Optimal trap velocity in a dynamic holographic optical trap using a nematic liquid crystal spatial light modulator

A dynamic holographic optical trap uses a dynamic diffractive optical element such as a liquid crystal spatial light modulator to realize one or more optical traps with independent controls. Such holographic optical traps provide a number of flexibilities and conveniences useful in various applications. One key requirement for such a trap is the ability to move the trapped microscopic object from one point to the other with the optimal velocity. In this paper we develop a nematic liquid crystal spatial light modulator based holographic optical trap and experimentally investigate the optimal velocity feasible for trapped beads of different sizes, in such a trap. Our results show that the achievable velocity of the trapped bead is a function of size of the bead, step size, interval between two steps and power carried by the laser beam. We observe that the refresh rate of a nematic liquid crystal spatial light modulator is sufficient to achieve an optimal velocity approaching the theoretical limit in the respective holographic trap for beads with radius larger than the wavelength of light.

Red blood cell in the field of a beam of optical tweezers

We consider the effect of a tightly focused laser beam with a wavelength of 1064 nm and a power from 10 to 160 mW on red blood cells during their optical trapping with optical tweezers. It is found that the shape of a red blood cell, which alters after optical trapping, ceases to change when the trapping duration is less than 5 min and the laser beam power is less than 60 mW. At a beam power above 80 mW, the red blood cell begins to fold at a trapping duration of about 1 min, and at powers above 100-150 mW, the red blood cell membrane ruptures in 1-3 min after optical trapping. It is also found that with repeated short-term capture of a red blood cell in an optical trap, the deformation properties of the membrane change: it becomes more rigid. The obtained results are important both for understanding the mechanisms of interaction of a laser beam with red blood cells and for optimising the technique of optical experiments, especially for measuring the deformation properties of a membrane using optical tweezers.

The story of the fibrin(ogen) αC-domains: evolution of our view on their structure and interactions

Although much has been established concerning the overall structure and function of fibrinogen, much less has been known about its two αC regions, each consisting of an αC-connector and αC-domain, but new information has been accumulating. This review summarizes the state of our current knowledge of the structure and interactions of fibrinogen’s αC regions. A series of studies with isolated αC regions and their fragments demonstrated that the αC-domain forms compact ordered structures consisting of N- and C-terminal sub-domains including β sheets and suggested that the αC-connector has a poly(L-proline) type II structure. Functionally, the αC-domains interact intramolecularly with each other and with the central region of the molecule, first demonstrated by electron microscopy and then quantified by optical trap force spectroscopy. Upon conversion of fibrinogen into fibrin, the αC-domains switch from intra- to intermolecular interactions to form ordered αC polymers. The formation of αC polymers occurs mainly through the homophilic interaction between the N-terminal sub-domains; interaction between the C-terminal sub-domains and the αC-connectors also contributes to this process. Considerable evidence supports the idea that the αC-regions accelerate fibrin polymerization and affect the final structure of fibrin clots. The interactions between αC-regions are important for the mechanical properties of clots, increasing their stiffness and extensibility. Conversion of fibrinogen into fibrin results in exposure of multiple binding sites in its αC regions, providing interaction of fibrin with different proteins and cell types during hemostasis and wound healing. This heretofore mysterious part of the fibrinogen molecule is finally giving up its secrets.

Numerical Analysis of Optical Trapping Force Affected by Lens Misalignments

Geometrical optics approximation is a classic method for calculating the optical trapping force on particles whose sizes are larger than the wavelength of the trapping light. In this study, the effect of the lens misalignment on optical force was analyzed in the geometrical optics regime. We used geometrical optics to analyze the influence of off-axis placement and the tilt of the lens on the trapping position and stiffness in an optical trap. Numerical calculation results showed that lens tilting has a greater impact on the optical trap force than the off-axis misalignments, and both misalignments will couple with each other and cause a shift of the equilibrium point and the asymmetry of the optical trap stiffness in different ways. Our research revealed the asymmetry in optical traps caused by lens misalignment and can provide guidance for optimize lens placement in future experiments.

The membrane-actin linker ezrin acts as a sliding anchor

Protein linkages to filamentous (F)-actin provide the cell membrane with mechanical resiliency and give rise to intricate membrane architectures. However, the actin cytoskeleton is highly dynamic, and undergoes rapid changes in shape during cell motility and other processes. The molecular mechanisms that underlie the mechanically robust yet fluid connection between the membrane and actin cytoskeleton remain poorly understood. Here, we used a single-molecule optical trap assay to examine how the prototypical membrane-actin linker ezrin acts to anchor F-actin to the cell membrane. Remarkably, we find that ezrin forms a complex that slides along F-actin over micron distances while resisting mechanical detachment. The ubiquity of ezrin and analogous proteins suggests that sliding anchors such as ezrin may constitute an important but overlooked element in the construction of the actin cytoskeleton.

Molecule-molecule and atom-molecule collisions with ultracold RbCs molecules

Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75\% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes or to laser-free decay pathways. For atom-molecule mixtures, RbCs+Rb and RbCs+Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom-molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.