Planar Optical Tweezer Trap (2D-LOT) System Realized by Light Sheet Illumination & Orthogonal Widefield Detection

Abstract

We report the realization of the first planar optical tweezer trap system by a sheet of light. To visualize the trapping of the target object (dielectric bead or live cell) in a plane, an orthogonal widefield detection is employed. The planar / two-dimensional lightsheet optical tweezer (2D-LOT) sub-system is realized in an inverted microscopy mode with illumination from the bottom. A 1064 nm laser (power ∼ 500mW) is expanded and directed to a combination of cylindrical lens and high NA objective lens to generate a tightly-focused diffraction-limited light sheet. The object to be trapped is injected in the specimen chamber (consists of two coverslips placed at a distance of ≈ 1mm) using a syringe. The solution containing the objects stayed in the chamber due to the surface tension of the fluid. The illumination of trap-laser light is along Z-direction (with coverslip along XZ-plane) whereas, the detection is achieved perpendicular to the coverslip (along Y-axis). The orthogonal detection is employed to directly visualize the trapping in a plane. To better visualize the specimen, a separate white light illumination sub-system is used. The characterization of system PSF estimates the size of light sheet trap PSF to be, 2073.84µm2which defines the active trap region / area. Beads are tracked on their way to the trap region for determining the trap stiffness along Z and X i.e,kz= 1.13 ± 0.034pN/µmandkx= 0.74±0.021pN/µm. Results (image and video) show real-time trapping of dielectric beads in the trap zone (2D plane) generated by the light sheet. The beads can be seen getting trapped from all directions in the XZ-plane. Prolonged exposure to the light sheet builds up a 2D array of beads in the trap zone. Similar experiments on live NIH3T3 cells show cells trapped in the 2D trap. The potential of the planar trap lies in its ability to confine objects in two dimensions, thereby opening new kinds of experiments in biophysics, atomic physics, and optical physics.Statement of SignificanceThe ability to trap and confine objects in two dimensions / a plane is an incredible feat that paves the way for new experiments in physical and biological sciences. Currently, no technique can achieve two-dimensional trapping of objects. Such a trap is realized by a sheet of light generating a near-rectangular potential trap-zone, enabling tweezing confined to a plane. This is unlike existing point-focus-based tweezers that are capable of point-potential well. The technique is expected to have widespread applications in science and engineering.