Friday, March 27, 2015

Dark-hollow optical beams with a controllable shape for optical trapping in air

A.P. Porfirev and R.V. Skidanov

A technique for generating dark-hollow optical beams (DHOBs) with a controllable cross-sectional intensity distribution is proposed and studied both theoretically and experimentally. Superimposed Bessel beams were used to generate such DHOBs. Variation of individual beam parameters enables the generation of Bessel-like non-diffracting beams. This technique allows the design of transmission functions for elements that shape both non-rotating and rotating DHOBs. We demonstrate photophoresis-based optical trapping and manipulation of absorbing air-borne nanoclusters with such beams.


Generation of microswimmers from passive Brownian particles in a spherically aberrated optical trap

Argha Mondal, Basudev Roy, and Ayan Banerjee

We induce spontaneous motion that is both directed and complex in micron-sized asymmetric Brownian particles in a spherically aberrated optical trap to generate microswimmers. The aberrated optical trap is prepared in a slightly modified optical tweezers configuration where we use a refractive index mismatched cover slip leading to the formation of an annular intensity distribution near the trap focal plane. Asymmetric scattering from a micro-particle trapped in this annular trap gives rise to a net tangential force on the particle causing it to revolve spontaneously in the intensity ring. The rate of revolution can be controlled from sub-Hz to a few Hz by changing the intensity of the trapping light. Theoretical simulations performed using finite-difference time-domain method verify the experimental observations. We also experimentally demonstrate simultaneous spin and revolution of a micro-swimmer which shows that complex motion can be achieved by designing a suitable shape of a micro-swimmer in the optical potential.


Thursday, March 26, 2015

Hamiltonian curl forces

M. V. Berry , Pragya Shukla

Newtonian forces depending only on position but which are non-conservative, i.e. whose curl is not zero, are termed ‘curl forces’. They are non-dissipative, but cannot be generated by a Hamiltonian of the familiar isotropic kinetic energy + scalar potential type. Nevertheless, a large class of such non-conservative forces (though not all) can be generated from Hamiltonians of a special type, in which kinetic energy is an anisotropic quadratic function of momentum. Examples include all linear curl forces, some azimuthal and radial forces, and some shear forces. Included are forces exerted on electrons in semiconductors, and on small particles by monochromatic light near an optical vortex. Curl forces imply restrictions on the geometry of periodic orbits, and non-conservation of Poincaré's integral invariant. Some fundamental questions remain, for example: how does curl dynamics generated by a Hamiltonian differ from dynamics under curl forces that are not Hamiltonian?


Optical transportation and controllable positioning of nanospheres using a microfiber

Yanjun Hu, Ying Li, Yonghe Deng and Ping Peng

We experimentally demonstrate an optical transportation and controllable positioning of polystyrene nanospheres using a 3 μm diameter microfiber. By placing the microfiber in a microfluidic channel and injecting a 980 nm laser light into the fiber, nanospheres suspended in the water were stably trapped to the microfiber and delivered along the direction of light propagation. Furthermore, by increasing the velocity of the fluid in the opposite direction of the laser light, it was found that, once the fluid velocity increased to 6 μm/s, spheres stopped their forward progress and halted on the microfiber, so the controllable positioning of spheres along the microfiber was realized.


Laser Trapping of Colloidal Metal Nanoparticles

Anni Lehmuskero, Peter Johansson, Halina Rubinsztein-Dunlop, Lianming Tong, and Mikael Käll

Optical trapping using focused laser beams (laser tweezers) has been proven extremely useful for contact-less manipulation of a variety of small objects, including biological cells, organelles within cells and a wide range of other dielectric micro/nano objects. Colloidal metal nanoparticles have drawn increasing attention in the field of optical trapping because of their unique interactions with electromagnetic radiation, caused by surface plasmon resonance effects, enabling a large number of nano-optical applications of high current interest. Here we try to give a comprehensive overview of the field of laser trapping and manipulation of metal nanoparticles based on results reported in the recent literature. We also discuss and describe the fundamentals of optical forces in the context of plasmonic nanoparticles, including effects of polarization, optical angular momentum and laser heating effects, as well as the various techniques that have been used to trap and manipulate metal nanoparticles. We conclude by suggesting possible directions for future research.


Wednesday, March 25, 2015

Divalent cations and molecular crowding buffers stabilize G-triplex at physiologically relevant temperatures

Hong-Xin Jiang, Yunxi Cui, Ting Zhao, Hai-Wei Fu, Deepak Koirala, Jibin Abraham Punnoose, De-Ming Kong & Hanbin Mao

G-triplexes are non-canonical DNA structures formed by G-rich sequences with three G-tracts. Putative G-triplex-forming sequences are expected to be more prevalent than putative G-quadruplex-forming sequences. However, the research on G-triplexes is rare. In this work, the effects of molecular crowding and several physiologically important metal ions on the formation and stability of G-triplexes were examined using a combination of circular dichroism, thermodynamics, optical tweezers and calorimetry techniques. We determined that molecular crowding conditions and cations, such as Na+, K+, Mg2+ and Ca2+, promote the formation of G-triplexes and stabilize these structures. Of these four metal cations, Ca2+ has the strongest stabilizing effect, followed by K+, Mg2+, and Na+ in a decreasing order. The binding of K+ to G-triplexes is accompanied by exothermic heats, and the binding of Ca2+ with G-triplexes is characterized by endothermic heats. G-triplexes formed from two G-triad layers are not stable at physiological temperatures; however, G-triplexes formed from three G-triads exhibit melting temperatures higher than 37°C, especially under the molecular crowding conditions and in the presence of K+ or Ca2+. These observations imply that stable G-triplexes may be formed under physiological conditions.


Radiative force on atoms from the view of photon emission

Zhuo Song, Yonggang Peng, and Yujun Zheng

In this Letter, we present a possible methodology to directly “read” the force on an atom via the photons emitted from the atom. In this methodology, the mean radiative force on an atom exerted by external fields can be expressed as a function of the average number of emitted photons 〈N〉 and its derivatives via the generating function approach developed by us recently.


From laser ultrasonics to optical manipulation

Tomaž Požar, Aleš Babnik, and Janez Možina
During the interaction of a laser pulse with the surface of a solid object, the object always gains momentum. The delivered force impulse is manifested as propulsion. Initially, the motion of the object is composed of elastic waves that carry and redistribute the acquired momentum as they propagate and reflect within the solid. Even though only ablation- and light-pressure-induced mechanical waves are involved in propulsion, they are always accompanied by the ubiquitous thermoelastic waves. This paper describes 1D elastodynamics of pulsed optical manipulation and presents two diametrical experimental observations of elastic waves generated in the confined ablation and in the radiation pressure regime.


Active diffusion positions the nucleus in mouse oocytes

Maria Almonacid, Wylie W. Ahmed, Matthias Bussonnier, Philippe Mailly, Timo Betz, Raphaël Voituriez, Nir S. Gov & Marie-Hélène Verlhac

In somatic cells, the position of the cell centroid is dictated by the centrosome. The centrosome is instrumental in nucleus positioning, the two structures being physically connected. Mouse oocytes have no centrosomes, yet harbour centrally located nuclei. We demonstrate how oocytes define their geometric centre in the absence of centrosomes. Using live imaging of oocytes, knockout for the ​formin 2 actin nucleator, with off-centred nuclei, together with optical trapping and modelling, we discover an unprecedented mode of nucleus positioning. We document how active diffusion of actin-coated vesicles, driven by ​myosin Vb, generates a pressure gradient and a propulsion force sufficient to move the oocyte nucleus. It promotes fluidization of the cytoplasm, contributing to nucleus directional movement towards the centre. Our results highlight the potential of active diffusion, a prominent source of intracellular transport, able to move large organelles such as nuclei, providing in vivo evidence of its biological function.


Nonlinear response and stability of a 2D rolling semi-cylinder during optical lift

Daniel G. Schuster Jr., Mario W. Gomes, Alexandra B. Artusio-Glimpse, Grover A. Swartzlander Jr.

In this paper, the response is found for a semi-cylindrical rod rocking on a level surface while subjected to forces from radiation pressure and gravity. Changes in the oscillation frequency of the rod as a function of light intensity are determined for both a mirrored and non-mirrored rod. The simulation results show that the equilibrium positions for the mirrored and non-mirrored rod exhibit a classic pitchfork and cusp catastrophe type bifurcation at critical laser intensities, respectively. By linearizing the systems equations of motion and sinusoidally modulating the laser intensity, the mathematical model for the rocking semi-cylinder could be transformed in the standard form of Mathieu’s equation. Inspired by the stability regions of the vertically oscillating inverted pendulum, a region of laser modulation parameters was determined, which could stabilize orientations of the lens which were unstable with constant laser intensity. Lastly, a comparison between the bifurcation point and change in natural frequency as functions of intensity between a previous analytical derivation and the full nonlinear model also showed that they agree closely for laser intensities near and below the critical intensity.