Axial Optical Traps: A New Direction for Optical Tweezers

Abstract

Optical tweezers have revolutionized our understanding of the microscopic world. Axial optical tweezers, which apply force to a surface-tethered molecule by directly moving either the trap or the stage along the laser beam axis, offer several potential benefits when studying a range of novel biophysical phenomena. This geometry, although it is conceptually straightforward, suffers from aberrations that result in variation of the trap stiffness when the distance between the microscope coverslip and the trap focus is being changed. Many standard techniques, such as back-focal-plane interferometry, are difficult to employ in this geometry due to back-scattered light between the bead and the coverslip, whereas the noise inherent in a surface-tethered assay can severely limit the resolution of an experiment. Because of these complications, precision force spectroscopy measurements have adapted alternative geometries such as the highly successful dumbbell traps. In recent years, however, most of the difficulties inherent in constructing a precision axial optical tweezers have been solved. This review article aims to inform the reader about recent progress in axial optical trapping, as well as the potential for these devices to perform innovative biophysical measurements.

Theory of optical-tweezers forces near a plane interface

“…Recent interest in axial optical trapping [39] considers a Hookean trapping force in a region around the beam focus. Our results shown in Fig.…”

Pinhole Optical Tweezers: Extending the Photobleaching Lifetime in the Presence of an Optical Trap by Wavefront Engineering

“…Here we introduce a new method that utilizes wavefront engineering to combine single-molecule fluorescence with axial optical tweezers 18 . By performing axial force spectroscopy with a carefully constructed Laguerre-Gausssian (LG) donut beam, a conical, 'dark' region of significantly lower intensity, infrared light is generated directly below the trapped microsphere.…”

Focused-Laguerre-Gaussian 3D-Trapping and Spanner at Low Numerical Aperture

“…Optical tweezers [1,8] with a single Gaussian beam for 3D trapping requires that the gradient force overcomes the scattering one, which needs a high numerical aperture microscope objective (typically NA>1) to satisfy. However, high NA brings a series of disadvantages such as large spherical aberrations, small field of view, high local heating of trapped sample and short working distance, crippling the potential for optical tweezers to perform relevant innovative biophysical measurements [9]. For the purpose of axial trapping with low NA objective lenses, different approaches have been demonstrated to date.…”

“…These contributions were significant, but more longer axial trapping range 3D trapping systems are needed for experiments, such as trapping objects at free liquid surface, reducing the influence of substrate while studying the nucleation of crystals and some other special processes. Our single optical trap of long axial trapping range and low-injury required power would be more convenient and flexible in some special biophysical experiments: the translocation of a protein, sensitivity study of protein-mediated DNA loops, single molecule fluorescence techniques [9,15,16]. In this letter, we demonstrate the stable 3D trapping and manipulation of polystyrene microspheres with diameter of 8m, using a single LG light beam focused through a microscope objective with low numerical aperture (NA=0.40), long working distance (WD=5.89mm) and low required trapping power (P=3.3mW).…”