Steerable dual-trap optical tweezers with confocal position detection using back-scattered light

Optical tweezers have emerged as a powerful tool in manipulating microscopic particles and in measuring weak forces of the order of a pico-Newton. As a result, it has found wide applications ranging from material science to biology. Dual-trap optical tweezers (DTOT) are of particular importance as they allow for two-point correlation measurements, such as in molecular force spectroscopy, two-point active micro-rheology, etc. Here, we report a design for a steerable DTOT setup that uses back-scattered light from the two traps for position detection. This is performed using a confocal scheme where the two detectors are placed at the conjugate points to the respective traps. This offers several significant advantages over current designs, such as zero cross-talk between signals, single module assembly, and robustness to thermal drift. Moreover, our design can be very easily integrated with standard microscopy techniques like phase contrast and differential interference contrast without modifying the microscope illumination unit.