Simultaneous three-dimensional tracking of individual signals from multi-trap optical tweezers using fast and accurate photodiode detection

Abstract

Multiple-beam optical traps facilitate advanced trapping geometries and exciting discoveries. However, the increased manipulation capabilities come at the price of more challenging position and force detection. Due to unrivaled bandwidth and resolution, photodiode based detection is preferred over camera based detection in most single/dual-beam optical traps assays. However, it has not been trivial to implement photodiode based detection for multiple-beam optical traps. Here, we present a simple and efficient method based on spatial filtering for parallel photodiode detection of multiple traps. The technique enables fast and accurate 3D force and distance detection of multiple objects simultaneously manipulated by multiple-beam optical tweezers.

Structure and dynamics of optically directed self-assembly of nanoparticles

“…4 . However, this not the case since the optical gradient force along the axial direction is the least among the three axes 24 25 . Due to the predominance of strong optical gradient force, all these trapped nanospheres try to come closer to the focal spot and hence form a three dimensional closed packed structure, which, would be a planer triangular cluster with three nanospheres sitting at the vertices of an equilateral triangle.…”

“…At the simplest level, it can be assumed that the focal spot is the potential energy minimum of the optical trap. Since the laser is linearly polarized along the x-axis, the gradient force along x, y and z axes are of the order: F x > F y > F z 24 25 . So two among three nanospheres would be placed along the x-axis and hence (c) and (d) configurations of Fig.…”

“…5 ). Since F x > F y > F z 24 25 , the short diagonal would be along the x-axis and the long diagonal would be along the y-axis. In case of the three-dimensional structure, the most closely packed structure would be triangular right pyramidal structure (panel (b) and (c) of Fig.…”

“…5 ). Orientation of the three-dimensional cluster would be: two nanospheres placed along the x-axis and the other two nanospheres placed along the y-axis as: F x > F y > F z 24 25 . Both panel (b) and (c) satisfies this criterion and hence both are equally probable.…”

High-Performance Image-Based Measurements of Biological Forces and Interactions in a Dual Optical Trap

“…The photodiode-based technique of back focal plane interferometry (BFPI) has been widely acknowledged as the only detection method suitable for attaining the highest level of optical trapping performance for biophysical studies [33][34][35] . This has persisted in the field, even after advances in camera technology have improved the precision and speed of image tracking, due to the inability of cameras to measure the relative displacement of a bead from its trap center with the same accuracy and speed as BFPI, particularly in the important case of mobile traps.…”