Thursday, June 30, 2011

Electric and magnetic dipolar response of germanium nanospheres: interference effects, scattering anisotropy, and optical forces

Raquel Gómez-Medina, Irene Suárez-Lacalle, Juan José Sáenz, Manuel Nieto-Vesperinas, Braulio García-Cámara, Francisco González, and Fernando Moreno

The coherent combination of electric and magnetic responses is the basis of the electromagnetic behavior of new engineered metamaterials. The basic constituents of their meta-atoms usually have metallic character and consequently high absorption losses. Based on standard “Mie” scattering theory, we found that there is a wide window in the near-infrared (wavelengths 1 to 3 μm), where light scattering by lossless submicrometer Ge spherical particles is fully described by their induced electric and magnetic dipoles. The interference between electric and magnetic dipolar fields is shown to lead to anisotropic angular distributions of scattered intensity, including zero backward and almost zero forward scattered intensities at specific wavelengths, which until recently was theoretically established only for hypothetically postulated magnetodielectric spheres. Although the scattering cross section at zero backward or forward scattering is exactly the same, radiation pressure forces are a factor of 3 higher in the zero forward condition.


Integral localized approximation description of ordinary Bessel beams and application to optical trapping forces

Leonardo A. Ambrosio and Hugo E. Hernández-Figueroa

Ordinary Bessel beams are described in terms of the generalized Lorenz-Mie theory (GLMT) by adopting, for what is to our knowledge the first time in the literature, the integral localized approximation for computing their beam shape coefficients (BSCs) in the expansion of the electromagnetic fields. Numerical results reveal that the beam shape coefficients calculated in this way can adequately describe a zero-order Bessel beam with insignificant difference when compared to other relative time-consuming methods involving numerical integration over the spherical coordinates of the GLMT coordinate system, or quadratures. We show that this fast and efficient new numerical description of zero-order Bessel beams can be used with advantage, for example, in the analysis of optical forces in optical trapping systems for arbitrary optical regimes.


Measurement of membrane rigidity on trapped unilamellar phospholipid vesicles by using differential confocal microscopy

Tze-Hsuan Liu, Jian-Long Xiao, Chau-Hwang Lee, and Jiunn-Yuan Lin

We use optical tweezers to trap a unilamellar phospholipid vesicle and measure the out-of-plane thermal fluctuations by using differential confocal microscopy. Bending moduli of the lipid membranes are calculated directly from the mean-square values of the fluctuation amplitudes. Owing to the refractive index contrast between the inner and outer solutions of the vesicle, optical tweezers trap the vesicle laterally and improve the reliability of the measured fluctuation amplitudes along the optical axis. Bending moduli of membranes in gel or fluid phases obtained by the combination of differential confocal microscopy and optical tweezers are close to those reported previously. We also obtain the bending modulus of sphingomyelin membranes in the gel phase, which was not reported previously.


Tuesday, June 28, 2011

Micromechanics of colloidal aggregates at the oil–water interface

Bum Jun Park and Eric M. Furst

The micromechanics of two-dimensional (2D) colloidal aggregates at the oil–water interface are measured using optical tweezers. Aggregates form from stable 2D suspensions after introducing either 0.25 M NaCl/0.1 mM SDS in the aqueous sub-phase or 25 μM sorbitan monooleate (SPAN 80) in the oil super-phase. Aggregates formed with NaCl/SDS have strong bond bending rigidities due to tangential forces between particles, leading to an average aggregate rigidity κa = 4.9 ± 3.1 mN m−1. Rigid aggregates are consistent with previously reported open microstructures and irreversible, diffusion-limited cluster aggregation kinetics. In contrast, aggregates formed by SPAN 80 exhibit weak bond rigidities (κa = 0.28 ± 0.31 mN m−1), enabling particle rearrangements that lead to a denser microstructure. The micromechanical properties of aggregates that constitute the macrocolloidal structure of 2D suspensions provide a critical link between their colloidal interactions and interfacial rheology.


Laser-trapping assembling dynamics of molecules and proteins at surface and interface

Hiroshi Masuhara, Teruki Sugiyama, Thitiporn Rungsimanon, Ken-ichi Yuyama, Atsushi Miura and Jing-Ru Tu

Laser trapping of molecules and proteins in solution at room temperature is made possible by irradiating 1064-nm continuous-wave (CW) laser with power around 1 W. Although conventional small molecules are not trapped at the focal point, molecules that can form clusters upon assembling and proteins whose size is close to 10 nm are gathered, giving unique assembly structure. Glycine in H2O shows crystallization, urea in D2O gives a millimeter-sized giant droplet, and cobalt oxide-filled ferritin protein confirms assembly followed by precipitation. Solute concentration, solvent, and laser power are important factors for determining trapping and assembling phenomena, and the laser focal position is very critical. These unique behaviors are realized by setting the irradiation at the air/solution surface, inside the solution, and at the glass/solution interface. Laser trapping-induced crystallization, liquid/liquid phase separation, and precipitation are compared with the previous results and considered. After summarizing the results, we describe our future perspective and plans


Monday, June 27, 2011

Targeted delivery and controllable release of nanoparticles using a defect-decorated optical nanofiber

Hongbao Xin and Baojun Li

Targeted drug delivery and controllable release are particularly beneficial in medical therapy. This work provides a demonstration of nanoparticles targeted delivery and controllable release using a defect-decorated optical nanofiber (NF). By using the NF, polystyrene particles (PSs) (713-nm diameter) suspended in water were successfully trapped, then delivered along the NF at an average velocity of 4.8 µm/s with the assistance of a laser beam of 980-nm wavelength at an optical power of 39 mW, and finally, assembled at the defect. Subsequently, by turning off the optical power, 90% of the assembled PSs can be released in 30 s. This method would be useful in targeted drug delivery and controllable release, and provide potential applications in targeted therapy.


Clustering of optically trapped large diameter plasmonic gold nanoparticles by laser beam of hybrid-TEM11* mode

Ranjeet Kumar, Dalip Singh Mehta, and Chandra Shakher

Multiple trapping and clustering of gold nanoparticles (Au-NPs) of 254- and 150-nm diameter was affected using optical tweezers near the plasmon excitation wavelength. To ensure that the gradient force exceeded the sum of multiply-enhanced destabilizing absorption and scattering forces originating from plasmon excitation, embedded intensity gradient regions of a spatially featured asymmetric (SFA) laser beam were exploited. Thus, an intra-cavity generated SFA beam, also referred as hybrid TEM11* mode, is an intermediate between pure TEM00 and TEM11 beams and was directly obtained from a diode-pumped solid state (Nd:YAG) laser resonator without introducing any external beam modulation devices. The parabolic Gaussian-ray model of a tightly focused laser beam was adopted to evaluate the radiation forces including the volume-correction factor raised from fractional polarization of such large diameter Au-NPs under laser illumination. Temperature rise of Au-NPs and its dissipation profile in surrounding medium has also been presented. This multiple trapping and clustering of Au-NPs at plasmon excitation wavelength using sufficiently low power could be realized due to embedded intensity gradients of the SFA beam. The study might be useful for understanding the light-matter interaction, improving the sensitivity of diagnostics, and safety and efficacy of therapeutic nanotechnologies in medicine, photothermolysis, and surface-enhanced Raman spectroscopy, etc.


Colloidal diffusion and hydrodynamic screening near boundaries

Pushkar P. Lele, James W. Swan, John F. Brady, Norman J. Wagner and Eric M. Furst

The hydrodynamic interactions between colloidal particles in small ensembles are measured at varying distances from a no-slip surface over a range of inter-particle separations. The diffusion tensor for motion parallel to the wall of each ensemble is calculated by analyzing thousands of particle trajectories generated by blinking holographic optical tweezers and by dynamic simulation. The Stokesian Dynamics simulations predict similar particle dynamics. By separating the dynamics into three classes of modes: self, relative and collective diffusion, we observe qualitatively different behavior depending on the relative magnitudes of the distance of the ensemble from the wall and the inter-particle separation. A simple picture of the pair-hydrodynamic interactions is developed, while many-body-hydrodynamic interactions give rise to more complicated behavior. The results demonstrate that the effect of many-body hydrodynamic interactions in the presence of a wall is much richer than the single particle behavior and that the multiple-particle behavior cannot be simply predicted by a superposition of pair interactions.


Single cell viability and impact of heating by laser absorption

Franziska Wetzel, Susanne Rönicke, Karla Müller, Markus Gyger, Daniel Rose, Mareike Zink and Josef Käs
Optical traps such as tweezers and stretchers are widely used to probe the mechanical properties of cells. Beyond their large range of applications, the use of infrared laser light in optical traps causes significant heating effects in the cell. This study investigated the effect of laser-induced heating on cell viability. Common viability assays are not very sensitive to damages caused in short periods of time or are not practicable for single cell analysis. We used cell spreading, a vital ability of cells, as a new sensitive viability marker. The optical stretcher, a two beam laser trap, was used to simulate heat shocks that cells typically experience during measurements in optical traps. The results show that about 60% of the cells survived heat shocks without vital damage at temperatures of up to 58 ± 2°C for 0.5 s. By varying the duration of the heat shocks, it was shown that 60% of the cells stayed viable when exposed to 48 ± 2°C for 5 s.


Friday, June 24, 2011

A dual optical tweezer for microrheology of bacterial suspensions

Yogesha , Raghu, A. , Nagesh, B.V. , Bhattacharya, S. , Mohana, D.C. , Ananthamurthy, S.

A dual optical tweezer has been built around an inverted microscope with high numerical aperture objective (N.A 1.4). The setup is versatile and can be used both as a single and a dual tweezer, and in the dual mode, enables us to optically trap two micron-sized latex beads within a few microns from each other in solution. Using this setup, we report measurements of the microrheological parameters of Pseudomonas fluorescens and Bacillus subtilis bacterial suspensions. We study the variation of viscoelastic moduli of these bacterial suspensions as a function of their cell count in solution. A comparison with inactive bacteria of corresponding cell count enables us to characterize the activity of the bacterial samples in terms of an average force that the bacteria exerts on the trapped bead. This work paves way for studies of interesting nonlinear rheological phenomena at small length scales.


Tuesday, June 21, 2011

Biophysics of Malarial Parasite Exit from Infected Erythrocytes

Rajesh Chandramohanadas, YongKeun Park, Lena Lui, Ang Li, David Quinn, Kingsley Liew, Monica Diez-Silva, Yongjin Sung, Ming Dao, Chwee Teck Lim, Peter Rainer Preiser, Subra Suresh

Upon infection and development within human erythrocytes, P. falciparum induces alterations to the infected RBC morphology and bio-mechanical properties to eventually rupture the host cells through parasitic and host derived proteases of cysteine and serine families. We used previously reported broad-spectrum inhibitors (E64d, EGTA-AM and chymostatin) to inhibit these proteases and impede rupture to analyze mechanical signatures associated with parasite escape. Treatment of late-stage iRBCs with E64d and EGTA-AM prevented rupture, resulted in no major RBC cytoskeletal reconfiguration but altered schizont morphology followed by dramatic re-distribution of three-dimensional refractive index (3D-RI) within the iRBC. These phenotypes demonstrated several-fold increased iRBC membrane flickering. In contrast, chymostatin treatment showed no 3D-RI changes and caused elevated fluctuations solely within the parasitophorous vacuole. We show that E64d and EGTA-AM supported PV breakdown and the resulting elevated fluctuations followed non-Gaussian pattern that resulted from direct merozoite impingement against the iRBC membrane. Optical trapping experiments highlighted reduced deformability of the iRBC membranes upon rupture-arrest, more specifically in the treatments that facilitated PV breakdown. Taken together, our experiments provide novel mechanistic interpretations on the role of parasitophorous vacuole in maintaining the spherical schizont morphology, the impact of PV breakdown on iRBC membrane fluctuations leading to eventual parasite escape and the evolution of membrane stiffness properties of host cells in which merozoites were irreversibly trapped, recourse to protease inhibitors. These findings provide a comprehensive, previously unavailable, body of information on the combined effects of biochemical and biophysical factors on parasite egress from iRBCs.


Monday, June 20, 2011

Optical forces between coupled plasmonic nanoparticles near metal surfaces and negative index material waveguides

C. Van Vlack, Peijun Yao, and S. Hughes

We present a study of light-induced forces between two coupled plasmonic nanoparticles above various slab geometries including a metallic half-space and a negative index material (NIM) slab waveguide. We investigate optical forces by nonperturbatively calculating the scattered electric field via a Green function technique which includes the particle interactions to all orders. For excitation frequencies near the surface plasmon polariton and slow-light waveguide modes of the metal and NIM, respectively, we find rich light-induced forces and significant dynamical back-action effects. Optical quenching is found to be important in both metal and NIM planar geometries, which reduces the spatial range of the achievable interparticle forces. However, reducing the loss in the NIM allows radiation to propagate through the slow-light modes more efficiently, thus causing the light-induced forces to be more pronounced between the two plasmonic particles. To highlight the underlying mechanisms by which the particles couple, we connect our Green function calculations to various familiar quantities in quantum optics.


External forces control mitotic spindle positioning

Jenny Fink, Nicolas Carpi, Timo Betz, Angelique Bétard, Meriem Chebah, Ammar Azioune, Michel Bornens, Cecile Sykes, Luc Fetler, Damien Cuvelier & Matthieu Piel
The response of cells to forces is essential for tissue morphogenesis and homeostasis. This response has been extensively investigated in interphase cells, but it remains unclear how forces affect dividing cells. We used a combination of micro-manipulation tools on human dividing cells to address the role of physical parameters of the micro-environment in controlling the cell division axis, a key element of tissue morphogenesis. We found that forces applied on the cell body direct spindle orientation during mitosis. We further show that external constraints induce a polarization of dynamic subcortical actin structures that correlate with spindle movements. We propose that cells divide according to cues provided by their mechanical micro-environment, aligning daughter cells with the external force field.


Experimental validation of free-energy-landscape reconstruction from non-equilibrium single-molecule force spectroscopy measurements

Amar Nath Gupta, Abhilash Vincent, Krishna Neupane, Hao Yu, Feng Wang & Michael T. Woodside
Free-energy-landscape formalisms provide the fundamental conceptual framework for physical descriptions of how proteins and nucleic acids fold into specific three-dimensional structures. Although folding landscapes are difficult to measure experimentally, recent theoretical work by Hummer and Szabo has shown that landscape profiles can be reconstructed from non-equilibrium single-molecule force spectroscopy measurements using an extension of the Jarzynski equality. This method has been applied to simulations and experiments but never validated experimentally. We tested it using force–extension measurements on DNA hairpins with distinct, sequence-dependent folding landscapes. Quantitative agreement was found between the landscape profiles obtained from the non-equilibrium reconstruction and those from equilibrium probability distributions. We also tested the method on a riboswitch aptamer with three partially folded intermediate states, successfully reconstructing the landscape but finding some states difficult to resolve owing to low occupancy or overlap of the potential wells. These measurements validate the landscape-reconstruction method and provide a new test of non-equilibrium work relations.


Time-averaged quadratic functionals of a Gaussian process

Denis S. Grebenkov

The characterization of a stochastic process from its single random realization is a challenging problem for most single-particle tracking techniques which survey an individual trajectory of a tracer in a complex or viscoelastic medium. We consider two quadratic functionals of the trajectory: the time-averaged mean-square displacement (MSD) and the time-averaged squared root mean-square displacement (SRMS). For a large class of stochastic processes governed by the generalized Langevin equation with arbitrary frictional memory kernel and harmonic potential, the exact formulas for the mean and covariance of these functionals are derived. The formula for the mean value can be directly used for fitting experimental data, e.g., in optical tweezers microrheology. The formula for the variance (and covariance) allows one to estimate the intrinsic fluctuations of measured (or simulated) time-averaged MSD or SRMS for choosing the experimental setup appropriately. We show that the time-averaged SRMS has smaller fluctuations than the time-averaged MSD, in spite of much broader applications of the latter one. The theoretical results are successfully confirmed by Monte Carlo simulations of the Langevin dynamics. We conclude that the use of the time-averaged SRMS would result in a more accurate statistical analysis of individual trajectories and more reliable interpretation of experimental data.


Thursday, June 16, 2011

Chromatin under mechanical stress: from single 30 nm fibers to single nucleosomes

Jan Bednar, Stefan Dimitrov

About a decade ago, the elastic properties of a single chromatin fiber and, subsequently, those of a single nucleosome started to be explored using optical and magnetic tweezers. These techniques have allowed direct measurements of several essential physical parameters of individual nucleosomes and nucleosomal arrays, including the forces responsible for the maintenance of the structure of both the chromatin fiber and the individual nucleosomes, as well as the mechanism of their unwinding under mechanical stress. Experiments on the assembly of individual chromatin fibers have illustrated the complexity of the process and the key role of certain specific components. Nevertheless a substantial disparity exists in the data reported from various experiments. Chromatin, unlike naked DNA, is a system which is extremely sensitive to environmental conditions, and studies carried out under even slightly different conditions are difficult to compare directly. In this review we summarize the available data and their impact on our knowledge of both nucleosomal structure and the dynamics of nucleosome and chromatin fiber assembly and organization.


Pattern formation in colloidal explosions

A. V. Straube, A. A. Louis, J. Baumgartl, C. Bechinger and R. P. A. Dullens

We study the non-equilibrium pattern formation that emerges when magnetically repelling colloids, trapped by optical tweezers, are abruptly released, forming colloidal explosions. For multiple colloids in a single trap we observe a pattern of expanding concentric rings. For colloids individually trapped in a line, we observe explosions with a zigzag pattern that persists even when magnetic interactions are much weaker than those that break the linear symmetry in equilibrium. Theory and computer simulations quantitatively describe these phenomena both in and out of equilibrium. An analysis of the mode spectrum allows us to accurately quantify the non-harmonic nature of the optical traps. Colloidal explosions provide a new way to generate well-characterized non-equilibrium behaviour in colloidal systems.


A simple dynamic optical manipulation technique for label-free detection of biological cells

Yuquan Zhang, Xin Wang, Yijia Wang, Siwei Zhu, Bruce Z. Gao, and X.-C. Yuan

A dynamic optical tweezers system is employed for generation of an optical trap in continuous rotation for manipulating a biological cell in an aqueous solution. When the rotating speed is increased, the trapped cell experiences an augmented viscous drag force, and eventually it escapes from the trap at the critical rotating speed: when the drag force is greater than the trapping force. With experimental verifications, the method can easily be employed to differentiate cells in terms of trapping forces due to different refractive indices. The proposed method is a simple, robust, accurate and noninvasive label-free technique for cell detection.


Tuesday, June 14, 2011

Microfluidic array cytometer based on refractive optical tweezers for parallel trapping, imaging and sorting of individual cells

Michael Werner, Fabrice Merenda, Joachim Piguet, René-Paul Salathé and Horst Vogel

Analysis of genetic and functional variability in populations of living cells requires experimental techniques capable of monitoring cellular processes such as cell signaling of many single cells in parallel while offering the possibility to sort interesting cell phenotypes for further investigations. Although flow cytometry is able to sequentially probe and sort thousands of cells per second, dynamic processes cannot be experimentally accessed on single cells due to the sub-second sampling time. Cellular dynamics can be measured by image cytometry of surface-immobilized cells, however, cell sorting is complicated under these conditions due to cell attachment. We here developed a cytometric tool based on refractive multiple optical tweezers combined with microfluidics and optical microscopy. We demonstrate contact-free immobilization of more than 200 yeast cells into a high-density array of optical traps in a microfluidic chip. The cell array could be moved to specific locations of the chip enabling us to expose in a controlled manner the cells to reagents and to analyze the responses of individual cells in a highly parallel format using fluorescence microscopy. We further established a method to sort single cells within the microfluidic device using an additional steerable optical trap. Ratiometric fluorescence imaging of intracellular pH of trapped yeast cells allowed us on the one hand to measure the effect of the trapping laser on the cells' viability and on the other hand to probe the dynamic response of the cells upon glucose sensing.


Friday, June 10, 2011

Nanopatterning on rough surfaces using optically trapped microspheres

Y.-C. Tsai, R. Fardel, and C. B. Arnold

While nanofabricated structures find an increasingly large number of applications, few techniques are able to pattern rough or uneven surfaces, or surfaces with pre-existing structure. In this letter we show that optical trap assisted nanopatterning (OTAN), a near-field laser based technique, is able to produce nanoscale features on surfaces with large roughness but without the need for focus adjustment. Patterning on model surfaces of polyimide with vertical steps greater than 0.5 μm shows a high degree of uniformity, demonstrating that OTAN is a suitable technique to pattern nontraditional surfaces for emerging technologies.


Irreversibility-to-reversibility crossover in transient response of an optically trapped particle

Manas Khan and A. K. Sood

We study the transient response of a colloidal bead which is released from different heights and allowed to relax in the potential well of an optical trap. Depending on the initial potential energy, the system's time evolution shows dramatically different behaviors. Starting from the short-time reversible to long-time irreversible transition, a stationary reversible state with zero net dissipation can be achieved as the release point energy is decreased. If the system starts with even lower energy, it progressively extracts useful work from thermal noise and exhibits an anomalous irreversibility. In addition, we have verified the Transient Fluctuation Theorem and the Integrated Transient Fluctuation Theorem even for the non-ergodic descriptions of our system.


Negative light pressure force between two metal bodies separated by a subwavelength slit

V. Nesterov, L. Frumin and E. Podivilov

An explicit analytical expression of a light-induced attractive force between two macroscopic metal bodies or films separated by a subwavelength slit has been derived. The analytical expression obtained agrees well with the numerical calculations for the main TM mode. This force, which acts as a force with negative light pressure, arises by the interaction of plasmon-polaritons excited at the surface of metal when light propagates through the subwavelength slit. Estimations of this light-induced attractive force show that the force is sufficient to enable measurements and practical applications.


Small-world rheology: an introduction to probe-based active microrheology

Laurence G. Wilson and Wilson C. K. Poon

We introduce active, probe-based microrheological techniques for measuring the flow and deformation of complex fluids. These techniques are ideal for mechanical characterization either when little sample is available, or when samples show significant spatial heterogeneity. We review recent results, paying particular attention to comparing and contrasting rheological parameters obtained from micro- and macro-rheological techniques.


Particles nanomanipulation by the enhanced evanescent field through a near-field scanning optical microscopy probe

B.H. Liu, L.J. Yang, Y. Wang and J.L. Cui

A near-field scanning optical microscopy (NSOM) probe and a polarized semiconductor laser (808 nm, cw) were applied to push the trapping resolution down to 120 nm on near-field optical manipulation. A multi-circular shape with a minimum size of 400 nm consisting of 120 nm polystyrene particles can be obtained. They are at a resolution of d (d: NSOM probe tip diameter) and λ/7 (λ: laser wavelength), respectively. It is proved that sample concentration and laser power can affect feature size of trapping patterns. In this paper, the effect of trapping forces acted on a nanoparticle along three axis directions on trapping positions is studied, and different trapping positions are generated: the aperture edge in polarization direction and center surface of the probe tip. The result indicates that the single mode NSOM fiber probe is able to trap nanoparticles in a circular shape with lower laser intensity than that required by conventional optical tweezers. The simulated trapping positions around the probe tip based on the conservation law of momentum are found to agree well with experimental results.


Stimulation of human red blood cells leads to Ca2+-mediated intercellular adhesion

Patrick Steffen, Achim Jung, Duc Bach Nguyen, Torsten Müller, Ingolf Bernhardt, Lars Kaestner and Christian Wagner

Red blood cells (RBCs) are a major component of blood clots, which form physiologically as a response to injury or pathologically in thrombosis. The active participation of RBCs in thrombus solidification has been previously proposed but not yet experimentally proven. Holographic optical tweezers and single-cell force spectroscopy were used to study potential cell–cell adhesion between RBCs. Irreversible intercellular adhesion of RBCs could be induced by stimulation with lysophosphatidic acid (LPA), a compound known to be released by activated platelets. We identified Ca2+ as an essential player in the signaling cascade by directly inducing Ca2+ influx using A23187. Elevation of the internal Ca2+ concentration leads to an intercellular adhesion of RBCs similar to that induced by LPA stimulation. Using single-cell force spectroscopy, the adhesion of the RBCs was identified to be approximately 100 pN, a value large enough to be of significance inside a blood clot or in pathological situations like the vasco-occlusive crisis in sickle cell disease patients.


Tuesday, June 7, 2011

Optical Reorientation and Trapping of Nematic Liquid Crystals Leading to the Formation of Micrometer-Sized Domain

Anwar Usman, Takayuki Uwada, and Hiroshi Masuhara

We report on observation of optical reorientation of homogeneous nematic liquid crystals (LCs) thin slab of 4′-penthyl-4-cyanobiphenyl induced by a tightly focused near-infrared laser beam. We found that for laser beam with intensity higher than 56 MW/cm^2 the optical reorientation unusually transforms to a new metastable domain, which grows with laser irradiation time up to a dimension much larger than that of the focal spot. The optically reoriented LCs with refractive index mismatch compared to the surroundings can act as a micrometer-sized “ghost particle”, leading to the generation of optical trapping and manipulation of the optically reoriented LCs. By using confocal Raman microspectroscopy, we show that the dichroic ratio in the confined focal volume changes upon the domain formation with an increase in the total Raman intensity, indicating that microscopic depolarization leading to deorientation or nematic → isotropic phase transition and possible microscopic densification takes place by the tightly focused laser beam.


Trapping two types of particles using a focused partially coherent elegant Laguerre–Gaussian beam

Chengliang Zhao and Yangjian Cai

The radiation forces on a Rayleigh dielectric sphere induced by a focused partially coherent elegant Laguerre–Gaussian (ELG) beam are investigated by using the Rayleigh scattering theory. It is found that a focused partially coherent ELG beam with suitable mode orders can be used to trap a Rayleigh particle whose refractive index is larger or smaller than that of the ambient by varying its initial spatial coherence width. Therefore, one can use one optical–trap system to trap two types of particles with different refractive indices.


Surface imaging using holographic optical tweezers

D B Phillips, J A Grieve, S N Olof, S J Kocher, R Bowman, M J Padgett, M J Miles and D M Carberry

We present an imaging technique using an optically trapped cigar-shaped probe controlled using holographic optical tweezers. The probe is raster scanned over a surface, allowing an image to be taken in a manner analogous to scanning probe microscopy (SPM), with automatic closed loop feedback control provided by analysis of the probe position recorded using a high speed CMOS camera. The probe is held using two optical traps centred at least 10 µm from the ends, minimizing laser illumination of the tip, so reducing the chance of optical damage to delicate samples. The technique imparts less force on samples than contact SPM techniques, and allows highly curved and strongly scattering samples to be imaged, which present difficulties for imaging using photonic force microscopy. To calibrate our technique, we first image a known sample—the interface between two 8 µm polystyrene beads. We then demonstrate the advantages of this technique by imaging the surface of the soft alga Pseudopediastrum. The scattering force of our laser applied directly onto this sample is enough to remove it from the surface, but we can use our technique to image the algal surface with minimal disruption while it is alive, not adhered and in physiological conditions. The resolution is currently equivalent to confocal microscopy, but as our technique is not diffraction limited, there is scope for significant improvement by reducing the tip diameter and limiting the thermal motion of the probe.


Monday, June 6, 2011

Longitudinal Lorentz force on a subwavelength-diameter optical fiber

Huakang Yu, Wei Fang, Fuxing Gu, Min Qiu, Zongyin Yang, and Limin Tong

We analyze the longitudinal Lorentz forces that a propagating continuous-wave light exerts on a subwavelength-diameter optical fiber. Our theoretical results show that, during the propagating process, the guided light exerts no net time-averaged force on the fiber. Via numerical simulation, we find a significant overall pull force of 0.4 pN/mW acting on a 450-nm-diam fiber tip at a wavelength of 980 nm due to the scattering of the end face and a calculated force distribution reveals the feature of a near-field accumulation. Our results may be helpful to the configuration of optomechanical components or devices based on these fibers.


Thermal processes in red blood cells exposed to infrared laser tweezers (λ = 1064 nm)

Ilya Krasnikov, Alexey Seteikin, Ingolf Bernhardt

Continuous-wave laser micro-beams are generally used as diagnostic tools in laser scanning microscopes or in the case of near-infrared (NIR) micro-beams, as optical traps for cell manipulation and force characterization. Because single beam traps are created with objectives of high numerical aperture, typical trapping intensities and photon flux densities are in the order of 10^6 W/cm^2 and 10^3 cm^(–2)s^(–1), respectively. The main idea of our theoretical study was to investigate the thermal reaction of RBCs irradiated by laser micro-beam. The study is supported by the fact that many experiments have been carried out with RBCs in laser NIR tweezers. In the present work it has been identified that the laser affects a RBC with a density of absorbed energy at approximately 10^7 J/cm^3, which causes a temperature rise in the cell of about 7–12 °C.


Folding and unfolding of a triple-branch DNA molecule with four conformational states

Sandra Engel; Anna Alemany; Nuria Forns; Philipp Maass; Felix Ritort

Single-molecule experiments provide new insights into biological processes hitherto not accessible by measurements performed on bulk systems. We report on a study of the kinetics of a triple-branch DNA molecule with four conformational states by pulling experiments with optical tweezers and theoretical modelling. Three distinct force rips associated with different transitions between the conformational states are observed in the folding and unfolding trajectories. By applying transition rate theory to a free energy model of the molecule, probability distributions for the first rupture forces of the different transitions are calculated. Good agreement of the theoretical predictions with the experimental findings is achieved. Furthermore, due to our specific design of the molecule, we found a useful method to identify permanently frayed molecules by estimating the number of opened base-pairs from the measured force jump values.


Comparison of interparticle force measurement techniques using optical trapping

Timothy P. Koehler, Christopher M. Brotherton and Anne M. Grillet

Optical trapping has become a powerful and common tool for sensitive determination of electrostatic interactions between colloidal particles. Two optical trapping based techniques, blinking laser tweezers and direct force measurements, have become increasingly prevalent in investigations of interparticle potentials. The blinking laser tweezers method repeatedly catches and releases a pair of particles to gather physical statistics of particle trajectories. Statistical analysis is used to determine drift velocities, diffusion coefficients, and ultimately colloidal forces as a function of the center–center separation of the particles. Direct force measurements monitor the position of a particle relative to the center of an optical trap as the separation distance between two continuously trapped particles is gradually decreased. As the particles near each other, the displacement from the trap center for each particle increases proportional to the interparticle force. Although these techniques are commonly employed in the investigation of interactions of colloidal particles, there exists no direct comparison of these experimental methods in the literature. In this study, we compare measurements of interparticle forces applying both methods to a model system of polystyrene particles in an aerosol-OT (AOT) hexadecane solution where the screening lengths are very large. We found that the interaction forces measured using the two techniques compare quantitatively with each other and Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Additionally, our studies show that direct force measurements can be far more sensitive than previous studies have reported and nearly as sensitive as the blinking method.


Force measurements of the disruption of the nascent polypeptide chain from the ribosome by optical tweezers

Alexandros Katranidis, Wilfried Grange, Ramona Schlesinger, Theodora Choli-Papadopoulou, Dorothea Brüggemann, Martin Hegner and Georg Büldt

We show that optical tweezers are a valuable tool to study the co-translational folding of a nascent polypeptide chain at the ribosome in real-time. The aim of this study was to demonstrate that a stable and intact population of ribosomes can be tethered to polystyrene beads and that specific hook-ups to the nascent polypeptide chain by dsDNA handles, immobilized on a second bead, can be detected. A rupture force of the nascent chain in the range of 10–50 pN was measured, which demonstrates that the system is anchored to the surface in a stable and specific way. This will allow in numerous future applications to follow protein folding using much lower forces.


Three-dimensional arrays of submicron particles generated by a four-beam optical lattice

B. N. Slama-Eliau and G. Raithel

Using an optical lattice formed by four laser beams, we obtain three-dimensional light-induced crystals of 490-nm-diameter polystyrene spheres in solution. The setup yields face-centered orthorhombic optical crystals of a packing density of about 40%. An alignment procedure is developed in which the crystals are first prepared near a sample wall, and then in the bulk of the sample. A series of tests is performed that demonstrate particle trapping in all three dimensions. For one case, the trapping force is measured, and good agreement with a simple theoretical model is found. Possible applications are discussed.


Effect of the size and shape of a red blood cell on elastic light scattering properties at the single-cell level

Matti Kinnunen, Antti Kauppila, Artashes Karmenyan, and Risto Myllylä

We demonstrate the use of a double-beam optical tweezers system to stabilize red blood cell (RBC) orientation in the optical tweezers during measurements of elastic light scattering from the trapped cells in an angle range of 5-30 degrees. Another laser (He-Ne) was used to illuminate the cell and elastic light scattering distribution from the single cell was measured with a goniometer and a photomultiplier tube. Moreover, CCD camera images of RBCs with and without laser illumination are presented as complementary information. Light scattering from a RBC was measured in different fixed orientations. Light scattering from cells was also measured when the length of the cell was changed in two different orientations. Light scattering measurements from spherical and crenate RBCs are described and the results are compared with other cell orientations. Analysis shows that the measured elastic light scattering distributions reveal changes in the RBC’s orientation and shape. The effect of stretching on the changes in scattering is larger in the case of face-on incidence of He-Ne laser light than in rim-on incidence. The scattering patterns from RBCs in different orientations as well as from a spherical RBC were compared with numerical results found in literature. Good correlation was found.


Under-filling trapping objectives optimizes the use of the available laser power in optical tweezers

Mohammed Mahamdeh, Citlali Pérez Campos, and Erik Schäffer

For optical tweezers, especially when used in biological studies, optimizing the trapping efficiency reduces photo damage or enables the generation of larger trapping forces. One important, yet not-well understood, tuning parameter is how much the laser beam needs to be expanded before coupling it into the trapping objective. Here, we measured the trap stiffness for 0.5–2 μm-diameter microspheres for various beam expansions. We show that the highest overall trapping efficiency is achieved by slightly under-filling a high-numerical aperture objective when using microspheres with a diameter corresponding to about the trapping-laser wavelength in the medium. The optimal filling ratio for the lateral direction depended on the microsphere size, whereas for the axial direction it was nearly independent. Our findings are in agreement with Mie theory calculations and suggest that apart from the choice of the optimal microsphere size, slightly under-filling the objective is key for the optimal performance of an optical trap.


Wednesday, June 1, 2011

Light control of silver nanoparticle’s diffusion

Silvia Albaladejo, Manuel I. Marqués, and Juan José Sáenz

The diffusion of silver nanoparticles in water at 298K inside an optical vortex lattice is analyzed in detail by numerical simulations. At power densities of the order of those used to trap nanoparticles with optical tweezers, the dynamic response shows three different regimes depending on the light wavelength. In the first one particles get trapped inside the light vortices following almost closed trajectories. In the second one, around the plasmon resonance, the diffusion constant is dramatically enhanced with respect to the Brownian motion. In the third one, at longer wavelengths, nanoparticles are confined during a few seconds in quasi-one-dimensional optical traps.