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 (Young's 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.

Light structuring for massively parallel optical trapping

Johann Rohner

Optical trapping, discovered in the 70's, allows moving and stabilizing small objects which sizes varies from atoms to particles of several microns. This technique, based on momentum conservation, is particularly well suited for manipulating biological matter (cells, organelles, vesicles, functionalized particles, etc.) and offers interesting potentialities for research in biotechnologies and biochemistry. The possibility to individually immobilize large numbers of microscopic objects opens new ways for the downscaling of analysis tools for drug screening, particles sorting or assessing statistical data. The combination of optical trapping with microfluidics greatly increases the prospect of the method. This PhD work takes place in a research aiming at creating large arrays of optical traps compatible with microfluidic devices in order to realize so-called lab-on-a-chip. These miniaturized systems allow recreating at smaller time scale, reduced resources and lower cost, experiments usually performed in a macroscopic environment. This study proposes solutions based on light interference and on landscaping of light intensity. Setups combining several laser beams are proposed to create interference patterns and various configuration of light potential wells. Increasing the number of interfering beams, in particular by using a multiple beams interferometer (Fizeau-Tolansky interferometer) leads to a raise of the light intensity gradient, further increasing the trapping efficiency. The quality of the optical traps is studied and discussed in comparison with conventional laser tweezers. More complex and original solutions using interference of electromagnetic fields are suggested. Namely, the light diffracted by the objects themselves is used to form new potential wells. Diffractive structures are devised to generate three-dimensional arrays of traps. The periodicity of those planar structures creates a self-imaging phenomenon, known as Talbot effect. The modulation of the field in the Fresnel zone, i.e. some tens of micrometers behind the diffractive element, reveals interesting properties for optical trapping, in particular local intensity amplification and gradient enhancement. When several particles are simultaneously immersed in an electromagnetic field, interaction effects arise, that link the particles. This phenomenon of optical binding, is studied and demonstrated here in the case of bidimensional optical crystals.

EPFL2007

Building optical matter with binding and trapping forces

Gerben Boer, Guy Delacrétaz, Jean-Marc Fournier, Johann Rohner, René Salathé

Very high frequency oscillations of intense light fields interact with micron-size dielectric objects to exert dc optical forces that allow polarizable particles to levitate, to be trapped and to be bound. Such optical forces are also suitable to arrange cold atoms in optical lattices. Various assemblages of optical traps, including periodic arrays, can be constructed either with independent lasers, or with a single laser beam split into different parts later recombined by interference, as well as through the use of diffractive elements. These optical-well arrays serve as templates for writing and erasing dynamic two-dimensional and three-dimensional "optical crystals", composed of mono-dispersed polystyrene spheres in water. Subsequently, the crystals become diffractive structures themselves. The association of micro-fluidics and optical trapping allows for the formation of optical traps into micro-channels. This leads to perform microchemistry experiments, such as fluorescence detection, on individual bodies attached to trapped particles. Self-trapping due to the optical binding force relates to the interaction between different dielectric objects located in an electromagnetic field; each one reacts not only to the field of the incident beam, but also to the induced fields radiated coherently by all other particles. Optical binding strongly influences the equilibrium state and the behavior of optical crystals. It must have the potential for creating collective effects.

2004

Optical Mirror from Laser-Trapped Mesoscopic Particles

Jean-Marc Fournier, Johann Rohner

Trapping of mesoscopic particles by optical forces usually relies on the gradient force, whereby particles are attracted into optical wells formed by landscaping the intensity of an optical field. This is most often achieved by optical Gaussian beams, interference patterns, general phase contrast methods, or other mechanisms. Hence, although the simultaneous trapping of several hundreds of particles can be achieved, these particles remain mostly independent with negligible interaction. Optical matter, however, relies on close packing and binding forces, with fundamentally different electrodynamic properties. In this Letter, we build ensembles of optically bound particles to realize a reflective surface that can be used to image an object or to focus a light beam. To our knowledge, this is the first experimental proof of the creation of a mirror by optical matter, and represents an important step toward the realization of a laser-trapped mirror (LTM) in space. From a theoretical point of view, optically bound close packing requires an exact solver of Maxwell's equations in order to precisely compute the field scattered by the collection of particles. Such rigorous calculations have been developed and are used here to study the focusing and resolving power of an LTM.

Amer Physical Soc2014

Light structuring for massively parallel optical trapping

Johann Rohner

EPFL2007