Enhancing the applied force and range of axial optical tweezers

Axial optical tweezers provide a natural geometry for performing biomechanical assays, such as rupture force measurements of protein binding. Axial traps, however, are typically weaker than their lateral counterparts and require high laser power to maintain a well-calibrated, linear restoring force. Here, we show how to extend the spatial range over which well-calibrated forces can be applied by considering aberration effects and extend the range of applied forces by accounting for the nonlinear response that appears when an optically trapped bead is moved far from the trap center. These refinements to the force calibration can be used to apply higher axial forces at reduced laser powers deeper into a sample. To illustrate the method, we reproduce both the linear extension regime and the overstretching transition observed in double-stranded DNA at significantly reduced laser powers.