High-resolution dual-trap optical tweezers with differential detection: instrument design.

Optical traps or "optical tweezers" have become an indispensable tool in understanding fundamental biological processes. In this article, we present the design of a high-resolution optical tweezers, which is designed to minimize sensitivity to environmental noise. When the tether is attached to a stage, resolution is typically limited by drift and fluctuations in the relative position of the stage and trap-forming objective, cued by environmental noise. In a dual-trap design, however, the biological system is decoupled entirely from the stage by utilizing two optical traps to hold both ends of the tether. By holding each end of the molecule in an identical fashion, such dual-trap optical tweezers are less sensitive to drift and fluctuations of mechanical components. In our instrument, we form two traps from two orthogonally polarized beams generated by a single laser. As a result, many optical components and much of the optical path traveled are common to both trapping beams, thus minimizing differential movements of one optical trap relative to the other. Additionally, this economical use of optics creates an instrument that is simple and compact, reducing the number of elements susceptible to environmental fluctuations.