Influence of time delay on trap stiffness in computer-controlled scanning optical tweezers

Abstract

In scanning optical tweezers, the positions of optically trapped particles are individually controlled by periodically updating the scan data, which make up the data set of time-shared positions of a single-beam optical trap. The scan data are usually generated from the computer and are then transferred to the controller that sends electrical signals to the scanner. During the data generation and transfer phases, a short but noticeable time delay, indicated by the updating of scan data, is incurred and the scanner momentarily stalls. In this paper, we investigate the influence of this time delay on the trapped beads in scanning optical tweezers having a low scanning frequency. To understand the influence of the time delay, we analyzed the motions of two beads, trapped by the scanning optical traps, having a finite distance between them. We found that the trapped beads undergo relatively large vibrations during scanning, and the trap stiffness for one of the two beads significantly decreases with the time delay. We anticipate that our observation is essential to design scanning optical traps that need to manipulate trapped beads in real-time.