Resonant optical trapping in microfluidic-integrated hollow photonic crystal cavities

The recent years have highlighted the potential of photonic nanostructures to extend the capabilities of optical tweezers and to perform advanced optical manipulations. In this context, two-dimensional photonic crystal cavities have been regarded as promising candidates owing to their high quality factors and low mode volumes. The work presented in this thesis reports the setting up of a complete experimental platform and the demonstration of optical trapping using the field confined in a planar photonic crystal cavity. In this work, cavities formed by a large circular defect, 700 nm in the diameter, in a triangular lattice are studied. These cavities possess a high quality factor (up to 7000 in air, around 2000 in water) and a central region where the confined field overlaps with the immersion medium. This feature offers a singular advantage for the maximization of the interaction between the cavity field and a particle as compared with standard photonic crystal cavity. An ultra-thin PDMS microfluidic membrane (≈170 μm) is developed for the reliable immersion of the hollow cavities and the control of the flow of nanoparticles to be trapped. The hollow cavities are then integrated within a microchannel within the membrane. The characterization and excitation of the cavities is then performed on the developed optical bench. Fluorescent imaging of the polystyrene particles is performed along with position tracking. The micro manipulation of the particles in the neighborhood of the cavity is realized through a combination of microfluidic valves and optical tweezers. Based on this experimental platform, the resonant optical trapping of 250 and 500 nm polystyrene particles is demonstrated. Trapping times of the order of several tens of minutes are obtained at sub-milliwatt optical powers. Further investigations are then performed on the perturbation of the cavity mode induced by the presence of a particle. This perturbation is experimentally observed in the form of a resonance wavelength shift of more than two cavity mode linewidth in the case of a 500 nm particle. Finally, the existence of back-action between the trapped particle and the cavity mode is experimentally evidenced. The later allows to observe the presence of two distinct trapping regimes. A qualitative understanding of these regimes is proposed based on a finite element method analysis.