# Above and beyond: Holographic tracking of axial displacements in holographic optical tweezers

## TL;DR

This paper reveals that colloidal particles in holographic optical tweezers move differently along the axial direction than the trap's programmed motion, with variations depending on particle properties, emphasizing the need for real-time feedback for precise 3D control.

## Contribution

It introduces a dipole-order theory to explain axial displacement variations and demonstrates holographic microscopy's capability for real-time feedback in optical trapping.

## Key findings

- Particles move farther along the axis than trap movement
- Different particles exhibit different axial displacements
- Holographic microscopy enables real-time feedback control

## Abstract

How far a particle moves along the optical axis in a holographic optical trap is not simply dictated by the programmed motion of the trap, but rather depends on an interplay of the trap's changing shape and the particle's material properties. For the particular case of colloidal spheres in optical tweezers, holographic video microscopy reveals that trapped particles tend to move farther along the axial direction than the traps that are moving them and that different kinds of particles move by different amounts. These surprising and sizeable variations in axial placement can be explained by a dipole-order theory for optical forces. Their discovery highlights the need for real-time feedback to achieve precise control of colloidal assemblies in three dimensions and demonstrates that holographic microscopy can meet that need.