Johann Rohner

Miniaturized high-NA focusing-mirror multiple optical tweezers

An array of high numerical aperture parabolic micromirrors (NA = 0.96) is used to generate multiple optical tweezers and to trap micron-sized dielectric particles in three dimensions within a fluidic device. The array of micromirrors allows generating arbitrarily large numbers of 3D traps, since the whole trapping area is not restricted by the field-of-view of the high-NA microscope objectives used in traditional tweezers arrangements. Trapping efficiencies of Q(max) r approximately = 0.22, comparable to those of conventional tweezers, have been measured. Moreover, individual fluorescence light from all the trapped particles can be collected simultaneously with the high-NA of the micromirrors. This is demonstrated experimentally by capturing more than 100 fluorescent micro-beads in a fluidic environment. Micromirrors may easily be integrated in microfluidic devices, offering a simple and very efficient solution for miniaturized optical traps in lab-on-a-chip devices.

Escape trajectories of single-beam optically trapped micro-particles in a transverse fluid flow

We have studied the transverse and axial equilibrium positions of dielectric micro-spheres trapped in a single-beam gradient optical trap and exposed to an increasing fluid flow transverse to the trapping beam axis. It is demonstrated that the axial equilibrium position of a trapped micro-sphere is a function of its transverse position in the trapping beam. Moreover, although the applied drag-force acts perpendicularly to the beam axis, reaching a certain distance r(0) from the beam axis (r(0)/a approximately 0.6, a being the sphere radius) the particle escapes the trap due to a breaking axial equilibrium before the actual maximum transverse trapping force is reached. The comparison between a theoretical model and the measurements shows that neglecting these axial equilibrium considerations leads to a theoretical overestimation in the maximal optical transverse trapping forces of up to 50%.

Multiple optical trapping in high gradient interference fringes

In biological investigations, many protocols using optical trapping call for the possibility to trap a large number of particles simultaneously. Interference fringes provide a solution for massively parallel micro-manipulation of mesoscopic objects. Concurrently, the strength of traps can be improved by raising the slope of fringe profiles, such as to create intensity gradients much higher than the ones formed by sinusoidal fringes (Youngs fringes). We use a multiple-beam interference system, derived from the classical Fizeau configuration, with semitransparent interfaces to generate walls of light with a very high intensity gradient (Tolansky fringes). These fringes are formed into a trapping set-up to produce new types of trapping templates. The possibility to build multiple trap arrays of various geometries is examined; a high number of particles can be trapped in those potential wells. The period of the fringes can easily be changed in order to fit traps sizes to the dimensions of the confined objects. This is achieved by modifying several parameters of the interferometer, such as the angle and/or the distance between the beam-splitter and the mirror. It is well known that optical trapping presents a great potential when used in conjunction with microfluidics for lab-on-a-chip applications. We present an original solution for multiple trapping integrated in a microfluidic device. This solution does not require high numerical aperture objectives.

Assembling mesoscopic particles by various optical schemes

Shaping optical fields is the key issue in the control of optical forces that pilot the manipulation of mesoscopic polarizable dielectric particles. The latter can be positioned according to endless configurations. The scope of this paper is to review and discuss several unusual designs which produce what we think are among some of the most interesting arrangements. The simplest schemes result from interference between two or several coherent light beams, leading to periodic as well as pseudo-periodic arrays of optical traps. Complex assemblages of traps can be created with holographic-type set-ups; this case is widely used by the trapping community. Clusters of traps can also be configured through interferometric-type set-ups or by generating external standing waves by diffractive elements. The particularly remarkable possibilities of the Talbot effect to generate three-dimensional optical lattices and several schemes of self-organization represent further very interesting means for trapping. They will also be described and discussed. in this paper. The mechanisms involved in those trapping schemes do not require the use of high numerical aperture optics; by avoiding the need for bulky microscope objectives, they allow for more physical space around the trapping area to perform experiments. Moreover, very large regular arrays of traps can be manufactured, opening numerous possibilities for new applications.

Comparison between various types of multiple optical tweezers

Many types of optical tweezers arrays have been proposed and developed for use in conjunction with microfluidics for bio-chemical essays. Trap arrays rely on different methods allowing various degrees of flexibility and relative trapping efficiencies. Among the different techniques currently employed, it is not simple to distinguish which ones produce adequate performances for a given task in bio-chemistry. Experimental results for trapping efficiently diverse biological specimens allow distinguishing between the properties of optical trap arrays based on techniques as different as interferometry, holography, Fresnel or Fraunhoffer diffraction of diffractive structures, generalized phase contrast, microlens assemblies, micro-mirrors matrices, and also clusters of individual tweezers. The bulkiness of those systems is another important factor in the design of labs-on-a-chip; in particular the use of cumbersome microscope objectives can be detrimental to chip optimization. Arrangements of tweezers produced with different concepts should be compared in terms of efficiency, ease of use, and number of traps simultaneously exploitable

Micro-optics for optical trapping in microfluidics

Micro-optical components offer several possibilities for creating large matrices of optical traps, either when working on inverted microscopes, or by directly integrating miniaturized optical components at the level of a micro-fluidic chip. In this article we focus on two particular configurations, both allowing to generate large arrays of 3D optical traps. The first configuration takes advantage of an array of refractive microlenses to generate multiple optical tweezers within the focal plane of a high-NA microscope objective. The second configuration relies on an array of focusing high-NA micromirrors which are directly integrated at the level of a micro-fluidic chip. We also present measurements of the maximal optical trapping forces that can be reached with several types of cells commonly employed in biology and biotechnology, and demonstrate that these forces are essentially related to the bulk refractive index of the cells.