Fashioning microscopic tools

By moulding optical fields, holographic optical tweezers are able to generate structured force fields with magnitudes and length scales of great utility for experiments in soft matter and biological physics. However, optically induced force fields are determined not only by the incident optical field, but by the shape and composition of the particles involved [1]. Indeed, there are desirable but simple attributes of a force field, such as orientational control, that cannot be introduced by sculpting optical fields alone. With this in mind, we show how relationships between force, displacement and function can be controlled by optimizing particle shapes. We experimentally demonstrate this with two example structures, fabricated using two photon polymerisation. In each case the geometry of the particle has been specifically designed to aid function: Firstly, we show a non-spherical probe with cylindrical optical trapping handles (Fig. 1a & 1b), enabling the imaging of surface topography with nanometre precision whilst applying ultra-low, femto-Newton sized forces [2]. The use of an extended structure has the additional advantages of ensuring that the trapping beams and tracking points remain removed from the sample under investigation, minimising erroneous tracking signals and potential photo-damage to the sample surface [3]. Secondly, we exhibit a passive force clamp requiring no optical feedback (and therefore acting as a broadband constant force optical spring), exploiting the effect of tapered trapping handles (Fig. 1c & 1d) [4].