize sperm sex sorting procedures. During the sorting and freezing-thawing processes, the two detection methods both indicated there were differences in mitochondrial activity and membrane integrity. Moreover, a dispersive-type NIR (Near Infrared Reflection) use of the Raman system resulted in the ability to detect a variety of sperm components, including relative DNA, lipid, carbohydrates and protein contents. The use of the Raman system allowed for PCA (principal components analysis) and DFA (discriminant function analysis) of fresh, stained, sorted and frozen-thawed sperm. The methodology, therefore, allows for distinguishing sperm from different samples (fresh, stained, sorted and frozen-thawed), and demonstrated the great discriminative power of ANN (artificial neural network) classification models for the differentiating sperm from different phases of the flow-sorting process. In conclusion, the damage induced by sperm sorting and freezing-thawing procedures can be quantified, and in the present research it is demonstrated that Raman spectroscopy is a valuable technology for assessing sperm quality.


Epoxidation of Alkenes on Soft Oxometalate (SOM) based Trails Patterned by using Laser of Optical Tweezers set up

Preethi Thomas, Subhrokoli Ghosh and Prof. Dr. Soumyajit Roy

We have developed a novel method of patterning ammonium heptamolybdate tetrahydrate based trails or arrays on a glass substrate using the laser beam of thermo-optical tweezers set up and use them for site specific catalysis. The trail of heptamolybdate primarily comprise of its polyoxometalate (POM) crystalline state formed from the phase transition of its dispersion phase soft oxometalate (SOM) upon exposure to laser beam of 1064 nm wavelength of thermo-optical tweezers set up. The supramolecular self-assembly of the SOMs facilitates their nucleation to POMs upon laser irradiation. The SOMs and SOM based trails have been characterized using various techniques such as FTIR spectroscopy, Raman spectroscopy, Thermogravimetric analysis and Atomic Force microscopy. These trails or arrays have been subsequently employed as catalytic avenues for site-specific catalysis of epoxidation of alkenes namely styrene, 1-octadecene, cis-cyclooctene and cyclohexene and studied using time resolved Raman spectroscopy. Site specificity here signifies the confinement of catalysis in a particular zone which in this work is the region where galleries of heptamolybdate have been etched on the glass slide.


Monday, April 18, 2016

Scattered field generation and optical forces in transformation optics

A V Novitsky

In this paper we develop an approach for making various scattered electromagnetic fields on the transformation-optics ground. To do so, we use the a special coordinate transformation from the a vacuum virtual space to physical space, which changes the boundary of the scattering device upon transformation. We explore this approach for small scatterers compared with radiation wavelength, which allows us to predict the arbitrarily directed optical forces. Obtaining scattered fields and optical forces can be useful in nano-optics and optical micromanipulation.


Nanomechanics of a fibroblast suspended using point-like anchors reveal cytoskeleton formation

Sabato Fusco, Pasquale Memmolo, Lisa Miccio, Francesco Merola, Martina Mugnano, Antonio Paciello, Pietro Ferraro and Paolo A. Netti

In an attempt to better elucidate the material–cytoskeleton crosstalk during the initial stage of cell adhesion, here we report how suspended cells anchored to point-like bonds are able to assemble their cytoskeleton when subjected to mechanical stress. The combination of holographic optical tweezers and digital holography gives the cell footholds for adhesion and mechanical stimulation, and at the same time, acts as a label-free, force-revealing system over time, detecting the cell nanomechanical response in the pN range. To confirm the formation of the cytoskeleton structures after the stimulation, a fluorescence imaging system was added as a control. The strategy here proposed portends broad applicability to investigate the correlation between the forces applied to cells and their cytoskeleton assembly process in this or other complex configurations with multiple anchor points.


Casimir and Optical Forces Acting on a Silicon NOEMS Device Based on Slot-Waveguide Structure

J. R. Rodrigues; F. S. S. Rosa ; V. R. Almeida

We investigate the effect of Casimir force on a silicon nano-opto-electro-mechanical system (NOEMS) device, based on a slot-waveguide structure and driven by optical forces. Light is strongly confined in the slot-waveguide, which enhances the optical force and reduces the optical power necessary to control the device. However, we predict that the Casimir force affects the device's critical points of stability, by reducing the critical optical power for the quasi-TE fundamental eigenmode, as well as imposing a critical power for the quasi-TM one. Casimir force also changes the initial NOEMS equilibrium position, pre-deforming the two sides of the slot-waveguide, even in the absence of optical forces; this establishes a minimal initial gap distance for the slot-waveguide, thus affecting the device's design parameters.


Thursday, April 14, 2016

Investigating the use of a hybrid plasmonic–photonic nanoresonator for optical trapping using finite-difference time-domain method

M. Mossayebi , A. J. Wright, A. Parini, M. G. Somekh, G. Bellanca, E. C. Larkins

We investigate the use of a hybrid nanoresonator comprising a photonic crystal (PhC) cavity coupled to a plasmonic bowtie nanoantenna (BNA) for the optical trapping of nanoparticles in water. Using finite-difference time-domain simulations, we show that this structure can confine light to an extremely small volume of ∼30,000nm3(∼30∼30,000nm3(∼30 zl) in the BNA gap whilst maintaining a high quality factor (5400–7700). The optical intensity inside the BNA gap is enhanced by a factor larger than 40 compared to when the BNA is not present above the PhC cavity. Such a device has potential applications in optical manipulation, creating high precision optical traps with an intensity gradient over a distance much smaller than the diffraction limit, potentially allowing objects to be confined to much smaller volumes and making it ideal for optical trapping of Rayleigh particles (particles much smaller than the wavelength of light).


α-SNAP Enhances SNARE Zippering by Stabilizing the SNARE Four-Helix Bundle

Lu Ma, Yuhao Kang, Junyi Jiao, Aleksander A. Rebane, Hyo Keun Cha, Zhiqun Xi, Hong Qu, Yongli Zhang

Intracellular membrane fusion is mediated by dynamic assembly and disassembly of soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors (SNAREs). α-SNAP guides NSF to disassemble SNARE complexes after membrane fusion. Recent experiments showed that α-SNAP also dramatically enhances SNARE assembly and membrane fusion. How α-SNAP is involved in these opposing activities is not known. Here, we examine the effect of α-SNAP on the stepwise assembly of the synaptic SNARE complex using optical tweezers. We found that α-SNAP destabilized the linker domain (LD) of the SNARE complex but stabilized its C-terminal domain (CTD) through a conformational selection mechanism. In contrast, α-SNAP minimally affected assembly of the SNARE N-terminal domain (NTD), indicating that α-SNAP barely bound the partially assembled trans-SNARE complex. Thus, α-SNAP recognizes the folded CTD for SNARE disassembly with NSF and subtly modulates membrane fusion by altering the stabilities of the SNARE CTD and LD.


Mechanics and activation of unconventional myosins

Christopher Batters and Claudia Veigel

Many types of cellular motility are based on the myosin family of motor proteins ranging from muscle contraction to exo- and endocytosis, cytokinesis, cell locomotion or signal transduction in hearing. At the centre of this wide range of motile processes lies the adaptation of the myosins for each specific mechanical task and the ability to coordinate the timing of motor protein mobilization and targeting. In recent years great progress has been made in developing single molecule technology to characterize the diverse mechanical properties of the unconventional myosins. Here we discuss the basic mechanisms and mechanical adaptations of unconventional myosins, and emerging principles regulating motor mobilization and targeting.


In-plane trapping and manipulation of ZnO nanowires by hybrid plasmonic field

Lichao Zhang, Xiujie Dou, Changjun Min, Yuquan Zhang, Luping Du, Zhenwei Xie, Junfeng Shen, Yujia Zeng and Xiaocong Yuan

In general, when a semiconductor nanowire is trapped by conventional laser beam tweezers, it tends to be aligned with the trapping beam axis rather than confined in the horizontal plane, and this limits the application of these nanowires in many in-plane nanoscale optoelectronic devices. In this work, we achieve the in-plane trapping and manipulation of a single ZnO nanowire by a hybrid plasmonic tweezer system on a flat metal surface. The gap between the nanowire and the metallic substrate leads to an enhanced gradient force caused by deep subwavelength optical energy confinement. As a result, the nanowire can be securely trapped in-plane at the center of the excited surface plasmon polariton field, and can also be dynamically moved and rotated by varying the position and polarization direction of the incident laser beam, which cannot be performed using conventional optical tweezers. The theoretical results show that the focused plasmonic field induces a strong in-plane trapping force and a high rotational torque on the nanowire, while the focused optical field produces a vertical trapping force to produce the upright alignment of the nanowire; this is in good agreement with the experimental results. Finally, some typical ZnO nanowire structures are built based on this technique, which thus further confirms the potential of this method for precise manipulation of components during the production of nanoelectronic and nanophotonic devices.


Tuesday, April 12, 2016

Man-Made Rotary Nanomotors: A Review of Recent Development

Kwanoh Kim, Jianhe Guo, Z. X. Liang, F. Q. Zhu and Donglei Emma Fan

The development rotary nanomotors is an essential step towards intelligent nanomachines and nanorobots. In this article, we review the concept, design, working mechanisms, and applications of the state-of-the-art rotary nanomotors made from synthetic nanoentities. The rotary nanomotors are categorized according to the energy sources employed to drive the rotary motion, including biochemical, optical, magnetic, and electric fields. The unique advantages and limitations for each type of rotary nanomachines are discussed. The advances of rotary nanomotors is pivotal for realizing dream nanomachines for myriad applications including microfluidics, biodiagnosis, nano-surgery, and biosubstance delivery.


Trapping in a Material World

Susan E. Skelton Spesyvtseva and Kishan Dholakia

The ability to manipulate small particles of matter using the forces of light, optical trapping forms the basis of a number of exciting research areas, spanning fundamental physics, applied chemistry and medicine and biology. Historically, a largely unexplored area has been the influence of the material properties of the particle on the optical forces. By taking a holistic approach in which the properties of the particle are considered alongside those of the light field, the force field on a particle can be optimized, allowing significant increases of the optical forces exerted and even the introduction of new forces, torques and other physical effects. Here we present an introduction to this newly emerging area, with a focus on high refractive index and anti-reflection coated particles, nanomaterials particles including metallic nanoparticles, optically anisotropic particles, and metamaterials. Throughout, we discuss future perspectives which will extend the capabilities and applications of optical trapping and shape future avenues of research in this burgeoning field.


A comprehensive strategy for the analysis of acoustic compressibility and optical deformability on single cells

Tie Yang, Francesca Bragheri, Giovanni Nava, Ilaria Chiodi, Chiara Mondello, Roberto Osellame, Kirstine Berg-Sørensen, Ilaria Cristiani & Paolo Minzioni

We realized an integrated microfluidic chip that allows measuring both optical deformability and acoustic compressibility on single cells, by optical stretching and acoustophoresis experiments respectively. Additionally, we propose a measurement protocol that allows evaluating the experimental apparatus parameters before performing the cell-characterization experiments, including a non-destructive method to characterize the optical force distribution inside the microchannel. The chip was used to study important cell-mechanics parameters in two human breast cancer cell lines, MCF7 and MDA-MB231. Results indicate that MDA-MB231 has both higher acoustic compressibility and higher optical deformability than MCF7, but statistical analysis shows that optical deformability and acoustic compressibility are not correlated parameters. This result suggests the possibility to use them to analyze the response of different cellular structures. We also demonstrate that it is possible to perform both measurements on a single cell, and that the order of the two experiments does not affect the retrieved values.


Radiation pressure cross section exerted on homogenous dielectric spherical particle by zeroth order Mathieu beams

A. Chafiq, A. Belafhal

In this paper, we present a full calculation of radiation pressure cross section and optical forces exerted by linearly polarized zeroth order Mathieu beams on homogenous dielectric spherical particle in the framework of generalized Lorenz–Mie theory (GLMT). In this theory, the scattered fields are dependent upon the Mie scattering coefficients and the beam shape coefficients. So a new optical property such as force and torque appears by changing the beam profile and the nature of particle. In this way, this work is devoted to the analysis of both transverse and longitudinal optical forces exerted on a simple dielectric spherical particle by zeroth order Mathieu beams and zeroth order Bessel (which is a particular case of the first beam). Thus, through numerical simulations, we show that zeroth order Mathieu beams can׳t trap this particle but Bessel beam presents some dimensional stable equilibrium points.


An optical detection method for analyzing the cellular response to paclitaxel at the single cell level

yijia wang, yuquan zhang, shiwu zhang, zhenying zhao, Changjun Min, jun liu and Xiaocong Yuan

The detection of cellular responses to drugs is important for biomedical research, but there is a lack of convenient label-free methods to analyze responses at the single cell level. The refractive index is a typical biophysical parameter reflecting cell status. An optical tweezers system was established to measure trapping efficiency, which relates to the refractive index. The response of two cancer cell lines and their paclitaxel resistant counterparts were measured using optical tweezers. Cyclin B1 expression, polymerized tubulin and caspase-3 activation were measured, compared and associated with the trapping efficiency. The results show that trapping efficiency declined sharply when caspase-3 was activated. These results suggest that optical tweezers can be used as an auxiliary method to analyze cellular responses to anti-tumor drugs.


Monday, April 11, 2016

Using optical trap to measure the refractive index of a single animal virus in culture fluid with high precision

Yuanjie Pang, Hanna Song, and Wei Cheng

The refractive index (RI) is a fundamental parameter of materials that can be used to distinguish and sort materials of different nature. Although the RI of a virus is required for many optics-based biosensing applications, RIs of animal viruses have never been measured. Here we have developed a technique that can measure the RI of individual viruses in aqueous media with high precision. This technique is based on optical trapping of single virions and works by relating the size and RI of a single virus to the stiffness of an optical trap. We have derived an analytic expression to quantitatively describe the optical trapping of these particles. We have validated this equation using nanoparticles of known RI, and measured the RI of individual human immunodeficiency viruses type-1, which yielded a value of 1.42 at 830 nm with less than 2% coefficient of variation. This value is much lower than the RI typically assumed for viruses, but very close to that of 2.0 M sucrose solution in water. To the best of our knowledge, this is the first report on the experimental measurement of the RI for a single animal virus in aqueous media. This technique does not require prior knowledge on the diameter of the nanoparticles, and can be applied to other viruses or nanoparticles for accurate measurement of RI that is critical for the label-free detection of these particles in various settings.


Direct observation of transition paths during the folding of proteins and nucleic acids

Krishna Neupane, Daniel A. N. Foster, Derek R. Dee, Hao Yu, Feng Wang, Michael T. Woodside

Transition paths, the fleeting trajectories through the transition states that dominate the dynamics of biomolecular folding reactions, encapsulate the critical information about how structure forms. Owing to their brief duration, however, they have not previously been observed directly. We measured transition paths for both nucleic acid and protein folding, using optical tweezers to observe the microscopic diffusive motion of single molecules traversing energy barriers. The average transit times and the shapes of the transit-time distributions agreed well with theoretical expectations for motion over the one-dimensional energy landscapes reconstructed for the same molecules, validating the physical theory of folding reactions. These measurements provide a first look at the critical microscopic events that occur during folding, opening exciting new avenues for investigating folding phenomena.


Role of Arbitrary Intensity Profile Laser Beam in Trapping of RBC for Phase-imaging

Kumar, Ranjeet; Srivastava, Vishal; Mehta, Dalip Singh; Shakher, Chandra;

Red blood cells (RBCs) are customarily adhered to a bio-functionalised substrate to make them stationary in interferometric phase-imaging modalities. This can make them susceptible to receive alterations in innate morphology due to their own weight. Optical tweezers (OTs) often driven by Gaussian profile of a laser beam is an alternative modality to overcome contact-induced perturbation but at the same time a steeply focused laser beam might cause photo-damage. In order to address both the photo-damage and substrate adherence induced perturbations, we were motivated to stabilize the RBC in OTs by utilizing a laser beam of ‘arbitrary intensity profile’ generated by a source having cavity imperfections per se. Thus the immobilized RBC was investigated for phase-imaging with sinusoidal interferograms generated by a compact and robust Michelson interferometer which was designed from a cubic beam splitter having one surface coated with reflective material and another adjacent coplanar surface aligned against a mirror. Reflected interferograms from bilayers membrane of a trapped RBC were recorded and analyzed. Our phase-imaging set-up is limited to work in reflection configuration only because of the availability of an upright microscope. Due to RBC’s membrane being poorly reflective for visible wavelengths, quantitative information in the signal is weak and therefore, the quality of experimental results is limited in comparison to results obtained in transmission mode by various holographic techniques reported elsewhere.


Optical-tweezing-based linear-optics nanoscopy

Omer Wagner, Moty Schultz, Yonatan Ramon, Eli Sloutskin, and Zeev Zalevsky

Previous works reported that linear optics could be used to observe sub-wavelength features with a conventional optical microscope. Yet, the ability to reach a sub-200 nm resolution with a visible light remains limited. We present a novel widely-applicable method, where particle trapping is employed to overcome this limit. The combination of the light scattered by the sample and by the trapped particles encodes super-resolution information, which we decode by post image processing, with the trapped particle locations predetermined. As the first proof of concept our method successfully resolved sample characteristic features down to 100 nm. Improved performance is achieved with the fluorescence of the trapped particles employed. Further improvement may be attained with trapped particles of a smaller size.


Friday, April 8, 2016

Electronic control of optical tweezers using space-time-wavelength mapping

Shah Rahman, Rasul Torun, Qiancheng Zhao, and Ozdal Boyraz

We present a new approach for electronic control of optical tweezers by using space-time-wavelength mapping (STWM), a technique that uses time-domain modulation to control local intensity values and hence the resulting optical force in space. The proposed technique enables direct control of magnitude, location, and polarity of force hot spots created by Lorentz force (gradient force). In this paper we develop an analytical formulation of the proposed STWM technique for optical tweezing. In the case study presented here we show that 150 fs optical pulses are dispersed in time and space to achieve a focused elliptical beam that is ∼20  μm long and ∼2  μm wide. By choosing the appropriate RF waveform and electro-optic modulator we can generate multiple hot spots with >200  pN force per pulse.


Trapping and manipulating nanoparticles in photonic nanojets

Haotian Wang, Xiang Wu, and Deyuan Shen

A novel optical manipulation system based on photonic nanojets (PNJs) is numerically investigated based on the finite element method. It is found that nanoscale particles can be trapped stably in a standing-wave PNJ generated by the constructive interference between two coherent PNJs. In particular, we show that the elongated standing-wave PNJs generated by using two-layer microcylinders or microspheres can provide larger manipulation platforms and stronger optical forces. To assess the trapping stability of the particle under the Brownian motion in the elongated PNJ, the relationship between the stability number and the particle size is studied. The simulation results show that the proposed elongated standing-wave PNJs can provide the stable and tunable manipulation for dielectric nanoparticles that are smaller than 100 nm.


Thursday, April 7, 2016

Mutually Exclusive Formation of G-Quadruplex and i-Motif Is a General Phenomenon Governed by Steric Hindrance in Duplex DNA

Yunxi Cui, Deming Kong, Chiran Ghimire, Cuixia Xu, and Hanbin Mao

G-Quadruplex and i-motif are tetraplex structures that may form in opposite strands at the same location of a duplex DNA. Recent discoveries have indicated that the two tetraplex structures can have conflicting biological activities, which poses a challenge for cells to coordinate. Here, by performing innovative population analysis on mechanical unfolding profiles of tetraplex structures in double-stranded DNA, we found that formations of G-quadruplex and i-motif in the two complementary strands are mutually exclusive in a variety of DNA templates, which include human telomere and promoter fragments of hINS and hTERT genes. To explain this behavior, we placed G-quadruplex- and i-motif-hosting sequences in an offset fashion in the two complementary telomeric DNA strands. We found simultaneous formation of the G-quadruplex and i-motif in opposite strands, suggesting that mutual exclusivity between the two tetraplexes is controlled by steric hindrance. This conclusion was corroborated in the BCL-2 promoter sequence, in which simultaneous formation of two tetraplexes was observed due to possible offset arrangements between G-quadruplex and i-motif in opposite strands. The mutual exclusivity revealed here sets a molecular basis for cells to efficiently coordinate opposite biological activities of G-quadruplex and i-motif at the same dsDNA location.


Dynamics analysis of microsphere in a dual-beam fiber-optic trap with transverse offset

Xinlin Chen, Guangzong Xiao, Hui Luo, Wei Xiong, and Kaiyong Yang

A comprehensive dynamics analysis of microsphere has been presented in a dual-beam fiber-optic trap with transverse offset. As the offset distance between two counterpropagating beams increases, the motion type of the microsphere starts with capture, then spiral motion, then orbital rotation, and ends with escape. We analyze the transformation process and mechanism of the four motion types based on ray optics approximation. Dynamic simulations show that the existence of critical offset distances at which different motion types transform. The result is an important step toward explaining physical phenomena in a dual-beam fiber-optic trap with transverse offset, and is generally applicable to achieving controllable motions of microspheres in integrated systems, such as microfluidic systems and lab-on-a-chip systems.


Combined Optical and Chemical Control of a Microsized Photofueled Janus Particle

Sabrina Simoncelli, Johannes Summer, Spas Nedev, Paul Kühler, Jochen Feldmann

A Au–silica Janus particle is elevated along the laser beam axis in an optical trap. The propulsion mechanism is based on the local temperature gradient created around the particle due to the photothermal conversion of the gold-coated hemisphere. The height of the particle and its motion-direction are tuned by the nature and the concentration of the electrolytes in the medium.


Destruction of a spherical polystyrene microparticle in pulsed ultraviolet beam for micromanipulation purpose

Roman V. Skidanov, Vadim S. Vasiliev
The modelling of heating a spherical polystyrene microparticle by the laser beam having the wavelength 355 nm and varying spot size is described. The simulation results are presented for two different values of the beam spot radius. Nature experiments are reported aimed at measuring the velocity of a particle with the radius 5µm accelerated by microexplosion of the neighbouring particle.


Fluorescence Detection of H5N1 Virus Gene Sequences Based on Optical Tweezers with Two-Photon Excitation Using a Single Near Infrared Nanosecond Pulse Laser

Cheng-Yu Li, Di Cao, Ya-Feng Kang, Yi Lin, Ran Cui, Dai-Wen Pang, and Hong-Wu Tang

We present an analytical platform by combining near-infrared optical tweezers with two-photon excitation for fluorescence detection of H5N1 virus gene sequences. A heterogeneous enrichment strategy, which involved polystyrene (PS) microsphere and quantum dots (QDs), was adopted. The final hybrid-conjugate microspheres were prepared by a facile one-step hybridization procedure by using PS microspheres capturing target DNA and QDs tagging, respectively. Quantitative detection was achieved by the optical tweezers setup with a low-cost 1064 nm nanosecond pulse laser for both optical trapping and two-photon excitation for the same hybrid-conjugate microsphere. The detection limits for both neuraminidase (NA) gene sequences and hemagglutinin (HA) gene sequences are 16–19 pM with good selectivity for one-base mismatch, which is approximately 1 order of magnitude lower than the most existing fluorescence-based analysis method. Besides, because of the fact that only signal from the trapped particle is detected upon two-photon excitation, this approach showed extremely low background in fluorescence detection and was successfully applied to directly detect target DNA in human whole serum without any separation steps and the corresponding results are very close to that in buffer solution, indicating the strong anti-interference ability of this method. Therefore, it can be expected to be an emerging alternative for straightforward detecting target species in complex samples with a simple procedure and high-throughput.


Monday, April 4, 2016

The Nanomechanical Properties of Lactococcus lactis Pili Are Conditioned by the Polymerized Backbone Pilin

Mickaël Castelain , Marie-Pierre Duviau, Alexis Canette, Philippe Schmitz, Pascal Loubière, Muriel Cocaign-Bousquet, Jean-Christophe Piard, Muriel Mercier-Bonin

Pili produced by Lactococcus lactis subsp. lactis are putative linear structures consisting of repetitive subunits of the major pilin PilB that forms the backbone, pilin PilA situated at the distal end of the pilus, and an anchoring pilin PilC that tethers the pilus to the peptidoglycan. We determined the nanomechanical properties of pili using optical-tweezers force spectroscopy. Single pili were exposed to optical forces that yielded force-versus-extension spectra fitted using the Worm-Like Chain model. Native pili subjected to a force of 0–200 pN exhibit an inextensible, but highly flexible ultrastructure, reflected by their short persistence length. We tested a panel of derived strains to understand the functional role of the different pilins. First, we found that both the major pilin PilB and sortase C organize the backbone into a full-length organelle and dictate the nanomechanical properties of the pili. Second, we found that both PilA tip pilin and PilC anchoring pilin were not essential for the nanomechanical properties of pili. However, PilC maintains the pilus on the bacterial surface and may play a crucial role in the adhesion- and biofilm-forming properties of L. lactis.


Structure and dynamics of optically directed self-assembly of nanoparticles

Debjit Roy, Dipankar Mondal & Debabrata Goswami

Self-assembly of nanoparticles leading to the formation of colloidal clusters often serves as the representative analogue for understanding molecular assembly. Unravelling the in situ structure and dynamics of such clusters in liquid suspensions is highly challenging. Presently colloidal clusters are first isolated from their generating environment and then their structures are probed by light scattering methods. In order to measure the in situ structure and dynamics of colloidal clusters, we have generated them using the high-repetition-rate femtosecond laser pulse optical tweezer. Since the constituent of our dimer, trimer or tetramer clusters are 250 nm radius two-photon resonant fluorophore coated nanospheres under the optical trap, they inherently produce Two-Photon Fluorescence, which undergo intra-nanosphere Fluorescence Energy Transfer. This unique energy transfer signature, in turn, enables us to visualize structures and orientations of these colloidal clusters during the process of their formation and subsequent dynamics in a liquid suspension. We also show that due to shape-birefringence, orientation and structural control of these colloidal clusters are possible as the polarization of the trapping laser is changed from linear to circular. We thus report important progress in sampling the smallest possible aggregates of nanoparticles, dimers, trimers or tetramers, formed early in the self-assembly process.


Developing a New Biophysical Tool to Combine Magneto-Optical Tweezers with Super-Resolution Fluorescence Microscopy

Zhaokun Zhou, Helen Miller, Adam J.M. Wollman and Mark C. Leake

We present a novel experimental setup in which magnetic and optical tweezers are combined for torque and force transduction onto single filamentous molecules in a transverse configuration to allow simultaneous mechanical measurement and manipulation. Previously we have developed a super-resolution imaging module which, in conjunction with advanced imaging techniques such as Blinking assisted Localisation Microscopy (BaLM), achieves localisation precision of single fluorescent dye molecules bound to DNA of ~30 nm along the contour of the molecule; our work here describes developments in producing a system which combines tweezing and super-resolution fluorescence imaging. The instrument also features an acousto-optic deflector that temporally divides the laser beam to form multiple traps for high throughput statistics collection. Our motivation for developing the new tool is to enable direct observation of detailed molecular topological transformation and protein binding event localisation in a stretching/twisting mechanical assay that previously could hitherto only be deduced indirectly from the end-to-end length variation of DNA. Our approach is simple and robust enough for reproduction in the lab without the requirement of precise hardware engineering, yet is capable of unveiling the elastic and dynamic properties of filamentous molecules that have been hidden using traditional tools.


Optical micromanipulation of nanoparticles and cells inside living zebrafish

Patrick Lie Johansen, Federico Fenaroli, Lasse Evensen, Gareth Griffiths & Gerbrand Koster

Regulation of biological processes is often based on physical interactions between cells and their microenvironment. To unravel how and where interactions occur, micromanipulation methods can be used that offer high-precision control over the duration, position and magnitude of interactions. However, lacking an in vivo system, micromanipulation has generally been done with cells in vitro, which may not reflect the complex in vivo situation inside multicellular organisms. Here using optical tweezers we demonstrate micromanipulation throughout the transparent zebrafish embryo. We show that different cells, as well as injected nanoparticles and bacteria can be trapped and that adhesion properties and membrane deformation of endothelium and macrophages can be analysed. This non-invasive micromanipulation inside a whole-organism gives direct insights into cell interactions that are not accessible using existing approaches. Potential applications include screening of nanoparticle-cell interactions for cancer therapy or tissue invasion studies in cancer and infection biology.


Low-Power Far Field Nanonewton Optical Force Trapping Based on Far-Field Nanofocusing Plasmonic Lens

Pengfei Cao, and Lin Cheng

In this article, we study the far-field trapping behavior of dielectric nanospheres with diameter of 200 nm by utilizing a plasmon enhanced far-field nanofocusing lens. Based on our high effects nanofocusing circular plasmonic lens, such a far-field plasmonic trap is constituted by illuminating with a laser to form a sharper focus (subwavelength) due to a constructive interference of cylindrical surface plasmon wave. The nanoparticles can be steadily trapped in the far-field focal region (4.4λ) with an optical force to nanonewton (−4.76 nN) order, and the required optical power is less than 0.5W. Compared with other surface plasmon tweezers, the proposed far-field plasmonic tweezers can not only avoid physical contact with the trapped particles to prevent contamination and reduce thermal damage effects due to metal absorption, but also enable the easy trapping and manipulation of nanosize dielectric particles owing to nanonewton scale forces.