Optical macro-tweezers: trapping of highly motile micro-organisms

Abstract

Optical micromanipulation stands for contact-free handling of microscopic particles bylight. Optical forces can manipulate non-absorbing objects in a large range of sizes,e.g., from biological cells down to cold atoms. Recently much progress has beenmade going from the micro- down to the nanoscale. Less attention has been paidto going the other way, trapping increasingly large particles. Optical tweezerstypically employ a single laser beam tightly focused by a microscope objectiveof high numerical aperture to stably trap a particle in three dimensions (3D).As the particle size increases, stable 3D trapping in a single-beam trap requiresscaling up the optical power, which eventually induces adverse biological effects.Moreover, the restricted field of view of standard optical tweezers, dictated by the useof high NA objectives, is particularly unfavorable for catching actively movingspecimens. Both problems can be overcome by traps with counter-propagating beams.Our ‘macro-tweezers’ are especially designed to trap highly motile organisms, asthey enable three-dimensional all-optical trapping and guiding in a volume of2 × 1 × 2 mm3. Here we report for the first time the optical trapping of large actively swimmingorganisms, such as for instance Euglena protists and dinoflagellates of up to70 µm length. Adverse bio-effects are kept low since trapping occurs outside high intensity regions, e.g.,focal spots. We expect our approach to open various possibilities in the contact-free handling of 50–100 µm sized objects that could hitherto not be envisaged, for instance all-optical holding ofindividual micro-organisms for taxonomic identification, selective collecting or tagging.