Highly efficient dual-fibre optical trapping with 3D printed diffractive Fresnel lenses

Highly efficient counter-propagating fibre-based optical traps are presented which utilize converging beams from fibres with 3D printed diffractive Fresnel lenses on their facet. The use of converging beam instead of diverging beam in dual fiber traps creates a strong trapping efficiency in both axial and transverse direction. Converging beams with a numerical aperture of up to 0.7 are produced by diffractive Fresnel lenses. These lenses also provide a large focal distance of up to 200um in a moderately high refractive index medium. Fabrication of such diffractive lenses with micro-sized features at the tip of a fibre is possible by femtosecond two photon lithography. In comparison to chemically etched fiber tips, the normalized trap stiffness of dual fiber tweezers is increased by an substantial factor of 35-50 when using converging beam produced by diffractive Fresnel lenses. The large focal length provided by these diffractive structures allows to work at large fibre-to-fibre distance which leads to la...