Abstract
In modern 3D microscopy, holding and orienting arbitrary biological objects with optical forces instead of using coverslips and gel cylinders is still a vision. Although optical trapping forces are strong enough and related photodamage is acceptable, the precise (re-) orientation of large specimen with multiple optical traps is difficult, since they grab blindly at the object and often slip off. Here, we present an approach to localize and track regions with increased refractive index using several holographic optical traps with a single camera in an off-focus position. We estimate the 3D grabbing positions around several trapping foci in parallel through analysis of the beam deformations, which are continuously measured by defocused camera images of cellular structures inside cell clusters. Although non-blind optical trapping is still a vision, this is an important step towards fully computer-controlled orientation and feature-optimized laser scanning of sub-mm sized biological specimen for future 3D light microscopy.
Highlights
When we buy fruits in a super-market, we often grab, rotate and squeeze the product with our hands to investigate it from all sides
We examined whether the optical forces generated by holographic optical tweezers are strong enough to lift or rotate large objects
The motivation of this study was to evaluate a technical approach based on optical forces that allow to flexibly hold and rotate biological specimen of a few 100 μm in size for taking 3D images—without using coverslips or gel cylinders for object mounting
Summary
When we buy fruits in a super-market, we often grab, rotate and squeeze the product with our hands to investigate it from all sides. With tens of micrometers in diameter, have been trapped and oriented by using low focused, counter-propagating laser beams enabling a large working range in combination with acoustic forces and conventional imaging[18] or with light-sheet fluorescence imaging[19]. In all these approaches, highly precise 3D control in position and orientation has not been possible, mainly because of a missing feedback, called force clamp. This was partly achieved in early approaches with elongated particles (diatoms) with optimized video control[20], for a position clamp and single-axis rotation[21] or counter-propagating beams[22]
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