Laser trapping of deformable objects

We report the trapping and manipulation of bubbles in viscous glass melts through the use of a laser. This phenomenon is observed in bubbles tens of micrometers in diameter under illumination by low numerical aperture (NA = 0.55). Once the bubble was centered on the optical axis, it was trapped and followed a lateral relocation of the laser beam. This phenomenon is explained by modifications of the bubble’s shape induced by axial heating and a decrease in surface tension. It is shown that formation of a concave dimple on the bubble’s front surface explains the observed laser trapping and manipulation. This mechanism of laser trapping is expected to take place in other deformable materials and can also be used to remove bubbles from melts or liquids. For this technique to be effective, the alteration of the bubble’s shape should be faster than its expulsion out of the laser’s point of focus

Laser manipulation based on a light-induced molecular reordering

We report on a novel principle of actuation of micrometer-sized liquid crystal droplets. It is based on a light-induced reordering of liquid crystal molecules inside the droplets. Polariscope imaging allowed to evaluate the birefringence change inside the micro-droplets. Directional actuation of the trapped droplet was achieved by cycling laser power with the direction defined by the polarization of the tweezing beam. Micro-actuation resulted from optically-induced birefringence; i.e., a nonlinear optical effect was utilized for mechanical manipulation of the micro-droplet. This principle of actuation can be used to induce molecular flows in sub-micrometer volumes

Laser manipulation and characterization of liquid crystal droplets

Abstract not reproduced here by request of the publisher. The text is available from http://dx.doi.org/10.1117/12.685058

Statics and dynamics of radial nematic liquid-crystal droplets manipulated by laser tweezers

Laser manipulation of trapped radial 4′-n-pentyl-4-cyanobiphenyl (5CB) nematic liquid-crystal droplets induced by molecular reordering is presented. We show experimentally that optical tweezers having linear, elliptical, or circular polarization can break the radial symmetry of the initial molecular organization inside a radial nematic droplet. Static distorted or twisted deformation modes and steady or unsteady nonlinear rotational dynamics are observed. Statics results are analyzed in terms of light-induced radial or left-right symmetry breaking effects associated with optical reorientation. The dynamical observations are compared with simulations from a slab analog model, for which orientational processes driven by optical nonlinearities can be accurately described. This study confirms that light-induced bulk reordering is an essential ingredient towards the understanding of the behavior of radial nematic liquid-crystal droplets in laser tweezers, as suggested by previous studies

Laser trapping of deformable objects

We report the trapping and manipulation of bubbles in viscous glass melts through the use of a laser. This phenomenon is observed in bubbles tens of micrometers in diameter under illumination by low numerical aperture (N = 0.55). Once the bubble was centered on the optical axis, it was trapped and followed a lateral relocation of the laser beam. This phenomenon is explained by modifications of the bubble's shape induced by axial heating and a decrease in surface tension. It is shown that formation of a concave dimple on the bubble's front surface explains the observed laser trapping and manipulation. This mechanism of laser trapping is expected to take place in other deformable materials and can also be used to remove bubbles from melts or liquids. For this technique to be effective, the alteration of the bubble's shape should be faster than its expulsion out of the laser's point of focus

Rheology measurement at liquid-crystal water interface using laser tweezers

We demonstrated that viscosity measurement using a spinning laser-trapped microsphere depends on the hydrophobicity or hydrophylicity of a spherical material. Reduction in viscous drag was observed in hydrophobic liquid crystal droplets. Rheology on a liquid crystal surface with D2O was demonstrated by spinning and stopping a laser tweezed droplet. The importance of nonslipping boundary conditions in viscosity measurements is discussed

Two-photon excitation of dye-doped liquid crystal by a CW-laser irradiation

We report on excitation of photoluminescence via two-photon absorption (2PA) in dye-doped liquid crystal droplets using tightly focused cw-laser illumination at 1064nm wavelength. The photoluminescence of the 2PA dye C40H54N2O2 (MBAPB) dispersed inside the 7CB liquid crystal host increases as square of the laser tweezers' irradiance. The 2PA cross-section of MBAPB was measured by femtosecond Z-scan method. The polarization and temperature dependence of the photoluminescence corroborated the presence on the dye's molecular alignment inside the nonpolar liquid crystal host. The molecular alignment via the host-dye interaction can be used for the laser manipulation of micro-objects (e.g., doped liquid crystal droplets) in micro-fluidic/mechanical applications

Optical Vortices from Liquid Crystal Droplets

We report on the generation of mono- and polychromatic optical phase singularities from micron-sized birefringent droplets. This is done experimentally by using liquid crystal droplets whose three dimensional architecture of the optical axis is controlled within the bulk by surfactant agents. Because of its microscopic size these optical vortex generators are optically trapped and manipulated at will, thus realizing a robust self-aligned micro-optical device for orbital angular momentum conversion. Experimental observations are supported by a simple model of optical spin-orbit coupling in uniaxial dielectrics that emphasizes the prominent role of the transverse optical anisotropy with respect to the beam propagation direction