Thursday, August 31, 2017

Substrate and Fano Resonance Effects on the Reversal of Optical Binding Force between Plasmonic Cube Dimers

M. R. C. Mahdy, Tianhang Zhang, Md. Danesh & Weiqiang Ding

The behavior of Fano resonance and the reversal of near field optical binding force of dimers over different substrates have not been studied so far. Notably, for particle clustering and aggregation, controlling the near filed binding force can be a key factor. In this work, we observe that if the closely located plasmonic cube homodimers over glass or high permittivity dielectric substrate are illuminated with plane wave, no reversal of lateral optical binding force occurs. But if we apply the same set-up over a plasmonic substrate, stable Fano resonance occurs along with the reversal of near field lateral binding force. It is observed that during such Fano resonance, stronger coupling occurs between the dimers and plasmonic substrate along with the strong enhancement of the substrate current. Such binding force reversals of plasmonic cube dimers have been explained based on the observed unusual behavior of optical Lorentz force during the induced stronger Fano resonance and the dipole-dipole resonance. Although previously reported reversals of near field optical binding forces were highly sensitive to particle size/shape (i.e. for heterodimers) and inter-particle distance, our configuration provides much relaxation of those parameters and hence could be verified experimentally with simpler experimental set-ups.


Saturated PID Control for the Optical Manipulation of Biological Cells

Mingyang Xie ; Xiaojian Li ; Yong Wang ; Yunhui Liu ; Dong Sun

This brief presents a saturated PID control scheme for cell manipulation by using robot-aided optical tweezers, with consideration of both translational and rotational control of the cell. By incorporating saturation functions into a PID controller and utilizing a velocity observer, cell position and orientation can asymptotically converge to the desired values. The proposed control approach also guarantees that the trapped cell can always stay within a small neighborhood around the centroid of the optical trap, thereby preventing the cell from escaping by optical trapping. The controller does not require the use of accurate dynamic model parameters, and hence can be implemented easily. Utilizing a velocity observer, velocity measurement by directly differentiating the measured position of the cell is not needed, which benefits noise reduction and system stability. The asymptotic stability of the closed-loop system is analyzed using Lyapunov's direct method. Experimental results are presented to demonstrate the effectiveness of the proposed approach.


Ultrahigh Purcell Factor, Improved Sensitivity, and Enhanced Optical Force in Dielectric Bowtie Whispering-Gallery-Mode Resonators

Qijing Lu ; Xiaogang Chen ; Hongqin Yang ; Xiang Wu ; Shusen Xie

We propose and theoretically investigate an all-dielectric bowtie whispering-gallery-mode (WGM) resonator which consists of two tip-to-tip coupled semiconductor nanorings with triangular cross section separated by low refractive index material gap. Mode splitting of symmetric WGM and antisymmetric WGM is observed and analyzed. Due to extremely field enhancements by the “slot” and “tip” effects, strong localization of light in the gap of symmetric WGMs leads to ultrasmall modal volume of 0.3μm3 . This value is two orders of magnitude smaller than that in toroidal microresonator which is the typical WGM microcavity with tight mode confinement. Importantly, large amount of light confined in the gap in bowtie WGM resonator not only suppresses the radiation loss (radiation-loss-related quality factor is above 108), but also makes it as an ideal platform for Purcell effect enhancement, ultrasensitive sensing, and optical trapping of nanoparticles. Calculation results show that Purcell factor can reach to 108 at room temperature when assuming quantum dots or atoms placed in the gap. Refractive index sensitivity is improved to 700 nm/RIU as compared with conventional slot waveguide with rectangular cross section. The optical gradient force is greatly enhanced and allows efficient trapping of single nanoscale particle with diameter of 5 nm even at a relatively large gap of 100 nm.


Heterogeneous interface adsorption of colloidal particles

Dong Woo Kang, Jin Hyun Lim and Bum Jun Park
The heterogeneous adsorption behaviors of charged colloidal particles to oil–water interfaces were quantitatively and statistically investigated. Using optical laser tweezers, the particles in a sessile water drop formed in an oil phase were laterally translated toward the slope of the oil–water interface and their attachment to the interface was attempted. The adsorption probability was found to logarithmically decrease as the ionic strength decreased and to depend on the holding time during which an optically trapped particle was held at the position closest to the interface. Non-unity of the adsorption probability at particular salt concentrations and the holding time dependence offer an important clue that the particle adsorption to the interface is not deterministic but stochastic. The stochastic adsorption process can be attributed to the surface heterogeneity of colloidal particles that consequently leads to changes in the electrostatic interactions between the particles and the interface. We also demonstrated that the salt dependence on the adsorption properties of the particles, as measured by optical laser tweezers, was consistent with their bulk behaviors with regard to the stability of particle-stabilized emulsions. Furthermore, we revealed the gravity-induced spontaneous adsorption of the particles to the interface under conditions of sufficiently strong ionic strength.


Detection and characterization of chemical aerosol using laser-trapping single-particle Raman spectroscopy

Aimable Kalume, Leonid A. Beresnev, Joshua Santarpia, and Yong-Le Pan

Detection and characterization of the presence of chemical agent aerosols in various complex atmospheric environments is an essential defense mission. Raman spectroscopy has the ability to identify chemical molecules, but there are limited numbers of photons detectable from single airborne aerosol particles as they are flowing through a detection system. In this paper, we report on a single-particle Raman spectrometer system that can measure strong spontaneous, stimulated, and resonance Raman spectral peaks from a single laser-trapped chemical aerosol particle, such as a droplet of the VX nerve agent chemical simulant diethyl phthalate. Using this system, time-resolved Raman spectra and elastic scattered intensities were recorded to monitor the chemical properties and size variation of the trapped particle. Such a system supplies a new approach for the detection and characterization of single airborne chemical aerosol particles.


Chirality and Chiroptical Effects in Metal Nanostructures: Fundamentals and Current Trends

Joel T. Collins, Christian Kuppe, David C. Hooper, Concita Sibilia, Marco Centini, Ventsislav K. Valev

Throughout the 19th and 20th century, chirality has mostly been associated with chemistry. However, while chirality can be very useful for understanding molecules, molecules are not well suited for understanding chirality. Indeed, the size of atoms, the length of molecular bonds and the orientations of orbitals cannot be varied at will. It is therefore difficult to study the emergence and evolution of chirality in molecules, as a function of geometrical parameters. By contrast, chiral metal nanostructures offer an unprecedented flexibility of design. Modern nanofabrication allows chiral metal nanoparticles to tune the geometric and optical chirality parameters, which are key for properties such as negative refractive index and superchiral light. Chiral meta/nano-materials are promising for numerous technological applications, such as chiral molecular sensing, separation and synthesis, super-resolution imaging, nanorobotics, and ultra-thin broadband optical components for chiral light. This review covers some of the fundamentals and highlights recent trends. We begin by discussing linear chiroptical effects. We then survey the design of modern chiral materials. Next, the emergence and use of chirality parameters are summarized. In the following part, we cover the properties of nonlinear chiroptical materials. Finally, in the conclusion section, we point out current limitations and future directions of development.


Wednesday, August 30, 2017

PFMCal : Photonic force microscopy calibration extended for its application in high-frequency microrheology

A. Butykai, P. Domínguez-García, F. M.Mor, R. Gaál, L. Forró, S. Jeney
The present document is an update of the previously published MatLab code for the calibration of optical tweezers in the high-resolution detection of the Brownian motion of non-spherical probes [1]. In this instance, an alternative version of the original code, based on the same physical theory [2], but focused on the automation of the calibration of measurements using spherical probes, is outlined. The new added code is useful for high-frequency microrheology studies, where the probe radius is known but the viscosity of the surrounding fluid maybe not. This extended calibration methodology is automatic, without the need of a user’s interface. A code for calibration by means of thermal noise analysis [3] is also included; this is a method that can be applied when using viscoelastic fluids if the trap stiffness is previously estimated [4]. The new code can be executed in MatLab and using GNU Octave.


Manipulating Living Cells to Construct a 3D Single-Cell Assembly without an Artificial Scaffold

Aoi Yoshida, Shoto Tsuji, Hiroaki Taniguchi, Takahiro Kenmotsu, Koichiro Sadakane and Kenichi Yoshikawa

Artificial scaffolds such as synthetic gels or chemically-modified glass surfaces that have often been used to achieve cell adhesion are xenobiotic and may harm cells. To enhance the value of cell studies in the fields of regenerative medicine and tissue engineering, it is becoming increasingly important to create a cell-friendly technique to promote cell–cell contact. In the present study, we developed a novel method for constructing stable cellular assemblies by using optical tweezers in a solution of a natural hydrophilic polymer, dextran. In this method, a target cell is transferred to another target cell to make cell–cell contact by optical tweezers in a culture medium containing dextran. When originally non-cohesive cells are held in contact with each other for a few minutes under laser trapping, stable cell–cell adhesion is accomplished. This method for creating cellular assemblies in the presence of a natural hydrophilic polymer may serve as a novel next-generation 3D single-cell assembly system with future applications in the growing field of regenerative medicine.


Multiple particles 3D trap based on all-fiber Bessel optical probe

Yaxun Zhang, Xiaoyun Tang, Yu Zhang, Zhihai Liu, Enming Zhao, Xinghua Yang, Jianzhong Zhang, Jun Yang, Libo Yuan

We propose and demonstrate an all-fiber Bessel optical tweezers for multiple microparticles (yeast cells) 3 dimensional trap. To the best knowledge of us, it is the first time to achieve the 3 dimensional stable non-contact multiple microparticles optical traps with long distance intervals by using a single all-fiber probe. The Bessel beam is produced by splicing coaxially a single mode fiber and a step index multimode fiber. The convergence of the output Bessel beam is performed by molding the tip of the multimode fiber into a special semi-ellipsoid shape. The effective trapping range of the all-fiber probe is 0 to 60μm, which is much longer than normal single fiber optical tweezers probes. The all-fiber Bessel optical probe is convenient to integrate and suitable for the lab on the chip. The structure of this fiber probe is simple, high-precision, low-cost, and small size, which provides new development for biological cells experiment and operation.


Ultra-high Q/V hybrid cavity for strong light-matter interaction

Donato Conteduca, Christopher Reardon, Mark G. Scullion, Francesco Dell’Olio, Mario N. Armenise, Thomas F. Krauss, and Caterina Ciminelli

The ability to confine light at the nanoscale continues to excite the research community, with the ratio between quality factor Q and volume V, i.e., the Q/V ratio, being the key figure of merit. In order to achieve strong light-matter interaction, however, it is important to confine a lot of energy in the resonant cavity mode. Here, we demonstrate a novel cavity design that combines a photonic crystal nanobeam cavity with a plasmonic bowtie antenna. The nanobeam cavity is optimised for a good match with the antenna and provides a Q of 1700 and a transmission of 90%. Combined with the bowtie, the hybrid photonic-plasmonic cavity achieves a Q of 800 and a transmission of 20%, both of which remarkable achievements for a hybrid cavity. The ultra-high Q/V of the hybrid cavity is of order of 106 (λ/n)−3, which is comparable to the state-of-the-art of photonic resonant cavities. Based on the high Q/V and the high transmission, we demonstrate the strong efficiency of the hybrid cavity as a nanotweezer for optical trapping. We show that a stable trapping condition can be achieved for a single 200 nm Au bead for a duration of several minutes (ttrap > 5 min) and with very low optical power (Pin = 190 μW).


Non-conservative optical forces

Sergey Sukhov and Aristide Dogariu

Undoubtedly, laser tweezers are the most recognized application of optically induced mechanical action. Their operation is usually described in terms of conservative forces originating from intensity gradients. However, the fundamental optical action on matter is non-conservative. We will review different manifestations of non-conservative optical forces (NCF) and discuss their dependence on the specific spatial properties of optical fields that generate them. New developments relevant to the NCF such as tractor beams and transversal forces are also discussed.


Tuesday, August 29, 2017

Displacement and localisation of a transparent nanosphere by light-pressure forces in the field of a focused laser beam

A.A. Afanas'ev and D.V. Novitsky

We report the results of a numerical simulation of the Langevin equation describing the motion of a transparent nanosphere under the action of a resulting light-pressure force in the field of a continuous focused Gaussian laser beam. The conditions for the localisation of the nanosphere near the focal waist of the beam focused by the lens are determined. An analytical solution of the approximate (truncated) equation of motion is found, which almost exactly coincides with the results of the numerical simulation of the initial equation.


Direct measurement of conformational strain energy in protofilaments curling outward from disassembling microtubule tips

Jonathan W Driver, Elisabeth A Geyer, Megan E Bailey, Luke M Rice, Charles L Asbury

Disassembling microtubules can generate movement independently of motor enzymes, especially at kinetochores where they drive chromosome motility. A popular explanation is the ‘conformational wave’ model, in which protofilaments pull on the kinetochore as they curl outward from a disassembling tip. But whether protofilaments can work efficiently via this spring-like mechanism has been unclear. By modifying a previous assay to use recombinant tubulin and feedback-controlled laser trapping, we directly demonstrate the spring-like elasticity of curling protofilaments. Measuring their mechanical work output suggests they carry ~25% of the energy of GTP hydrolysis as bending strain, enabling them to drive movement with efficiency similar to conventional motors. Surprisingly, a β-tubulin mutant that dramatically slows disassembly has no effect on work output, indicating an uncoupling of disassembly speed from protofilament strain. These results show the wave mechanism can make a major contribution to kinetochore motility and establish a direct approach for measuring tubulin mechano-chemistry.


Multibeam interferometric optical tweezers

Mohammadbagher Mohammadnezhad; Abdollah Hassanzadeh

Using the interference of N collimated laser beams, optical lattices with N -fold rotational symmetry are generated over the interface of two semi-infinite dielectric media. The interaction of small dielectric particles with these interference patterns is investigated using Rayleigh approximation. The polarization state of the interfering beams considerably influences the interference patterns and potential landscapes. Therefore, both parallel and perpendicular polarized interfering beams are considered and the corresponding potential profiles are compared and analyzed. We also study how the number of interfering waves, incident and azimuth angles, and initial phases of the incident beams influence optical lattices and potential profiles. It is found that the ring-shaped patterns with good confinement properties can be achieved by increasing the number of incident beams. In addition, by increasing the number of incident beams one can make an optical trap with sharper intensity gradient and deeper potential well, which is an advantage for trapping small Rayleigh particles. The lattices resulting from the interference of N incident waves with different incident angles are also investigated. Furthermore, the effects of changing the azimuth angles between two adjacent incident wave vectors on the intensity patterns are studied. The proposed configuration and the numerical results can find numerous applications in particle arrangement, particle sorting, and the creation of quasicrystals. We believe that interference approaches have many potential capabilities for molding light wavefronts and creating multiple traps with sophisticated patterns.


In Vivo Manipulation of Single Biological Cells With an Optical Tweezers-Based Manipulator and a Disturbance Compensation Controller

Xiaojian Li ; Chichi Liu ; Shuxun Chen ; Yong Wang ; Shuk Han Cheng ; Dong Sun

In vivo manipulation of biological cells has attracted considerable attention in recent years. This process is particularly useful for precision medicine, such as cancer target therapy. Robotics technology is becoming necessary to stably and effectively manipulate and control single target cells in a complex in vivo environment. This paper presents a robot-aided optical tweezers-based manipulation technology that serves a function in the transport of single biological cells in vivo. An enhanced disturbance compensation controller is developed to minimize the effect of fluids (e.g., blood flow) on the cell. The method has exhibited advantages of flexibility in adjusting cell tracking trajectory online and the capability to minimize steady-state error and eliminate overshoot. Simulations and experiments of tracking single target cells in living zebrafish embryos have demonstrated the effectiveness of the proposed approach in a dynamic in vivo environment.


An early mechanical coupling of planktonic bacteria in dilute suspensions

Simon Sretenovic, Biljana Stojković, Iztok Dogsa, Rok Kostanjšek, Igor Poberaj & David Stopar

It is generally accepted that planktonic bacteria in dilute suspensions are not mechanically coupled and do not show correlated motion. The mechanical coupling of cells is a trait that develops upon transition into a biofilm, a microbial community of self-aggregated bacterial cells. Here we employ optical tweezers to show that bacteria in dilute suspensions are mechanically coupled and show long-range correlated motion. The strength of the coupling increases with the growth of liquid bacterial culture. The matrix responsible for the mechanical coupling is composed of cell debris and extracellular polymer material. The fragile network connecting cells behaves as viscoelastic liquid of entangled extracellular polymers. Our findings point to physical connections between bacteria in dilute bacterial suspensions that may provide a mechanistic framework for understanding of biofilm formation, osmotic flow of nutrients, diffusion of signal molecules in quorum sensing, or different efficacy of antibiotic treatments at low and high bacterial densities.


Monday, August 28, 2017

Drying-mediated optical assembly of silica spheres in a symmetrical metallic waveguide structure

Tian Xu, Cheng Yin, XueFen Kan, TingChao He, Qingbang Han, Zhuangqi Cao, and Xianfeng Chen

We describe the optical trapping application of a simple metallic slab optical waveguide structure, and demonstrate the influence of the excited guided modes on the aggregation behavior of silica particles during the irreversible evaporation process. Periodic horizontal stripes are formed by the highly ordered assemblies of the silica spheres, which is explained via the interference effect between the forward propagating modes and its reflection at the solvent surface. Particularly, several layers consisting of high-density particles are discernible in the stripe zones due to the optical binding, while no particles locate between these stripes. Completely different from the self-assembly patterns in the evaporating solvent without excitation of optical modes, this Letter demonstrates the versatility in the possible patterns of the optical assembly by a coupling waveguide with more complex structures.


Biphasic Effect of Profilin Impacts the Formin mDia1 Force-Sensing Mechanism in Actin Polymerization

Hiroaki Kubota, Makito Miyazaki, Taisaku Ogawa, Togo Shimozawa, Kazuhiko Kinosita Jr., Shin’ichi Ishiwata

Formins are force-sensing proteins that regulate actin polymerization dynamics. Here, we applied stretching tension to individual actin filaments under the regulation of formin mDia1 to investigate the mechanical responses in actin polymerization dynamics. We found that the elongation of an actin filament was accelerated to a greater degree by stretching tension for ADP-G-actin than that for ATP-G-actin. An apparent decrease in the critical concentration of G-actin was observed, especially in ADP-G-actin. These results on two types of G-actin were reproduced by a simple kinetic model, assuming the rapid equilibrium between pre- and posttranslocated states of the formin homology domain two dimer. In addition, profilin concentration dramatically altered the force-dependent acceleration of actin filament elongation, which ranged from twofold to an all-or-none response. Even under conditions in which actin depolymerization occurred, applications of a several-piconewton stretching tension triggered rapid actin filament elongation. This extremely high force-sensing mechanism of mDia1 and profilin could be explained by the force-dependent coordination of the biphasic effect of profilin; i.e., an acceleration effect masked by a depolymerization effect became dominant under stretching tension, negating the latter to rapidly enhance the elongation rate. Our findings demonstrate that the biphasic effect of profilin is controlled by mechanical force, thus expanding the function of mDia1 as a mechanosensitive regulator of actin polymerization.


Quantification of Chemical and Mechanical Effects on the Formation of the G-Quadruplex and i-Motif in Duplex DNA

Sangeetha Selvam, Shankar Mandal, and Hanbin Mao

The formation of biologically significant tetraplex DNA species, such as G-quadruplexes and i-motifs, is affected by chemical (ions and pH) and mechanical [superhelicity (σ) and molecular crowding] factors. Because of the extremely challenging experimental conditions, the relative importance of these factors on tetraplex folding is unknown. In this work, we quantitatively evaluated the chemical and mechanical effects on the population dynamics of DNA tetraplexes in the insulin-linked polymorphic region using magneto-optical tweezers. By mechanically unfolding individual tetraplexes, we found that ions and pH have the largest effects on the formation of the G-quadruplex and i-motif, respectively. Interestingly, superhelicity has the second largest effect followed by molecular crowding conditions. While chemical effects are specific to tetraplex species, mechanical factors have generic influences. The predominant effect of chemical factors can be attributed to the fact that they directly change the stability of a specific tetraplex, whereas the mechanical factors, superhelicity in particular, reduce the stability of the competing species by changing the kinetics of the melting and annealing of the duplex DNA template in a nonspecific manner. The substantial dependence of tetraplexes on superhelicity provides strong support that DNA tetraplexes can serve as topological sensors to modulate fundamental cellular processes such as transcription.


Recent advances in studying single bacteria and biofilm mechanics

Even C, Marlière C, Ghigo JM, Allain JM, Marcellan A, Raspaud E

Bacterial biofilms correspond to surface-associated bacterial communities embedded in hydrogel-like matrix, in which high cell density, reduced diffusion and physico-chemical heterogeneity play a protective role and induce novel behaviors. In this review, we present recent advances on the understanding of how bacterial mechanical properties, from single cell to high-cell density community, determine biofilm tri-dimensional growth and eventual dispersion and we attempt to draw a parallel between these properties and the mechanical properties of other well-studied hydrogels and living systems.


Effect of directional pulling on mechanical protein degradation by ATP-dependent proteolytic machines

Adrian O. Olivares, Hema Chandra Kotamarthi, Benjamin J. Stein, Robert T. Sauer and Tania A. Baker

AAA+ proteases and remodeling machines couple hydrolysis of ATP to mechanical unfolding and translocation of proteins following recognition of sequence tags called degrons. Here, we use single-molecule optical trapping to determine the mechanochemistry of two AAA+ proteases, Escherichia coli ClpXP and ClpAP, as they unfold and translocate substrates containing multiple copies of the titinI27 domain during degradation initiated from the N terminus. Previous studies characterized degradation of related substrates with C-terminal degrons. We find that ClpXP and ClpAP unfold the wild-type titinI27 domain and a destabilized variant far more rapidly when pulling from the N terminus, whereas translocation speed is reduced only modestly in the N-to-C direction. These measurements establish the role of directionality in mechanical protein degradation, show that degron placement can change whether unfolding or translocation is rate limiting, and establish that one or a few power strokes are sufficient to unfold some protein domains.


Light-driven micro- and nanomotors for environmental remediation

M. Safdar, J. Simmchen and J. Jänis

Synthetic micro- and nanomotors (MNMs) have emerged as a vibrant research field in multidisciplinary nanotechnology with proof-of-concept applications in various disciplines. In this tutorial review, an overview of the latest achievements towards light-driven MNMs is given and their propulsion mechanisms are introduced. The focus of the paper is on the autonomously propelled MNMs that exploit light-induced physical effects or chemical reactions. Light-induced body deformation, as a completely different, nature inspired concept that is found mostly in soft, polymeric MNMs, is also reviewed. In the end, a few applications of photocatalytic and light-driven MNMs for environmental remediation are presented and their potential is critically discussed.


Friday, August 25, 2017

Controlled shaping of lipid vesicles in a microfluidic diffusion chamber

M. Mally, B. Božič, S. Vrhovec Hartman, U. Klančnik, M. Mur, S. Svetina and J. Derganc

Synthetic lipid vesicles represent an important model system for studying membrane processes, which often depend on membrane shape, but controlled shaping of vesicles remains a challenging experimental task. Here, we present a novel method for shaping giant lipid vesicles by independently regulating osmotic conditions and the concentration of membrane-shaping molecules, which intercalate into the membrane and drive membrane bending. The method is based on the microfluidic diffusion chamber, where the solution around the vesicles can be repeatedly exchanged solely by diffusion, without any hydrodynamic flow that could deform the membrane. By using lipopolysaccharide (LPS) as a vesicle shape-modifying molecule, we demonstrate controlled and reversible transformations across three shape classes, from invaginated to evaginated vesicles. We show that extensive shape transformations can lead to shapes that are assumed to comprise narrow membrane necks that hinder equilibration of the membrane and the vesicle interior. All the observed shapes are in good agreement with the predictions of the area-difference-elasticity model applied to the vesicles that were denser than their surrounding solution. Our results validate the microfluidic diffusion chamber as a universal framework for membrane shaping that could also pave the way towards controlled fabrication of synthetic membranes resembling cell-compartments with large surface-to-volume ratios.


Using back focal plane interferometry to probe the influence of Zernike aberrations in optical tweezers

Thomas F. Dixon, Lachlan W. Russell, Ana Andres-Arroyo, and Peter J. Reece

We experimentally investigate the influence of geometric aberrations in optical tweezers using back focal plane interferometry. We found that the introduction of coma aberrations causes significant modification to the Brownian motion of the trapped particle, producing an apparent cross-coupling between the in-plane aberrated axis and the weaker propagation axis. This coupling is evidenced by the emergence of a second dominant low frequency Lorentzian feature in the position power spectral density. The effect on Brownian motion was confirmed using a secondary unaberrated probe beam, ruling out the possibility of systematic optical effects related to the detection system.


Automated Transportation of Biological Cells for Multiple Processing Steps in Cell Surgery

Hao Yang ; Xiangpeng Li ; Yunhui Liu ; Dong Sun

Most studies on automated cell transportation are single-task oriented. Results from these investigations hardly meet the increasing demand for emerging cell surgery operations that usually require a series of manipulation tasks with multiple processing steps. In this paper, automated cell transportation to accomplish a multistep process in cell surgery was investigated. A novel control system that can manipulate grouped cells to move into different task regions sequentially and continuously without interruption was developed based on a robot-aided optical tweezers manipulation system. A potential field-based controller was designed to achieve multistep processing control, where the new concepts of contractive coalition and switching region were incorporated into tweezers-cell coalition. The success of this controller lies in simultaneously controlling the positions of the optical tweezers, trapping multiple cells effectively, and avoiding collisions in a unified manner. Simulations and experiments of transferring a group of cells to a number of task regions were performed to demonstrate the effectiveness of the proposed approach.


Optomechanical soft metamaterials

Xiangjun Peng, Wei He, Yifan Liu, Fengxian Xin, Tian Jian Lu

We present a new type of optomechanical soft metamaterials, which is different from conventional mechanical metamaterials, in that they are simple isotropic and homogenous materials without resorting to any complex nano/microstructures. This metamaterial is unique in the sense that its responses to uniaxial forcing can be tailored by programmed laser inputs to manifest different nonlinear constitutive behaviors, such as monotonic, S-shape, plateau, and non-monotonic snapping performance. To demonstrate the novel metamaterial, a thin sheet of soft material impinged by two counterpropagating lasers along its thickness direction and stretched by an in-plane tensile mechanical force is considered. A theoretical model is formulated to characterize the resulting optomechanical behavior of the thin sheet by combining the nonlinear elasticity theory of soft materials and the optical radiation stress theory. The optical radiation stresses predicted by the proposed model are validated by simulations based on the method of finite elements. Programmed optomechanical behaviors are subsequently explored using the validated model under different initial sheet thicknesses and different optical inputs, and the first- and second-order tangential stiffness of the metamaterial are used to plot the phase diagram of its nonlinear constitutive behaviors. The proposed optomechanical soft metamaterial shows great potential in biological medicine, microfluidic manipulation, and other fields.


Stochastic analysis of time series for the spatial positions of particles trapped in optical tweezers

S. M. Mousavi, S. N. Seyed Reihani, G. Anvari, M. Anvari, H. G. Alinezhad & M. Reza Rahimi Tabar

We propose a nonlinear method for the analysis of the time series for the spatial position of a bead trapped in optical tweezers, which enables us to reconstruct its dynamical equation of motion. The main advantage of the method is that all the functions and parameters of the dynamics are determined directly (non-parametrically) from the measured series. It also allows us to determine, for the first time to our knowledge, the spatial-dependence of the diffusion coefficient of a bead in an optical trap, and to demonstrate that it is not in general constant. This is in contrast with the main assumption of the popularly-used power spectrum calibration method. The proposed method is validated via synthetic time series for the bead position with spatially-varying diffusion coefficients. Our detailed analysis of the measured time series reveals that the power spectrum analysis overestimates considerably the force constant.


Dynamic fatigue measurement of human erythrocytes using dielectrophoresis

Qiang Y, Liu J, Du E

Erythrocytes must undergo severe deformation to pass through narrow capillaries and submicronic splenic slits for several hundred thousand times in their normal lifespan. Studies of erythrocyte biomechanics have been mainly focused on cell deformability and rheology measured from a single application of stress and mostly under a static or quasi-static state using classical biomechanical techniques, such as optical tweezers and micropipette aspiration. Dynamic behavior of erythrocytes in response to cyclic stresses that contributes to the membrane failure in blood circulation is not fully understood. This paper presents a new experimental method for dynamic fatigue analysis of erythrocytes, using amplitude modulated electrokinetic force field in a microfluidic platform. We demonstrate the capability of this new technique using a low cycle fatigue analysis of normal human erythrocytes and ATP-depleted erythrocytes. Cyclic tensile stresses are generated to induce repeated uniaxial stretching and extensional recovery of single erythrocytes. Results of morphological and biomechanical parameters of individually tracked erythrocytes show strong correlations with the number of the loading cycles. Under a same strength of electric field, after 180 stress cycles, for normal erythrocytes, maximum stretch ratio decreases from 3.80 to 2.86, characteristic time of cellular extensional recovery increases from 0.16s to 0.37s, membrane shear viscosity increases from 1.0(µN/m)s to 1.6(µN/m)s. Membrane deformation in a small number of erythrocytes becomes irreversible after large deformation for about 200 cyclic loads. ATP-depleted cells show similar trends in decreased deformation and increased characteristic time with the loading cycles. These results show proof of concept of the new microfluidics technique for dynamic fatigue analysis of human erythrocytes.


Thursday, August 24, 2017

One-dimensional photonic crystals bound by light

Liyong Cui, Xiao Li, Jun Chen, Yongyin Cao, Guiqiang Du, and Jack Ng
Through rigorous simulations, the light scattering induced optical binding of one-dimensional (1D) dielectric photonic crystals is studied. The optical forces corresponding to the pass band, band gap, and band edge are qualitatively different. It is shown that light can induce self-organization of dielectric slabs into stable photonic crystals, with its lower band edge coinciding with the incident light frequency. Incident light at normal and oblique incidence and photonic crystals with parity-time symmetry are also considered.


Trapping two types of particles using a focused partially coherent circular edge dislocations beam

Hanghang Zhang, Jinhong Li, Ke Cheng, Meiling Duan, Zhifang Feng

A focused partially coherent circular edge dislocations beam used to trap Rayleigh dielectric sphere with different refractive indices is studied. The dependence of radiation forces on the number of circular edge dislocations p, the spatial correlation length σ0, relative refractive index nr, and particle radius a are analyzed and illustrated by numerical examples. It is shown that the focused partially coherent circular edge dislocations beam can be used to trap high index of refraction particles at focus F and bright ring R2, and simultaneously to capture low index of refraction particles at dark ring R1. It is much easier to capture the high index of refraction particles at focus F and the low index of refraction particles at dark ring as for the larger number of circular edge dislocations p and the spatial correlation length σ0, therefore it is necessary to optimally choose on p and σ0 for obtaining an optimal optical guiding. The ranges of the radius for two types of particles stably captured also have been determined. The obtained results are useful for analyzing the trapping efficiency of circular edge dislocations beams applied in micromanipulation technology.


Chiral Rayleigh particles discrimination in dynamic dual optical traps

Luis Carretero, Pablo Acebal, Salvador Blaya

A chiral optical conveyor belt for enantiomeric separation of nanoparticles is numerically demonstrated by using different types of counter propagating elliptical Laguerre Gaussian beams with different beam waist and topological charge. The analysis of chiral resolution has been made for particles immersed in water demonstrating that in the analyzed conditions one type of enantiomer is trapped in a deep potential and the others are transported by the chiral conveyor toward another trap located in a different geometrical region.


Light Shaping with Holography, GPC and Holo-GPC

Andrew Bañas, Jesper Glückstad

Light shaping techniques based on phase-only modulation offer multiple advantages over amplitude modulation. This review examines and compares the merits of two phase modulation techniques; phase-only computer generated holography and Generalized Phase Contrast (GPC). Both techniques are briefly presented while recent developments in GPC will also be covered. Furthermore, novel hybrid schemes that inherit merits from both holography and GPC are also covered. In particular, our most recent technique coined “Holo-GPC” will be discussed in addition to earlier hybrid techniques. We will discuss how Holo-GPC utilizes the simplicity of GPC in forming well-defined speckle-free shapes and the versatility of holography in distributing these shaped beams over an extended 3D volume. To conclude, we cite applications where the combined strengths of the two photon-efficient phase-only light shaping techniques open new possibilities.


A stretched conformation of DNA with a biological role?

Niklas Bosaeus, Anna Reymer, Tamás Beke-Somfai, Tom Brown, Masayuki Takahashi, Pernilla Wittung-Stafshede, Sandra Rocha and Bengt Nordén

We have discovered a well-defined extended conformation of double-stranded DNA, which we call Σ-DNA, using laser-tweezers force-spectroscopy experiments. At a transition force corresponding to free energy change ΔG = 1·57 ± 0·12 kcal (mol base pair)−1 60 or 122 base-pair long synthetic GC-rich sequences, when pulled by the 3′−3′ strands, undergo a sharp transition to the 1·52 ± 0·04 times longer Σ-DNA. Intriguingly, the same degree of extension is also found in DNA complexes with recombinase proteins, such as bacterial RecA and eukaryotic Rad51. Despite vital importance to all biological organisms for survival, genome maintenance and evolution, the recombination reaction is not yet understood at atomic level. We here propose that the structural distortion represented by Σ-DNA, which is thus physically inherent to the nucleic acid, is related to how recombination proteins mediate recognition of sequence homology and execute strand exchange. Our hypothesis is that a homogeneously stretched DNA undergoes a ‘disproportionation’ into an inhomogeneous Σ-form consisting of triplets of locally B-like perpendicularly stacked bases. This structure may ensure improved fidelity of base-pair recognition and promote rejection in case of mismatch during homologous recombination reaction. Because a triplet is the length of a gene codon, we speculate that the structural physics of nucleic acids may have biased the evolution of recombinase proteins to exploit triplet base stacks and also the genetic code.


Wednesday, August 23, 2017

Levitation and propulsion of a Mie-resonance particle by a surface plasmon

A. V. Maslov

It is predicted that the optical force induced by a surface plasmon can form a stable equilibrium position for a resonant particle at a finite distance from the surface. The levitated particle can be efficiently propelled along the surface without touching it. The levitation originates from the strong interaction of the particle with the surface.


The effect of saliva on the fate of nanoparticles

Birgit J. Teubl, Biljana Stojkovic, Dominic Docter, Elisabeth Pritz, Gerd Leitinger, Igor Poberaj, Ruth Prassl, Roland H. Stauber, Eleonore Fröhlich, Johannes G. Khinast, Eva Roblegg

The design of nanocarriers for local drug administration to the lining mucosa requires a sound knowledge of how nanoparticles (NPs) interact with saliva. This contact determines whether NPs agglomerate and become immobile due to size- and interaction-filtering effects or adsorb on the cell surface and are internalized by epithelial cells. The aim of this study was to examine the behavior of NPs in saliva considering physicochemical NP properties. The salivary pore–size distribution was determined, and the viscosity of the fluid inside of the pores was studied with optical tweezers. Distinct functionalized NPs (20 and 200 nm) were dispersed in saliva and salivary buffers and characterized, and surface-bound MUC5B and MUC7 were analyzed by 1D electrophoresis and immunoblotting. NP mobility was recorded, and cellular uptake studies were performed with TR146 cells. The mode diameter of the salivary mesh pores is 0.7 μm with a peak width of 1.9 μm, and pores are filled with a low-viscosity fluid. The physicochemical properties of the NPs affected the colloidal stability and mobility: compared with non-functionalized particles, which did not agglomerate and showed a cellular uptake rate of 2.8%, functionalized particles were immobilized, which was correlated with agglomeration and increased binding to mucins. The present study showed that the salivary microstructure facilitates NP adsorption. However, NP size and surface functionalization determine the colloidal stability and cellular interactions.


Equilibrium and out-of-equilibrium mechanics of living mammalian cytoplasm

Gupta, Satish Kumar; Guo, Ming

Living cells are intrinsically non-equilibrium systems. They are driven out of equilibrium by the activity of the molecular motors and other enzymatic processes. This activity along with the ever present thermal agitation results in intracellular fluctuations inside the cytoplasm. In analogy to Brownian motion, the material property of the cytoplasm also influences the characteristics of these fluctuations. In this paper, through a combination of experimentation and theoretical analysis, we show that intracellular fluctuations are indeed due to non-thermal forces at relatively long time-scales, however, are dominated solely by thermal forces at relatively short time-scales. Thus, the cytoplasm of living mammalian cells behaves as an equilibrium material at short time-scales. The mean square displacement of these intracellular fluctuations scales inversely with the cytoplasmic shear modulus in this short time-scale equilibrium regime, and is inversely proportional to the square of the cytoplasmic shear modulus in the long time-scale out-of-equilibrium regime. Furthermore, we deploy passive microrheology based on these fluctuations to extract the mechanical property of the cytoplasm at the high-frequency regime. We show that the cytoplasm of living mammalian cells is a weak elastic gel in this regime; this is in an excellent agreement with an independent micromechanical measurement using optical tweezers.


Laser-Printing and 3D Optical-Control of Untethered Microrobots

Ebubekir Avci, Maria Grammatikopoulou, Guang-Zhong Yang

The two-photon photo-polymerization (2PP) method is used to manufacture an articulated micro-robot for indirect manipulation of cellular structures under laser light. To tackle the stickiness issue between the components of the proposed mechanism, optimizing the contact surface areas is carried out. A step-by-step procedure to manufacture and control an untethered articulated micro-robot under laser light is demonstrated. The manufacture and optical control of a floating multi-component micro-mechanism has not been achieved before. This is significant because it is anticipated that the articulated microrobots could be used in complex biomedical applications where control in 3D space is required such as single-cell analysis, embryo injection, polar-body biopsy, nuclear transplantation, and multi-dimensional imaging for microsurgery.


Dielectrophoretic force measurement of red blood cells exposed to oxidative stress using optical tweezers and a microfluidic chip

Hee-Jae Jeon, Hyungbeen Lee, Dae Sung Yoon, Beop-Min Kim

Red blood cell (RBC) dysfunction is often associated with a pathological intervention, and it has been proposed as a critical risk factor for certain lethal diseases. Examining the cell viability of RBCs under various physiological conditions is essential and of importance for precise diagnosis and drug discovery in the field of medicine and pharmacy. In this paper, we report a new analytical method that employs dielectrophoretic (DEP) force measurements in absolute units to assess the viability, and potentially the functionality of RBCs. We precisely quantify the frequency-dependent DEP forces of the RBCs by using a micro-electrode embedded chip combined with optical tweezers. DEP characteristics are known to be well-correlated with the viability of biological cells, and DEP forces are measured in both fresh and long-term stored RBCs to investigate the effect that the storage period has on the cell viability. Moreover, we investigate the DEP behavior of RBCs when exposed to oxidative stress and verify whether EDTA protects the RBCs from an oxidant. From the experiments, it is found that the fresh RBCs without oxidative stress display very high DEP forces over the entire frequency range, exhibiting two cutoff frequencies. However, both the RBCs stored for the long-term period and exposed to oxidative stress reveals that there exist no significant DEP forces over the frequency range. The results indicate that the DEP forces can serve as a useful parameter to verify whether the RBCs in certain blood are fresh and not exposed to oxidative stress. Therefore, it is believed that our system can be applied to a diagnostic system to monitor the cell viability of the RBCs or other types of cells.


Tuesday, August 22, 2017

Single-molecule imaging reveals the translocation and DNA looping dynamics of hepatitis C virus NS3 helicase

Chang-Ting Lin, Felix Tritschler, Kyung Suk Lee, Meigang Gu, Charles M. Rice, Taekjip Ha

Non-structural protein 3 (NS3) is an essential enzyme and a therapeutic target of hepatitis C virus (HCV). Compared to NS3-catalyzed nucleic acids unwinding, its translation on single stranded nucleic acids have received relatively little attention. To investigate the NS3h translocation with single-stranded nucleic acids substrates directly, we have applied a hybrid platform of single-molecule fluorescence detection combined with optical trapping. With the aid of mechanical manipulation and fluorescence localization, we probed the translocase activity of NS3h on laterally stretched, kilobase-size single-stranded DNA and RNA. We observed that the translocation rate of NS3h on ssDNA at a rate of 24.4 nucleotides per second, and NS3h translocates about three time faster on ssRNA, 74 nucleotides per second. The translocation speed was minimally affected by the applied force. A subpopulation of NS3h underwent a novel translocation mode on ssDNA where the stretched DNA shortened gradually and then recovers its original length abruptly before repeating the cycle repetitively. The speed of this mode of translocation was reduced with increasing force. With corroborating data from single-molecule fluorescence resonance energy transfer (smFRET) experiments, we proposed that NS3h can cause repetitive looping of DNA. The smFRET dwell time analysis showed similar translocation time between sole translocation mode versus repetitive looping mode, suggesting that the motor domain exhibits indistinguishable enzymatic activities between the two translocation modes. We propose a potential secondary nucleic acids binding site at NS3h which might function as an anchor point for translocation-coupled looping.


Optical spin torque induced by vector Bessel (vortex) beams with selective polarizations on a light-absorptive sphere of arbitrary size

Renxian Li, Chunying Ding, F.G. Mitri

The optical spin torque (OST) induced by vector Bessel (vortex) beams can cause a particle to rotate around its center of mass. Previous works have considered the OST on a Rayleigh absorptive dielectric sphere by a vector Bessel (vortex) beam, however, it is of some importance to analyze the OST components for a sphere of arbitrary size. In this work, the generalized Lorenz-Mie theory (GLMT) is used to compute the OST induced by vector Bessel (vortex) beams on an absorptive dielectric sphere of arbitrary size, with particular emphasis on the beam order, the polarization of the plane wave component forming the beam, and the half-cone angle. The OST is expressed as the integration of the moment of the time-averaged Maxwell stress tensor, and the beam shape coefficients (BSCs) are calculated using the angular spectrum decomposition method (ASDM). Using this theory, the OST exerted on the light-absorptive dielectric sphere in the Rayleigh, Mie or the geometrical optics regimes can be considered. The axial and transverse OSTs are numerically calculated with particular emphasis on the sign reversal of the axial OST and the vortex-like character of the transverse OST, and the effects of polarization, beam order, and half-cone angle are discussed in detail. Numerical results show that by choosing an appropriate polarization, order and half-cone angle, the sign of the axial OST can be reversed, meaning that the sphere would spin in opposite handedness of the angular momentum carried by the incident beam. The vortex-like structure of the total transverse OSTs can be observed for all cases. When the sphere moves radially away from the beam axis, it may rotate around its center of mass in either the counter-clockwise or the clockwise direction. Conditions are also predicted where the absorptive sphere experiences no spinning. Potential applications in particle manipulation and rotation in optical tweezers and tractor beams would benefit from the results.


Guiding cellular activity with polarized light

Colin Constant, Andrea Bergano, Kiminobu Sugaya, Aristide Dogariu

Actin, cytoskeleton protein forming microfilaments, play a crucial role in cellular motility. Here we show that exposure to very low levels of polarized light guide their orientation in-vivo within the live cell. Using a simple model to describe the role of actin-filament orientation in directional cellular motion, we demonstrate that the actin polymerization/depolymerization mechanism develops primarily along this direction and, under certain conditions, can lead to guidance of the cell movement. Our results also show a dose dependent increase in actin activity in direct correspondence to the level of laser irradiance. We found that total expression of Tau protein, which stabilize microtubules, was decreased by the irradiance, indicating that exposure to the light may change the activity of kinase, leading to increased cell activity.


Investigating the effect of cell substrate on cancer cell stiffness by optical tweezers

Muhammad Sulaiman Yousafzai, Giovanna Coceano, Serena Bonin, Joseph Niemela, Giacinto Scoles, Dan Cojoc

The mechanical properties of cells are influenced by their microenvironment. Here we report cell stiffness alteration by changing the cell substrate stiffness for isolated cells and cells in contact with other cells. Polydimethylsiloxane (PDMS) is used to prepare soft substrates with three different stiffness values (173, 88 and 17 kPa respectively). Breast cancer cells lines, namely HBL-100, MCF-7 and MDA-MB-231 with different level of aggressiveness are cultured on these substrates and their local elasticity is investigated by vertical indentation of the cell membrane. Our preliminary results show an unforeseen behavior of the MDA-MB-231 cells. When cultured on glass substrate as isolated cells, they are less stiff than the other two types of cells, in agreement with the general statement that more aggressive and metastatic cells are softer. However, when connected to other cells the stiffness of MDA-MB-231 cells becomes similar to the other two cell lines. Moreover, the stiffness of MDA-MB-231 cells cultured on soft PDMS substrates is significantly higher than the stiffness of the other cell types, demonstrating thus the strong influence of the environmental conditions on the mechanical properties of the cells.


Scanning holographic optical tweezers

L. A. Shaw, Robert M. Panas, C. M. Spadaccini, and J. B. Hopkins

The aim of this Letter is to introduce a new optical tweezers approach, called scanning holographic optical tweezers (SHOT), which drastically increases the working area (WA) of the holographic-optical tweezers (HOT) approach, while maintaining tightly focused laser traps. A 12-fold increase in the WA is demonstrated. The SHOT approach achieves its utility by combining the large WA of the scanning optical tweezers (SOT) approach with the flexibility of the HOT approach for simultaneously moving differently structured optical traps in and out of the focal plane. This Letter also demonstrates a new heuristic control algorithm for combining the functionality of the SOT and HOT approaches to efficiently allocate the available laser power among a large number of traps. The proposed approach shows promise for substantially increasing the number of particles that can be handled simultaneously, which would enable optical tweezers additive fabrication technologies to rapidly assemble microgranular materials and structures in reasonable build times.


Monday, August 21, 2017

Brownian fluctuations of an optically rotated nanorod

Faegheh Hajizadeh, Lei Shao, Daniel Andrén, Peter Johansson, Halina Rubinsztein-Dunlop, and Mikael Käll

Gold nanorods can be optically trapped in aqueous solution and forced to rotate at kilohertz rates by circularly polarized laser light. This enables detailed investigations of local environmental parameters and processes, such as medium viscosity and nanoparticle–molecule reactions. Future applications may include nanoactuation and single-cell analysis. However, the influence of photothermal heating on the nanoparticle dynamics needs to be better understood in order to realize widespread and quantitative use. Here we analyze the hot Brownian motion of a rotating gold nanorod trapped in two dimensions by an optical tweezers using experiments and stochastic simulations. We show that, for typical settings, the effective rotational and translational Brownian temperatures are drastically different, being closer to the nanorod surface temperature and ambient temperature, respectively. Further, we show that translational dynamics can have a non-negligible influence on the rotational fluctuations due to the small size of a nanorod in comparison to the focal spot. These results are crucial for the development of gold nanorods into generic and quantitative optomechanical sensor and actuator elements.


Fabrication of Fresnel plates on optical fibres by FIB milling for optical trapping, manipulation and detection of single cells

Rita S. Rodrigues Ribeiro, Pabitra Dahal, Ariel Guerreiro, Pedro A. S. Jorge & Jaime Viegas

The development of economical optical devices with a reduced footprint foreseeing manipulation, sorting and detection of single cells and other micro particles have been encouraged by cellular biology requirements. Nonetheless, researchers are still ambitious for advances in this field. This paper presents Fresnel zone and phase plates fabricated on mode expanded optical fibres for optical trapping. The diffractive structures were fabricated using focused ion beam milling. The zone plates presented in this work have focal distance of ~5 µm, while the focal distance of the phase plates is ~10 µm. The phase plates are implemented in an optical trapping configuration, and 2D manipulation and detection of 8 µm PMMA beads and yeast cells is reported. This enables new applications for optical trapping setups based on diffractive optical elements on optical fibre tips, where feedback systems can be integrated to automatically detect, manipulate and sort cells.


Mechanosensing drives acuity of αβ T-cell recognition

Yinnian Feng, Kristine N. Brazin, Eiji Kobayashi, Robert J. Mallis, Ellis L. Reinherz and Matthew J. Lang

T lymphocytes use surface <mml:math><mml:mrow><mml:mi>α</mml:mi><mml:mi>β</mml:mi></mml:mrow></mml:math>αβ T-cell receptors (TCRs) to recognize peptides bound to MHC molecules (pMHCs) on antigen-presenting cells (APCs). How the exquisite specificity of high-avidity T cells is achieved is unknown but essential, given the paucity of foreign pMHC ligands relative to the ubiquitous self-pMHC array on an APC. Using optical traps, we determine physicochemical triggering thresholds based on load and force direction. Strikingly, chemical thresholds in the absence of external load require orders of magnitude higher pMHC numbers than observed physiologically. In contrast, force applied in the shear direction (<mml:math><mml:mo>∼</mml:mo></mml:math>∼10 pN per TCR molecule) triggers T-cell Ca2+ flux with as few as two pMHC molecules at the interacting surface interface with rapid positional relaxation associated with similarly directed motor-dependent transport via <mml:math><mml:mo>∼</mml:mo></mml:math>∼8-nm steps, behaviors inconsistent with serial engagement during initial TCR triggering. These synergistic directional forces generated during cell motility are essential for adaptive T-cell immunity against infectious pathogens and cancers.


Nonlinear Self-Action of Light through Biological Suspensions

Anna Bezryadina, Tobias Hansson, Rekha Gautam, Benjamin Wetzel, Graham Siggins, Andrew Kalmbach, Josh Lamstein, Daniel Gallardo, Edward J. Carpenter, Andrew Ichimura, Roberto Morandotti, and Zhigang Chen
It is commonly thought that biological media cannot exhibit an appreciable nonlinear optical response. We demonstrate, for the first time to our knowledge, a tunable optical nonlinearity in suspensions of cyanobacteria that leads to robust propagation and strong self-action of a light beam. By deliberately altering the host environment of the marine bacteria, we show experimentally that nonlinear interaction can result in either deep penetration or enhanced scattering of light through the bacterial suspension, while the viability of the cells remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces (including both gradient and forward-scattering forces) acting on the bacteria explains our experimental observations.

Membrane tension controls adhesion positioning at the leading edge of cells

Bruno Pontes, Pascale Monzo, Laurent Gole, Anabel-Lise Le Roux, Anita Joanna Kosmalska, Zhi Yang Tam, Weiwei Luo, Sophie Kan, Virgile Viasnoff, Pere Roca-Cusachs, Lisa Tucker-Kellogg, Nils C. Gauthier

Cell migration is dependent on adhesion dynamics and actin cytoskeleton remodeling at the leading edge. These events may be physically constrained by the plasma membrane. Here, we show that the mechanical signal produced by an increase in plasma membrane tension triggers the positioning of new rows of adhesions at the leading edge. During protrusion, as membrane tension increases, velocity slows, and the lamellipodium buckles upward in a myosin II–independent manner. The buckling occurs between the front of the lamellipodium, where nascent adhesions are positioned in rows, and the base of the lamellipodium, where a vinculin-dependent clutch couples actin to previously positioned adhesions. As membrane tension decreases, protrusion resumes and buckling disappears, until the next cycle. We propose that the mechanical signal of membrane tension exerts upstream control in mechanotransduction by periodically compressing and relaxing the lamellipodium, leading to the positioning of adhesions at the leading edge of cells.


Friday, August 18, 2017

Rapid Characterization of a Mechanically Labile α-Helical Protein Enabled by Efficient Site-Specific Bioconjugation

Robert Walder, Marc-André LeBlanc, William J. Van Patten, Devin T. Edwards, Jacob A. Greenberg, Ayush Adhikari, Stephen R. Okoniewski, Ruby May A. Sullan, David Rabuka, Marcelo C. Sousa, and Thomas T. Perkins

Atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) is a powerful yet accessible means to characterize mechanical properties of biomolecules. Historically, accessibility relies upon the nonspecific adhesion of biomolecules to a surface and a cantilever and, for proteins, the integration of the target protein into a polyprotein. However, this assay results in a low yield of high-quality data, defined as the complete unfolding of the polyprotein. Additionally, nonspecific surface adhesion hinders studies of α-helical proteins, which unfold at low forces and low extensions. Here, we overcame these limitations by merging two developments: (i) a polyprotein with versatile, genetically encoded short peptide tags functionalized via a mechanically robust Hydrazino-Pictet-Spengler ligation and (ii) the efficient site-specific conjugation of biomolecules to PEG-coated surfaces. Heterobifunctional anchoring of this polyprotein construct and DNA via copper-free click chemistry to PEG-coated substrates and a strong but reversible streptavidin–biotin linkage to PEG-coated AFM tips enhanced data quality and throughput. For example, we achieved a 75-fold increase in the yield of high-quality data and repeatedly probed the same individual polyprotein to deduce its dynamic force spectrum in just 2 h. The broader utility of this polyprotein was demonstrated by measuring three diverse target proteins: an α-helical protein (calmodulin), a protein with internal cysteines (rubredoxin), and a computationally designed three-helix bundle (α3D). Indeed, at low loading rates, α3D represents the most mechanically labile protein yet characterized by AFM. Such efficient SMFS studies on a commercial AFM enable the rapid characterization of macromolecular folding over a broader range of proteins and a wider array of experimental conditions (pH, temperature, denaturants). Further, by integrating these enhancements with optical traps, we demonstrate how efficient bioconjugation to otherwise nonstick surfaces can benefit diverse single-molecule studies.


Direct Measurement of Photoacoustic Signal Sensitivity to Aerosol Particle Size

Johannes W. Cremer, Paul A. Covert, Evelyne A. Parmentier, and Ruth Signorell

Continuing efforts to quantify the influence of aerosol light absorption upon global heat budgets rely on high-quality measurements of aerosol optical properties. Of the available methods, photoacoustic spectroscopy stands out as a sensitive method for measurements of aerosol absorption with minimal sample modification. Theoretical treatments of photoacoustic aerosol detection have predicted size-dependent damping of the photoacoustic signal as a result of particle thermal inertia. We provide experimental confirmation of this prediction using a single-particle photoacoustic spectrometer, which allows us to measure photoacoustic signals with high sensitivity and size-specificity. Both the magnitude and phase of the photoacoustic response follow the linearized description of the heat flux. The quantification of this effect provides a basis for future, system-specific case studies.


Toward Controlled Photothermal Treatment of Single Cell: Optically Induced Heating and Remote Temperature Monitoring In Vitro through Double Wavelength Optical Tweezers

Sławomir Drobczyński, Katarzyna Prorok, Konstantin Tamarov, Kamila Duś-Szachniewicz, Vesa-Pekka Lehto, and Artur Bednarkiewicz
Cancer treatment based on hyperthermia (HT) relies on exposing the malignant cells to elevated local temperature. Although the procedure has been successfully applied in clinics, the fundamental aspects of HT are not yet fully understood. In order to verify the susceptibility of single cells in vitro to raised temperature, we have developed novel nano- and microtools. In particular, an optical double-trap system utilizing combined galvano-mirror scanning and spatial light phase modulator was devised to manipulate several micron-sized objects simultaneously. The manipulation comprised both optical trapping and translocating, on demand photoactivated heating, and simultaneous remote temperature readout of living cells, infrared activated heaters and optical thermometers, respectively. Mesoporous silicon microparticles were used as an infrared absorber to generate an increased temperature of about 100 °C with 0.4 W laser power. The optical micron-sized thermometer was based on up-converting Yb–Er codoped nanocrystalline particles encapsulated in amorphous silica shells produced with yeast cells as the templates. These hybrid particles displayed a relative sensitivity of 0.28%/K, an accuracy of 0.1 °C (at 32 °C), spatial resolution of <10 μm, and a temporal response of 50 ms/acquisition to record the temperature changes in specified areas in real time. The system was utilized in monitoring the stepwise cell death of individual diffuse large B-cell lymphoma (DLBCL) cells due to locally induced excessive heating induced by the absorber localized in the vicinity of the cell.


Photon mass drag and the momentum of light in a medium

Mikko Partanen, Teppo Häyrynen, Jani Oksanen, and Jukka Tulkki

Conventional theories of electromagnetic waves in a medium assume that the energy propagating with the light pulse in the medium is entirely carried by the field. Thus, the possibility that the optical force field of the light pulse would drive forward an atomic mass density wave (MDW) and the related kinetic and elastic energies is neglected. In this work, we present foundations of a covariant theory of light propagation in a medium by considering a light wave simultaneously with the dynamics of the medium atoms driven by optoelastic forces between the induced dipoles and the electromagnetic field. We show that a light pulse having a total electromagnetic energy ℏω propagating in a nondispersive medium transfers a mass equal to δm=(n2−1)ℏω/c2, where n is the refractive index. MDW, which carries this mass, consists of atoms, which are more densely spaced inside the light pulse as a result of the field-dipole interaction. We also prove that the transfer of mass with the light pulse, the photon mass drag effect, gives an essential contribution to the total momentum of the light pulse, which becomes equal to the Minkowski momentum pM=nℏω/c. The field's share of the momentum is the Abraham momentum pA=ℏω/(nc), while the difference pM−pA is carried by MDW. Due to the coupling of the field and matter, only the total momentum of the light pulse and the transferred mass δm can be directly measured. Thus, our theory gives an unambiguous physical meaning to the Abraham and Minkowski momenta. We also show that to solve the centenary Abraham-Minkowski controversy of the momentum of light in a nondispersive medium in a way that is consistent with Newton's first law, one must account for the mass transfer effect. We derive the photon mass drag effect using two independent but complementary covariant models. In the mass-polariton (MP) quasiparticle approach, we consider the light pulse as a coupled state between the photon and matter, isolated from the rest of the medium. The momentum and the transferred mass of MP follow unambiguously from the Lorentz invariance and the fundamental conservation laws of nature. To enable the calculation of the mass and momentum distribution of a light pulse, we have also generalized the electrodynamics of continuous media to account for the space- and time-dependent optoelastic dynamics of the medium driven by the field-dipole forces. In this optoelastic continuum dynamics (OCD) approach, we obtain with an appropriate space-time discretization a numerically accurate solution of the Newtonian continuum dynamics of the medium when the light pulse is propagating in it. The OCD simulations of a Gaussian light pulse propagating in a diamond crystal give the same momentum pM and the transferred mass δm for the light pulse as the MP quasiparticle approach. Our simulations also show that, after photon transmission, some nonequilibrium of the mass distribution is left in the medium. Since the elastic forces are included in our simulations on equal footing with the optical forces, our simulations also depict how the mass and thermal equilibria are reestablished by elastic waves. In the relaxation process, a small amount of photon energy is dissipated into lattice heat. We finally discuss a possibility of an optical waveguide setup for experimental measurement of the transferred mass of the light pulse. Our main result that a light pulse is inevitably associated with an experimentally measurable mass is a fundamental change in our understanding of light propagation in a medium.


Friction Mediates Scission of Tubular Membranes Scaffolded by BAR Proteins

Mijo Simunovic, Jean-Baptiste Manneville, Henri-François Renard, Emma Evergren, Krishnan Raghunathan, Dhiraj Bhatia, Anne K. Kenworthy, Gregory A. Voth, Jacques Prost, Harvey T. McMahon, Ludger Johannes, Patricia Bassereau, Andrew Callan-Jones

Membrane scission is essential for intracellular trafficking. While BAR domain proteins such as endophilin have been reported in dynamin-independent scission of tubular membrane necks, the cutting mechanism has yet to be deciphered. Here, we combine a theoretical model, in vitro, and in vivo experiments revealing how protein scaffolds may cut tubular membranes. We demonstrate that the protein scaffold bound to the underlying tube creates a frictional barrier for lipid diffusion; tube elongation thus builds local membrane tension until the membrane undergoes scission through lysis. We call this mechanism friction-driven scission (FDS). In cells, motors pull tubes, particularly during endocytosis. Through reconstitution, we show that motors not only can pull out and extend protein-scaffolded tubes but also can cut them by FDS. FDS is generic, operating even in the absence of amphipathic helices in the BAR domain, and could in principle apply to any high-friction protein and membrane assembly.


A foam model highlights the differences of the macro- and microrheology of respiratory horse mucus

Gross A, Torge A, Schaefer UF, Schneider M, Lehr CM, Wagner C

Native horse mucus is characterized with micro- and macrorheology and compared to hydroxyethylcellulose (HEC) gel as a model. Both systems show comparable viscoelastic properties on the microscale and for the HEC the macrorheology is in good agreement with the microrheology. For the mucus, the viscoelastic moduli on the macroscale are several orders of magnitude larger than on the microscale. Large amplitude oscillatory shear experiments show that the mucus responds nonlinearly at much smaller deformations than HEC. This behavior fosters the assumption that the mucus has a foam like structure on the microscale compared to the typical mesh like structure of the HEC, a model that is supported by cryogenic-scanning-electron-microscopy (CSEM) images. These images allow also to determine the relative amount of volume that is occupied by the pores and the scaffold. Consequently, we can estimate the elastic modulus of the scaffold. We conclude that this particular foam like microstructure should be considered as a key factor for the transport of particulate matter which plays a central role in mucus function with respect to particle penetration.


Thursday, August 17, 2017

Negative optical radiation force and spin torques on subwavelength prolate and oblate spheroids in fractional Bessel–Gauss pincers light-sheets

F. G. Mitri

Fractional Bessel–Gauss light-sheets [J. Opt. 19, 055602 (2017) [CrossRef] ], which correspond to finite optical “slices” in 2D and possess asymmetric slit openings and bending characteristics, are examined from the standpoint of optical radiation force and spin torque theories for a subwavelength spheroid with arbitrary orientation in space. The vector angular spectrum decomposition method in addition to the Lorenz gauge condition and Maxwell’s equations are used to determine the Cartesian components of the incident radiated electric field of the Bessel–Gauss light-sheets. In the framework of the dipole approximation, the numerical results for the Cartesian components of the optical radiation force and spin torque vectors show that negative forces (oriented in the opposite direction of wave motion) and spin torques arise depending on the beam parameters, the orientation of the subwavelength spheroid in 3D space, and its aspect ratio (i.e., prolate versus oblate). The spin torque sign reversal reveals that counter-clockwise or clockwise rotations around the center of mass of the spheroid can occur. The results find important applications in the application of auto-focusing light-sheets in particle manipulation, rotation, and optical sorting devices.


Review of optical detection of single molecules beyond the diffraction and diffusion limit using plasmonic nanostructures

Farzia Karim; Todd B. Smith; Chenglong Zhao

Single-molecule detection has become a unique and indispensable tool for the study of molecular motions and interactions at the single-molecule level. Unlike ensemble measurement where the information is averaged, single-molecule analysis yields invaluable information on both the individual molecular properties and their microenvironment. Among the various technologies for the detection of single molecules, the detection with optical methods has many advantages in terms of its high sensitivity, electrical passiveness, and robustness. The recent advances in the engineering of either the excitation light or the solution of the molecules have paved the way for enhanced single-molecule detection. We present recent developments and future perspectives for single-molecule detection in the following three regimes: on a dry surface, in solutions at ultralow concentrations, and in solutions at native physiological concentrations.


Coordination among tertiary base pairs results in an efficient frameshift-stimulating RNA pseudoknot

Yu-Ting Chen Kai-Chun Chang Hao-Teng Hu Yi-Lan Chen You-Hsin Lin Chiung-Fang Hsu Cheng-Fu Chang Kung-Yao Chang Jin-Der Wen

Frameshifting is an essential process that regulates protein synthesis in many viruses. The ribosome may slip backward when encountering a frameshift motif on the messenger RNA, which usually contains a pseudoknot structure involving tertiary base pair interactions. Due to the lack of detailed molecular explanations, previous studies investigating which features of the pseudoknot are important to stimulate frameshifting have presented diverse conclusions. Here we constructed a bimolecular pseudoknot to dissect the interior tertiary base pairs and used single-molecule approaches to assess the structure targeted by ribosomes. We found that the first ribosome target stem was resistant to unwinding when the neighboring loop was confined along the stem; such constrained conformation was dependent on the presence of consecutive adenosines in this loop. Mutations that disrupted the distal base triples abolished all remaining tertiary base pairs. Changes in frameshifting efficiency correlated with the stem unwinding resistance. Our results demonstrate that various tertiary base pairs are coordinated inside a highly efficient frameshift-stimulating RNA pseudoknot and suggest a mechanism by which mechanical resistance of the pseudoknot may persistently act on translocating ribosomes.


Dyadic Green's function formalism for photoinduced forces in tip-sample nanojunctions

Faezeh Tork Ladani and Eric Olaf Potma

A comprehensive theoretical analysis of photoinduced forces in an illuminated nanojunction, formed between an atomic force microscopy tip and a sample, is presented. The formalism is valid within the dipolar approximation and includes multiple scattering effects between the tip, sample, and a planar substrate through a dyadic Green's function approach. This physically intuitive description allows a detailed look at the quantitative contribution of multiple scattering effects to the measured photoinduced force, effects that are typically unaccounted for in simpler analytical models. Our findings show that the presence of the planar substrate and anisotropy of the tip have a substantial effect on the magnitude and the spectral response of the photoinduced force exerted on the tip. Unlike previous models, our calculations predict photoinduced forces that are within range of experimentally measured values in photoinduced force microscopy (PiFM) experiments.


Controlling the net charge on a nanoparticle optically levitated in vacuum

Martin Frimmer, Karol Luszcz, Sandra Ferreiro, Vijay Jain, Erik Hebestreit, and Lukas Novotny

Optically levitated nanoparticles in vacuum are a promising model system to test physics beyond our current understanding of quantum mechanics. Such experimental tests require extreme control over the dephasing of the levitated particle's motion. If the nanoparticle carries a finite net charge, it experiences a random Coulomb force due to fluctuating electric fields. This dephasing mechanism can be fully excluded by discharging the levitated particle. Here, we present a simple and reliable technique to control the charge on an optically levitated nanoparticle in vacuum. Our method is based on the generation of charges in an electric discharge and does not require additional optics or mechanics close to the optical trap.


Biophysics and Biofluiddynamics of Primary Cilia: evidence for and against the flow-sensing function

Subhra Nag, Andrew Resnick

Primary cilia have been called "the forgotten organelle" for over 20 years. As cilia now have their own Journal and several books devoted to their study, perhaps it's time to reconsider the moniker "forgotten organelle". In fact, during the drafting of this review, 12 relevant publications have been issued- we therefore apologize in advance for any relevant work we inadvertently omitted. What purpose is yet another ciliary review? The primary goal of this review is to specifically examine the evidence for and against the hypothesized flow-sensing function of primary cilia expressed by differentiated epithelia within a kidney tubule, bringing together differing disciplines and their respective conceptual and experimental approaches. We will show that understanding the biophysics/biomechanics of primary cilia provides essential information for understanding any potential role of ciliary function in disease. We will summarize experimental and mathematical models used to characterize renal fluid flow and incident force on primary cilia, to characterize the mechanical response of cilia to an externally applied force and discuss possible ciliary-mediated cell signaling pathways triggered by flow. Throughout, we stress the importance of separating the effects of fluid shear and stretch from the action of hydrodynamic drag.


Wednesday, August 16, 2017

On the validity of the integral localized approximation for Bessel beams and associated radiation pressure forces

Leonardo A. Ambrosio, Jiajie Wang, and Gérard Gouesbet

In this paper we investigate the integral version of the localized approximation (ILA)—a powerful technique for evaluating the beam shape coefficients in the framework of the generalized Lorenz–Mie theory—as applied to ideal scalar Bessel beams (BBs). Originally conceived for arbitrary shaped beams with a propagating factor exp(±𝑖𝑘𝑧)exp(±ikz), it has recently been shown that care must be taken when applying the ILA for the case of ideal scalar BBs, since they carry a propagating factor exp(±𝑖𝑘𝑧 cos 𝛼)exp(±ikz cos α), with 𝛼α being the axicon angle, which cannot be smoothly accommodated into its mathematical formalism. Comparisons are established between the beam shape coefficients calculated from both ILA and exact approaches, assuming paraxial approximation and both on- and off-axis beams. Particular simulations of radiation pressure forces are provided based on the existing data in the literature. This work helps us in elucidating that ILA provides adequate beam shape coefficients and descriptions of ideal scalar BBs up to certain limits and, even when it fails to do so, reliable information on the physical optical properties of interest can still be inferred, depending on specific geometric and electromagnetic aspects of the scatterer.


Radiation Forces on a Cluster of Spherical Nanoparticles in Visible Light Spectrum

A. N. Moqadam, A. Pourziad, and S. Nikmehr

The scattering of the electromagnetic waves by the spherical particles is discussed. Nanometer-sized dielectric spheres confined in a cluster are devoted to investigate the effect of the EM radiation on them. Incident wave is considered to be in visible light spectrum which facilitates multiple scattering calculation for nanoparticles. Radiation forces are discussed in terms of scattering pressure and Lorentz force, hence Discrete Dipole Approximation (DDA) and classical Mie theory is employed in radiation force computation and electromagnetic random multiple scattering analysis. Electric momentum of dipoles is defined in the term of A-1 term method. The radiation forces on particles are accurately calculated with computer codes. Extracted results can be applied to conscious deviation of spherical nanoparticles in clean rooms or similar mediums. The effect of the incident wave parameters and the orientation of spherical profile and particles in the cluster are predicted through various simulations.


Interaction of Bessel pincers light-sheets with an absorptive subwavelength sphere coated by a plasmonic layer

F. G. Mitri

The interaction of Bessel pincers light-sheets (i.e., finite optical beam “slices” in 2D) with a light-absorptive subwavelength sphere coated by a plasmonic layer in vacuum is examined in the framework of the electric dipole approximation. The vector angular spectrum decomposition method in addition to the Lorenz gauge condition and Maxwell’s equations are used to determine the Cartesian components of the incident radiated electric field of the Bessel pincers light-sheets. Two main effects are thoroughly investigated from the standpoint of the optical radiation force and spin torque theories. The numerical results for the optical radiation force and spin torque components show that a plasmonic layer of optimal thickness coating a small sphere can enhance the longitudinal and transverse force components as well as the axial spin torque induced by Bessel pincers light-sheets at the plasmonic resonance. Moreover, sign reversal of the longitudinal force component is manifested off the plasmonic resonance, resulting in particle trapping in a confined region in space. An axial spin torque sign reversal also occurs, which induces counterclockwise or clockwise rotation around the center of mass of the coated absorptive sphere depending on its position in the cross-sectional transverse plane. Furthermore, based on Newton’s second law of motion, a particle dynamics numerical analysis is performed, which reveals the emergence of retrograde motions of the coated sphere off-resonance as well as complex non-rectilinear/wiggly trajectories depending on its position in the transverse cross-sectional plane. At the plasmonic resonance, the coated sphere is propelled away from the source in the transverse cross-sectional plane along quasi-rectilinear trajectories. The results find related applications using autofocusing Bessel pincers light-sheets in particle manipulation and optical sorting devices.


On the origin of the driving force in the Marangoni propelled gas bubble trapping mechanism

A. Miniewicz, C. Quintard, H. Orlikowska and S. Bartkiewicz

Gas bubbles can be trapped and then manipulated with laser light. In this report, we propose the detailed optical trapping mechanism of gas bubbles confined inside a thin light-absorbing liquid layer between two glass plates. The necessary condition of bubble trapping in this case is the direct absorption of light by the solution containing a dye. Due to heat release, fluid whirls propelled by the surface Marangoni effect at the liquid/gas interface emerge and extend to large distances. We report the experimental microscopic observation of the origin of whirls at an initially flat liquid/air interface as well as at the curved interface of a liquid/gas bubble and support this finding with advanced numerical simulations using the finite element method within the COMSOL Multiphysics platform. The simulation results were in good agreement with the observations, which allowed us to propose a simple physical model for this particular trapping mechanism, to establish the origin of forces attracting bubbles toward a laser beam and to predict other phenomena related to this effect.


Scanning dimensional measurement using laser-trapped microsphere with optical standing-wave scale

Masaki Michihata; Shin-ichi Ueda; Satoru Takahashi; Kiyoshi Takamasu; Yasuhiro Takaya

We propose a laser trapping-based scanning dimensional measurement method for free-form surfaces. We previously developed a laser trapping-based microprobe for three-dimensional coordinate metrology. This probe performs two types of measurements: a tactile coordinate and a scanning measurement in the same coordinate system. The proposed scanning measurement exploits optical interference. A standing-wave field is generated between the laser-trapped microsphere and the measured surface because of the interference from the retroreflected light. The standing-wave field produces an effective length scale, and the trapped microsphere acts as a sensor to read this scale. A horizontal scan of the trapped microsphere produces a phase shift of the standing wave according to the surface topography. This shift can be measured from the change in the microsphere position. The dynamics of the trapped microsphere within the standing-wave field was estimated using a harmonic model, from which the measured surface can be reconstructed. A spherical lens was measured experimentally, yielding a radius of curvature of 2.59 mm, in agreement with the nominal specification (2.60 mm). The difference between the measured points and a spherical fitted curve was 96 nm, which demonstrates the scanning function of the laser trapping-based microprobe for free-form surfaces.


Focal and optical trapping behaviors of radially polarized vortex beam with broken axial symmetry

Zhongsheng Man, Luping Du, Yuquan Zhang, Changjun Min, Shenggui Fu, and Xiaocong Yuan

We explore a radially polarized vortex beam with broken axial symmetry under tight focusing conditions. The beam’s three mutually orthogonal polarization components (radial, azimuthal, and longitudinal) are all rotated by an angle of π/2 with respect to the input field in the focal plane. We validate this effect experimentally. Finally, we prove that this effect can be used to transport nanoparticles.


Tuesday, August 15, 2017

Assessment of the “cross-bridge”-induced interaction of red blood cells by optical trapping combined with microfluidics

Kisung Lee; Christian Wagner; Alexander V. Priezzhev

Red blood cell (RBC) aggregation is an intrinsic property of the blood that has a direct effect on the blood viscosity and circulation. Nevertheless, the mechanism behind the RBC aggregation has not been confirmed and is still under investigation with two major hypotheses, known as “depletion layer” and “cross-bridging.” We aim to ultimately understand the mechanism of the RBC aggregation and clarify both models. To measure the cell interaction in vitro in different suspensions (including plasma, isotonic solution of fibrinogen, isotonic solution of fibrinogen with albumin, and phosphate buffer saline) while moving the aggregate from one solution to another, an approach combining optical trapping and microfluidics has been applied. The study reveals evidence that RBC aggregation in plasma is at least partly due to the cross-bridging mechanism. The cell interaction strength measured in the final solution was found to be significantly changed depending on the initial solution where the aggregate was formed.


Mapping the restoring force of an optical trap in three dimensions using a two laser dual-trap approach

A. Raudsepp, M. A. K. Williams & S. B. Hall

A two laser optical tweezers set-up is developed and used to measure deflections of a microsphere trapped in a calibrated spatial light modulator steered probe trap as it is stepped through a three dimensional grid about a fixed test trap. These measurements are used to map the restoring force of the test trap on the microsphere in three dimensions. Results are validated over a common range by comparison to drag force measurements for both silica and polystyrene microspheres.


Composite device for interfacing an array of atoms with a single nanophotonic cavity mode

Mark Sadgrove and Kali P Nayak

We propose a method of trapping atoms in arrays near to the surface of a composite nanophotonic device with optimal coupling to a single cavity mode. The device, comprised of a nanofiber mounted on a grating, allows the formation of periodic optical trapping potentials near to the nanofiber surface along with a high cooperativity nanofiber cavity. We model the device analytically and find good agreement with numerical simulations. We numerically demonstrate that for an experimentally realistic device, an array of traps can be formed whose centers coincide with the antinodes of a single cavity mode, guaranteeing optimal coupling to the cavity. Additionally, we simulate a trap suitable for a single atom within 100 nm of the fiber surface, potentially allowing larger coupling to the nanofiber than found using typical guided mode trapping techniques.


Enhancing Optical Forces in InP-Based Waveguides

Mohammad Esmail Aryaee Panah, Elizaveta S. Semenova & Andrei V. Lavrinenko

Cantilever sensors are among the most important microelectromechanical systems (MEMS), which are usually actuated by electrostatic forces or piezoelectric elements. Although well-developed microfabrication technology has made silicon the prevailing material for MEMS, unique properties of other materials are overlooked in this context. Here we investigate optically induced forces exerted upon a semi-insulating InP waveguide suspended above a highly doped InP:Si substrate, in three different regimes: the epsilon-near-zero (ENZ), with excitation of surface plasmon polaritons (SPPs) and phonons excitation. An order of magnitude amplification of the force is observed when light is coupled to SPPs, and three orders of magnitude amplification is achieved in the phonon excitation regime. In the ENZ regime, the force is found to be repulsive and higher than that in a waveguide suspended above a dielectric substrate. Low losses in InP:Si result in a big propagation length. The induced deflection can be detected by measuring the phase change of the light when passing through the waveguide, which enables all-optical functioning, and paves the way towards integration and miniaturization of micro-cantilevers. In addition, tunability of the ENZ and the SPP excitation wavelength ranges, via adjusting the carrier concentration, provides an extra degree of freedom for designing MEMS devices.


Dynamic plasmonic nano-traps for single molecule surface-enhanced Raman scattering

Yuquan Zhang, Junfeng Shen, Zhenwei Xie, Xiujie Dou, Changjun Min Ting Lei, Jun Liu, Siwei Zhu and Xiaocong Yuan

Intense electric fields at the nanoscale are essential for single molecule surface-enhanced Raman scattering (SERS) detection. Such fields can be achieved in plasmonic nano-gaps between nanoparticles and metal films through hybridization of surface plasmons. The nano-gaps could be formed and dynamically controlled by using plasmonic tweezers; however, the aggregation of particles in the plasmonic field degrades each particle's enhancement and spoils the nanosized-spatial resolution. Here, dual-plasmonic tweezers are proposed and demonstrated to accurately control the number of nano-gaps and enhancement by tailoring a crater-shaped potential well in the nano-trap system. As the electric field in the nano-gap is intense, SERS spectral signatures of a single molecular level are probed simultaneously. These advantages point towards the implementation of enhanced Raman spectra, and broad applications in optical molecular detection.