Abstract
In optical trapping systems the trap stiffness, or spring constant, deteriorates dramatically with trap depth due to optical aberrations and system misalignment. This can severely hamper studies that employ optical tweezers to make accurate quantitative measurements. Here, a deformable membrane mirror is used, in conjunction with a random search algorithm, to correct for these aberrations by optimizing on a merit factor that is directly proportional to the trap stiffness. Previous studies have sought to address this issue but none have used a merit factor that is directly proportional to the trap stiffness. We demonstrate that the lateral trap stiffness, measured with and without aberration correction at increasing depths, improves throughout the trapping range of a conventional trap and allows us to extend the maximum depth at which we can trap from 136 to 166 μm. At a depth of 131 μm, trap stiffness improved by factors of 4.37 and 3.31 for the x- and y-axes respectively. The aberration correction resulted in deformable membrane mirror shapes where a single shape could be applied throughout a wide range of trap depths, showing significant improvement, and had the added benefit of making the lateral trapping forces more uniform in x and y.
Highlights
The incorporation of adaptive optics (AO) in microscopy applications allows the user to compensate optical aberrations at large imaging depths and to restore resolution and signal strength to surface quality [1]
The first row shows the scatter plot of the Brownian motion of a particle which was trapped with the flat deformable membrane mirror (DMM) shape at depths of 44 μm, 88 μm and 131 μm respectively while the second row shows the scatter plot when the particle was trapped with a DMM shape that had been optimized for each respective depth
We have demonstrated the use of a DMM to increase the lateral trapping force of an optical trap
Summary
The incorporation of adaptive optics (AO) in microscopy applications allows the user to compensate optical aberrations at large imaging depths and to restore resolution and signal strength to surface quality [1]. The principle of adaptive optics involves generating a pre-shaped wavefront with equal but opposite distortion to those introduced by the system and the sample by means of an active element, for example a deformable membrane mirror (DMM) or spatial light modulator (SLM). Optical trapping (OT) is the confinement in three dimensions of microscopic particles through the forces exerted by the intensity gradients of a strongly focused laser beam [2, 3]. The net force is directed towards the highest intensity region of the beam. Light is manipulated and relatively noninvasive which makes optical traps especially suited for mechanical measurements of biological systems. Single molecule mechanical measurements using OT, including biological motor motility, protein–protein unbinding and protein unfolding, have attracted tremendous interest in recent years [4, 5]
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