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Soutenance de thèse

Mardi 20 octobre à 14h00,
Salle Nevill Mott, D420

Orateur : Jean-Baptiste Decombe
"Development and application of nanostructured fibers optical tweezer"

Abstract :

Optical tweezers allow to trap and manipulate objects without any mechanical contact with light and with an extreme accuracy. This non-invasive and non-destructive technique is of large interest in many scientific domains such as biophysics and medicine. Conventional optical tweezers use a laser beam which is strongly focalised by a microscope objective.

The use of optical fibers attracts increasing attention as highly flexible and compact tools for particle trapping. Fiber-based optical tweezers do not require bulky optics and require only little alignments.

In this context, the objective of this thesis was to develop a dual fiber nano-tip optical tweezers in order to trap particles with micro and nano-meter sizes. Our tweezers consist of two chemically etched optical fiber tips placed in front of each other with typical gaps from 20 nm to 20 μm. This dual contra-propagative beams configuration allow to cancel light radiation pressure. Efficient trapping can thus be obtained at relative low light intensities. Moreover, strong focusing is not required. Our device present an high flexibility due to in situ optimization and control of the fibre positions and individual particle manipulation without any substrate.

During our work, we experimentally demonstrated stable and reproducible trapping of one or several particles in suspension. Various dielectric particles were trapped, from one micrometer polystyrene beads to luminescent YAG:Ce3+ particles with diameters down to 60 nm. During this thesis, the latter were specifically elaborated and optimized for the optical trapping. We also measured optical forces applied to trapped particles by analyzing their residual Brownian motion. We showed the trapping potential is of harmonic shape, allowing to define its optical stiffness.

Finally, by modifying the emitted optical beam shape, we were able to improve specific tweezers characteristics. On one hand, nondiffracting quasi-Bessel beams allow us to get a stable trapping at large fiber-to-fiber distances. On the other hand, the use of metallised fiber tips allows to improve the beam confinement and enhance optical forces while reducing light intensity. We proved the near-field coupling between two metallised tips which were especially elaborated in this work. Those last results open promising perspectives for the development of plasmonic tweezers working in the near-field, which are especially well adapted for nano-particles trapping.

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