Conical diffraction of linearly polarised light controls the angular position of a microscopic object

D. P. O’Dwyer, C. F. Phelan, K. E. Ballantine, Y. P. Rakovich, J. G. Lunney, and J. F. Donegan

Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity profile. Rotation of the plane of polarisation produces the unique effect of spatially moving the crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them at any angular position on the ring. Continuous motion around the circle is also demonstrated. This crescent beam does not require an interferometeric arrangement to form it, nor does it carry optical angular momentum. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of light suggests that this optical process should find application in the manipulation and actuation of micro- and nano-scale physical and biological objects.

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Control of micro-cantilever using passive optical feedback for force microscopy

Hao Fu, Yong Liu, Jian Chen and Gengyu Cao

We demonstrate the use of bolometric force backaction in lever-based miniature Fabry–Pérot (FP) optical cavity for micro-cantilever control. In our experiment, low finesse FP microcavity is formed by polished fiber end and mass loaded micro-cantilever. We show that the dynamics of micro-cantilever can be deeply modified when photon intensity stored in the FP microcavity is sufficiently large. For blue microcavity detuning, we confirm this control mechanism can optimize the dynamics of micro-cantilever and dramatically reduce its Brownian motion amplitude without deteriorating force resolution. This effective and low-cost control method can be simply realized in most optical detection force microscopes.

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Optical orbital angular momentum from the curl of polarization

Xi-Lin Wang, Jing Chen, Yongnan Li, Jianping Ding, Cheng-Shan Guo, and Hui-Tian Wang

We predict a new category of optical orbital angular momentum that is associated with the curl of polarization and a kind of vector field with radial-variant hybrid states of polarization that can carry such novel optical orbital angular momentum. We present a scheme for creating the desired vector fields. Optical trapping experiments validate that the vector fields, which have no additional phase vortex, exert torques to drive the orbital motion of the trapped isotropic microspheres.

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A microfluidic system with optical laser tweezers to study mechanotransduction and focal adhesion recruitment

Peyman Honarmandi, Hyungsuk Lee, Matthew J. Lang and Roger D. Kamm

We present a new method to locally apply mechanical tensile and compressive force on single cells based on integration of a microfluidic device with an optical laser tweezers. This system can locate a single cell within customized wells exposing a square-like membrane segment to a functionalized bead. Beads are coated with extracellular matrix (ECM) proteins of interest (e.g. fibronectin) to activate specific membrane receptors (e.g.integrins). The functionalized beads are trapped and manipulated by optical tweezers to apply mechanical load on the ECM-integrin-cytoskeleton linkage. Activation of the receptor is visualized by accumulation of expressed fluorescent proteins. This platform facilitates isolation of single cells and excitation by tensile/compressive forces applied directly to the focal adhesion via specific membrane receptors. Protein assembly or recruitment in a focal adhesion can then be monitored and identified using fluorescent imaging. This platform is used to study the recruitment of vinculin upon the application of external tensile force to single endothelial cells. Vinculin appears to be recruited above the forced bead as an elliptical cloud, centered 2.1 ± 0.5 μm from the 2 μm bead center. The mechanical stiffness of the membrane patch inferred from this measurement is 42.9 ± 6.4 pN μm−1 for a 5 μm × 5 μm membrane segment. This method provides a foundation for further studies of mechanotransduction and tensile stiffness of single cells.

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Resolving interparticle position and optical forces along the axial direction using optical coherence gating

T. H. Chow, W. M. Lee, K. M. Tan, B. K. Ng, and C. J. R. Sheppard

In this paper, we demonstrate the use of coherence gating to resolve particle positions and forces in the axial direction. Through coherence gating, particle displacements and interparticle separations can be resolved with a high signal-to-noise ratio. We achieved both high depth resolvability (10−6 m) and weak optical force (10−15 N) measurements in an optical trapping system using a low coherence interferometry system. Trap stiffness as low as 1.46 fN μm−1 was measured. This technique is well-suited for the direct visualization of interparticle optical-mechanical interactions.

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Determining the binding mode and binding affinity constant of tyrosine kinase inhibitor PD153035 to DNA using optical tweezers

Chih-Ming Cheng, Wei-Ting Wang, Chien-Ting Hsu, Jing-Shin Tsai, Chien-Ming Wu and Tzu-Sen Yang
Accurately predicting binding affinity constant (KA) is highly required to determine the binding energetics of the driving forces in drug–DNA interactions. Recently, PD153035, brominated anilinoquinazoline, has been reported to be not only a highly selective inhibitor of epidermal growth factor receptor but also a DNA intercalator. Here, we use a dual-trap optical tweezers to determining KA for PD153035, where KA is determined from the changes in B-form contour length (L) of PD153035–DNA complex. Here, L is fitted using a modified wormlike chain model. We found that a noticeable increment in L in 1 mM sodium cacodylate was exhibited. Furthermore, our results showed that KA = 1.18(±0.09) × 104 (1/M) at 23 ± 0.5 °C and the minimum distance between adjacent bound PD153035 ≈ 11 bp. We anticipate that by using this approach we can determine the complete thermodynamic profiles due to the presence of DNA intercalators.

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Plasmon Hybridization Reveals the Interaction between Individual Colloidal Gold Nanoparticles Confined in an Optical Potential Well

Lianming Tong, Vladimir D. Miljković, Peter Johansson, and Mikael Käll

The understanding of interaction forces between nanoparticles in colloidal suspension is central to a wide range of novel applications and processes in science and industry. However, few methods are available for actual characterization of such forces at the single particle level. Here we demonstrate the first measurements of colloidal interactions between two individual diffusing nanoparticles using a colorimetric assay based on plasmon hybridization, that is, strong near-field coupling between localized surface plasmon resonances. The measurements are possible because individual gold nanoparticle pairs can be loosely confined in an optical potential well created by a laser tweezers. We quantify the degree of plasmon hybridization for a large number of individual particle pairs as a function of increasing salt concentration. The data reveal a considerable heterogeneity at the single particle level but the estimated average surface separations are in excellent agreements with predictions based on the classical theory of Derjaguin, Landau, Verwey, and Overbeek.

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On-the-fly cross flow laser guided separation of aerosol particles based on size, refractive index and density–theoretical analysis

A. A. Lall, A. Terray, and S. J. Hart

Laser separation of particles is achieved using forces resulting from the momentum exchange between particles and photons constituting the laser radiation. Particles can experience different optical forces depending on their size and/or optical properties, such as refractive index. Thus, particles can move at different speeds in the presence of an optical force, leading to spatial separations. In this paper, we present a theoretical analysis on laser separation of non-absorbing aerosol particles moving at speeds (1-10 cm/sec) which are several orders of magnitude greater than typical particle speeds used in previous studies in liquid medium. The calculations are presented for particle deflection by a loosely focused Gaussian 1064 nm laser, which simultaneously holds and deflects particles entrained in flow perpendicular to their direction of travel. The gradient force holds the particles against the viscous drag for a short period of time. The scattering force simultaneously pushes the particles, perpendicular to the flow, during this period. Our calculations show particle deflections of over 2500 µm for 15 µm aerosol particles, and a separation of over 1500 µm between 5 µm and 10 µm particles when the laser is operated at 10W. We show that a separation of about 421 µm can be achieved between two particles of the same size (10 µm) but having a refractive index difference of 0.1. Density based separations are also possible. Two 10 µm particles with a density difference of 600 kg/m3 can be separated by 193 µm. Examples are shown for separation distances between polystyrene, poly(methylmethacrylate), silica and water particles. These large laser guided deflections represent a novel achievement for optical separation in the gas phase.

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Nanomanipulation of single influenza virus using dielectrophoretic concentration and optical tweezers for single virus infection to a specific cell on a microfluidic chip

Hisataka Maruyama, Kyosuke Kotani, Taisuke Masuda, Ayae Honda, Tatsuro Takahata and Fumihito Arai
A major problem when analyzing bionanoparticles such as influenza viruses (approximately 100 nm in size) is the low sample concentrations. We developed a method for manipulating a single virus that employs optical tweezers in conjunction with dielectrophoretic (DEP) concentration of viruses on a microfluidic chip. A polydimethylsiloxane microfluidic chip can be used to stably manipulate a virus. The chip has separate sample and analysis chambers to enable quantitative analysis of the virus functions before and after it has infected a target cell. The DEP force in the sample chamber concentrates the virus and prevents it from adhering to the glass substrate. The concentrated virus is transported to the sample selection section where it is trapped by optical tweezers. The trapped virus is transported to the analysis chamber and it is brought into contact with the target cell to infect it. This paper describes the DEP virus concentration for single virus infection of a specific cell. We concentrated the influenza virus using the DEP force, transported a single virus, and made it contact a specific H292 cell.

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Out-of-equilibrium microrheology using optical tweezers to probe directional viscoelastic properties under shear

Manas Khan and A. K. Sood

Many wormlike micellar systems exhibit appreciable shear thinning due to shear-induced alignment. As the micelles get aligned introducing directionality in the system, the viscoelastic properties are no longer expected to be isotropic. An optical-tweezers–based active microrheology technique enables us to probe the out-of-equilibrium rheological properties of a wormlike micellar system simultaneously along two orthogonal directions —parallel to the applied shear, as well as perpendicular to it. While the displacements of a trapped bead in response to active drag force carry signature of conventional shear thinning, its spontaneous position fluctuations along the perpendicular direction manifest an orthogonal shear thickening, an effect hitherto unobserved.

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