Optical setup

The optical setup used in the experiments is shown in figures 1, 2 and 3. An Nd:YVO4 infrared laser (1064 nm) is located on an auxiliary shelf below the optical table to avoid noise from its cooling fans. The beam is brought to the desired traveling height by means of a periscope system (P). A half-wave plate (HWP) provides the right polarization to the beam incident on the spatial light modulator (SLM, Hamamatsu X10468-03), which generates steerable holographic traps. Two sets of telescopes (lenses L1 and L2, and L3 and L4) allow us to adjust the beam width to the size of the SLM display and to overfill the entrance pupil of the objective in which we generate the image of the modulator. We take advantage of the objective lens (Nikon Plan Apo 60x 1.2 NA, water-immersion and Nikon Plan Fluor 100x 1.3 NA, oil-immersion, phase contrast, Fig. 1) of the microscope (Nikon, Eclipse TE-2000E) to tightly focus the laser beam on the sample and to simultaneously observe the cells with a CCD camera (QImaging, QICAM). A dichroic mirror inserted in between, before the objective lens, redirects the beam so the same path can be used for both the laser and the illumination light coming from the condenser. The light passes through the mirror and reaches the CCD camera at the bottom of the microscope. Finally, the objective focuses the laser on a tiny spot at the specimen plane to generate the optical trap. By means of a user-friendly software we can dynamically create, move and remove traps to perform the experiments conveniently.