Tuesday, September 28, 2010

Structured light spots projected by a Dammann grating with high power efficiency and uniformity for optical sorting

Xiaodong Sun, Yuyang Sun, Jing Bu, Siwei Zhu, and X.-C. Yuan

We report a method for microfluidic multiple trapping and continuous sorting of microparticles using an optical potential landscape projected by a Dammann grating, enabling a high power-efficient approach to forming a composite two-dimensional spots array with high uniformity. The Dammann grating is fabricated in a photoresist by optical lithography. It is employed to create an optical lattice for multiple optical trapping and sorting in a mixture of polymer particles (n=1.59) and silica particles (n=1.42) with the same diameters of 3.1μm. In addition to the exponential selectivity by the projected optical landscapes, the proposed microfluidic sorting system has advantages in terms of high power efficiency and high uniformity due to the Dammann grating.


Stability Analysis of Particle Trapping in Time-Division Optical Tweezers by the Generalized Lorentz–Mie Theory

Johtaro Yamamoto and Toshiaki Iwai

Time-division multiplexing in the proposed holographic optical tweezers (HOT) has been used to quasi-simultaneously generate two different intensity patterns, a carrier beam spot and a beam array, by alternately feeding the corresponding hologram patterns to a spatial light modulator (SLM). Since the switching of the input holograms degrades the spatial stability of trapping a Brownian particle within the generated intensity spot area, it is necessary to numerically investigate the conditions in the time-division multiplexing for a particle to be stably trapped by a focused Gaussian beam. The Smoluchowski equation based on the generalized Lorentz–Mie theory (GLMT) model is evaluated numerically by an explicit method to estimate the relationship among particle size, switching rate, and focused laser beam power. Finally, the validity of the numerical analysis in this work is confirmed by experiments.


Friday, September 24, 2010

Optical tweezer for micro and nano scale rheology of biomaterials

Raghu A, Yogesha and Sharath Ananthamurthy

Since the first proposal and demonstration of an optical trap to hold micron sized particles by Ashkin in 1986, optical tweezer-based techniques for control and manipulation of particles has seen tremendous progress and enabled the exploration of the structure and dynamics of materials at the nano length scale. In biophysical applications, these techniques have enabled a range of activities from measurements of very small forces (at sub-pN levels) at operation within cells thus assisting in the elucidation of intracellular transport mechanisms, to obtaining elastic parameters of biomolecules. The techniques have helped in gaining insights on the viscoelastic properties at micro and nano length scales of a range of soft materials such as colloids, polymer melts, bio membranes etc.
The paper describes some of the studies carried out by us to understand polymer structure and elasticity of these materials in solution. The domains of "passive" and "active" rheology are explained with examples of studies undertaken to address these domains. The work using video microscopy to study silk fibroin solutions is used to exemplify the domain of passive rheology. We present recent work on bacterial solutions wherein live bacteria influence the optical trap region and thereby help us study the local viscoelasticity of such a medium.


Thursday, September 23, 2010

Light-controlled movement of isotropic droplets in smectic films

M. Conradi

This paper reports on optical trapping of micrometre-sized isotropic inclusions in free-standing smectic A* films. Droplet manipulation and trapping potential in such a two-dimensional anisotropic system show that optical trapping has two distinct regimes with unique separation dependence, governed by long-range and short-range trapping forces and enhanced diffusivity at the free surfaces. Molecular ordering in the surface layers of isotropic inclusions, at the liquid crystal-air interface, in addition leads to a new field of light-controlled particle dynamics. For low laser powers, translational motion of a droplet along the laser polarisation is observed. Above the threshold laser power, the transfer of optical angular momentum to the inclusion via linearly polarised light leads to circular-like motion. As the optical torque for a given intensity is counterbalanced by the elastic torque of the smectic film, this motion results in finite angle steps.


Wednesday, September 22, 2010

Optical trapping and propulsion of red blood cells on waveguide surfaces

Balpreet Singh Ahluwalia, Peter McCourt, Thomas Huser, and Olav Gaute Hellesø

We have studied optical trapping and propulsion of red blood cells in the evanescent field of optical waveguides. Cell propulsion is found to be highly dependent on the biological medium and serum proteins the cells are submerged in. Waveguides made of tantalum pentoxide are shown to be efficient for cell propulsion. An optical propulsion velocity of up to 
6 µm/s on a waveguide with a width of ~6 µm is reported. Stable optical trapping and propulsion of cells during transverse flow is also reported.


Friday, September 17, 2010

Optical manipulation and transport of microparticles on silicon nitride microring-resonator-based add–drop devices

Hong Cai and Andrew W. Poon

We demonstrate optical manipulation and transport of 1μm sized polystyrene particles on silicon nitride microring-resonator-based add–drop devices in an integrated optofluidic chip. By tuning the input laser wavelength and upon certain resonance quality factors, we observe microparticles (i) transported to the throughput port, (ii) routed to the microring and trapped in round trips, and (iii) transported to the drop port. We investigate the microparticle velocity at various laser wavelengths and in various spatial regions of the devices with different resonance quality factors. Such a device can act as a particle add–drop filter for “particle circuits” in lab-on-a-chip applications.


Thursday, September 16, 2010

Measurement of the atom number distribution in an optical tweezer using single-photon counting

A. Fuhrmanek, Y. R. P. Sortais, P. Grangier, and A. Browaeys 

We demonstrate in this paper a method to reconstruct the atom number distribution of a cloud containing a few tens of cold atoms. The atoms are first loaded from a magneto-optical trap into a microscopic optical dipole trap and then released in a resonant light probe where they undergo a Brownian motion and scatter photons. We count the number of photon events detected on an image intensifier. Using the response of our detection system to a single atom as a calibration, we extract the atom number distribution when the trap is loaded with more than one atom. The atom number distribution is found to be compatible with a Poisson distribution.


Scannable Plasmonic Trapping Using a Gold Stripe

Kai Wang, Ethan Schonbrun, Paul Steinvurzel and Kenneth B. Crozier

Using counterpropagating surface plasmon polaritons (SPPs) on a gold stripe, we demonstrate a scannable integrated optical tweezer. We demonstrate the trapping of individual fluorescent beads on the stripe, which supports a single quasi-transverse magnetic (TM) mode at the metal−water interface. The beads are localized to the stripe center, with a standard deviation of 51 nm transverse to the stripe, corresponding to a trap stiffness of 1.7 pN/μm. The localization along the stripe is achieved by balancing the scattering forces from the two counterpropagating SPPs excited by prism coupling. The particle position along the stripe can be controlled by varying the relative intensity of the two input beams. This work adds an important new capability to plasmonic optical tweezers, that of scanning. We anticipate that this will broaden the range of applications of plasmonic optical manipulation.


Dielectrophoretically assembled particles: feasibility for optofluidic systems

Khashayar Khoshmanesh, Chen Zhang, Jos L. Campbell, Aminuddin A. Kayani, Saeid Nahavandi, Arnan Mitchell and Kourosh Kalantar-zadeh

This work presents the dielectrophoretic manipulation of sub-micron particles suspended in water and the investigation of their optical responses using a microfluidic system. The particles are made of silica and have different diameters of 600, 450, and 250 nm. Experiments show a very interesting feature of the curved microelectrodes, in which the particles are pushed toward or away from the microchannel centerline depending on their levitation heights, which is further analyzed by numerical simulations. In doing so, applying an AC signal of 12 Vp–p and 5 MHz across the microelectrodes along with a flow rate of 1 μl/min within the microchannel leads to the formation of a tunable band of particles along the centerline. Experiments show that the 250 nm particles guide the longitudinal light along the microchannel due to their small scattering. This arrangement is employed to study the feasibility of developing an optofluidic system, which can be potentially used for the formation of particles-core/liquid-cladding optical waveguides.


Wednesday, September 15, 2010

Single-molecule derivation of salt dependent base-pair free energies in DNA

Josep M. Huguet, Cristiano V. Bizarro, Núria Forns, Steven B. Smith, Carlos Bustamante, and Felix Ritort
Accurate knowledge of the thermodynamic properties of nucleic acids is crucial to predicting their structure and stability. To date most measurements of base-pair free energies in DNA are obtained in thermal denaturation experiments, which depend on several assumptions. Here we report measurements of the DNA base-pair free energies based on a simplified system, the mechanical unzipping of single DNA molecules. By combining experimental data with a physical model and an optimization algorithm for analysis, we measure the 10 unique nearest-neighbor base-pair free energies with 0.1 kcal mol-1 precision over two orders of magnitude of monovalent salt concentration. We find an improved set of standard energy values compared with Unified Oligonucleotide energies and a unique set of 10 base-pair-specific salt-correction values. The latter are found to be strongest for AA/TT and weakest for CC/GG. Our unique energy values and salt corrections improve predictions of DNA unzipping forces and are fully compatible with melting temperatures for oligos. The method should make it possible to obtain free energies, enthalpies, and entropies in conditions not accessible by bulk methodologies.


Monday, September 13, 2010

Analysis of optical trapping and propulsion of Rayleigh particles using Airy beam

Hua Cheng, Weiping Zang, Wenyuan Zhou, and Jianguo Tian

The radiation forces and trajectories of Rayleigh dielectric particles induced by one-dimensional Airy beam were numerically analyzed. Results show that the Airy beam drags particles into the optical intensity peaks, and guides particles vertically along parabolic trajectories. Viscosity of surrounding medium significantly affects the trajectories. Random Brownian force affects the trajectories. Meanwhile, trapping potential depths and minimum trapping particle radii in different potential wells were also discussed. The trapping stability could be improved by increasing either the input peak intensity or the particle radius.


Analysis of transverse and longitudinal optical trap stiffness and its application in a novel measurement method of micro-particle refractive index

Jixiang Li, Xiaolin Zhang, Youli Yu, Wenjing Li, Peng Liu

Based on the model of the single TEM00 Gaussian beam optical tweezers, the transverse and longitudinal optical trapping forces of micro-particles are calculated by ray-optics theory. The similarities and differences of the longitudinal optical trap shapes, which are computed by two different methods, are analyzed. We studied the relation between the optical trap stiffness and the particle parameters which include the size and refractive index (RI), and found that the ratio of the transverse and longitudinal stiffness is a constant when the refractive indices of the particles are the same. Through the analysis, the results show that there is an important relationship between the optical stiffness and the product of particle RI and radius. With this relationship, we can obtain the particle RI by measuring the optical stiffness and the particle radius. This novel micro-particle RI measurement method is of great usefulness in atmospheric environmental science, polymer chemistry science, identification of mineral science and bio-medical science.


Optical trapping of particles at the air/water interface for studies in Langmuir monolayers

Andrea Gutiérrez-Campos and Rolando Castillo

Optical experimental devices allow to observe and manipulate several micron-sized objects, for instance, condensed phase domains formed in Langmuir monolayers at the air/water interface. In this paper, an experimental instrument designed to trap and manipulate those domains is presented. This instrument consists of optical tweezers, colloidal beads as handles and Brewster angle microscopy (BAM) to observe the trapped domains. With this instrument it was possible to trap and observe small domains (20 - 30 mu m) of nervonic acid monolayers.


Giant Optical Manipulation

Vladlen G. Shvedov, Andrei V. Rode, Yana V. Izdebskaya, Anton S. Desyatnikov, Wieslaw Krolikowski, and Yuri S. Kivshar

We demonstrate a new principle of optical trapping and manipulation increasing more than 1000 times the manipulation distance by harnessing strong thermal forces while suppressing their stochastic nature with optical vortex beams. Our approach expands optical manipulation of particles into a gas media and provides a full control over trapped particles, including the optical transport and pinpoint positioning of ∼100  μm objects over a meter-scale distance with ±10  μm accuracy.


Thursday, September 9, 2010

Forty Years of Optical Manipulation

David McGloin and J.P. Reid

This year, as the laser celebrates its 50th anniversary, a field that was made possible through laser technology reaches an important milestone as well. Over the past 40 years, optical manipulation research has deepened our understanding of physics and biology, and it has yielded the optical-tweezer technique that is used across all the sciences.


Transiently crosslinked F-actin bundles

Dan Strehle, Jörg Schnauß, Claus Heussinger, José Alvarado, Mark Bathe, Josef Käs and Brian Gentry

F-actin bundles are prominent cytoskeletal structures in eukaryotes. They provide mechanical stability in stereocilia, microvilli, filopodia, stress fibers and the sperm acrosome. Bundles are typically stabilized by a wide range of specific crosslinking proteins, most of which exhibit off-rates on the order of 1s−1. Yet F-actin bundles exhibit structural and mechanical integrity on time scales that are orders of magnitude longer. By applying large deformations to reconstituted F-actin bundles using optical tweezers, we provide direct evidence of their differential mechanical response in vitro: bundles exhibit fully reversible, elastic response on short time scales and irreversible, elasto-plastic response on time scales that are long compared to the characteristic crosslink dissociation time. Our measurements show a broad range of characteristic relaxation times for reconstituted F-actin bundles. This can be reconciled by considering that bundle relaxation behavior is also modulated by the number of filaments, crosslinking type and occupation number as well as the consideration of defects due to filament ends.


Momentum of Light in a Dielectric Medium

Peter W. Milonni and Robert W. Boyd

We review different expressions that have been proposed for the stress tensor and for the linear momentum of light in dielectric media, focusing on the Abraham and Minkowski forms. Analyses of simple models and consideration of available experimental results support the interpretation of the Abraham momentum as the kinetic momentum of the field, while the Minkowski momentum is the recoil momentum of absorbing or emitting guest atoms in a host dielectric. Momentum conservation requires consideration not only of the momentum of the field and of recoiling guest atoms, but also of the momentum the field imparts to the medium. Different model assumptions with respect to electrostriction and the dipole force lead to different expressions for this momentum. We summarize recent work on the definition of the canonical momentum for the field in a dielectric medium.


Metal particle manipulation by laser irradiation in borosilicate glass

Hirofumi Hidai, Takato Yamazaki, Sho Itoh, Kuniaki Hiromatsu, and Hitoshi Tokura

We propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. A 1-µm-thick platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. Platinum particles with diameters of 3 to 50 μm migrated at speeds up to 10 mm/s. In addition to platinum particles, nickel and austenitic stainless steel (SUS304) particles can be implanted.


Cell deformation cytometry using diode-bar optical stretchers

Ihab Sraj, Charles D. Eggleton, Ralph Jimenez, Erich Hoover and Jeff Squier

The measurement of cell elastic parameters using optical forces has great potential as a reagent-free method for cell classification, identification of phenotype, and detection of disease; however, the low throughput associated with the sequential isolation and probing of individual cells has significantly limited its utility and application. We demonstrate a single-beam, high-throughput method where optical forces are applied anisotropically to stretch swollen erythrocytes in microfluidic flow. We also present numerical simulations of model spherical elastic cells subjected to optical forces and show that dual, opposing optical traps are not required and that even a single linear trap can induce cell stretching, greatly simplifying experimental implementation. Last, we demonstrate how the elastic modulus of the cell can be determined from experimental measurements of the equilibrium deformation. This new optical approach has the potential to be readily integrated with other cytometric technologies and, with the capability of measuring cell populations, enabling true mechanical-property-based-cell cytometry.


Wednesday, September 8, 2010

Rapid construction of mechanically- confined multi- cellular structures using dendrimeric intercellular linker

Mo, XJ, Li, QS, Lui, LWY, Zheng, BX, Kang, CH, Nugraha, B, Yue, ZL, Jia, RR, Fu, HX, Choudhury, D, Arooz, T, Yan, J, Lim, CT, Shen, SL, Tan, CH , Yu, H

Tissue constructs that mimic the in vivo cell-cell and cell-matrix interactions are especially useful for applications involving the cell- dense and matrix- poor internal organs. Rapid and precise arrangement of cells into functional tissue constructs remains a challenge in tissue engineering. We demonstrate rapid assembly of C3A cells into multi- cell structures using a dendrimeric intercellular linker. The linker is composed of oleyl- polyethylene glycol (PEG) derivatives conjugated to a 16 arms- polypropylenimine hexadecaamine (DAB) dendrimer. The positively charged multivalent dendrimer concentrates the linker onto the negatively charged cell surface to facilitate efficient insertion of the hydrophobic oleyl groups into the cellular membrane. Bringing linker- treated cells into close proximity to each other via mechanical means such as centrifugation and micromanipulation enables their rapid assembly into multi- cellular structures within minutes. The cells exhibit high levels of viability, proliferation, three- dimensional (3D) cell morphology and other functions in the constructs. We constructed defined multi- cellular structures such as rings, sheets or branching rods that can serve as potential tissue building blocks to be further assembled into complex 3D tissue constructs for biomedical applications.


Monday, September 6, 2010

Volumetric multiple optical traps produced by Devil's lenses

W. D. Furlan, F. Gimenez, A. Calatayud, L. Remon, J. A. Monsoriu
We propose the use of a new diffractive optical element coined Devil's Vortex-Lens (DVL) to produce optical tweezers. In its more general form it results as the combination of a Devil’s lens and a helical vortex phase mask. It is shown that under monochromatic illumination a DVL generates a focal volume with several concatenated doughnut modes that are axially distributed according to the self-similarity of the lens. The orbital angular momentum associated to each link in the chain is investigated.


Particle jumps between optical traps in a one-dimensional (1D) optical lattice

Martin Šiler and Pavel Zemánek

We address the problem of stochastic particle transitions between stable positions in a one-dimensional (1D) periodic potential profile. With respect to experimental realization, such stable positions are represented by the optical traps formed in an evanescent standing wave. The behaviour of sub-micrometre-sized particles in this 'optical potential energy landscape' is analysed theoretically and experimentally, and the emphasis is put on particle jumps between neighbouring optical traps. Our theoretical model assumes overdamped stochastic motion of a particle in a finite-depth potential well. Subsequently, the mean first passage time is utilized to express the new quantity called the mean optical trap escape time (MOTET), which describes the mean time of the particle escape to a neighbouring stable position (optical trap). Theoretical predictions of the MOTET are compared with the Monte-Carlo simulations and with the experimental results for similar parameters of the potential energy profile. This comparison reveals that the properties of the optical traps (trap stiffness and depth) can be obtained from the analysis of the MOTET for the experimentally observed particle jumps only if high-speed video microscopy is used and the surface–particle distance is known.


Saturday, September 4, 2010

Light fields with an axially expanded intensity distribution for stable three-dimensional optical trapping

Susanne Zwick, Christian Schaub, Tobias Haist, and Wolfgang Osten

We introduce a new kind of light field to improve and simplify the trapping process of axially displaced particles. To this end we employ a light field with an axially expanded intensity distribution, which at the same time enables stable axial trapping. We present simulations of the axial intensity distribution of the novel trapping field and first experimental results, which demonstrate the improvement of the reliability of the axial trapping process. The method can be used to automate trapping of particles that are located outside of the focal plane of the microscope.


Thursday, September 2, 2010

Stochastic Simulations With Graphics Hardware: Characterization of Accuracy and Performance

Arvind Balijepalli, Thomas W. LeBrun, Satyandra K. Gupta

Methods to implement stochastic simulations on the graphics processing unit (GPU) have been developed. These algorithms are used in a simulationof microassembly and nanoassembly with optical tweezers, but are alsodirectly compatible with simulations of a wide variety of assemblytechniques using either electrophoretic, magnetic, or other trapping techniques. Significant speedup is possible for stochastic particle simulations when using the GPU, included in most personal computers (PCs), rather than the central processing unit (CPU) that handles most calculations. However, a careful analysis of the accuracy and precision when using the GPU in stochastic simulations is lacking and is addressed here. A stochastic simulation for spherical particles has been developed andmapped onto stages of the GPU hardware that provide the best performance. The results from the CPU and GPU implementation are then compared with each other and with well-established theory. The error in the mean ensemble energy and the diffusion constant is measured for both the CPU and the GPU implementations. The time taken to complete several simulation experiments on each platform has also been measured and the speedup attained by the GPU is then calculated.


Wednesday, September 1, 2010

Ion-Dependent Dynamics of DNA Ejections for Bacteriophage λ

David Wu, David Van Valen, Qicong Hu and Rob Phillips

We studied the control parameters that govern the dynamics of in vitro DNA ejection in bacteriophage λ. Previous work demonstrated that bacteriophage DNA is highly pressurized, and this pressure has been hypothesized to help drive DNA ejection. Ions influence this process by screening charges on DNA; however, a systematic variation of salt concentrations to explore these effects has not been undertaken. To study the nature of the forces driving DNA ejection, we performed in vitro measurements of DNA ejection in bulk and at the single-phage level. We present measurements on the dynamics of ejection and on the self-repulsion force driving ejection. We examine the role of ion concentration and identity in both measurements, and show that the charge of counterions is an important control parameter. These measurements show that the mobility of ejecting DNA is independent of ionic concentrations for a given amount of DNA in the capsid. We also present evidence that phage DNA forms loops during ejection, and confirm that this effect occurs using optical tweezers.