Positional Imprinting of Optical Traps Corrects for Mechanical Drift in High-Resolution Instruments

Abstract

The performance of high-resolution optical traps typically depends on the passive mechanical stability of several critical optical components. Passive stability demands stringent requirements for the laboratory environment to limit instrumental drift arising from vibrations, acoustics, and temperature variations. To follow the real-time position of biological motors taking base pair steps along DNA, angstrom resolution is needed. Limiting the positional drift of an optical trap to one angstrom requires that a collimated beam be held angularly stable to 40 nanoradians. To minimize instrumental drift, our lab uses a dual-beam setup wherein two traps are used to perform measurements in solution, isolated from the microscope stage. Consequently, angular drift from optical elements interacting with both beams is correlated and therefore cancelled. Even in this dual-beam configuration, however, four optical elements can introduce relative angular drift between the beams, resulting in uncorrelated motions that are indistinguishable from single molecule activity. We have identified this necessary portion of all dual-beam instruments as the primary source of relative positional drift between the traps. Unfortunately, even stringent environmental controls are expected to be insufficient to prevent nanoradian-level motions of these optical components. We therefore seek instead to measure and correct for drift that inevitably occurs. Here, we present a new advance in trap design that enables us to correct for the relative positional instability of dual optical traps in real-time using a novel positional imprinting technique. We show that our new optical trap design is capable of virtually eliminating mechanical drift, consistently yielding noise characteristics under high-noise conditions that are comparable to the quietest conditions we can achieve in our laboratory. This technology will potentially reduce the engineering controls necessary for high-resolution optical trapping, making the technique available to research labs lacking specialized facilities.