Light structuring for massively parallel optical trapping

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.

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