Force Clamping in Arbitrary Directions

Introduction

Ever since the discovery and description of optical trapping by Arthur Ashkin in the 1970s [1], this technique has been used in various applications from the well-defined manipulation of micrometer-sized particles and cells to the highly precise detection of forces and molecular dynamics in the course of biochemical processes. The underlying physics is the same for all applications: In an electromagnetic gradient, dielectric particles with a higher refractive index than the surrounding medium will experience a force pointing towards regions with higher light intensity. This phenomenon is the basic principle of optical tweezing where a tightly focused laser beam generates a three-dimensional potential well resulting in a restoring force on optically dense objects. These are thus 'trapped' in the center of the beam.

While simple manipulation and force measurements have been sufficient for many applications in the past, the growing interest, for example, in single molecule studies calls for new modes of trapping and tracking. These experiments include protein unfolding [2] or the precise tracking of molecular motors [3], [4].

In order to fulfill the high accuracy and stability requirements of these studies, the NanoTracker™ 2 system has been equipped with the ability to hold particles under defined forces in arbitrary directions within the focal spot over a specific period of time.