Thursday, January 19, 2017

Rotation of single live mammalian cells using dynamic holographic optical tweezers

Bin Cao, Laimonas Kelbauska, Samantha Chan, Rishabh M. Shetty, Dean Smith, Deirdre R. Meldrum

We report on a method for rotating single mammalian cells about an axis perpendicular to the optical system axis through the imaging plane using dynamic holographic optical tweezers (HOTs). Two optical traps are created on the opposite edges of a mammalian cell and are continuously transitioned through the imaging plane along the circumference of the cell in opposite directions, thus providing the torque to rotate the cell in a controlled fashion. The method enables a complete 360° rotation of live single mammalian cells with spherical or near-to spherical shape in 3D space, and represents a useful tool suitable for the single cell analysis field, including tomographic imaging.


Mapping intracellular mechanics on micropatterned substrates

Kalpana Mandal, Atef Asnacios, Bruno Goud, and Jean-Baptiste Manneville

The mechanical properties of cells impact on their architecture, their migration, intracellular trafficking, and many other cellular functions and have been shown to be modified during cancer progression. We have developed an approach to map the intracellular mechanical properties of living cells by combining micropatterning and optical tweezers-based active microrheology. We optically trap micrometer-sized beads internalized in cells plated on crossbow-shaped adhesive micropatterns and track their displacement following a step displacement of the cell. The local intracellular complex shear modulus is measured from the relaxation of the bead position assuming that the intracellular microenvironment of the bead obeys power-law rheology. We also analyze the data with a standard viscoelastic model and compare with the power-law approach. We show that the shear modulus decreases from the cell center to the periphery and from the cell rear to the front along the polarity axis of the micropattern. We use a variety of inhibitors to quantify the spatial contribution of the cytoskeleton, intracellular membranes, and ATP-dependent active forces to intracellular mechanics and apply our technique to differentiate normal and cancer cells.


Extraordinary Optical Transmission: Fundamentals and Applications

Sergio G. Rodrigo; Fernando de León-Pérez; Luis Martín-Moreno

Extraordinary optical transmission (EOT) is a term that refers to electromagnetic resonances through sets of subwavelength apertures in either a flat or a corrugated metal film, providing a larger transmission of electromagnetic fields than would be expected from the small aperture size. Since its discovery in 1998, EOT has been a very active research field, leading both to the discovery of new ways of enhancing optical transmission and to its application to sensing, color filters, metamaterials, lenses, optical trapping, enhancement of nonlinear effects, among others. This paper reviews the different mechanisms that lead to EOT, paying special attention to the new research areas and applications that have appeared in the last few years.


Roadmap on structured light

Halina Rubinsztein-Dunlop, Andrew Forbes, M V Berry, M R Dennis, David L Andrews, Masud Mansuripur, Cornelia Denz, Christina Alpmann, Peter Banzer, Thomas Bauer, Ebrahim Karimi, Lorenzo Marrucci, Miles Padgett, Monika Ritsch-Marte, Natalia M Litchinitser, Nicholas P Bigelow, C Rosales-Guzmán, A Belmonte, J P Torres, Tyler W Neely, Mark Baker, Reuven Gordon, Alexander B Stilgoe, Jacquiline Romero, Andrew G White, Robert Fickler, Alan E Willner, Guodong Xie, Benjamin McMorran and Andrew M Weiner

Structured light refers to the generation and application of custom light fields. As the tools and technology to create and detect structured light have evolved, steadily the applications have begun to emerge. This roadmap touches on the key fields within structured light from the perspective of experts in those areas, providing insight into the current state and the challenges their respective fields face. Collectively the roadmap outlines the venerable nature of structured light research and the exciting prospects for the future that are yet to be realized.


Nonequilibrium dissipation in living oocytes

É. Fodor, W. W. Ahmed, M. Almonacid, M. Bussonnier, N. S. Gov, M.-H. Verlhac, T. Betz, P. Visco and F. van Wijland

Living organisms are inherently out-of-equilibrium systems. We employ recent developments in stochastic energetics and rely on a minimal microscopic model to predict the amount of mechanical energy dissipated by such dynamics. Our model includes complex rheological effects and nonequilibrium stochastic forces. By performing active microrheology and tracking micron-sized vesicles in the cytoplasm of living oocytes, we provide unprecedented measurements of the spectrum of dissipated energy. We show that our model is fully consistent with the experimental data, and we use it to offer predictions for the injection and dissipation energy scales involved in active fluctuations.


Wednesday, January 18, 2017

Optical dipole forces: Working together

Clarice D. Aiello

Strength lies in numbers and in teamwork: tens of thousands of artificial atoms tightly packed in a nanodiamond act cooperatively, enhancing the optical trapping forces beyond the expected classical bulk polarizability contribution.


Thermal fluctuation analysis of singly optically trapped spheres in hollow photonic crystal cavities

M. Tonin, F. M. Mor, L. Forró, S. Jeney, and R. Houdré

We report on the behaviour of singly optically trapped nanospheres inside a hollow, resonant photonic crystal cavity and measure experimentally the trapping constant using back-focal plane interferometry. We observe two trapping regimes arising from the back-action effect on the motion of the nanosphere in the optical cavity. The specific force profiles from these trapping regimes is measured.


Self-accelerating fan-shaped beams along arbitrary trajectories: a new tool for optical manipulation

Xiaolin Sui, Juanying Zhao, Bo Liu, Ziheng Yan, Changdong Cao and Shouhuan Zhou

We demonstrate, both theoretically and experimentally, a kind of fan-shaped optical beam propagating along the arbitrary trajectories (such as parabolic, hyperbolic and three-dimensional spiraling trajectories). With a controlled profile, this fan-shaped optical beam can be obtained from superposition of the Bessel-like beam and vortex Bessel-like beam. Also, the ability of guiding and transporting microparticles along its lobes is explored. These beams may find a variety of applications in optical trapping and manipulation.


Trapping and rotating of a metallic particle trimer with optical vortex

Z. Shen, L. Su, X.-C. Yuan, and Y.-C. Shen

We have experimentally observed the steady rotation of a mesoscopic size metallic particle trimer that is optically trapped by tightly focused circularly polarized optical vortex. Our theoretical analysis suggests that a large proportion of the radial scattering force pushes the metallic particles together, whilst the remaining portion provides the centripetal force necessary for the rotation. Furthermore, we have achieved the optical trapping and rotation of four dielectric particles with optical vortex. We found that, different from the metallic particles, instead of being pushed together by the radial scattering force, the dielectric particles are trapped just outside the maximum intensity ring of the focused field. The radial gradient force attracting the dielectric particles towards the maximum intensity ring provides the centripetal force for the rotation. The achieved steady rotation of the metallic particle trimer reported here may open up applications such as the micro-rotor.


Single-Molecule Force Spectroscopy Trajectories of a Single Protein and Its Polyproteins Are Equivalent: A Direct Experimental Validation Based on A Small Protein NuG2

Hai Lei, Dr. Chengzhi He, Prof. Dr. Chunguang Hu, Jinliang Li, Prof. Dr. Xiaodong Hu, Prof. Dr. Xiaotang Hu, Prof. Dr. Hongbin Li

Single-molecule force spectroscopy (SMFS) has become a powerful tool in investigating the mechanical unfolding/folding of proteins at the single-molecule level. Polyproteins made of tandem identical repeats have been widely used in atomic force microscopy (AFM)-based SMFS studies, where polyproteins not only serve as fingerprints to identify single-molecule stretching events, but may also improve statistics of data collection. However, the inherent assumption of such experiments is that all the domains in the polyprotein are equivalent and one SMFS trajectory of stretching a polyprotein made of n domains is equivalent to n trajectories of stretching a single domain. Such an assumption has not been validated experimentally. Using a small protein NuG2 and its polyprotein (NuG2)4 as model systems, here we use optical trapping (OT) to directly validate this assumption. Our results show that OT experiments on NuG2 and (NuG2)4 lead to identical parameters describing the unfolding and folding kinetics of NuG2, demonstrating that indeed stretching a polyprotein of NuG2 is equivalent to stretching single NuG2 in force spectroscopy experiments and thus validating the use of polyproteins in SMFS experiments.