Simultaneous detection of rotational and translational motion in optical tweezers by measurement of backscattered intensity

TL;DR

This paper presents a straightforward optical method to simultaneously measure both translational and rotational motions of microscopic particles in optical tweezers using backscattered intensity differences on a quadrant photodiode, enabling high-precision detection.

Contribution

The authors introduce a novel technique that accurately detects both translational and rotational motions simultaneously, improving measurement sensitivity and eliminating cross-talk.

Findings

Achieved detection sensitivity of around 50 mdeg for angular motion.

Successfully measured Brownian motion components in an unperturbed trap.

Demonstrated precise measurement of translation-rotation coupling in micro-objects.

Abstract

We describe a simple yet powerful technique of simultaneously measuring both translational and rotational motion of mesoscopic particles in optical tweezers by measuring the backscattered intensity on a quadrant photodiode (QPD). While the measurement of translational motion by taking the difference of the backscattered intensity incident on adjacent quadrants of a QPD is well-known, we demonstrate that rotational motion can be measured very precisely by taking the difference between the diagonal quadrants. The latter measurement eliminates the translational component entirely, and leads to a detection sensitivity of around 50 mdeg at S/N of 2 for angular motion of a driven micro-rod. The technique is also able to resolve the translational and rotational Brownian motion components of the micro-rod in an unperturbed trap, and can be very useful in measuring translation-rotation coupling of micro-objects induced by hydrodynamic interactions.